HYPOID GEAR DESIGN METHOD AND HYPOID GEAR

Abstract

A degree of freedom of a hypoid gear is improved. An instantaneous axis in a relative rotation of a gear axis and a pinion axis, a line of centers, an intersection between the instantaneous axis and the line of centers, and an inclination angle of the instantaneous axis with respect to the rotation axis of the gear are calculated based on a shaft angle, an offset, and a gear ratio of a hypoid gear. Based on these variables, base coordinate systems are determined, and the specifications are calculated using these coordinate systems. For the spiral angles, pitch cone angles, and reference circle radii of the gear and pinion, one of the values for the gear and the pinion is set and a design reference point is calculated. Based on the design reference point and a contact normal of the gear, specifications are calculated. The pitch cone angle of the gear or the pinion can be freely selected.

Description

TECHNICAL FIELD

The present invention relates to a method of designing a hypoid gear.

BACKGROUND ART

A design method of a hypoid gear is described in Ernest Wildhaber, Basic Relationship of Hypoid Gears, American Machinist, USA, Feb. 14, 1946, p. 108-111 and in Ernest Wildhaber, Basic Relationship of Hypoid Gears II, American Machinist, USA, Feb. 28, 1946, p. 131-134. In these references, a system of eight equations is set and solved (for cone specifications that contact each other) by setting a spiral angle of a pinion and an equation of a radius of curvature of a tooth trace, in order to solve seven equations with nine variables which are obtained by setting, as design conditions, a shaft angle, an offset, a number of teeth, and a ring gear radius. Because of this, the cone specifications such as the pitch cone angle Γ_gw depend on the radius of curvature of the tooth trace, and cannot be arbitrarily determined.

In addition, in the theory of gears in the related art, a tooth trace is defined as “an intersection between a tooth surface and a pitch surface”. However, in the theory of the related art, there is no common geometric definition of a pitch surface for all kinds of gears. Therefore, there is no common definition of the tooth trace and of contact ratio of the tooth trace for various gears from cylindrical gears to hypoid gears. In particular, in gears other than the cylindrical gear and a bevel gear, the tooth trace is not clear.

In the related art, the contact ratio m_f of tooth trace is defined by the following equation for all gears.

m _f =F tan*ψ₀/p

where, p represents the circular pitch, F represents an effective face width, and ψ₀ represents a spiral angle.

Table 1 shows an example calculation of a hypoid gear according to the Gleason method. As shown in this example, in the Gleason design method, the tooth trace contact ratios are equal for a drive-side tooth surface and for a coast-side tooth surface. This can be expected because of the calculation of the spiral angle ψ₀ as a virtual spiral bevel gear with ψ₀=(ψ_pw+ψ_gw)/2 (refer to FIG. 9).

The present inventors, on the other hand, proposed in Japanese Patent No. 3484879 a method for uniformly describing the tooth surface of a pair of gears. In other word, a method for describing a tooth surface has been shown which can uniformly be used in various situations from a pair of gears having parallel axes, which is the most widely used configuration, to a pair of gears whose axes do not intersect and are not parallel with each other (skew position).

There is a desire to determine the cone specifications independent from the radius of curvature of the tooth trace, and to increase the degree of freedom of the design.

In addition, in a hypoid gear, the contact ratio and the transmission error based on the calculation method of the related art are not necessarily correlated to each other. Of the contact ratios of the related art, the tooth trace contact ratio has the same value between the drive-side and the coast-side, and thus the theoretical basis is brought into question.

An advantage of the present invention is that a hypoid gear design method is provided which uses the uniform describing method of the tooth surface described in JP 3484879, and which has a high degree of freedom of design.

Another advantage of the present invention is that a hypoid gear design method is provided in which a design reference body of revolution (pitch surface) which can be applied to the hypoid gear, the tooth trace, and the tooth trace contact ratio are newly defined using the uniform describing method of the tooth surface described in JP 3484879, and the newly defined tooth trace contact ratio is set as a design index.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a design method of a hypoid gear wherein an instantaneous axis S which is an axis of a relative angular velocity of a first gear and a second gear, a line of centers v_c which is common to rotation axes of the first gear and the second gear, an intersection C_s between the instantaneous axis S and the line of centers v_c, and an inclination angle Γ_s of the instantaneous axis S with respect to the rotation axis of the second gear are calculated based on a shaft angle Σ, an offset E, and a gear ratio i_o of a hypoid gear, basic coordinate systems C₁, C₂, and C_s are determined from these variables, and specifications are calculated based on the coordinate systems. In particular, the specifications are calculated by setting a common point of contact of pitch cones of the first gear and the second gear as a design reference point P_w.

When an arbitrary point (design reference point) P_w is set in a static space, six cone specifications which are in contact at the point P_w are represented by coordinates (u_cw, v_ow, z_cw) of the point P_w based on a plane (pitch plane) S_t defined by a peripheral velocity V_1w and a peripheral velocity V_2w at the point P_w and a relative velocity V_rsw. Here, the cone specifications refer to reference circle radii R_1w and R_2w of the first gear and the second gear, spiral angles ψ_pw and ψ_gw of the first gear and the second gear, and pitch cone angles γ_pw and Γ_gw of the first gear and the second gear. When three of these cone specifications are set, the point P_w is set, and thus the remaining three variables are also set. In other words, in various aspects of the present invention, the specifications of cones which contact each other are determined based merely on the position of the point P_w regardless of the radius of curvature of the tooth trace.

Therefore, it is possible to set a predetermined performance as a design target function, and select the cone specifications which satisfy the target function with a high degree of freedom. Examples of the design target function include, for example, a sliding speed of the tooth surface, strength of the tooth, and the contact ratio. The performance related to the design target function is calculated while the cone specification, for example the pitch cone angle Γ_gw, is changed, and the cone specification is changed and a suitable value is selected which satisfies the design request.

According to one aspect of the present invention, an contact ratio is employed as the design target function, and there is provided a method of designing a hypoid gear wherein a pitch cone angle Γ_gcone of one gear is set, an contact ratio is calculated, the pitch cone angle Γ_gcone is changed so that the contact ratio becomes a predetermined value, a pitch cone angle Γ_gw is determined, and specifications are calculated based on the determined pitch cone angle Γ_gw. As described above, the contact ratio calculated by the method of the related art does not have a theoretical basis. In this aspect of the present invention, a newly defined tooth trace and an contact ratio related to the tooth trace are calculated, to determine the pitch cone angle. The tooth surface around a point of contact is approximated by its tangential plane, and a path of contact is made coincident to an intersection of the surface of action (pitch generating line L_pw), and a tooth trace is defined as a curve on a pitch hyperboloid obtained by transforming the path of contact into a coordinate system which rotates with each gear. Based on the tooth trace of this new definition, the original contact ratio of the hypoid gear is calculated and the contact ratio can be used as an index for design. A characteristic of the present invention is in the definition of the pitch cone angle related to the newly defined tooth trace.

According to another aspect of the present invention, it is preferable that, in the hypoid gear design method, the tooth trace contact ratio is assumed to be 2.0 or more in order to achieve constant engagement of two gears with two or more teeth.

When the pitch cone angle Γ_gw is set to an inclination angle Γ_s of an instantaneous axis, the contact ratios of the drive-side and the coast-side can be set approximately equal to each other. Therefore, it is preferable for the pitch cone angle to be set near the inclination angle of the instantaneous axis. In addition, it is also preferable to increase one of the contact ratios of the drive-side or coast-side as required. In this process, first, the pitch cone angle is set at the inclination angle of the instantaneous axis and the contact ratio is calculated, and a suitable value is selected by changing the pitch cone angle while observing the contact ratio. It is preferable that a width of the change of the pitch cone angle be in a range of ±5° with respect to the inclination angle Γ_s of the instantaneous axis. This is because if the change is out of this range, the contact ratio of one of the drive-side and the coast-side will be significantly reduced.

More specifically, according to one aspect of the present invention, a hypoid gear is designed according to the following steps:

(a) setting a shaft angle Σ, an offset E, and a gear ratio i₀ of a hypoid gear;

(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i₀, an instantaneous axis S which is an axis of a relative angular velocity of a first gear and a second gear, a line of centers v_c with respect to rotation axes of the first gear and the second gear, an intersection C_s between the instantaneous axis S and the line of centers v_c, and an inclination angle Γ_s of the instantaneous axis S with respect to the rotation axis of the second gear, and determining coordinate systems C₁, C₂, and C_s for calculation of specifications;

(c) setting three variables including one of a reference circle radius R_1w of the first gear and a reference circle radius R_2w of the second gear, one of a spiral angle ψ_pw of the first gear and a spiral angle ψ_gw of the second gear, and one of a pitch cone angle γ_p, of the first gear and a pitch cone angle Γ_gw of the second gear;

(d) calculating the design reference point P_w, which is a common point of contact of pitch cones of the first gear and the second gear, and the three other variables which are not set in the step (c), based on the three variables which are set in the step (c);

(e) setting a contact normal g_wD of a drive-side tooth surface of the second gear;

(f) setting a contact normal g_wC of a coast-side tooth surface of the second gear; and

(g) calculating specifications of the hypoid gear based on the design reference point P_w, the three variables which are set in the step (c), the contact normal g_wD of the drive-side tooth surface of the second gear, and the contact normal g_wC of the coast-side tooth surface of the second gear.

According to another aspect of the present invention, a hypoid gear is designed according to the following steps:

(a) setting a shaft angle Σ, an offset E, and a gear ratio i₀ of a hypoid gear;

(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i₀, an instantaneous axis S which is an axis of a relative angular velocity of a first gear and a second gear, a line of centers v_c with respect to rotation axes of the first gear and the second gear, an intersection C_s between the instantaneous axis S and the line of centers v_c, and an inclination angle Γ_s of the instantaneous axis S with respect to the rotation axis of the second gear, and determining coordinate systems C₁, C₂, and C_s for calculation of specifications;

(c) setting three variables including one of a reference circle radius R_1w of the first gear and a reference circle radius R_2w of the second gear, one of a spiral angle ψ_pw of the first gear and a spiral angle ψ_gw of the second gear, and one of a pitch cone angle γ_pw of the first gear and a pitch cone angle Γ_gw of the second gear;

(d) calculating the design reference point P_w, which is a common point of contact of pitch cones of the first gear and the second gear, and the three other variables which are not set in the step (c), based on the three variable which are set in the step (c);

(e) calculating a pitch generating line L_pw which passes through the design reference point P_w and which is parallel to the instantaneous axis S;

(f) setting an internal circle radius R_2t and an external circle radius R_2h of the second gear;

(g) setting a contact normal g_wD of a drive-side tooth surface of the second gear;

(h) calculating an intersection P_0D between a reference plane S_H which is a plane orthogonal to the line of centers v_c and passing through the intersection C_s and the contact normal g_wD and a radius R_20D of the intersection P_0D around a gear axis;

(i) calculating an inclination angle φ_s0D of a surface of action S_wD which is a plane defined by the pitch generating line L_pw and the contact normal g_wD with respect to the line of centers v_c, an inclination angle ψ_sw0D of the contact normal g_wD on the surface of action S_wD with respect to the instantaneous axis S, and one pitch P_gwD on the contact normal g_wD;

(j) setting a provisional second gear pitch cone angle Γ_gcone, and calculating an contact ratio m_fconeD of the drive-side tooth surface based on the internal circle radius R_2t and the external circle radius R_2h;

(k) setting a contact normal g_wC of a coast-side tooth surface of the second gear;

(l) calculating an intersection P_0c between the reference plane S_H which is a plane orthogonal to the line of centers v_c and passing through the intersection C_s and the contact normal g_wC and a radius R_20c of the intersection P_0C around the gear axis;

(m) calculating an inclination angle φ_s0C of a surface of action S_wC which is a plane defined by the pitch generating line L_pw and the contact normal g_wC with respect to the line of centers v_c, an inclination angle ψ_sw0C of the contact normal g_wC on the surface of action S_wC with respect to the instantaneous axis S, and one pitch P_gwC on the contact normal g_wC;

(n) setting a provisional second gear pitch cone angle Γ_gcone, and calculating an contact ratio m_fconeC of the coast-side tooth surface based on the internal circle radius R_2t and the external circle radius R_2h;

(o) comparing the contact ratio m_fconeD of the drive-side tooth surface and the contact ratio m_fconeC of the coast-side tooth surface, and determining whether or not these contact ratios are predetermined values;

(p) when the contact ratios of the drive-side and the coast-side are the predetermined values, replacing the provisional second gear pitch cone angle Γ_gcone with the second gear pitch cone angle Γ_gw obtained in the step (c) or in the step (d);

(q) when the contact ratios of the drive-side and the coast-side are not the predetermined values, changing the provisional second gear pitch cone angle Γ_gcone and re-executing from step (g);

(r) re-determining the design reference point P_w, the other one of the reference circle radius R_1w of the first gear and the reference circle radius R_2w of the second gear which is not set in the step (c), the other one of the spiral angle ψ_pw of the first gear and the spiral angle ψ_gw of the second gear which is not set in the step (c), and the first gear pitch cone angle γ_pw based on the one of the reference circle radius R_1w of the first gear and the reference circle radius R_2w of the second gear which is set in the step (c), the one of the spiral angle ψ_pw of the first gear and the spiral angle ψ_gw of the second gear which is set in the step (c), and the second gear pitch cone angle Γ_gw which is replaced in the step (p), and

(s) calculating specifications of the hypoid gear based on the specifications which are set in the step (c), the specifications which are re-determined in the step (r), the contact normal g_wD of the drive-side tooth surface of the second gear, and the contact normal g_wC of the coast-side tooth surface of the second gear.

According to another aspect of the present invention, a hypoid gear is designed according to the following steps:

(a) setting a shaft angle Σ, an offset E, and a gear ratio i₀ of a hypoid gear;

(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i₀, an instantaneous axis S which is an axis of a relative angular velocity of a first gear and a second gear, a line of centers v_c with respect to rotation axes of the first gear and the second gear, an intersection C_s between the instantaneous axis S and the line of centers v_c, and an inclination angle Γ_s of the instantaneous axis S with respect to the rotation axis of the second gear, to determine coordinate systems C₁, C₂, and C_s for calculation of specifications;

(c) setting three variables including one of a reference circle radius R_1w of the first gear and a reference circle radius R_2w of the second gear, one of a spiral angle ψ_pw of the first gear and a spiral angle ψ_gw of the second gear, and one of a pitch cone angle γ_pw of the first gear and a pitch cone angle Γ_gw of the second gear;

(d) calculating the design reference point P_w, which is a common point of contact of pitch cones of the first gear and the second gear, and the three other variables which are not set in the step (c), based on the three variables which are set in the step (c);

(e) calculating a pitch generating line L_pw which passes through the design reference point P, and which is parallel to the instantaneous axis S;

(f) setting an internal circle radius R_2t and an external circle radius R_2h of the second gear;

(g) setting a contact normal g_wD of a drive-side tooth surface of the second gear;

(h) calculating an intersection P_0D between a reference plane S_H which is a plane orthogonal to the line of centers v_c and passing through the intersection C_s and the contact normal g_wD and a radius R_20D of the intersection P_0D around a gear axis;

(i) calculating an inclination angle φ_s0D of a surface of action S_wD which is a plane defined by the pitch generating line L_pw and the contact normal g_wD with respect to the line of centers v_c, an inclination angle ψ_sw0D of the contact normal g_wD on the surface of action S_wD with respect to the instantaneous axis S, and one pitch P_gwD on the contact normal g_wD;

(j) setting a provisional second gear pitch cone angle Γ_gcone, and calculating an contact ratio m_fconeD of the drive-side tooth surface based on the internal circle radius R_2t and the external circle radius R_2h;

(k) setting a contact normal g_wC of a coast-side tooth surface of the second gear;

(l) calculating an intersection P_0C between the reference plane S_H which is a plane orthogonal to the line of centers v_c and passing through the intersection C_s and the contact normal g_wC and a radius R_20c of the intersection P_0C around the gear axis;

(m) calculating an inclination angle φ_s0C of a surface of action S_wC which is a plane defined by the pitch generating line L_pw and the contact normal g_wC with respect to the line of centers v_c, an inclination angle ψ_sw0C of the contact normal g_wC on the surface of action S_wC with respect to the instantaneous axis S, and one pitch P_gwC on the contact normal g_wC;

(n) setting a provisional second gear pitch cone angle Γ_gcone, and calculating an contact ratio m_fconeC of the coast-side tooth surface based on the internal circle radius R_2t and the external circle radius R_2h;

(o) comparing the contact ratio m_fconeD of the drive-side tooth surface and the contact ratio m_fconeC of the coast-side tooth surface, and determining whether or not these contact ratios are predetermined values;

(p) changing, when the contact ratios of the drive-side and the coast-side are not the predetermined values, the provisional second gear pitch cone angle Γ_gcone and re-executing from step (g);

(q) defining, when the contact ratios of the drive-side and the coast-side are the predetermined values, a virtual cone having the provisional second gear pitch cone angle Γ_gcone as a cone angle;

(r) calculating a provisional virtual cone γ_pcone the first gear based on the determined Γ_gcone; and

(s) calculating specifications of the hypoid gear based on the design reference point P_w, the reference circle radius R_1w of the first gear and the reference circle radius R_2w of the second gear which are set in the step (c) and the step (d), the spiral angle ψ_pw of the first gear and the spiral angle ψ_gw of the second gear which are set in the step (c) and the step (d), the cone angle Γ_gcone of the virtual cone and the cone angle γ_gcone of the virtual cone which are defined in the step (q) and the step (r), the contact normal g_wD of the drive-side tooth surface of the second gear, and the contact normal g_wC of the coast-side tooth surface of the second gear.

