Patent Application
20180195602
The present invention relates to improvements on a worm reducer which is to be used in various mechanical apparatuses such as an electric power steering device.
When applying a steering angle to steered wheels (generally, front wheels except for a special vehicle such as a forklift), a power steering device has been widely used as a device for reducing a force necessary for a driver to operate a steering wheel.
As such power steering device, there has been recently used an electric power steering device configured to use an electric motor as an auxiliary power source. The electric power steering device is advantageous as compared to a hydraulic power steering device in that the electric power steering device can be made smaller and lighter, an amount (torque) of auxiliary power can be easily controlled, and a power loss of an engine is smaller, for example.
Various structures have been known for the electric power steering device. In any structure, auxiliary power of the electric motor is applied via a reducer to a steering shaft configured to rotate in accordance with an operation of the steering wheel or a member (a pinion shaft or a rack shaft configuring a steering gear unit) configured to be displaced in accordance with rotation of the steering shaft. As the reducer, a worm reducer is generally used. In the case of the electric power steering device using the worm reducer, a worm shaft configured to rotate by the electric motor and a worm wheel configured to rotate together with a rotary shaft, which is an output part of the worm reducer, are meshed with each other such that the auxiliary power of the electric motor can be transmitted to the rotary shaft.
For example, Patent Document 1 discloses an electric power steering device as shown in
In the worm reducer including the worm wheel 4 and the worm shaft 5, a backlash exists inevitably at a meshing portion between the worm wheel 4 and the worm teeth 6. The backlash is caused by a dimensional error and a mounting error of each member configuring the worm reducer and is also caused as tooth surfaces of the worm wheel 4 and the worm teeth 6 are worn. If the backlash exists at the meshing portion, when changing a rotating direction of the rotary shaft 2, uncomfortable gear striking noise may be generated at the meshing portion. Therefore, in the shown structure, a pressing piece 11 is externally fitted to a part of the tip end portion of the worm shaft 5, which protrudes beyond the tip end-side bearing 8, and an elastic member such as a coil spring 12 is provided between the pressing piece 11 and the housing 3. The worm teeth 6 of the worm shaft 5 are pressed by the coil spring 12 towards the worm wheel 4 via the pressing piece 11. By this configuration, the backlash between the worm teeth 6 and the worm wheel 4 is suppressed, so that the gear striking noise is suppressed.
According to the above conventional structure, the worm shaft 5 has the part, to which the pressing piece 11 is externally fitted, at the further tip end-side than a part which is supported by the tip end-side bearing 8. Therefore, an axial dimension of the worm shaft 5 is longer for that length, so that there is a room for improvement.
Patent Document 1: JP-B-4381024
The present invention has been made in view of the above circumstances, and an object is to provide a worm reducer including an urging unit configured to urge a worm shaft in a meshing direction so as to suppress backlash at a meshing portion between a worm wheel and worm teeth and having a structure capable of reducing an axial dimension of a tip end-side portion of the worm shaft.
A worm reducer of the present invention includes a housing, a worm wheel, a worm shaft, a tip end-side bearing, a rocking member, an urging member and a guide member.
The worm wheel is rotatably supported to the housing.
The worm shaft rotatably supported to the housing with worm teeth provided on an axially intermediate part thereof being meshed with the worm wheel, and a rotational force from a power source is transmitted to a base end portion of the worm shaft.
The tip end-side bearing is configured to support a tip end-side portion of the worm shaft rotatably with respect to the housing;
The rocking member is provided around the tip end-side bearing inside the housing and is configured to rock about a rocking shaft extending in a direction perpendicular to a center axis of the worm shaft.
The urging member is configured to urge the rocking member towards one side in a rocking direction about the rocking shaft.
The guide member is provided to allow the worm shaft to move in a meshing direction in which the worm shaft comes close to the worm wheel, and which is urged in the meshing direction as the rocking member is urged to the one side in the rocking direction about the rocking shaft by the urging member, thereby urging the worm shaft in the meshing direction via the tip end-side bearing.
In the meantime, when implementing the present invention, the guide member may be provided integrally with the tip end-side bearing (for example, it may be provided integrally with an outer ring configuring the tip end-side bearing).
