Patent Application
20090062058
1. Field of the Invention
The present invention relates generally to a power transmission, particularly to a planetary gear set having helical gear teeth.
2. Description of the Prior Art
Due to noise associated with the operation of spur gears, it has become conventional for gear assemblies of automotive transmissions to include helical gearing to provide quieter operation. However, a recognized design constraint associated with helical planetary gear sets is the axial thrust component of the tangential loads that transmit torsion between the mating gear teeth. The axial component is caused by the helix angles of the mating gear teeth.
In a planetary gear unit, a sun gear and ring gear mesh with each planet pinion. Thrust loads, applied to each planet pinion in opposite axial directions at the lines of contact due to its two meshing engagements, induce an overturning moment on each planet pinion. The reaction to the overturning moment causes additional loading of the bearing, as well as increasing the probability of end loading of the needles on which each planet pinion is supported on its pinion shaft. The additional loading reduces bearing life. In some cases, pinions are added to the assembly to increase bearing life even though the gear life is adequate.
The axial forces cause unwanted loading of other components both inside and outside of the planetary assembly Within the assembly, the axial/moment loads produce an adverse effect on bearings, washers, pinion shafts, carrier surfaces, and pinion bores caused by the overturning moment. Multiple thrust bearings, which are added to most transmission assemblies to react the thrust loads, increase the package space required for the assembly.
In addition, the thrust loads must be reacted outside the assembly, thereby requiring use of expensive external thrust bearings. These requirements increase the manufacturing and assembly cost of the transmission assembly and increase the length of the assembly, which is at a premium particularly in vehicles having front wheel drive, in which the transmission and engine are arranged laterally with respect to the longitudinal axis of the vehicle.
In the automotive industry, it is known that herringbone gears can be used to address the thrust loads associated with conventional helical gearing. Because herringbone/double helical gears are difficult and costly to manufacture, as well as difficult to assemble, they are rarely used in automotive drive trains, transmissions or transfer cases.
A need exists for a technique to limit or avoid use of trust bearings in transmission assemblies and to eliminate the overturning moment induced in planetary transmission planet pinions due to the thrust loads, yet maintain the quiet gear operation associated with helical gear teeth.
The planetary gear set includes a sun gear including a first set of gear teeth having a first helix angle and second set of gear teeth formed as a single unit with the first set of gear teeth and having a second helix angle of opposite hand relative to the first helix angle. A ring gear includes a third set of gear teeth having a third helix angle, and fourth set of gear teeth having a fourth helix angle of opposite hand relative to the third helix angle. Planet pinions, each include a fifth set of gear teeth having a fifth helix angle, and sixth set of gear teeth formed as a single unit with the fifth set of gear teeth and having a sixth helix angle of opposite hand relative to the fifth helix angle.
The axial force component or thrust forces developed at each mesh on the planet pinions have equal magnitude and opposite direction. The thrust force component developed at a mesh on each planet pinion is cancelled within the respective planet pinion by the thrust force component developed at the other mesh on the respective planet pinion. Therefore, there is substantially no unbalanced net thrust force component on any planet pinion that requires a reaction force to place the pinion in structural equilibrium, and no provision for a reaction force need be provided at the pinions.
Similarly, the overturning moment that is induced by the axial force component developed at each mesh is cancelled within the respective pinion by the overturning moment developed at the other mesh on the respective planet pinion. Therefore, there is substantially no unbalanced net overturning moment on any planet pinion that would requires a reaction moment/load to place the pinion in structural equilibrium. Consequently, the bearing that supports a respective pinion on the pinion shaft has reduced loading due to the elimination of the overturning moment caused by the axial load component of the mesh helix angle and reducing the risk of end loading as well.
The size of the gear set is smaller and its weight is less compared to those of a conventional planetary gear set having the same torque capacity, yet it compares favorably with respect to noise, vibration and harshness. The costs to manufacture and assemble the gear set are lower than those costs of a conventional planetary gear set.
The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art, such as both simple and compound Ravigneaux carriers.
These and other advantages will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which:
Referring now to
A helix is a curve wound around the outer surface of a cylinder or cone that advances uniformly along the axis of the cylinder or cone as it winds around. A helix angle is the angle that a straight tangent to the helix at any point makes with an element of the cylinder or cone, such as a diametric plane perpendicular to the axis or a diametric plane parallel to the axis.
Sun gear 20 is an integral unit, i.e., a unitary component formed in one piece without any connections, such as mechanical attachments, frictional engagements, interference fits or chemical bonds among constituent parts. Preferably sun gear 20 is formed as an integral unit of sintered powdered metal.
Each planet pinion 26 is an integral unit, i.e., a unitary component formed in one piece without any connections, such as mechanical attachments, frictional engagements, interference fits or chemical bonds among constituent parts. Preferably each planet pinion 26 is formed as an integral unit of sintered powdered metal.