According to another aspect of the present invention, in a method of designing a hypoid gear, a pitch cone angle of one gear is set equal to an inclination angle of an instantaneous axis, and the specifications are calculated. When the pitch cone angle is set equal to the inclination angle of the instantaneous axis, the contact ratios of the drive-side tooth surface and the coast-side tooth surface become almost equal to each other. Therefore, a method is provided in which the pitch cone angle is set to the inclination angle of the instantaneous axis in a simple method, that is, without reviewing the contact ratios in detail.

More specifically, according to another aspect of the present invention, the hypoid gear is designed according to the following steps:

(a) setting a shaft angle Σ, an offset E, and a gear ratio i₀ of a hypoid gear;

(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i₀, an instantaneous axis S which is an axis of a relative angular velocity of a first gear and a second gear, a line of centers v_c with respect to rotational axes of the first gear and the second gear, an intersection C_s between the instantaneous axis S and the line of centers v_c, and an inclination angle Γ_s of the instantaneous axis with respect to the rotational axis of the second gear;

(c) determining the inclination angle Γ_s of the instantaneous axis as a second gear pitch cone angle Γ_gw; and

(d) calculating specifications of the hypoid gear based on the determined second gear pitch cone angle Γ_gw.

According to another aspect of the present invention, in a method of designing a hypoid gear, a design reference point P_w is not set as a point of contact between the pitch cones of the first gear and second gear, but is determined based on one of reference circle radii R_1w and R_2w of the first gear and the second gear, a spiral angle ψ_rw, and a phase angle β_w of the design reference point, and the specifications are calculated.

More specifically, according to another aspect of the present invention, a hypoid gear is designed according to the following steps:

(a) setting a shaft angle Σ, an offset E, and a gear ratio i₀ of a hypoid gear;

(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i₀, an instantaneous axis S which is an axis of a relative angular velocity of a first gear and a second gear, a line of centers v_c with respect to rotation axes of the first gear and the second gear, an intersection C_s between the instantaneous axis S and the line of centers v_c, and an inclination angle Γ_s of the instantaneous axis S with respect to the rotation axis of the second gear, to determine coordinate systems C₁, C₂, and C_s for calculation of specifications;

(c) setting one of a reference circle radius R_1w of the first gear and a reference circle radius R_2w of the second gear, a spiral angle ψ_rw, and a phase angle β_w of a design reference point P_w, to determine the design reference point;

(d) calculating the design reference point P_w and a reference circle radius which is not set in the step (c) from a condition where the first gear and the second gear share the design reference point P_w, based on the three variables which are set in the step (c);

(e) setting one of a reference cone angle γ_pw of the first gear and a reference cone angle Γ_gw of the second gear;

(f) calculating a reference cone angle which is not set in the step (e), based on the shaft angle Σ and the reference cone angle which is set in the step (e);

(g) setting a contact normal g_wD of a drive-side tooth surface of the second gear;

(h) setting a contact normal g_wC of a coast-side tooth surface of the second gear; and

(i) calculating specifications of the hypoid gear based on the design reference point P_w, the reference circle radii R_1w and R_2w, and the spiral angle ψ_rw which are set in the step (c) and the step (d), the reference cone angles γ_pw and Γ_gw which are set in the step (e) and the step (f), and the contact normals g_wC and g_wD which are set in the step (g) and the step (h).

The designing steps of these two aspects of the present invention can be executed by a computer by describing the steps with a predetermined computer program. A unit which receives the gear specifications and variables is connected to the computer and a unit which provides a design result or a calculation result at an intermediate stage is also connected to the computer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing external appearance of a hypoid gear.

FIG. 1B is a diagram showing a cross sectional shape of a gear.

FIG. 1C is a diagram showing a cross sectional shape of a pinion.

FIG. 2 is a diagram schematically showing appearance of coordinate axes in each coordinate system, a tooth surface of a gear, a tooth profile, and a path of contact.

FIG. 3 is a diagram showing a design reference point P₀ and a path of contact g₀ for explaining a variable determining method, with coordinate systems C₂′, O_q2, C₁′, and O_q1.

FIG. 4 is a diagram for explaining a relationship between gear axes I and II and an instantaneous axis S.

FIG. 5 is a diagram showing a relative velocity V_s at a point C_s.

FIG. 6 is a diagram showing, along with planes S_H, S_s, S_p, and S_n, a reference point P₀, a relative velocity V_rs0, and a path of contact g₀.

FIG. 7 is a diagram showing a relationship between a relative velocity V_rs and a path of contact g₀ at a point P.

FIG. 8 is a diagram showing a relative velocity V_rs0 and a path of contact g₀ at a reference point P₀, with a coordinate system C_s.

FIG. 9 is a diagram showing coordinate systems C₁, C₂, and C_s of a hypoid gear and a pitch generating line L_pw.

FIG. 10 is a diagram showing a tangential cylinder of a relative velocity V_rsw.

FIG. 11 is a diagram showing a relationship between a pitch generating line L_pw, a path of contact g_w, and a surface of action S_w at a design reference point P_w.

FIG. 12 is a diagram showing a surface of action using coordinate systems C_s, C₁, and C₂ in the cases of a cylindrical gear and a crossed helical gear.

FIG. 13 is a diagram showing a surface of action using coordinate systems C_s, C₁, and C₂ in the cases of a bevel gear and a hypoid gear.

FIG. 14 is a diagram showing a relationship between a contact point P_w and points O_1nw and O_2nw.

FIG. 15 is a diagram showing a contact point P_w and a path of contact g_w in planes S_tw, S_nw, and G_2w.

FIG. 16 is a diagram showing a transmission error of a hypoid gear manufactured as prototype that uses current design method.

FIG. 17 is an explanatory diagram of a virtual pitch cone.

FIG. 18 is a diagram showing a definition of a ring gear shape.

FIG. 19 is a diagram showing a definition of a ring gear shape.

FIG. 20 is a diagram showing a state in which an addendum is extended and a tip cone angle is changed.

FIG. 21 is a diagram showing a transmission error of a hypoid gear manufactured as prototype using a design method of a preferred embodiment of the present invention.

FIG. 22 is a schematic structural diagram of a system which aids a design method for a hypoid gear.

FIG. 23 is a diagram showing a relationship between a gear ratio and a tip cone angle of a uniform tooth depth hypoid gear designed using a current method.

FIG. 24 is a diagram showing a relationship between a gear ratio and a tip cone angle of a tapered tooth depth hypoid gear designed using a current method.

FIG. 25 is a diagram showing a relationship between a tooth trace curve and a cutter radius of a uniform-depth tooth.

FIG. 26 is a diagram showing a relationship between a tooth trace curve and a cutter radius of a tapered-depth tooth.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described with reference to the drawings.

1. Coordinate System of Hypoid Gear

1.1 Coordinate Systems C₁, C₂, C_q1, and C_q2

In the following description, a small diameter gear in a pair of hypoid gears is referred to as a pinion, and a large diameter gear is referred to as a ring gear. In addition, in the following, the descriptions may be based on the tooth surface, tooth trace, etc. of the ring gear, but because the pinion and the ring gear are basically equivalent, the description may similarly be based on the pinion. FIG. 1A is a perspective view showing external appearance of a hypoid gear. The hypoid gear is a pair of gears in which a rotational axis (pinion axis) I of the pinion 10 and the rotational axis (gear axis) II of the ring gear 14 are not parallel and do not intersect. A line of centers v_c of the pinion axis and the gear axis exists, and a distance (offset) between the two axes on the line of centers v_c is set as E, an angle (shaft angle) between the pinion axis and the gear axis projected onto a plane orthogonal to the line of centers v_c is set as E, and a gear ratio is set as i₀. FIG. 1B is a cross sectional diagram at a plane including the axis II of the ring gear 14. An angle between a pitch cone element pc_g passing at a design reference point P_w, to be described later, and the axis II is shown as a pitch cone angle Γ_gw. A distance between the design reference point P_w and the axis II is shown as a reference circle radius R_2w. FIG. 1C is a cross sectional diagram at a plane including the axis I of the pinion 10. An angle between a pitch cone element pc_p passing at the design reference point P_w and the axis I is shown as a pitch cone angle γ_pw. In addition, a distance between the design reference point P_w and the axis I is shown as a reference circle radius R_1w.

FIG. 2 shows coordinate systems C₁ and C₂. A direction of the line of centers v_c is set to a direction in which the direction of an outer product ω_2i×ω_1i of angular velocities ω_1i and ω_2i of the pinion gear axis I and the ring gear axis II is positive. The intersection points of the pinion and ring gear axes I, II and the line of centers v_c are designated by C₁ and C₂ and a situation where C₂ is above C₁ with respect to the line of centers v_c will be considered in the following. A case where C₂ is below C₁ would be very similar. A distance between C₁ and C₂ is the offset E. A coordinate system C₂ of a ring gear 14 is defined in the following manner. The origin of the coordinate system C₂ (u_2c, v_2c, z_2c) is set at C₂, a z_2c axis of the coordinate system C₂ is set to extend in the ω₂₀ direction on the ring gear axis II, a v_2c axis of the coordinate system C₂ is set in the same direction as that of the line of centers v_c, and a u_2c axis of the coordinate system C₂ is set to be normal to both the axes to form a right-handed coordinate system. A coordinate system C₁ (u_1c, v_1c, z_1c) can be defined in a very similar manner for the pinion 10.

FIG. 3 shows a relationship between the coordinate systems C₁, C₂, C_q1, and C_q2 in gears I and II. The coordinate systems C₂ and C_q2 of the gear II are defined in the following manner. The origin of the coordinate system C₂ (u_2c, v_2c, z_2c) is set at C₂, a z_2c axis of the coordinate system C₂ is set to extend in the ω₂₀ direction on the ring gear axis II, a v_2c axis of the coordinate system C₂ is set in the same direction as that of the line of centers v_c, and a u_2c axis of the coordinate system C₂ is set to be normal to both the axes to form a right-handed coordinate system. The coordinate system C_q2 (q_2c, v_q2c, z_2c) has the origin C₂ and the z_2c axis in common, and is a coordinate system formed by the rotation of the coordinate system C₂ around the z_2c axis as a rotational axis by χ₂₀ (the direction shown in the figure is positive) such that the plane v_2c=0 is parallel to the plane of action G₂₀. The u_2c axis becomes a q_2c axis, and the v_2c axis becomes a v_q2c axis.

The plane of action G₂₀ is expressed by v_q2c=−R_b2 using the coordinate system C_q2. In the coordinate system C₂, the inclination angle of the plane of action G₂₀ to the plane v_2c=0 is the angle χ₂₀, and the plane of action G₂₀ is a plane tangent to the base cylinder (radius R_b20).

The relationships between the coordinate systems C₂ and C_q2 become as follows because the z_2c axis is common.

u _2c =q _2c cos χ₂₀−v_q2c sin χ₂₀

v _2c =q _2c sin χ₂₀+v_q2c cos χ₂₀

Because the plane of action G₂₀ meets V_q2c=−R_b20, the following expressions (1), are satisfied if the plane of action G₂₀ is expressed by the radius R_b20 of the base cylinder.

u _2c =q _2c cos χ₂₀+R_b20 sin χ₂₀

v _2c =q _2c sin χ₂₀−R_b20 cos χ₂₀

z_2c=z_2c  (1)

If the line of centers g₀ is defined to be on the plane of action G₂₀ and also defined such that the line of centers g₀ is directed in the direction in which the q_2c axis component is positive, an inclination angle of the line of centers g₀ from the q_2c axis can be expressed by ψ_b20 (the direction shown in the figure is positive). Accordingly, the inclination angle of the line of centers g₀ in the coordinate system C₂ is defined to be expressed in the form of g₀ (φ₂₀, ψ₂₀) with the inclination angle φ₂₀ (the complementary angle of the χ₂₀) of the plane of action G₂₀ with respect to the line of centers v_c, and ψ_b2.

As for the gear I, coordinate systems C₁ (u_1C, v_1c, z_1c) and C_q1 (q_1C, v_q1c, z_1c) a plane of action G₁₀, a radius R_b1 of the base cylinder, and the inclination angle g₀ (φ₁₀, ψ_b10) of the line of centers g₀ can be similarly defined. Because the systems share a common z_1c axis, the relationship between the coordinate systems C₁ and C_q1 can also be expressed by the following expressions (2).

u _1c =q _1c cos χ₁₀+R_b10 sin χ₁₀

v _1c =q _1c sin χ₁₀−R_b10 cos χ₁₀

z_1c=z_1c  (2)

The relationship between the coordinate systems C₁ and C₂ is expressed by the following expressions (3).

u _1c =−u _2c cos Σ−z_2c sin Σ

v _1c =v _2c +E

z _1c =u _2c sin Σ−z_2c cos Σ  (3)

1.2 Instantaneous Axis (Relative Rotational Axis) S

FIG. 4 shows a relationship between an instantaneous axis and a coordinate system C_S. If the orthogonal projections of the two axes I (ω₁₀ and II (ω₂₀) to the plane S_H are designated by I_s (ω₁₀″) and II_s (ω₂₀″), respectively, and an angle of I_s with respect to II_s when the plane S_H is viewed from the positive direction of the line of centers v_c to the negative direction thereof is designated by Ω, I_s is in a zone of 0≦Q≦π (the positive direction of the angle Ω is the counterclockwise direction) with respect to II_s in accordance with the definition of ω₂₀×ω₁₀. If an angle of the instantaneous axis S (ω_r) to the II_s on the plane S_H is designated by ω_S (the positive direction of the angle Ω_s is the counterclockwise direction), the components of ω₁₀″ and ω₂₀″ that are orthogonal to the instantaneous axis on the plane S_H must be equal to each other in accordance with the definition of the instantaneous axis (ω_r=ω₁₀−ω₂₀). Consequently, Ω_s satisfies the following expressions (4):

sin Ω_s/sin(Ω_s−Ω)=ω₁₀/ω₂₀; or

sin Γ_s/sin(Σ−Γ_s)=ω₁₀/ω₂₀  (4)

wherein Z=π−Ω (shaft angle) and Γ_s=π−Ω_s. The positive directions are shown in the figure. In other words, the angle Γ_s is an inclination of the instantaneous axis S with respect to the ring gear axis II_s on the plane S_H, and the angle Γ_s will hereinafter be referred to as an inclination angle of the instantaneous axis.

The location of C_s on the line of centers v_c can be obtained as follows. FIG. 5 shows a relative velocity V_s (vector) of the point C_s. In accordance with the aforesaid supposition, C₁ is located under the position of C₂ with respect to the line of centers v_c and ω₁₀≧ω₂₀. Consequently, C_S is located under C₂. If the peripheral velocities of the gears I, II at the point C_s are designated by V_s1 and V_s2 (both being vectors), respectively, because the relative velocity V_s (=V_s1−V_s2) exists on the instantaneous axis S, the components of V_s1 and V_s2 (existing on the plane S_H) orthogonal to the instantaneous axis must always be equal to each other. Consequently, the relative velocity V_s (=V_s1−V_s2) at the point C_s would have the shapes as shown in the same figure on the plane S_H according to the location (Γ₅) of the instantaneous axis S, and the distance C₂C_s between C₂ and C_s can be obtained by the following expression (5). That is,

C ₂ C _s =E tan Γ_s/{tan(ΣΓ_s)+tan Γ_s}  (5).

The expression is effective within a range of 0≦Γ_s≦π, and the location of C_s changes together with Γ_s, and the location of the point C_s is located above C₁ in the case of 0≦Γ_s≦π/2, and the location of the point C_s is located under C₁ in the case of π/2≦Γ_s≦π.

1.3 Coordinate System C_s

Because the instantaneous axis S can be determined in a static space in accordance with the aforesaid expressions (4) and (5), the coordinate system C_s is defined as shown in FIG. 4. The coordinate system C_s (u_c, v_c, z_c) is composed of C_s as its origin, the directed line of centers v_c as its v_c axis, the instantaneous axis S as its z_c axis (the positive direction thereof is the direction of ω_r), and its u_c axis taken to be normal to both the axes as a right-handed coordinate system. Because it is assumed that a pair of gears being objects transmit a motion of a constant ratio of angular velocity, the coordinate system C_s becomes a coordinate system fixed in the static space, and the coordinate system C_s is a basic coordinate system in the case of treating a pair of gears performing the transmission of the motion of constant ratio of angular velocity together with the previously defined coordinate systems C₁ and C₂.

1.4 Relationship Among Coordinate Systems C₁, C₂, and C_s

If the points C₁ and C₂ are expressed to be C₁ (0, v_cs1, 0) and C₂ (0, v_cs2, 0) by the use of the coordinate system C_s, v_cs1 and v_cs2 are expressed by the following expressions (6).

$\begin{array}{cc}\begin{array}{c}{v}_{\mathrm{cs}2}=C_SC_2\\ =E\tan {\Gamma }_s/\left\{\tan \left(\Sigma -{\Gamma }_s\right)+\tan {\Gamma }_s\right\}\end{array}\begin{array}{c}{v}_{\mathrm{cs}1}=C_SC_1\\ =v_{\mathrm{cs}2}-E\\ =-E\tan \left(\Sigma -{\Gamma }_s\right)/\left\{\tan \left(\Sigma -{\Gamma }_s\right)+\tan {\Gamma }_s\right\}\end{array}& \left(6\right)\end{array}$

If it is noted that C₂ is always located above C_s with respect to the v_c axis, the relationships among the coordinate system C_s and the coordinate systems C₁ and C₂ can be expressed as the following expressions (7) and (8) with the use of v_cs1/v_cs2, Σ, and Γ_s.

u _1c =u _c cos(Σ−Γ_s)+z_c sin(Σ−Γ_s)

v _1c =v _c −v _cs1

z _1c =−u _c sin(Σ−Γ_s)+z_c cos(Σ−Γ_s)  (7)

u _2c =−u _c cos Γ_s+z_c sin Γ_s

v _2c =v _c −v _cs2

z _2c =−u _c sin Γ_s−z_c cos Γ_s  (8)

The relationships among the coordinate system C_s and the coordinate systems C₁ and C₂ are conceptually shown in FIG. 6.

2. Definition of Path of Contact g₀ by Coordinate System C_s 2.1 Relationship between Relative Velocity and Path of Contact g₀

FIG. 7 shows a relationship between the set path of contact g₀ and a relative velocity V_rs (vector) at an arbitrary point P on g₀. Incidentally, a prime sign (′) and a double-prime sign (″) in the figure indicate orthogonal projections of a point and a vector on the target plane. If the position vector of the P from an arbitrary point on the instantaneous axis S is designated by r when a tooth surface contacts at the arbitrary point P on the path of contact g₀, the relative velocity V_rs at the point P can be expressed by the following expression (9).

v _rs=ω_r×r+v_s  (9)

where

ω_r=ω₁₀−ω₂₀

ω_r=ω₂₀ sin Σ/sin(Σ−Γ_s)=ω₁₀ sin Σ/sin Γ_s

V _s=ω₁₀×[C₁C_s]−ω₂₀×[C₂C_s]

V _s=ω₂₀E sin Γ_s=ω₁₀E sin(Σ−Γ_s).

Here, [C₁C_s] indicates a vector having C₁ as its starting point and C_s as its endpoint, and [C₂C_s] indicates a vector having C₂ as its starting point and C_s as its end point.

The relative velocity V_rs exists on a tangential plane of the surface of a cylinder having the instantaneous axis S as an axis, and an inclination angle ψ relative to V_s on the tangential plane can be expressed by the following expression (10).

cos ψ=|V_s|V_rs|  (10)

Because the path of contact g₀ is also the line of centers of a tooth surface at the point of contact, g₀ is orthogonal to the relative velocity V_rs at the point P. That is,

V _rs ·g ₀=0

Consequently, g₀ is a directed straight line on a plane N normal to V_rs at the point P. If the line of intersection of the plane N and the plane S_H is designated by H_n, H_n is in general a straight line intersecting with the instantaneous axis S, with g₀ necessarily passing through the H_n if an infinite intersection point is included. If the intersection point of g₀ with the plane S_H is designated by P₀, then P₀ is located on the line of intersection H_n, and g₀ and P₀ become as follows according to the kinds of pairs of gears.

(1) Case of Cylindrical Gears or Bevel Gears (Σ=0, π or E=0)

Because V_s=0, V_rs simply means a peripheral velocity around the instantaneous axis S. Consequently, the plane N includes the S axis. Hence, H_n coincides with S, and the path of contact g₀ always passes through the instantaneous axis S. That is, the point P₀ is located on the instantaneous axis S. Consequently, for these pairs of gears, the path of contact g₀ is an arbitrary directed straight line passing at the arbitrary point P₀ on the instantaneous axis.

(2) Case of Gear Other than that Described Above (Σ≠0, π or E≠0)

In the case of a hypoid gear, a crossed helical gear or a worm gear, if the point of contact P is selected at a certain position, the relative velocity V_rs, the plane N, and the straight line H_n, all peculiar to the point P, are determined. The path of contact g₀ is a straight line passing at the arbitrary point P₀ on H_n, and does not, in general, pass through the instantaneous axis S. Because the point P is arbitrary, g₀ is also an arbitrary directed straight line passing at the point P₀ on a plane normal to the relative velocity V_rs0 at the intersection point P₀ with the plane S_H, That is, the aforesaid expression (9) can be expressed as follows.

V _rs =V _rs0+ω_r×[P₀P]·g₀

Here, [P₀P] indicates a vector having P₀ as its starting point and the P as its end point. Consequently, if V_rs0·g₀=0, V_rs·g₀=0, and the arbitrary point P on g₀ is a point of contact.

2.2 Selection of Reference Point

Among pairs of gears having two axes with known positional relationship and the angular velocities, pairs of gears with an identical path of contact g₀ have an identical tooth profile corresponding to g₀, with the only difference between them being which part of the tooth profile is used effectively. Consequently, in design of a pair of gears, the position at which the path of contact g₀ is disposed in a static space determined by the two axes is important. Further, because a design reference point is only a point for defining the path of contact g₀ in the static space, the position at which the design reference point is selected on the path of contact g₀ does not cause any essential difference. When an arbitrary path of contact g₀ is set, the g₀ necessarily intersects with a plane S_H including the case where the intersection point is located at an infinite point. Thus, the path of contact g₀ is determined with the point P₀ on the plane S_H (on an instantaneous axis in the case of cylindrical gears and bevel gears) as the reference point.

FIG. 8 shows the reference point P₀ and the path of contact g₀ by the use of the coordinate system C_s. When the reference point expressed by means of the coordinate system C_s is designated by P₀ (u_c0, v_c0, z_c0) each coordinate value can be expressed as follows.

u_c0=O_sP₀

v_c0=0

z_c0=C_sO_s

For cylindrical gears and bevel gears, u_c0=0. Furthermore, the point O_s is the intersection point of a plane S_s, passing at the reference point P₀ and being normal to the instantaneous axis S, and the instantaneous axis S.

2.3 Definition of Inclination Angle of Path of Contact g₀

The relative velocity V_rs0 at the point P₀ is concluded as follows with the use of the aforesaid expression (9).

V _rs0=ω_r×[u_c0]+V_s

where, [u_c0] indicates a vector having O_s as its starting point and P₀ as its end point. If a plane (u_c=u_c0) being parallel to the instantaneous axis S and being normal to the plane S_H at the point P₀ is designated by S_p, V_rs0 is located on the plane S_p, and the inclination angle ψ₀ of V_rs0 from the plane S_H (V_c=0) can be expressed by the following expression (11) with the use of the aforesaid expression (10).

$\begin{array}{cc}\begin{array}{c}\tan {\psi }_0={\omega }_ru_{c0}/V_s\\ =u_{c0}\sin \Sigma /\left\{E\sin \left(\Sigma -{\Gamma }_s\right)\sin {\Gamma }_s\right\}\end{array}& \left(11\right)\end{array}$

Incidentally, ψ₀ is assumed to be positive when u_c0≧0, and the direction thereof is shown in FIG. 8.

If a plane passing at the point P₀ and being normal to V_rs0 is designated by S_n, the plane S_n is a plane inclining to the plane S_s by the ψ₀, and the path of contact g₀ is an arbitrary directed straight line passing at the point P_o and located on the plane S_n. Consequently, the inclination angle of g₀ in the coordinate system C_s can be defined with the inclination angle ψ₀ of the plane S_n from the plane S_s (or the v_c axis) and the inclination angle φ_n0 from the plane S_p on the plane S_n, and the defined inclination angle is designated by g₀ (ψ₀, φ_n0). The positive direction of φ_n0 is the direction shown in FIG. 8.

2.4. Definition of g₀ by Coordinate System C_s

FIG. 6 shows relationships among the coordinate system C_s, the planes S_H, S_s, S_p and S_n, P_o and g₀ (ψ₀, φn₀). The plane S_H defined here corresponds to a pitch plane in the case of cylindrical gears and an axial plane in the case of a bevel gear according to the current theory. The plane S_s is a transverse plane, and the plane S_p corresponds to the axial plane of the cylindrical gears and the pitch plane of the bevel gear. Furthermore, it can be considered that the plane S_n is a normal plane expanded to a general gear, and that φ_n0 and ψ₀ also are a normal pressure angle and a spiral angle expanded to a general gear, respectively. By means of these planes, pressure angles and spiral angles of a pair of general gears can be expressed uniformly to static spaces as inclination angles to each plane of line of centers (g₀'s in this case) of points of contact. The planes S_n, φ_n0, and ψ₀ defined here coincide with those of a bevel gear of the current theory, and differ for other gears because the current theory takes pitch planes of individual gears as standards, and then the standards change to a static space according to the kinds of gears. With the current theory, if a pitch body of revolution (a cylinder or a circular cone) is determined, it is sufficient to generate a mating surface by fixing an arbitrary curved surface to the pitch body of revolution as a tooth surface, and in the current theory, conditions of the tooth surface (a path of contact and the normal thereof) are not limited except for the limitations of manufacturing. Consequently, the current theory emphasizes the selection of P₀ (for discussions about pitch body of revolution), and there has been little discussion concerning design of g₀ (i.e. a tooth surface realizing the g₀) beyond the existence of a tooth surface.

For a pair of gears having the set shaft angle Σ thereof, the offset E thereof, and the directions of angular velocities, the path of contact g₀ can generally be defined in the coordinate system C_s by means of five independent variables of the design reference point P₀ (u_c0, v_c0, z_c0) and the inclination angle g₀ (ψ₀, φ_n0). Because the ratio of angular velocity i₀ and v_c0=0 are set as design conditions in the present embodiment, there are three independent variables of the path of contact g₀. That is, the path of contact g₀ is determined in a static space by the selections of the independent variables of two of (z_c0), φ_n0, and ψ₀ in the case of cylindrical gears because z_c0 has no substantial meaning, three of z_c0, φ_n0, and ψ₀ in the case of a bevel gear, or three of z_c0, φ_n0, and ψ₀ (or u_c0) in the case of a hypoid gear, a worm gear, or a crossed helical gear. When the point P₀ is set, ψ₀ is determined at the same time and only φ_n0 is a freely selectable variable in the case of the hypoid gear and the worm gear. However, in the case of the cylindrical gears and the bevel gear, because P₀ is selected on an instantaneous axis, both of ψ₀ and φ_n0 are freely selectable variables.

3. Pitch Hyperboloid

3.1 Tangential Cylinder of Relative Velocity

FIG. 9 is a diagram showing an arbitrary point of contact P_w, a contact normal g_w thereof, a pitch plane S_tw, the relative velocity V_rsw, and a plane S_nw which is normal to the relative velocity V_rsw of a hypoid gear, along with basic coordinate systems C₁, C₂, and C_s. FIG. 10 is a diagram showing FIG. 9 drawn from a positive direction of the z_c axis of the coordinate system C_s. The arbitrary point P_w and the relative velocity V_rsw are shown with cylindrical coordinates P_w (r_w, β_w, z_cw: C_s). The relative velocity V_rsw is inclined by ψ_rw from a generating line L_pw on the tangential plane S_pw of the cylinder having the z_c axis as its axis, passing through the arbitrary point P_w, and having a radius of r_w.

When the coordinate system C_s is rotated around the z_c axis by β_w, to realize a coordinate system C_rs (u_rc, v_rc, z_c: C_rs), the tangential plane S_pw can be expressed by u_rc=r_w, and the following relationship is satisfied between u_rc=r_w and the inclination angle ψ_rw of V_rsw.

$\begin{array}{cc}\begin{array}{c}{u}_{\mathrm{rc}}=r_w\\ =V_s\tan {\psi }_{\mathrm{rw}}/{\omega }_r\\ =E\tan {\psi }_{\mathrm{rw}}\times \sin \left(\Sigma -{\Gamma }_s\right)\sin {\Gamma }_s/\sin \Sigma \end{array}& \left(12\right)\end{array}$

where Vs represents a sliding velocity in the direction of the instantaneous axis and ω_r represents a relative angular velocity around the instantaneous axis.

The expression (12) shows a relationship between r_w of the arbitrary point P_w (r_w, β_w, z_cw: C_s) and the inclination angle ψ_rw of the relative velocity V_rsw thereof. In other words, when ψ_rw is set, r_w is determined. Because this is true for arbitrary values of β_w and z_cw, P_w with a constant ψ_rw defines a cylinder with a radius r_w. This cylinder is called the tangential cylinder of the relative velocity.

3.2 Pitch Generating Line and Surface of Action

When r_w (or ψ_rw) and β_w are set, P_w is determined on the plane z_c=z_cw. Because this is true for an arbitrary value of z_cw, points P_w having the same r_w (or ψ_rw) and the same β_w draw a line element of the cylinder having a radius r_w. This line element is called a pitch generating line L_pw. A directed straight line which passes through a point P_w on a plane S_nw orthogonal to the relative velocity V_rsw at the arbitrary point P_w on the pitch generating line L_pw satisfies a condition of contact, and thus becomes a contact normal.