Similarly,
The first set of gear teeth 60 of ring gear 22 are preferably indexed or offset one-half tooth pitch with respect to the teeth of the second set 64 uniformly about the axis 32 of ring gear 22. The pitch offset between the sets of teeth 60, 64 may be uniform about axis 32, but different than one-half tooth pitch, or the teeth of gear sets 60, 64 may be aligned mutually with no pitch offset. The teeth of gear set 50 mesh with and engage the teeth of gear set 60 on ring gear 22, and the teeth of gear set 52 mesh with and engage the teeth of gear set 62 on the sun gear.
As
The outer surface of a circular cylinder 82 of the first ring gear portion 70 is formed with a series of angularly-spaced, radial teeth 78 and radial slots 80 angularly spaced about axis 32, each slot being located between adjacent teeth 78.
Cylinder 82 extends along axis 32 from the radial slots 80 and radial teeth 78 to a series of axial teeth 84 and spaces 86, each space being located between adjacent teeth 84 and having on open axial end. The profile of each tooth 84 is that of a trapezoid having an axial surface 88 directed along axis 32, a surface 90 extending from an axial edge 92 of surface 82 and inclined axially and circumferentially, and a circumferential surface 92 connecting surfaces 88 and 90 at the base of each tooth 84.
The outer surface of a circular cylinder 100 of the second ring gear portion 74 is formed with a series of axial teeth 102 and axial recesses 104, angularly-spaced about axis 32. The profile of each recess 104 is complementary to the profile of each tooth 84 on the first ring gear portion 70 and has an open end 106 for accepting a tooth 84 when it is inserted axially into a recess 104. Similarly, the profile of each axial tooth 102 is complementary to the profile of each axial space 86 on the first ring gear portion 70. Preferably, the number of recesses 104 is equal to the number of teeth 84, the number of spaces 86 is equal to the number of teeth 102, each axial recesses 104 is aligned with a corresponding axial tooth 84, and each axial space 86 is aligned with a corresponding axial tooth 102. When the recesses 104, spaces 86 and teeth 84, 102 engage mutually they form a drive connection between the portions 70 and 74 of ring gear 22. That connection provides torsion continuity between portions 70, 74, permits their mutual disengagement, and is indexed such that the gear teeth sets 50-60 and 52-66 are engaged with the proper pitch offset, if a pitch offset is employed.
The left-hand or right-hand helix angle of the helical gear sets 40, 50, 60 are reversed with respect to that of the corresponding helical gear sets 42, 52, 64, and they are phased or indexed, i.e., circumferentially offset mutually. For example, if the helix angle is right-handed for gear teeth 40, then the helix orientation is left-handed for gear set 42. The helical gear teeth 50 of pinions 26 are circumferentially indexed by one-half tooth pitch relative to the helical gear teeth 52 of pinions 26, and the helical gear teeth 40 of sun gear 20 are circumferentially indexed by one-half tooth pitch relative to the helical gear teeth 42 of sun gear 20. Similarly, helical gear teeth 60 of ring gear 22 are circumferentially indexed by one-half tooth pitch relative to the helical gear teeth 64 of the ring gear 22.
The axial force or thrust force component transmitted to the gear sets 5052 of the planet pinions 26 due to their engagement with the ring gear 22 and sun gear 20 are of equal magnitude and opposite direction. For example, the thrust force component on each gear set 50, 52 of planet pinion 26 is cancelled within the respective planet pinion by the thrust force component developed at the other gear set on the respective planet pinion. Therefore, substantially no unbalanced net thrust force component is present on any planet pinion that requires a reaction force to place the pinion in structural equilibrium, and no provision for a reaction force is provided at the pinions 26.
Similarly, any overturning moment that is induced by the thrust force component transmitted to each gear set 50, 52 of pinions 26 is cancelled within the respective pinion by the overturning moment transmitted to other gear set on the respective planet pinion. Therefore, substantially no unbalanced net overturning moment is applied to any planet pinion 26 that would require a reaction moment to place the pinion in structural equilibrium. Consequently, the bearing that supports a pinion 26 on the pinion shaft 30 has virtually no end loading due to an unbalanced overturning moment on the pinion.
Sun gear 20 and ring 48 are alternately held on case 12 against rotation and released by a hydraulically-actuated brake 120, which is actuated by a servo 122 that includes a cylinder 124, a piston 124 located in the cylinder, return spring 128 for restoring the piston to the disengaged position of
Similarly a drive connection of sun gear 20 and ring 48 to another component 138 of transmission 10 can be opened and closed alternately by a hydraulically-actuated clutch 140, which is actuated by a servo 142 that includes the cylinder 134, a piston 146 located in the cylinder, return spring 148 for restoring the piston to the disengaged position of
In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.