FIG. 11 is a diagram conceptually drawing relationships among the pitch generating line L_pw, directed straight line g_w, surface of action S_w, contact line w, and a surface of action S_wc (dotted line) on the side C to be coast. A plane having an arbitrary directed straight line g_w on the plane S_nw passing through the point P_w as a normal is set as a tooth surface W. Because all of the relative velocity V_rsw at the arbitrary point P_w on the pitch generating line L_pw are parallel and the orthogonal planes S_nw are also parallel, of the normals of the tooth surface W, any normal passing through the pitch generating line L_pw becomes a contact normal, and a plane defined by the pitch generating line L_pw and the contact normal g_w becomes the surface of action S_w and an orthogonal projection of the pitch generating line L_pw to the tooth surface W becomes the contact line w. Moreover, because the relationship is similarly true for another normal g_wc on the plane S_nw passing through the point P_w and the surface of action S_wc thereof, the pitch generating line L_pw is a line of intersection between the surfaces of action of two tooth surfaces (on the drive-side D and coast-side C) having different contact normals on the plane S_nw.

3.3 Pitch Hyperboloid

The pitch generating line L_pw is uniquely determined by the shaft angle Σ, offset E, gear ratio i₀, inclination angle ψ_rw of relative velocity V_rsw, and rotation angle β_w from the coordinate system C_s to the coordinate system C_rs. A pair of hyperboloids which are obtained by rotating the pitch generating line L_pw around the two gear axes, respectively, contact each other in a line along L_pw, and because the line L_pw is also a line of intersection between the surfaces of action, the drive-side D and the coast-side C also contact each other along the line L_pw. Therefore, the hyperboloids are suited as revolution bodies for determining the outer shape of the pair of gears. In the present invention, the hyperboloids are set as the design reference revolution bodies, and are called the pitch hyperboloids. The hyperboloids in the related art are revolution bodies in which the instantaneous axis S is rotated around the two gear axes, respectively, but in the present invention, the pitch hyperboloid is a revolution body obtained by rotating a parallel line having a distance r_w from the instantaneous axis.

In the cylindrical gear and the bevel gear, L_pw coincides with the instantaneous axis S or z_c (r_w→0) regardless of ψ_rw and β_w, because of special cases of the pitch generating line L_pw (V_s→0 as Σ→0 or E→0 in the expression (12)). The instantaneous axis S is a line of intersection of the surfaces of action of the cylindrical gear and the bevel gear, and the revolution bodies around the gear axes are the pitch cylinder of the cylindrical gear and the pitch cone of the bevel gear.

For these reasons, the pitch hyperboloids which are the revolution bodies of the pitch generating line L_pw have the common definition of the expression (12) from the viewpoint that the hyperboloid is a “revolution body of line of intersection of surfaces of action” and can be considered to be a design reference revolution body for determining the outer shape of the pair of gears which are common to all pairs of gears.

3.4 Tooth Trace (New Definition of Tooth Trace)

In the present invention, a curve on the pitch hyperboloid (which is common to all gears) obtained by transforming a path of contact to a coordinate system which rotates with the gear when the tooth surface around the point of contact is approximated with its tangential plane and the path of contact is made coincident with the line of intersection of the surfaces of action (pitch generating line L_pw) is called a tooth trace (curve). In other words, a tooth profile, among arbitrary tooth profiles on the tooth surface, in which the path of contact coincides with the line of intersection of the surface of action is called a tooth trace. The tooth trace of this new definition coincides with the tooth trace of the related art defined as an intersection between the pitch surface (cone or cylinder) and the tooth surface in the cylindrical gears and the bevel gears and differs in other gears. In the case of the current hypoid gear, the line of intersection between the selected pitch cone and the tooth surface is called a tooth trace.

3.5 Contact Ratio

A total contact ratio m is defined as a ratio of a maximum angular displacement and an angular pitch of a contact line which moves on an effective surface of action (or effective tooth surface) with the rotation of the pair of gears. The total contact ratio m can be expressed as follows in terms of the angular displacement of the gear.

m=(θ_2max−θ_2min)/(2θ_2p)

where θ_2max and θ_2min represent maximum and minimum gear angular displacements of the contact line and 2θ_2p represents a gear angular pitch.

Because it is very difficult to represent the position of the contact line as a function of a rotation angle except for special cases (involute helicoid) and it is also difficult to represent such on the tooth surface (curved surface) in the stage of design, the surface of action has been approximated with a plane in a static space, a path of contact has been set on the surface of action, and an contact ratio has been determined and set as an index along the path of contact.

FIGS. 12 and 13 show the surface of action conceptually shown in FIG. 11 in more detail with reference to the coordinate systems C_s, C₁, and C₂. FIG. 12 shows a surface of action in the cases of the cylindrical gear and a crossed helical gear, and FIG. 13 shows a surface of action in the cases of the bevel gear and the hypoid gear. FIGS. 12 and 13 show surfaces of action with the tooth surface (tangential plane) when the intersection between g_w and the reference plane S_H (v_c=0) is set as P₀ (u_c0, v_c0=0, z_c0: C_s), the inclination angle of g_w is represented in the coordinate system C_s, and the contact normal g_w is set g_w=g₀ (ψ₀, φ_n0: C_s). A tooth surface passing through P₀ is shown as W₀, a tooth surface passing through an arbitrary point P_d on g_w=g₀ is shown as W_d, a surface of action is shown by S_w=S_w0, and an intersection between the surface of action and the plane S_H is shown with L_pw0 (which is parallel to L_pw). Because planes are considered as the surface of action and the tooth surface, the tooth surface translates on the surface of action. The point P_w may be set at any point, but because the static coordinate system C_s has its reference at the point P₀ on the plane S_H, the contact ratio is defined with an example configuration in which P_w is set at P₀.

The contact ratio of the tooth surface is defined in the following manner depending on how the path of contact passing through P_w=P₀ is defined on the surface of action S_w=S_w0:

(1) Contact Ratio m_z Orthogonal Axis

This is a ratio between a length separated by an effective surface of action (action limit and the tooth surface boundary) of lines of intersection h_1z and h_2z (P₀P_z1sw and P₀P_z2sw in FIGS. 12 and 13) between the surface of action S_w0 and the planes of rotation Z₁₀ and Z₂₀ and a pitch in this direction;

(2) Tooth Trace Contact Ratio m_f

This is a ratio between a length of L_pw0 which is parallel to the instantaneous axis separated by the effective surface of action and a pitch in this direction;

(3) Transverse Contact Ratio m_s

This is a ratio between a length separated by an effective surface of action of a line of intersection (P₀P_ssw in FIGS. 12 and 13) between a plane S_s passing through P₀ and normal to the instantaneous axis and S_w0 and a pitch in this direction;

(4) Contact Ratio in Arbitrary Direction

This includes cases where the path of contact is set in a direction of g₀ (P₀P_Gswn in FIG. 13) and cases where the path of contact is set in a direction of a line of intersection (P_wP_gcon in FIGS. 12 and 13) between an arbitrary conical surface and S_w0;

(5) Total Contact Ratio

This is a sum of contact ratios in two directions (for example, (2) and (3)) which are normal to each other on the surface of action, and is used as a substitute for the total contact ratio.

In addition, except for points on g_w=g₀, the pitch (length) would differ depending on the position of the point, and the surface of action and the tooth surface are actually not planes. Therefore, only an approximated value can be calculated for the contact ratio. Ultimately, a total contact ratio determined from the angular displacement must be checked.

3.6 General Design Method of Gear Using Pitch Hyperboloid

In general, a gear design can be considered, in a simple sense, to be an operation, in a static space (coordinate system C_s) determined by setting the shaft angle Σ, offset E, and gear ratio i₀, to:

(1) select a pitch generating line and a design reference revolution body (pitch hyperboloid) by setting a design reference point P_w(r_w(ψ_rw), β_w, z_cw: C_rs); and(2) set a surface of action (tooth surface) having g_w by setting an inclination angle (ψ_rw, φ_nrw: C_rs) of a tooth surface normal g_w passing through P_w.

In other words, the gear design method (selection of P_w and g_w) comes down to selection of four variables including r_w (normally, ψ_rw is set) β_w, z_cw (normally, R_2w (gear pitch circle radius) is set in place of z_cw), and φ_nrw. A design method for a hypoid gear based on the pitch hyperboloid when Σ, E, and i₀ are set will be described below.

3.7 Hypoid Gear (−π/2<β_w<π/2)

(1) Various hypoid gears can be realized depending on how β_w is selected, even with set values for ψ_rw (r_w) and z_cw (R_2w).

(a) From the viewpoint of the present invention, the Wildhaber (Gleason) method is one method of determining P_w by determining β_w through setting of a constraint condition to “make the radius of curvature of a tooth trace on a plane (FIG. 9) defined by peripheral velocities of a pinion and ring gear at P_w coincide with the cutter radius”. However, because the tooth surface is possible as long as an arbitrary curved surface (therefore, arbitrary radius of curvature of tooth trace) having g_w passing through P_w has a mating tooth surface, this condition is not necessarily a requirement even when a conical cutter is used. In addition, although this method employs circular cones which circumscribe at P_w, the pair of gears still contact on a surface of action having the pitch generating line L_pw passing through P_w regardless of the cones. Therefore, the line of intersection between the pitch cone circumscribing at P_w and the surface of action determined by this method differs from the pitch generating line L_pw (line of intersection of surfaces of action). When g_w is the same, the inclination angle of the contact line on the surface of action and L_pw are equal to each other, and thus the pitch in the direction of the line of intersection between the surface of action and the pitch cone changes according to the selected pitch cone (FIG. 11). In other words, a large difference in the pitch is caused between the drive-side and the coast-side in the direction of the line of intersection between the pitch cone and the surface of action (and, consequently, the contact ratios in this direction). In the actual Wildhaber (Gleason) method, two cones are determined by giving pinion spiral angle and an equation of radius of curvature of tooth trace for contact equations of the two cones (seven equations having nine unknown variables), and thus the existence of the pitch generating line and the pitch hyperboloid is not considered.

(b) In a preferred embodiment described in section 4.2A below, β_w is selected by giving a constraint condition that “a line of intersection between a cone circumscribing at P_w and the surface of action is coincident with the pitch generating line L_pw”. As a result, as will be described below, the tooth trace contact ratios on the drive-side and the coast-side become approximately equal to each other.

(2) Gear radius R_2w, β_w, and ψ_rw are set and a design reference point P_w (u_cw, v_cw, z_cw: C_s) is determined on the pitch generating line L_pw. The pitch hyperboloids can be determined by rotating the pitch generating line L_pw around each tooth axis. A method of determining the design reference point will be described in section 4.2B below.

(3) A tooth surface normal g_w passing through P_w is set on a plane S_nw normal to the relative velocity V_rsw of P_w. The surface of action S_w is determined by g_w and the pitch generating line L_pw.

4. Design Method for Hypoid Gear

A method of designing a hypoid gear using the pitch hyperboloid will now be described in detail.

4.1 Coordinate Systems C_s, C₁, and C₂ and Reference Point P_w

When the shaft angle Σ, offset E, and gear ratio i₀ are set, the inclination angle Γ_s of the instantaneous axis, and the origins C₁(0, v_cs1, 0: C_s) and C₂(0, V_cs2, 0: C_s) of the coordinate systems C₁ and C₂ are represented by the following expressions.

sin Γ_s/sin(Σ−Γ_s)=i₀

v _cs2 =E tan Γ_s/{tan(Σ−Γ_s)+tan Γ_s}

v _cs1 =v _cs2 −E

The reference point P_w is set in the coordinate system C_s as follows.

P_w(u_cw,v_cw,z_cw:C_s)

If P_w is set as P_w(r_w, β_w, z_cw: C_s) by representing P_w with the cylindrical radius r_w of the relative velocity and the angle β_w from the u_c axis, the following expressions hold.

u_cw=r_w cos β_w

v_cw=r_w sin β_w

The pitch generating line L_pw is determined as a straight line which passes through the reference point P_w and which is parallel to the instantaneous axis (inclination angle Γ_s), and the pitch hyperboloids are determined as revolution bodies of the pitch generating line L_pw around the gear axes.

If the relative velocity of P_w is V_rsw, the angle ψ_rw between V_rsw and the pitch generating line L_pw is, based on expression (12),

tan ψ_rw=r_w sin Σ/{E sin(Σ−Γ_s)sin Γ_s}

Here, ψ_rw is the same anywhere on the same cylinder of the radius r_w.

When transformed into coordinate systems C₁ and C₂, P_w(u_1cw, v_1cw, z_1cw: C₁) P_w(u_2cw, v_2cw, z_2cw: C₂) and pinion and ring gear reference circle radii R_1w and R_2w can be expressed with the following expressions.

u _1cw =u _cw cos(Σ−Γ_s)+z_cw sin(Σ−Γ_s)

v _1cw =v _cw −v _cs1

z _1cw =−u _cw sin(Σ−Γ_s)+z_cw cos(Σ−Γ_s)

u _2cw =−u _cw cos Γ_s+z_cw sin Γ_s

v _2cw =v _cw −v _cs2

z _2cw =−u _cw sin Γ_s−z_cw cos Γ_s

R _1w ² =u _1cw ² +v _1cw ²

R _2w ² =u _2cw ² +v _2cw ²  (13)

4.2A Cones Passing through Reference Point P_w

A pitch hyperboloid which is a geometric design reference revolution body is difficult to manufacture, and thus in reality, in general, the gear is designed and manufactured by replacing the pitch hyperboloid with a pitch cone which passes through the point of contact P_w. The replacement with the pitch cones is realized in the present embodiment by replacing with cones which contact at the point of contact P_w.

The design reference cone does not need to be in contact at P_w, but currently, this method is generally practiced. When β_w is changed, the pitch angle of the cone which contacts at P_w changes in various manners, and therefore another constraint condition is added for selection of the design reference cone (β_w). The design method would differ depending on the selection of the constraint condition. One of the constraint conditions is the radius of curvature of the tooth trace in the Wildhaber (Gleason) method which is already described. In the present embodiment, β_w is selected with a constraint condition that a line of intersection between the cone which contacts at P_w and the surface of action coincides with the pitch generating line L_pw.

As described, there is no substantial difference caused by where on the path of contact g₀ the design reference point is selected. Therefore, a design method of a hypoid gear will be described in which the point of contact P_w is set as the design reference point and circular cones which contact at P_w are set as the pitch cones.

4.2A.1 Pitch Cone Angles

Intersection points between a plane S_nw normal to the relative velocity V_rsw of the reference point P_w and the gear axes are set as O_1nw and O_2nw (FIG. 9). FIG. 14 is a diagram showing FIG. 9 viewed from the positive directions of the tooth axes z_1c and z_2c, and intersections O_1nw and O_2nw can be expressed by the following expressions.

O_1nw(0,0,−E/(tan ε_2w sin Σ):C₁)

O_2nw(0,0,−E/(tan ε_1w sin Σ):C₂)

where sin ε_1w=v_1cw/R_1w and sin ε_2w=v_2cw/R_2w.

In addition, O_1nwP_w and O_2nwP_w can be expressed with the following expressions.

O _1nw P _w ={R _1w ²+(−E/(tan ε_2w sin Σ)−z_1cw)²}^1/2

O _2nw P _w ={R _2w ²+(−E/(tan ε_1w sin Σ)−z_2cw)²}^1/2

Therefore, the cone angles γ_pw and Γ_gw of the pinion and ring gear can be determined with the following expressions, taking advantage of the fact that O_1nwP_w and O_2nwP_w are back cone elements:

cos γ_pw=R_1w/O_1nwP_w

cos Γ_gw=R_2w/O_2nwP_w  (14).

The expression (14) sets the pitch cone angles of cones having radii of R_1w and R_2w and contacting at P_w.

4.2A.2 Inclination Angle of Relative Velocity at Reference Point P_w

The relative velocity and peripheral velocity are as follows.

V _rsw/ω₂₀={(E sin Γ_s)²+(r_w sin Σ/sin(Σ−Γ_s))²}^1/2

V _1w/ω₂₀=i₀R_1w

V _2w/ω₂₀=R_2w

When a plane defined by peripheral velocities V_1w and V_2w is S_tw, the plane S_tw is a pitch plane. If an angle formed by V_1w and V_2w is ψ_v12w and an angle formed by V_rsw and V_1w is ψ_vrs1w (FIG. 9),

cos(ψ_v12w)=(V_1w^2+V_2w²−V_rsw²)/(2V_1w×V_2w)

cos(ψ_vrs1w)=(V_rsw²+V_1w²−V_2w²)/(2V_1w×V_rsw)

If the intersections between the plane S_tw and the pinion and gear axes are O_1w and O_2w, the spiral angles of the pinion and the ring gear can be determined in the following manner as inclination angles on the plane S_tw from P_wO_1w and P_wO_2w (FIG. 9).

ψ_pw=π/2−ψ_vrs1w

ψ_gw=π/2−ψ_v12w−ψ_vrs1w  (15)

When a pitch point P_w, (r_w, β_w, z_cw: C_s) is set, specifications of the cones contacting at P_w and the inclination angle of the relative velocity V_rsw can be determined based on expressions (13), (14) and (15). Therefore, conversely, the pitch point P_w and the relative velocity V_rsw can be determined by setting three variables (for example, R_2w, ψ_pw, Γ_gw) from among the cone specifications and the inclination angle of the relative velocity V_rsw. Each of these three variables may be any variable as long as the variable represents P_w, and the variables may be, in addition to those described above, for example, a combination of a ring gear reference radius R_2w, a ring gear spiral angle ψ_gw, and a gear pitch cone angle Γ_gw, or a combination of the pinion reference radius R_1w, the ring gear spiral angle ψ_pw, and Γ_gw.

4.2A.3 Tip Cone Angle

Normally, an addendum a_G and an addendum angle α_G=a_G/O_2wP_w are determined and the tip cone angle is determined by Γ_gf=Γ_s+α_G. Alternatively, another value may be arbitrarily chosen for the addendum angle α_G.

4.2A.4 Inclination Angle of Normal g_w at Reference Point P_w

FIG. 15 shows the design reference point P_w and the contact normal g_w on planes S_tw, S_nw, and G_2w.

(1) Expression of Inclination Angle of g_w in Coordinate System C_s

An intersection between g_w passing through P_w (u_cw, v_cw, z_cw: C_s) and the plane S_H(β_w=0) is set as P₀ (u_c0, 0, z_c0: C_s) and the inclination angle of g_w is represented with reference to the point P₀ in the coordinate system C_s, by g_w (ψ₀, φ_n0: C_s). The relationship between P₀ and P_w is as follows (FIG. 11):

u _c0 =u _cw+(v_cw/cos ψ₀)tan φ_n0

z _c0 =z _cw −v _cw tan ψ₀  (16)

(2) Expression of Inclination Angle of g_w on Pitch Plane S_tw and Plane S_nw (FIG. 9)

When a line of intersection between the plane S_nw and the pitch plane S_tw is g_tw, an inclination angle on the plane S_nw from g_tw is set as φ_nw. The inclination angle of g_w is represented by g_w(ψ_gw, φ_Pnw) using the inclination angle ψ_gw of V_rsw from P_wO_2w on the pitch plane S_tw and φ_nw.

(3) Transformation Equation of Contact Normal g_w

In the following, transformation equations from g_w(ψ_gw, φ_nw) to g_w(ψ₀, φ_n0: C_s) will be determined.

FIG. 15 shows g_w(ψ_gw, φ_nw) with g_w(φ_2w, ψ_b2w: C₂). In FIG. 15, g_w is set with P_wA, and projections of point A are sequentially shown with B, C, D, and E. In addition, the projection points to the target sections are shown with prime signs (′) and double-prime signs (″). The lengths of the directed line segments are determined in the following manner, with P_wA=L_g:

$\begin{array}{cc}{A}^{\prime }A=L_g\sin {\varphi }_{\mathrm{nw}}B^{\prime }B=L_g\cos {\varphi }_{\mathrm{nw}}\cos {\psi }_{\mathrm{gw}}C^{\prime }C=A^{\prime }AP_wC^{\prime }=L_g\cos {\varphi }_{\mathrm{nw}}\sin {\psi }_{\mathrm{gw}}C^{\prime }K=P_wC^{\prime }/\tan {\Gamma }_{\mathrm{gw}}\begin{array}{c}{C}^{″}C=C^{\prime }C-C^{\prime }K)\sin {\Gamma }_{\mathrm{gw}}\\ =L_g\left(\sin {\varphi }_{\mathrm{nw}}-\cos {\varphi }_{\mathrm{nw}}\sin {\psi }_{\mathrm{gw}}/\tan {\Gamma }_{\mathrm{gw}}\right)\sin {\Gamma }_{\mathrm{gw}}\end{array}D^{\prime }D=B^{\prime }BP_wE=P_wAE^{\prime }E=C^{″}C\begin{array}{c}\sin {\psi }_{b2w}=E^{\prime }E/P_wE\\ =C^{″}C/L_g\\ =\left(\sin {\varphi }_{\mathrm{nw}}-\cos {\varphi }_{\mathrm{nw}}\sin {\psi }_{\mathrm{gw}}/\tan {\Gamma }_{\mathrm{gw}}\right)\sin {\Gamma }_{\mathrm{gw}}\end{array}& \left(17\right)\\ \tan {\eta }_{x2w}=C^{\prime }C/P_wC^{\prime }=\tan {\varphi }_{nw}/\sin {\psi }_{\mathrm{gw}}\begin{array}{c}{P}_wC={\left(P_wC^{\prime 2}+C^{\prime }C^2\right)}^{1/2}\\ =L_g\times {\left\{{\left(\cos {\varphi }_{\mathrm{nw}}\sin {\psi }_{\mathrm{gw}}\right)}^2+{\left(\sin {\varphi }_{\mathrm{nw}}\right)}^2\right\}}^{1/2}\end{array}\begin{array}{c}{P}_wC^{″}=P_wC\cos \left\{{\eta }_{x2w}-\left(\pi /2-{\Gamma }_{\mathrm{gw}}\right)\right\}\\ =P_wC\sin \left({\eta }_{\mathrm{xw}2}+{\Gamma }_{\mathrm{gw}}\right)\end{array}\begin{array}{c}\tan \left({\chi }_{2w}-{\varepsilon }_{2w}\right)=D^{\prime }D/P_wC^{″}\\ =\cos {\varphi }_{\mathrm{nw}}\cos {\psi }_{\mathrm{gw}}/[\{{\left(\cos {\varphi }_{\mathrm{nw}}\sin {\psi }_{\mathrm{gw}}\right)}^2+\\ {{\left(\sin {\varphi }_{\mathrm{nw}}\right)}^2\}}^{1/2}\times \sin \left({\eta }_{x2w}+{\Gamma }_{\mathrm{gw}}\right)]\end{array}{\varphi }_{2w}=\pi /2-{\chi }_{2w}& \left(18\right)\end{array}$

When g_w(φ_2w, ψ_b2w: C₂) is transformed from the coordinate system C₂ to the coordinate system C_s, g_w(ψ₀, φ_n0: C_s) can be represented as follows:

sin φ_n0=cos ψ_b2w sin φ_2w cos Γ_s+sin ψ_b2w sin Γ_s

tan ψ₀=tan φ_2w sin Γ_s−tan ψ_b2w cos Γ_s/cos φ_2w  (19)

With the expressions (17), (18), and (19), g_w(ψ_gw, φ_nw) can be represented by g_w(ψ₀, φ_n0: C_s).4.2B Reference Point P_w Based on R_2w, β_w, ψ_rw

As described above at the beginning of section 4.2A, the pitch cones of the pinion and the gear do not have to contact at the reference point P_w. In this section, a method is described in which the reference point P_w is determined on the coordinate system C_s without the use of the pitch cone, and by setting the gear reference radius R_2w, a phase angle β_w, and a spiral angle ψ_rw of the reference point.

The reference point P_w is set in the coordinate system C_s as follows:

P_w(u_cw,v_cw,z_cw:C_s)

When P_w is represented with the circle radius r_w of the relative velocity, and an angle from the u_c axis β_w, in a form of P_w(r_w, β_w, z_cw: C_s),

u_cw=r_w cos β_w

v_cw=r_w sin β_w

In addition, as the phase angle β_w of the reference point and the spiral angle ψ_rw are set based on expression (12) which represents a relationship between a radius r_w around the instantaneous axis of the reference point P_w and the inclination angle ψ_rw of the relative velocity,

r _w =E tan ψ_rw×sin(Σ−Γ_s)sin Γ_s/sin Σ

u_cw and v_cw are determined accordingly.

Next, P_w(u_cw, v_cw, z_cw: C_s) is converted to the coordinate system C₂ of rotation axis of the second gear. This is already described as expression (13).

u _2cw =−u _cw cos Γ_s+z_cw sin Γ_s

v _2cw =v _cw −v _cs2

z _2cw =−u _cw sin Γ_s−z_cw cos Γ_s  (13a)

Here, as described in section 4.1, v_cs2=E tan Γ_s/{tan(Σ−Γ_s)+tan Γ_s}. In addition, there is an expression in expression (13) describing:

R _2w ² =u _2cw ² +v _2cw ²  (13b)

Thus, by setting the gear reference radius R_2w, z_cw is determined based on expressions (13a) and (13b), and the coordinate of the reference point P_w in the coordinate system C_s is calculated.

Once the design reference point P_w is determined, the pinion reference circle radius R_1w can also be calculated based on expression (13).

Because the pitch generating line L_pw passing at the design reference point P_w is determined, the pitch hyperboloid can be determined. Alternatively, it is also possible to determine a design reference cone in which the gear cone angle Γ_gw is approximated to be a value around Γ_s, and the pinion cone angle γ_pw is approximated by Σ−Γ_gw. Although the reference cones share the design reference point P_w, the reference cones are not in contact with each other. The tip cone angle can be determined similarly to as in section 4.2A.3.

A contact normal g_w is set as g_w(ψ_rw, φ_nrw; C_rs) as shown in FIG. 10. The variable φ_nrw represents an angle, on the plane S_nw, between an intersecting line between the plane S_nw and the plane S_pw and the contact normal g_w. The contact normal g_w can be converted to g_w(ψ₀, φ_n0; C_s) as will be described later. Because ψ_pw and ψ_gw can be determined based on expression (15), the contact normal g_w can be set as g_w(ψ_pw, φ_nw; S_nw) similar to section 4.2A.4.

Conversion of the contact normal from the coordinate system C_rs to the coordinate system C_s will now be described.

(1) A contact normal g_w(ψ_rw, φ_nrw; C_rs) is set.

(2) When the displacement on the contact normal g_w is L_g, the axial direction components of the displacement L_g on the coordinate system C_rs are:

L _urs =−L _g sin φ_nrw

L _vrs =L _g cos φ_nrw·cos ψ_rw

L _zrs =L _g cos φ_nrw·sin ψ_rw

(3) The axial direction components of the coordinate system C_s are represented with (L_urs, L_vrs, L_zrs) as:

L _uc =L _urs·cos β_w−L_vrs·sin β_w

L _vc =L _urs·sin β_w+L_vrs·cos β_w

L_zc=L_zrs

(4) Based on these expressions,

$\begin{array}{c}{L}_{\mathrm{uc}}=-\left(L_g\sin {\varphi }_{\mathrm{nrw}}\right)\cos {\beta }_w-\left(L_g\cos {\varphi }_{\mathrm{nrw}}·\cos {\psi }_{\mathrm{rw}}\right)\sin {\beta }_w\\ =-L_g\left(\sin {\varphi }_{\mathrm{nrw}}·\cos {\beta }_w+\cos {\varphi }_{\mathrm{nrw}}·\cos {\psi }_{\mathrm{rw}}·\sin {\beta }_w\right)\end{array}$ $\begin{array}{c}{L}_{\mathrm{vc}}=-\left(L_g\sin {\varphi }_{\mathrm{nrw}}\right)\sin {\beta }_w+\left(L_g\cos {\varphi }_{\mathrm{nrw}}·\cos {\psi }_{\mathrm{rw}}\right)\cos {\beta }_w\\ =L_g\left(-\sin {\varphi }_{\mathrm{nrw}}·\sin {\beta }_w+\cos {\varphi }_{\mathrm{nrw}}·\cos {\psi }_{\mathrm{rw}}·\cos {\beta }_w\right)\end{array}$

(5) From FIG. 6, the contact normal g_w(ψ₀, φ_n0; C_s) is:

$\begin{array}{c}\tan {\psi }_0=L_{\mathrm{zc}}/L_{\mathrm{vc}}\\ =\cos {\varphi }_{\mathrm{nrw}}·\sin {\psi }_{\mathrm{rw}}/\left(-\sin {\varphi }_{\mathrm{nrw}}·\sin {\beta }_w+\cos {\varphi }_{\mathrm{nrw}}·\cos {\psi }_{\mathrm{rw}}·\cos {\beta }_w\right)\end{array}$ $\begin{array}{c}\sin {\varphi }_{n0}=-L_{\mathrm{uc}}/L_g\\ =\sin {\varphi }_{\mathrm{nrw}}·\cos {\beta }_w+\cos {\varphi }_{\mathrm{nrw}}·\cos {\psi }_{\mathrm{rw}}·\sin {\beta }_w\end{array}$

(6) From FIG. 11, the contact normal g_w(φ_s0, ψ_sw0; C₅) is:

tan φ_s0=−L_uc/L_vc=sin φ_nrw·cos β_w+cos φ_nrw·cos ψ_rw·sin β_w)/(−sin φ_nrw·sin β_w+cos φ_nrw·cos ψ_rw·cos β_w)

sin ψ_sw0=L_zc/L_g=cos φ_nrw·cos ψ_rw

The simplest practical method is a method in which the design reference point P_w is determined with β_w set as β_w=0, and reference cones are selected in which the gear cone angle is around Γ_gw=Γ_s and the pinion cone angle is around γ_pw=Σ−Γ_gw. In this method, because β_w=0, the contact normal g_w is directly set as g_w(ψ₀, φ_n0; C_s).

4.3 Tooth Trace Contact Ratio

4.3.1 General Equation of Tooth Trace Contact Ratio

An contact ratio m_f along L_pw and an contact ratio m_fcone along a direction of a line of intersection (P_wP_gcone in FIG. 13) between an arbitrary cone surface and S_w0 are calculated with an arbitrary point P_w on g_w=g₀ as a reference. The other contact ratios m_z and m_s are similarly determined.

Because the contact normal g_w is represented in the coordinate system C_s with g_w=g₀(ψ₀, φ_n0: C_s), the point P_w(u_2cw, v_2cw, z_2cw: C₂) represented in the coordinate system C₂ is converted into the point P_w(q_2cw, −R_b2w, z_2cw: C_q2) on the coordinate system C_q2 in the following manner:

$\begin{array}{cc}{q}_{2\mathrm{cw}}=u_{2\mathrm{cw}}\cos {\chi }_{20}+v_{2\mathrm{cw}}\sin {\chi }_{20}\begin{array}{c}{R}_{b2w}=u_{2\mathrm{cw}}\sin {\chi }_{20}-v_{2\mathrm{cw}}\cos {\chi }_{20}\\ =R_{2w}\cos \left({\varphi }_{20}+{\varepsilon }_{2w}\right)\end{array}{\chi }_{20}=\pi /2-{\varphi }_{20}\tan {\varepsilon }_{2w}=v_{2\mathrm{cw}}/u_{2\mathrm{cw}}R_{2w}={\left(u_{2\mathrm{cw}}^2+v_{2\mathrm{cw}}^2\right)}^{1/2}& \left(20\right)\end{array}$

The inclination angle g₀(φ₂₀, ψ_b20: C₂) of the contact normal g_w=g_o, the inclination angle φ_s0 of the surface of action S_w0, and the inclination angle ψ_sw0 of g₀ (=P₀P_Gswn) on S_w0 (FIGS. 12 and 13) are determined in the following manner:

(a) For Cylindrical Gears, Crossed Helical Gears, and Worm Gears

tan φ₂₀=tan φ_n0 cos(Γ_s−ψ₀)

sin ψ_b20=sin φ_n0 sin(Γ_s−ψ₀)

tan φ_s0=tan φ_n0 cos Γ₀

tan ψ_sw0=tan ψ₀ sin φ_s0

or sin ψ_sw0=sin φ_n0 sin ψ₀  (20a)

(b) For Bevel Gears and Hypoid Gears

tan φ₂₀=tan φ_n0 cos Γ_s/cos ψ₀+tan ψ₀ sin Γ_s

sin ψ_b20=sin φ_n0 sin Γ_s−cos φ_n0 sin ψ₀ cos Γ_s

tan φ_s0=tan φ_n0/cos ψ₀

tan ψ_sw0=tan ψ₀ cos φ_s0  (20b)

The derivation of φ_s0 and ψ_sw0 are detailed in, for example, Papers of Japan Society of Mechanical Engineers, Part C, Vol. 70, No. 692, c2004-4, Third Report of Design Theory of Power Transmission Gears.

In the following, a calculation is described in the case where the path of contact coincides with the contact normal g_w=g_o. If it is assumed that with every rotation of one pitch P_w moves to P_g, and the tangential plane W translates to W_g, the movement distance P_wP_g can be represented as follows (FIG. 11):

P _w P _g =P _gw =R _b2w(2θ_2p)cos ψ_b20  (21)

where P_gw represents one pitch on g₀ and 2θ_2p represents an angular pitch of the ring gear.

When the intersection between L_pw and W_g is P_1w, one pitch P_fw=P_1wP_w on the tooth trace L_pw is:

P _fw =P _gw/sin ψ_sw0  (22)

The relationship between the internal and external circle radii of the ring gear and the face width of the ring gear is:

R _2t =R _2h −F _g/sin Γ_gw

where R_2t and R_2h represent internal and external circle radii of the ring gear, respectively, F_g represents a gear face width on the pitch cone element, and Γ_gw represents a pitch cone angle.

Because the effective length F_lwp of the tooth trace is a length of the pitch generating line L_pw which is cut by the internal and external circles of the ring gear:

F _lwp={(R_2h²−v_2pw²)^1/2−(R_2t²−v_2pw²)^1/2}/sin Γ_s  (23)

Therefore, the general equation for the tooth trace contact ratio m_f would be:

m _f =F _lwp /P _fw  (24)

4.3.2 For Cylindrical Gear (FIG. 12)

The pitch generating line L_pw coincides with the instantaneous axis (Γ_s=0), and P_w may be anywhere on L_pw. Normally, P_w is taken at the origin of the coordinate system C_s, and, thus, P_w(u_cw, v_cw, z_cw: C_s) and the contact normal g_w=g₀(ψ₀, φ_n0: C_s) can be simplified as follows, based on expressions (20) and (20a):

P_w(0,0,0:C_s),P_w(0,−v_cs20:C₂)

P ₀(q_2pw=−v_cs2 sin χ₂₀,−R_b2w=−v_cs2 cos χ₂₀,0:C_q2)

φ₂₀=φ_s0,ψ_b20=−ψ_sw0

tan ψ_b20=−tan ψ_sw0=−tan ψ₀ sin φ₂₀

In other words, the plane S_w0 and the plane of action G₂₀ coincide with each other. It should be noted, however, that the planes are viewed from opposite directions from each other.

These values can be substituted into expressions (21) and (22) to determine the tooth trace contact ratio m_f with the tooth trace direction pitch P_fw and expression (24):

$\begin{array}{cc}{P}_{\mathrm{gw}}=R_{b2w}\left(2{\theta }_{2p}\right)\cos {\psi }_{b20}P_{\mathrm{fw}}=P_{\mathrm{gw}}/\sin {\psi }_{\mathrm{sw}0}=R_{b2w}\left(2{\theta }_{2p}\right)/\tan {\psi }_{b20}\begin{array}{c}{m}_f=F_{1\mathrm{wp}}/P_{\mathrm{fw}}\\ =F\tan {\psi }_0/R_{2w}\left(2{\theta }_{2p}\right)\\ =F\tan {\psi }_0/p\end{array}& \left(25\right)\end{array}$

where R_2w=R_b2w/sin φ₂₀ represents a radius of a ring gear reference cylinder, p=R_2w(2θ_2p) represents a circular pitch, and F=F_lwp represents the effective face width.

The expression (25) is a calculation equation of the tooth trace contact ratio of the cylindrical gear of the related art, which is determined with only p, F, and ψ₀ and which does not depend on φ_n0. This is a special case, which is only true when Γ_s=0, and the plane S_w0 and the plane of action G₂₀ coincide with each other.

4.3.3 For Bevel Gears and Hypoid Gears

For the bevel gears and the hypoid gears, the plane S_w0 does not coincide with G₂₀ (S_w0≠G₂₀), and thus the tooth trace contact ratio m_f depends on φ_n0, and would differ between the drive-side and the coast-side. Therefore, the tooth trace contact ratio m_f of the bevel gear or the hypoid gear cannot be determined with the currently used expression (25). In order to check the cases where the currently used expression (25) can hold, the following conditions (a), (b), and (c) are assumed:

(a) the gear is a bevel gear; therefore, the pitch generating line L_pw coincides with the instantaneous axis and the design reference point is P_w(0, 0, z_cw: C_s);

(b) the gear is a crown gear; therefore, Γ_s=π/2; and

(c) the path of contact is on the pitch plane; therefore, φ_n0=0.

The expressions (20), and (20b)-(24) can be transformed to yield:

φ₂₀=ψ₀, ψ_b20=0, φ_s0=0, ψ_sw0=ψ₀

R_b2w=R_2w cos φ₂₀=R_2w cos ψ₀

P _gw =R _b2w(2θ_2p)cos ψ_b20=R_2w(2θ_2p)cos ψ₀

P _fw =|P _gw/sin ψ_sw0|=|R_2w(2θ_2p)/tan ψ₀|

m _f =P _lwp /P _fw =F tan ψ₀/R_2w(2θ_2p)=F tan ψ₀/p  (26)

The expression (26) is identical to expression (25). In other words, the currently used expression (25) holds in bevel gears which satisfy the above-described conditions (a), (b), and (c). Therefore,

(1) strictly, the expression cannot be applied to normal bevel gears having Γ_s different from n/2 (Γ_s≠π/2) and φ_n0 different from 0 (φ_n0≠0); and

(2) in a hypoid gear (E≠0), the crown gear does not exist and ε_2w differs from 0 (ε_2w≠0).

For these reasons, the tooth trace contact ratios of general bevel gears and hypoid gears must be determined with the general expression (24), not the expression (26).

4.4 Calculation Method of Contact Ratio m_fcone Along Line of Intersection of Gear Pitch Cone and Surface of Action S_w0

The tooth trace contact ratios of the hypoid gear (Gleason method) is calculated based on the expression (26), with an assumption of a virtual spiral bevel gear of ψ₀=(ψ_pw+ψ_gw)/2 (FIG. 9), and this value is assumed to be sufficiently practical. However, there is no theoretical basis for this assumption. In reality, because the line of intersection of the gear pitch cone and the tooth surface is assumed to be the tooth trace curve, the contact ratio is more properly calculated along the line of intersection of the gear pitch cone and the surface of action S_w0 in the static coordinate system. In the following, the contact ratio m_fcone of the hypoid gear is calculated from this viewpoint.

FIG. 13 shows a line of intersection P_wP_gcone with an arbitrary cone surface which passes through P_w on the surface of action S_w0. Because P_wP_gcone is a cone curve, it is not a straight line in a strict sense, but P_wP_gcone is assumed to be a straight line here because the difference is small. When the line of intersection between the surface of action S_w0 and the plane v_2c=0 is P_sswP_gswn, the line of intersection P_sswP_gswn and an arbitrary cone surface passing through P_w have an intersection P_gcone, which is expressed in the following manner:

P_gcone(u_cgcone,v_cs2,z_cgcone:C_s)

P_gcone(u_2cgcone,0,z_c2gcone:C₂)

where

u _cgcone =u _cw+(v_cw−v_cs2)tan φ_s0

z _cgcone={(v_cs2−v_cw)/cos φ_s0} tan ψ_gcone+z_cw

u _2cgcone =−u _cgcone cos Γ_s+z_cgcone sin Γ_s

z _2cgcone =−u _cgcone sin Γ_s−z_cgcone cos Γ_s

ψ_gcone represents an inclination angle of P_wP_gcone from P₀P_ssw on S_w0.

Because P_gcone is a point on a cone surface of a cone angle Γ_gcone passing through P_w, the following relationship holds.

u _2cgcone −R _2w=−(z_2cgcone−z_2cw)tan Γ_gcone  (27)

When a cone angle Γ_gcone is set, ψ_gcone can be determined through expression (27). Therefore, one pitch P_cone along P_wP_gcone is:

P _cone =P _gw/cos(ψ_gcone−ψ_sw0)  (28)

The contact length F_lwpcone along P_wP_gcone can be determined in the following manner.

In FIG. 11, if an intersection between P_wP_gcone and L_pw0 is P_ws(u_cws, 0, z_cws: C_s),

u _cws =u _cw +v _cw tan φ_s0

z _cws =z _cw−(v_cw/cos φ_s0)tan ψ_gcone

If an arbitrary point on the straight line P_wP_gcone is set as Q(u_cq, v_cq, z_cq: C_s) (FIG. 11), u_cq and v_cq can be represented as functions of z_cq:

v _cq={(z_cq−z_cws)/tan ψ_gcone} cos φ_s0

u _cq =u _cws −v _cq tan φ_s0

If the point Q is represented in the coordinate system C₂ using expression (13), to result in Q(u_2cq, v_2cq, z_2cq: C₂), the radius R_2cq of the point Q is:

u _2cq =−u _cq cos Γ_s+z_cq sin Γ_s

v _2cq =v _cq −v _cs2

R _2cq=√(u_2cq²+v_2cq²)

If the values of z_cq where R_2cq=R_2h and R_2cq=R_2t are z_cqh and z_cqt, the contact length F_lwpcone is:

F _lwpcone=(z_cqh−z_cqt)/sin ψ_gcone  (29)

Therefore, the contact ratio m_fcone along P_wP_gcone is:

m _fcone =F _lwpcone /P _cone  (30)

The value of m_fcone where ψ_gcone→π/2 (expression (30)) is the tooth trace contact ratio m_f (expression (24)).

5. EXAMPLES

Table 1 shows specifications of a hypoid gear designed through the Gleason method. The pitch cone is selected such that the radius of curvature of the tooth trace=cutter radius R_c=3.75″. In the following, according to the above-described method, the appropriateness of the present embodiment will be shown with a test result by:

(1) first, designing a hypoid gear having the same pitch cone and the same contact normal as Gleason's and calculating the contact ratio m_fcone in the direction of the line of intersection of the pitch cone and the surface of action, and

(2) then, designing a hypoid gear with the same ring gear reference circle radius R_2w, the same pinion spiral angle ψ_pw, and the same inclination angle φ_nw of the contact normal, in which the tooth trace contact ratio on the drive-side and the coast-side are approximately equal to each other.

5.1 Uniform Coordinate Systems C_s, C₁, and C₂, Reference Point P_w and Pitch Generating Line L_pw

When values of a shaft angle Σ=90°, an offset E=28 mm, and a gear ratio i₀=47/19 are set, the intersection C, between the instantaneous axis and the line of centers and the inclination angle Γ_s of the instantaneous axis are determined in the following manner with respect to the coordinate systems C₁ and C₂:

C_s(0,24.067,0:C₂), C_s(0,−3.993,0:C₁), Γ_s=67.989°

Based on Table 1, when values of a ring gear reference circle radius R_2w=89.255 mm, a pinion spiral angle ψ_pw=46.988°, and a ring gear pitch cone angle Γ_gw=62.784° are set, the system of equations based on expressions (13), (14), and (15) would have a solution:

r_w=9.536 mm, β_w=11.10°, z_cw=97.021

Therefore, the pitch point P_w is:

P_w(9.358,1.836,97.021:C_s)

The pitch generating line L_pw is determined on the coordinate system C_s as a straight line passing through the reference point P_w and parallel to the instantaneous axis (Γ_s=67.989°).

In the following calculations, the internal and external circle radii of the ring gear, R_2t=73.87 and R_2h=105 are set to be constants.

5.2 Contact Ratio m_fconeD of Tooth Surfaced (Represented with Index of D) with Contact Normal g_wD

Based on Table 1, when g_wD is set with g_wD(ψ_gw=30.859°, φ_nwD=15°), g_wD can be converted into coordinate systems C_s and C₂ with expressions (17), (18), and (19), to yield:

g _wD(φ_20D=48.41°,ψ_b20D=0.20°:C₂)

g _wD(ψ_0D=46.19°,φ_n0D=16.48°:C_s)

The surface of action S_wD can be determined on the coordinate system C_s by the pitch generating line L_pw and g_wD. In addition, the intersection P_0d between g_wD and the plane S_H and the radius R_20D around the gear axis are, based on expression (16):

P_0D(10.142,0,95.107:C_s), R_20D=87.739 mm

The contact ratio m_fconeD in the direction of the line of intersection between the pitch cone and the surface of action is determined in the following manner.

The inclination angle φ^s0D of the surface of action S_wD, the inclination angle ψ_sw0D of g_wD on S_wD, and one pitch P_gwD on g_wD are determined, based on expressions (20), (20b), and (21), as:

φ_s0D=23.13°, ψ_sw0D=43.79°, P_gwD=9.894

(1) When Γ_gw=Γ_gcone=62.784° is set, based on expressions (27)-(30),

ψ_gcone63D=74.98°, P_cone63D=20.56,

F_lwpcone63D=34.98, m_fcone63D=1.701.

(2) When Γ_gcone=Γ_s=67.989° is set, similarly,

ψ_gcone68D=−89.99°, P_cone68D=14.30,

F_lwpcone68D=34.70, m_fcone68D=2.427.

(3) When Γ_gcone=72.0° is set, similarly,

ψ_gcone72D=78.88°, P_cone72D=12.09,

F_lwpcone72D=36.15, m_fcone72D=2.989.

5.3 Contact Ratio m_fconeC of Tooth Surface C (Represented with Index C) with Contact Normal g_wC

When g_wC(ψ_gw=30.859°, φ_nwC=−27.5°) is set, similar to the tooth surface D,

g _wC(φ_20C=28.68°,ψ_b20C=−38.22°:C₂)

g _wC(ψ_0c=40.15°,φ_n0c=−25.61°:C_s)

P _0C(8.206,0,95.473:C_s), R_20C=88.763 mm

The inclination angle φ_s0C of the surface of action S_wC, the inclination angle ψ_sw0C of g_wC on S_wc, and one pitch P_gwC on g_wC are, based on expressions (20), (20b), and (21):

φ_s0C=−32.10°, ψ_sw0C=35.55°, P_gwC=9.086

(1) When Γ_gw=Γ_gcone=62.784° is set, based on expressions (27)-(30),

ψ_gcone63C=81.08°, P_cone63C=12.971,

F_lwpcone63C=37.86, m_fcone63C=2.919.

(2) When Γ_gcone=Γ_s=67.989° is set, similarly,

ψ_gcone68C=−89.99°, P_cone68C=15.628,

F_lwpcone68C=34.70, m_fcone68C=2.220.

(3) When Γ_gcone=72° is set, similarly,

ψ_gcone72C=−82.92°, P_cone72C=19.061,

F_lwpcone72C=33.09, m_fcone72C=1.736.

According to the Gleason design method, because Γ_gw=Γ_gcone=62.784°, the contact ratio along the line of intersection between the pitch cone and the surface of action are m_fcone63D=1.70 and m_fcone63C=2.92, which is very disadvantageous for the tooth surface D. This calculation result can be considered to be explaining the test result of FIG. 16.

In addition, when the ring gear cone angle Γ_gcone=Γ_s=67.989°, ψ_gcone=−89.99° in both the drive-side and the coast-side. Thus, the line of intersection between the cone surface and the surface of action coincides with the pitch generating line L_pw, the tooth trace contact ratio of the present invention is achieved, and the contact ratio is approximately equal between the drive-side and the coast-side. Because of this, as shown in FIG. 17, a virtual pitch cone C_pv passing through a reference point P_w determined on the pitch cone angle of Γ_gw=62.784° and having the cone angle of Γ_gcone=67.989° and a pinion virtual cone (not shown) having the cone angle γ_pcone=Σ−γ_gcone=22.02° can be defined, and the addendum, addendum angle, dedendum, and dedendum angle of the hypoid gear can be determined according to the following standard expressions of gear design, with reference to the virtual pitch cone. In the gear determined as described above, the tooth trace contact ratio of the present embodiment can be realized along the virtual pitch cone angle.

α_g=Σδ_t×a_g/(a_g+a_p)  (31)

a _g +a _p =h _k (action tooth size)  (32)

where Σδ_t represents a sum of the ring gear addendum angle and the ring gear dedendum angle (which changes depending on the tapered tooth depth), α_g represents the ring gear addendum angle, a_g represents the ring gear addendum, and a_p represents the pinion addendum.

The virtual pitch cones C_pv of the ring gear and the pinion defined here do not contact each other, although the cones pass through the reference point P_w1.

The addendum and the addendum angle are defined as shown in FIGS. 18 and 19. More specifically, the addendum angle α_g of a ring gear 100 is a difference between cone angles of a pitch cone 102 and a cone 104 generated by the tooth tip of the ring gear, and the dedendum angle β_g is similarly a difference between cone angles of the pitch cone 102 and a cone 106 generated by the tooth root of the ring gear. An addendum a_g of the ring gear 100 is a distance between the design reference point P_w and the gear tooth tip 104 on a straight line which passes through the design reference point P_w and which is orthogonal to the pitch cone 102, and the dedendum b_g is similarly a distance between the design reference point P_w and the tooth root 106 on the above-described straight line. Similar definitions apply for a pinion 110.

By changing the pitch cone angle such that, for example, Γ_gw=72°>Γ_s, it is possible to design the tooth trace contact ratio to be larger on the tooth surface D and smaller on the tooth surface C. Conversely, by changing the pitch cone angle such that, for example, Γ_gw=62.784°<Γ_s, the tooth trace contact ratio would be smaller on the tooth surface D and larger on the tooth surface C.

A design method by the virtual pitch cone C_pv will now be additionally described. FIG. 17 shows a pitch cone C_p1 having a cone angle Γ_gw=62.784° according to the Gleason design method and the design reference point P_w1. As described above, in the Gleason design method, the drive-side tooth surface is disadvantageous in view of the contact ratio. When, on the other hand, the gear is designed with a pitch cone C_p2 having a cone angle Γ_gw=67.989°, the contact ratio can be improved. The design reference point P_w2 in this case is a point of contact between the pitch cones of the ring gear and the pinion. In other words, the design reference point is changed from the reference point P_w1 determined based on the Gleason design method to the reference point P_w2 so that the design reference point is at the point of contact between pitch cones of the ring gear and the pinion.

As already described, if the surface of action intersects the cone surface having the cone angle of Γ_gw=67.989° over the entire face width, the above-described tooth trace contact ratio can be realized. In other words, in FIG. 17, when the element of the cone (virtual pitch cone) passing through the design reference point P_w1 and having the cone angle of 67.989° exists in the gear tooth surface in the face width of the ring gear, the above-described contact ratio can be realized. In order to realize this, a method may be considered in which the addendum angle and the dedendum angle are changed according to the current method. However, this method cannot be realized due to the following reason.

In order for the cone surface having the cone angle of 67.989° (approximately 68°) and the surface of action to intersect over the entire face width without a change in the pitch cone C_p1, the ring gear addendum angle α_g may be increased so that the tip cone angle Γ_f is 68°. As shown in FIG. 20, by setting the gear addendum angle α_g to 5.216°, the tip cone angle Γ_f=68° is realized and the above-described tooth trace contact ratio is achieved along the tooth tip. However, if the tooth is designed according to the standard expressions (31) and (32), almost no dedendum of the ring gear exists, and the pinion would consist mostly of the dedendum. In this case, the pinion would have negative addendum modification, sufficient effective tooth surface cannot be formed, and the strength of the tooth of the pinion is reduced. Thus, such a configuration cannot be realized.

5.4 Hypoid Gear Specifications and Test Results when Γ_gw is Set Γ_gw=Γ_s=67.989°

Table 2 shows hypoid gear specifications when Γ_gw is set Γ_gw=Γ_s=67.989°. Compared to Table 1, identical ring gear reference circle radius R_2w=89.255 mm and pinion spiral angle ψ_pw=46.988° are employed, and the ring gear pitch cone angle is changed from Γ_gw=62.784° to 67.989°. As a result, P_w and Γ_gw differ as shown in FIG. 17 and, as will be described below, the other specifications are also different. The pitch cone of the gear is in contact with the pitch cone of the pinion at the reference point P_w.

Design reference point P_w(9.607, 0.825, 96.835: C_s)

Pinion cone radius R_1w=45.449 mm

Ring gear pitch cone angle Γ_gw=67.989°

Pinion pitch cone angle γ_pw=21.214°

Spiral angle on ring gear pitch plane ψ_gw=30.768°.

With the pressure angles φ_nwD and φ_nwC identical to Table 1, if g_wD(30.768°, 15°) and g_wC(30.768°, −27.5°) are set, the inclination angles would differ, in the static coordinate system C_s, from g_wD and g_wC of Table 1:

g _wD(ψ_0D=45.86°,φ_n0D=19.43°:C_s)

g _wC(ψ_0C=43.17°,φ_n0C=−22.99°:C_s)

The inclination angles of g_wD and g_wC on the surface of action, and one pitch are:

φ_s0D=26.86°, ψ_sw0D=42.59°, P_gwD=9.903

φ_s0C=−30.19°, ψ_sw0C=39.04°, P_gwC=9.094

The tooth trace contact ratios are calculated in the following manner based on expressions (22), (23), and (24):

Drive-side: P_fwD=14.63, F_lwpD=34.70, m_fD=2.371

Coast-side: P_fwC=14.44, F_lwpC=34.70, m_fC=2.403

FIG. 21 shows a test result of the specifications of Table 2, and it can be seen that, based on a comparison with FIG. 16, the transmission error is approximately equal between the drive-side and the coast-side, corresponding to the tooth trace contact ratios.

5.5 Specifications of Hypoid Gear when β_w=0

Table 3 shows specifications of a hypoid gear when β_w is set to 0 (β_w=0) in the method of determining the design reference point P_w based on R_2w, β_w, and ψ_rw described in section 4.2B.

6. COMPUTER AIDED DESIGN SYSTEM

In the above-described design of hypoid gears, the design is aided by a computer aided system (CAD) shown in FIG. 22. The CAD system comprises a computer 3 having a processor 1 and a memory 2, an inputting device 4, an outputting device 5, and an external storage device 6. In the external storage device 6, data is read and written from and to a recording medium. On the recording medium, a gear design program for executing the design method of the hypoid gear as described above is recorded in advance, and the program is read from the recording medium as necessary and executed by the computer.

The program can be briefly described as follows. First, a design request value of the hypoid gear and values of variables for determining a tooth surface are acquired. A pitch cone angle Γ_gcone of one gear is provisionally set and used along with the acquired values of the variables, and an contact ratio m_fconeD of the drive-side tooth surface and an contact ratio m_fconeC of the coast-side tooth surface based on the newly defined tooth trace as described above are calculated. The pitch cone angle Γ_gcone is changed and the calculation is repeatedly executed so that these contact ratios become predetermined values. When the contact ratios of the tooth surfaces become predetermined values, the pitch cone angle at this point is set as a design value Γ_gw, and the specifications of the hypoid gear are calculated. The predetermined value of the contact ratio designates a certain range, and values in the range. Desirably, the range of the contact ratio is greater than or equal to 2.0. The range may be changed between the drive-side and the coast-side. The initial value of the pitch cone angle Γ_gcone to be provisionally set is desirably set to the inclination angle Γ_s of the instantaneous axis S.

Another program calculates the gear specifications by setting the pitch cone angle Γ_gw to the inclination angle Γ_s of the instantaneous axis from the first place, and does not re-adjust the pitch cone angle according to the contact ratio. Because it is known that the contact ratios of the tooth surfaces become approximately equal to each other when the pitch cone angle Γ_gw is set to the inclination angle Γ_s of the instantaneous axis, such a program is sufficient as a simple method.

FIG. 23 shows a gear ratio and a tip cone angle (face angle) Γ_f of a uniform tooth depth in which a tooth depth is constant along a face width direction, manufactured through a face hobbing designed by the current method. The uniform tooth depth hypoid gear is a gear in which both the addendum angle α_g and the dedendum angle β_g of FIG. 19 are 0°, and, consequently, the tip cone angle Γ_f is equal to the pitch cone angle Γ_gw. The specifications of the tooth are determined by setting addendum and dedendum in the uniform tooth depth hypoid gear. In a uniform tooth depth hypoid gear, as shown in FIG. 25, while a cutter revolves around the center axis II of the ring gear 14, the cutter rotates with a cutter center cc as the center of rotation. With this motion, the edge of the cutter moves in an epicycloidal shape and the tooth trace curve is also in an epicycloidal shape. In hypoid gears having a ratio r_c/D_g0 between a cutter radius r_c and an outer diameter D_g0 of less than or equal to 0.52, a ratio E/D_g0 between an offset E and the outer diameter D_g0 of greater than or equal to 0.111, and a gear ratio of greater than or equal to 2 and less than or equal to 5, as shown in FIG. 23, hypoid gears around the inclination angle Γ_s of the instantaneous axis S are not designed. On the other hand, with the design method of the cone angle according to the present embedment, hypoid gears having the tip cone angle Γ_f which is around Γ_s can be designed.

FIG. 24 shows a gear ratio and a tip cone angle (face angle) Γ_f of a tapered tooth depth gear in which a tooth depth changes along a face width direction, manufactured through a face milling designed by the current method. As shown in FIG. 19, the tip cone angle Γ_f is a sum of the pitch cone angle Γ_gw and the addendum angle α_g, and is a value determined by a sum of the ring gear addendum angle and the ring gear dedendum angle, the ring gear addendum, and the pinion addendum, as shown in expression (31). In a tapered tooth depth hypoid gear, as shown in FIG. 26, a radius of curvature of the tooth trace of the ring gear 14 is equal to the cutter radius r_c. In hypoid gears having a ratio r_c/D_g0 between a cutter radius r_c and an outer diameter D_g0 of less than or equal to 0.52, a ratio E/D_g0 between an offset E and the outer diameter D_g0 of greater than or equal to 0.111, and a gear ratio of greater than or equal to 2 and less than or equal to 5, as shown in FIG. 24, hypoid gears of greater than or equal to the inclination angle Γ_s of the instantaneous axis S are not designed. On the other hand, the hypoid gear according to the present embedment has a tip cone angle of greater than or equal to Γ_s although the variables are within the above-described range, as shown with reference “A” in FIG. 24. Therefore, specifications that depart from the current method are designed.

	TABLE 1
	PINION	RING GEAR
SHAFT ANGLE Σ	90°
OFFSET E	28
NUMBER OF TEETH N1, N2	19	47
INCLINATION ANGLE Γs OF	67.989°
INSTANTANEOUS AXIS
CUTTER RADIUS Rc (RADIUS OF	3.75″
CURVATURE OF GEAR TOOTH
TRACE)
REFERENCE CIRCLE RADIUS	45.406	89.255
R1w, R2w
PITCH CONE ANGLE γpw, Γgw	26.291°	62.784°
SPIRAL ANGLE ON PITCH PLANE	46.988°	30.858°
φpw, φgw
TIP CONE ANGLE	30.728°	63.713°
INTERNAL AND EXTERNAL RADII		73.9, 105, (35)
OF GEAR (FACE WIDTH) R2t,
R2h (Fq)
GEAR ADDENDUM		1.22
GEAR DEDENDUM		6.83
GEAR WORKING DEPTH		7.15
CONTACT RATIO	DRIVE-SIDE	COAST-SIDE
(GLEASON METHOD)
PRESSURE ANGLE φnw	15°	−27.5°
TRANSVERSE CONTACT RATIO	1.13	0.78
TRACE CONTACT RATIO	2.45	2.45
(NEW CALCULATION METHOD	(1.70)	(2.92)
mfcone)

	TABLE 2
	PINION	RING GEAR
SHAFT ANGLE Σ	90°
OFFSET E	28
NUMBER OF TEETH N1, N2	19	47
INCLINATION ANGLE Γs OF	67.989°
INSTANTANEOUS AXIS
CUTTER RADIUS Rc (RADIUS OF	ARBITRARY
CURVATURE OF GEAR TOOTH
TRACE)
REFERENCE CIRCLE RADIUS	45.449	89.255
R1w, R2w
PITCH CONE ANGLE γpw, Γgw	21.214°	67.989°
SPIRAL ANGLE ON PITCH PLANE	46.988°	30.768°
φpw, φgw
TIP CONE ANGLE	25.267°	68.850°
INTERNAL AND EXTERNAL RADII		73.9, 105
OF GEAR R2t, R2h
GEAR ADDENDUM		1.22
GEAR DEDENDUM		6.83
GEAR WORKING DEPTH		7.15
CONTACT RATIO	DRIVE-SIDE	COAST-SIDE
(NEW CALCULATION METHOD)
PRESSURE ANGLE φnw	15°	−27.5°
TRANSVERSE CONTACT RATIO	1.05	0.85
ms
TOOTH TRACE CONTACT RATIO	2.37	2.40
mf

	TABLE 3
	PINION	RING GEAR
SHAFT ANGLE Σ	90°
OFFSET E	28
NUMBER OF TEETH N1, N2	19	47
INCLINATION ANGLE Γs OF	67.989°
INSTANTANEOUS AXIS
CUTTER RADIUS Rc (RADIUS OF	ARBITRARY
CURVATURE OF GEAR TOOTH
TRACE)
DESIGN REFERENCE POINT Pw	(9.73, 0, 96.64)
REFERENCE CIRCLE RADIUS	45.41	89.255
R1w , R2w
PITCH CONE ANGLE γpw , Γgw	22°	68°
SPIRAL ANGLE ψrw = ψ0	45°
TIP CONE ANGLE	22°	68°
INTERNAL AND EXTERNAL RADII		73.9, 105
OF GEAR (FACE WIDTH) R2t , R2h
GEAR ADDENDUM		1.22
GEAR DEDENDUM		6.83
GEAR WORKING TOOTH DEPTH		7.15
CONTACT RATIO	DRIVE-SIDE	COAST-SIDE
(NEW CALCULATION METHOD)
PRESSURE ANGLE φn0D , φn0C	18°	−20°
TRANSVERSE CONTACT RATIO ms	1.34	0.63
TOOTH TRACE CONTACT RATIO	2.43	2.64
mf

Claims

1. A method of designing a hypoid gear comprising a pair of gears including a first gear and a second gear, the method comprising: (a) setting a shaft angle Σ, an offset E, and a gear ratio i0 of a hypoid gear;(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i0, an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear, a line of centers vc with respect to rotational axes of the first gear and the second gear, an intersection Cs between the instantaneous axis S and the line of centers vc, and an inclination angle Γs of the instantaneous axis S with respect to the rotational axis of the second gear, to determine coordinate systems C1, C2, and Cs for calculation of specifications;(c) setting three variables including one of a reference circle radius R1w of the first gear and a reference circle radius R2w of the second gear, one of a spiral angle ψpw of the first gear and a spiral angle Γvgw of the second gear, and one of a pitch cone angle γpw of the first gear and a pitch cone angle Γgw of the second gear;(d) calculating a design reference point Pw, which is a common point of contact of pitch cones of the first gear and the second gear, and the three other variables which are not set in the step (c), based on the three variables which are set in the step (c);(e) setting a contact normal gwD of a drive-side tooth surface of the second gear;(f) setting a contact normal gwC of a coast-side tooth surface of the second gear; and(g) calculating specifications of the hypoid gear based on the design reference point Pw, the three variables which are set in the step (c), the contact normal gwD of the drive-side tooth surface of the second gear, and the contact normal gwC of the coast-side tooth surface of the second gear.

2. A method of designing a hypoid gear comprising a pair of gears including a first gear and a second gear, the method comprising: (a) setting a shaft angle Σ, an offset E, and a gear ratio i0 of a hypoid gear;(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i0, an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear, a line of centers vc with respect to rotational axes of the first gear and the second gear, an intersection Cs between the instantaneous axis S and the line of centers vc, and an inclination angle Γs of the instantaneous axis S with respect to the rotational axis of the second gear, to determine coordinate systems C1, C2, and Cs for calculation of specifications;(c) setting three variables including one of a reference circle radius R1w of the first gear and a reference circle radius R2w of the second gear, one of a spiral angle ψpw of the first gear and a spiral angle ψgw of the second gear, and one of a pitch cone angle γpw of the first gear and a pitch cone angle Γgw of the second gear;(d) calculating a design reference point Pw, which is a common point of contact of pitch cones of the first gear and the second gear and the three other variables which are not set in the step (c), based on the three variables which are set in the step (c);(e) calculating a pitch generating line Lpw which passes through the design reference point Pw and which is parallel to the instantaneous axis S;(f) setting an internal circle radius R2t and an external circle radius R2h of the second gear;(g) setting a contact normal gwD of a drive-side tooth surface of the second gear;(h) calculating an intersection P0D between a reference plane SH which is a plane orthogonal to the line of centers vc and passing through the intersection Cs and the contact normal gwD and a radius R20D of the intersection P0D around a gear axis;(i) calculating an inclination angle φs0D of a surface of action SwD which is a plane defined by the pitch generating line Lpw and the contact normal gwD with respect to the line of centers vc, an inclination angle ψsw0D of the contact normal gwD on the surface of action SwD with respect to the instantaneous axis S, and one pitch PgwD on the contact normal gwD;(j) setting a provisional second gear pitch cone angle Γgcone, and calculating an contact ratio mfconeD of the drive-side tooth surface based on the internal circle radius R2t and the external circle radius R2h;(k) setting a contact normal gwC of a coast-side tooth surface of the second gear;(l) calculating an intersection P0C between the reference plane SH which is a plane orthogonal to the line of centers vc and passing through the intersection Cs and the contact normal gwC and a radius R20C of the intersection P0C around the gear axis;(m) calculating an inclination angle φs0C of a surface of action SwC which is a plane defined by the pitch generating line Lpw and the contact normal gwC with respect to the line of centers vc, an inclination angle ψsw0C of the contact normal gwC on the surface of action SwC with respect to the instantaneous axis S, and one pitch PgwC on the contact normal gwC;(n) setting a provisional second gear pitch cone angle Γgcone, and calculating an contact ratio mfconeC of the coast-side tooth surface based on the internal circle radius R2t and the external circle radius R2h;(o) comparing the contact ratio mfconeD of the drive-side tooth surface and the contact ratio mfconeC of the coast-side tooth surface, and determining whether or not these contact ratios are predetermined values;(p) when the contact ratios of the drive-side and the coast-side are the predetermined values, replacing the provisional second gear pitch cone angle Γgcone with the second gear pitch cone angle Γgw obtained in the step (c) or in the step (d);(q) when the contact ratios of the drive-side and the coast-side are not the predetermined values, changing the provisional second gear pitch cone angle Γgcone and re-executing from the step (g);(r) re-determining the design reference point Pw, the other one of the reference circle radius R1w of the first gear and the reference circle radius R2w of the second gear which is not set in the step (c), the other one of the spiral angle ψpw of the first gear and the spiral angle ψgw of the second gear which is not set in the step (c), and the first gear pitch cone angle γpw based on the one of the reference circle radius R1w of the first gear and the reference circle radius R2w of the second gear which is set in the step (c), the one of the spiral angle ψpw of the first gear and the spiral angle ψgw of the second gear which is set in the step (c), and the second gear pitch cone angle Γgw which is replaced in the step (p), and(s) calculating specifications of the hypoid gear based on the specifications which are set in the step (c), the specifications which are re-determined in the step (r), the contact normal gwD of the drive-side tooth surface of the second gear, and the contact normal gwC of the coast-side tooth surface of the second gear.

3. A method of designing a hypoid gear comprising a pair of gears including a first gear and a second gear, the method comprising: (a) setting a shaft angle Σ, an offset E, and a gear ratio i0 of a hypoid gear;(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i0, an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear, a line of centers vc with respect to rotational axes of the first gear and the second gear, an intersection Cs between the instantaneous axis S and the line of centers vc, and an inclination angle Γs of the instantaneous axis S with respect to the rotational axis of the second gear, to determine coordinate systems C1, C2, and Cs for calculation of specifications;(c) setting three variables including one of a reference circle radius R1w of the first gear and a reference circle radius R2w of the second gear, one of a spiral angle ψpw of the first gear and a spiral angle ψgw of the second gear, and one of a pitch cone angle γpw of the first gear and a pitch cone angle Γgw of the second gear;(d) calculating a design reference point Pw, which is a common point of contact of pitch cones of the first gear and the second gear, and the three other variables which are not set in the step (c), based on the three variables which are set in the step (c);(e) calculating a pitch generating line Lpw which passes through the design reference point Pw and which is parallel to the instantaneous axis S;(f) setting an internal circle radius R2t and an external circle radius R2h of the second gear;(g) setting a contact normal gwD of a drive-side tooth surface of the second gear;(h) calculating an intersection P0D between a reference plane SH which is a plane orthogonal to the line of centers vc and passing through the intersection Cs and the contact normal gwD and a radius R20D of the intersection P0D around a gear axis;(i) calculating an inclination angle φs0D of a surface of action SwD which is a plane defined by the pitch generating line Lpw and the contact normal gwD with respect to the line of centers vc, an inclination angle ψsw0D of the contact normal gwD on the surface of action SwD with respect to the instantaneous axis S, and one pitch PgwD on the contact normal gwD;(j) setting a provisional second gear pitch cone angle Γgcone, and calculating an contact ratio mfconeD of the drive-side tooth surface based on the internal circle radius R2t and the external circle radius R2h;(k) setting a contact normal gwC of a coast-side tooth surface of the second gear;(l) calculating an intersection P0C between a reference plane SH which is a plane orthogonal to the line of centers vc and passing through the intersection Cs and the contact normal gwC and a radius R20C of the intersection P0C around the gear axis;(m) calculating an inclination angle φs0C of a surface of action SwC which is a plane defined by the pitch generating line Lpw and the contact normal gwC with respect to the line of centers vc, an inclination angle ψsw0C of the contact normal gwC on the surface of action SwC with respect to the instantaneous axis S, and one pitch PgwC on the contact normal gwC;(n) setting a provisional second gear pitch cone angle Γgcone, and calculating an contact ratio mfconeC of the coast-side tooth surface based on the internal circle radius R2t and the external circle radius R2h;(o) comparing the contact ratio mfconeD of the drive-side tooth surface and the contact ratio mfconeC of the coast-side tooth surface, and determining whether or not these contact ratios are predetermined values;(p) when the contact ratios of the drive side and the coast-side are not the predetermined values, changing the provisional second gear pitch cone angle Γgcone and re-executing from the step (g);(q) when the contact ratios of the drive side and the coast-side are the predetermined values, defining a virtual cone having the provisional second gear pitch cone angle Γgcone as a cone angle;(r) calculating a provisional pitch cone angle γpcone of the virtual cone of the first gear based on the determined pitch cone angle Γgcone; and(s) calculating specifications of the hypoid gear based on the design referenced point Pw, the reference circle radius R1w of the first gear and the reference circle radius R2w of the second gear which are set in the step (c) and the step (d), the spiral angle ψpw of the first gear and the spiral angle ψgw of the second gear which are set in the step (c) and the step (d), the cone angle Γgcone of the virtual cone and the cone angle γpcone of the virtual cone which are defined in the step (q) and the step (r), the contact normal gwD of the drive-side tooth surface of the second gear, and the contact normal gwC of the coast-side tooth surface of the second gear.

4. The method of designing a hypoid gear according to claim 2, wherein in the step (j) and the step (n), the provisional second gear pitch cone angle Γgcone which is initially set is the inclination angle Γs of the instantaneous axis S.

5. The method of designing a hypoid gear according to claim 2, wherein the predetermined values of the contact ratios in the step (o) are tooth trace contact ratios of greater than or equal to 2.0.

6. A method of designing a hypoid gear comprising a pair of gears including a first gear and a second gear, the method comprising: (a) setting a shaft angle Σ, an offset E, and a gear ratio i0 of a hypoid gear;(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i0, an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear, a line of centers vc with respect to rotational axes of the first gear and the second gear, an intersection Cs between the instantaneous axis S and the line of centers vc, and an inclination angle Γs of the instantaneous axis with respect to the rotational axis of the second gear, and to determine coordinate systems C1, C2, and Cs for calculation of specifications;(c) determining the inclination angle Γs of the instantaneous axis as a second gear pitch cone angle Γgw; and(d) calculating specifications of the hypoid gear based on the determined second gear pitch cone angle Γgw.

7. A method of designing a hypoid gear comprising a pair of gears including a first gear and a second gear, the method comprising: (a) setting a shaft angle Σ, an offset E, and a gear ratio i0 of a hypoid gear;(b) calculating, based on the shaft angle Σ, the offset E, and the gear ratio i0, an instantaneous axis S which is an axis of a relative angular velocity of a first gear and a second gear, a line of centers vc with respect to rotation axes of the first gear and the second gear, an intersection Cs between the instantaneous axis S and the line of centers vc, and an inclination angle Γs of the instantaneous axis S with respect to the rotation axis of the second gear, to determine coordinate systems C1, C2, and Cs for calculation of specifications;(c) setting one of a reference circle radius R1w of the first gear and a reference circle radius R2w of the second gear, a spiral angle ψrw, and a phase angle βw of a design reference point Pw, to determine the design reference point;(d) calculating the design reference point Pw and a reference circle radius which is not set in the step (c) from a condition where the first gear and the second gear share the design reference point Pw, based on the three variables which are set in the step (c);(e) setting one of a reference cone angle γpw of the first gear and a reference cone angle Γgw of the second gear;(f) calculating a reference cone angle which is not set in the step (e), based on the shaft angle Σ and the reference cone angle which is set in the step (e);(g) setting a contact normal gwD of a drive-side tooth surface of the second gear;(h) setting a contact normal gwC of a coast-side tooth surface of the second gear; and(i) calculating specifications of the hypoid gear based on the design reference point Pw, the reference circle radii R1w and R2w and the spiral angle ψrw which are set in the step (c) and the step (d), the reference cone angles γpw and Γgw which are set in the step (e) and the step (f), and the contact normals gwC and gwD which are set in the step (g) and the step (h).

8. A hypoid gear designed by the design method of hypoid gear according to claim 1.

9. A hypoid gear comprising a pair of gears including a first gear and a second gear, wherein a tip cone angle Γf of the second gear is set at a value in a predetermined range around an inclination angle Γs of an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear with respect to a rotational axis of the second gear, and the predetermined range is a range in which each of tooth trace contact ratios of a drive-side tooth surface and a coast-side tooth surface is greater than or equal to 2.0.

10. A hypoid gear comprising a pair of gears including a first gear and a second gear, wherein a tip cone angle Γf of the second gear is set at a value in a predetermined range around an inclination angle Γs of an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear with respect to a rotational axis of the second gear.

11. A hypoid gear comprising a pair of gears including a first gear and a second gear, wherein a tip cone angle Γf of the second gear is set at a value which is greater than or equal to an inclination angle Γs of an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear with respect to a rotational axis of the second gear.

12. A uniform tooth depth hypoid gear comprising a pair of gears including a first gear and a second gear and in which a tooth depth is constant along a face width direction and a tooth trace shape is such that a radius of curvature of a tooth trace is not constant, wherein a ratio rc/Dg0 between a cutter radius r, and an outer diameter Dg0 is less than or equal to 0.52, a ratio E/Dg0 between an offset E and the outer diameter Dg0 is greater than or equal to 0.111, and a gear ratio is greater than or equal to 2 and less than or equal to 5, anda tip cone angle Γf of the second gear is set at a value which is within a range of ±5° of an inclination angle Γs of an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear with respect to a rotational axis of the second gear.

13. The uniform tooth depth hypoid gear according to claim 12, wherein the tooth trace shape is an epicycloidal shape.

14. An tapered tooth depth hypoid gear comprising a pair of gears including a first gear and a second gear and in which a tooth depth changes along a face width direction, wherein a ratio rc/Dg0 between a radius of curvature rc of a tooth trace of the second gear and an outer diameter Dg0 is less than or equal to 0.52, a ratio E/Dg0 between an offset E and the outer diameter Dg0 is greater than or equal to 0.111, and a gear ratio is greater than or equal to 2 and less than or equal to 5, anda tip cone angle Γr of the second gear is set at a value which is greater than or equal to an inclination angle Γs, of an instantaneous axis S which is an axis of a relative angular velocity of the first gear and the second gear with respect to a rotational axis of the second gear, and less than or equal to (Γs+5)°.

15. (canceled)

16. A storage medium which stores therein a program which, when executed, causes a computer to execute a design method of hypoid gear according to claim 1.

17. The method of designing a hypoid gear according to claim 3, wherein in the step (j) and the step (n), the provisional second gear pitch cone angle Γgcone which is initially set is the inclination angle Γs of the instantaneous axis S.

18. The method of designing a hypoid gear according to claim 3, wherein the predetermined values of the contact ratios in the step (o) are tooth trace contact ratios of greater than or equal to 2.0.

19. The method of designing a hypoid gear according to claim 4, wherein the predetermined values of the contact ratios in the step (o) are tooth trace contact ratios of greater than or equal to 2.0.

20. The method of designing a hypoid gear according to claim 17, wherein the predetermined values of the contact ratios in the step (o) are tooth trace contact ratios of greater than or equal to 2.0.

21. A hypoid gear designed by the design method of hypoid gear according to claim 2.

22. A hypoid gear designed by the design method of hypoid gear according to claim 3.

23. A hypoid gear designed by the design method of hypoid gear according to claim 6.

24. A hypoid gear designed by the design method of hypoid gear according to claim 7.

25. A storage medium which stores therein a program which, when executed, causes a computer to execute a design method of hypoid gear according to claim 2.

26. A storage medium which stores therein a program which, when executed, causes a computer to execute a design method of hypoid gear according to claim 3.

27. A storage medium which stores therein a program which, when executed, causes a computer to execute a design method of hypoid gear according to claim 6.

28. A storage medium which stores therein a program which, when executed, causes a computer to execute a design method of hypoid gear according to claim 7.