When implementing the present invention, the rocking member may include a part which can be located at a position further than a center axis of the rocking shaft in an opposite direction to the meshing direction and include a wedge portion of which a radial thickness increases from one side in the rocking direction about the rocking shaft towards the other side. The guide member may be urged in the meshing direction based on the increase of the radial thickness of a part of the wedge portion which is interposed between inner peripheral surface of the housing and an outer peripheral surface of the guide member as the rocking member is urged to the one side in the rocking direction about the rocking shaft by the urging member.
Further, when implementing the present invention, the guide member may be further urged in the meshing direction by a reactive force which is to be applied to the guide member when the urging member urges the rocking member.
Further, when implementing the present invention, an outer peripheral surface of the rocking member may include an outer spherical convex surface part, and an inner peripheral surface of the rocking member may include an inner spherical concave surface part with which the guide member is in sliding contact, and a center of curvature of the outer spherical convex surface part may be located at a position offset from a center of curvature of the inner spherical concave surface part.
Further, when implementing the present invention, one of the rocking member and the guide member may be provided with the rocking shaft, and the other of the rocking member and the guide member may be provided with a pivotal support part configured to pivotably support the rocking shaft.
Further, when implementing the present invention, an outer peripheral surface of the guide member may include an inner spherical convex surface part having a center at a point on the center axis of the rocking shaft, and an inner peripheral surface of the rocking member may include an inner spherical concave surface part which is to be spherically fitted with the inner spherical convex surface part.
Further, when implementing the present invention, the rocking member may include a rocking-side engaging part, and the guide member may include a guide-side engaging part, and a rocking range of the rocking member about the rocking shaft may be regulated based on engagement of both the engaging parts.
Further, when implementing the present invention, an elastic member may be interposed between an inner peripheral surface of the housing and an outer peripheral surface of the rocking member, and the rocking member may be urged in the meshing direction by an elastic force of the elastic member.
Also, when implementing the present invention, the rocking member may be supported inside the housing without radial rattling by the elastic force of the elastic member.
Further, when implementing the present invention, the urging member may include a coil spring, and both end portions of the coil spring may be engaged to the rocking member while an inner peripheral surface thereof is contacted to an outer peripheral surface of the guide member, so that the coil spring urges the rocking member to the one side in the rocking direction about the rocking shaft and applies a reactive force in the meshing direction, which is to be applied to the outer peripheral surface of the guide member by urging of the rocking member.
Further, when implementing the present invention, one end-side portion of the urging member may be configured to urge the guide member in the meshing direction and the other end-side portion of the urging member may be configured to urge the rocking member to the one side in the rocking direction about the rocking shaft.
According to the worm reducer of the present invention configured as described above, the guide member is configured such that when the rocking member rocks to one side in the rocking direction about the rocking shaft, the guide member is moved in the meshing direction, which is the direction in which the worm shaft comes close to the worm wheel, thereby urging the worm shaft in the meshing direction via the tip end-side bearing. For this reason, it is not necessary to provide the tip end-side portion of the worm shaft with a part, which is to further extend towards the tip end-side than a part which is supported by the tip end-side bearing. Therefore, it is possible to reduce an axial dimension of the tip end-side portion of the worm shaft for that amount.
A first embodiment of the present invention will be described with reference to
A worm reducer of the first embodiment is to be used with being incorporated to an electric power steering device and includes a housing 3a, and a worm wheel 4a, a worm shaft 5a and an urging unit 13, which are accommodated in the housing 3a.
The worm wheel 4a is rotatably supported to the housing 3a. The worm shaft 5a is rotatably supported to the housing 3a at a base end portion (a right end portion in
The urging unit 13 is provided around the tip end-side bearing 8a in the holding part 17. The worm shaft 5a is urged by the urging unit 13 in a meshing direction (a direction facing from the upper towards the lower in
In the below descriptions, regarding the urging unit 13, a left side in
The guide member 18 is formed to have a circular ring shape having a crank section (refer to
The guide member 18 is supported to the outer ring 16 concentrically with the outer ring 16 by externally fitting and fixing the fitting cylinder part 22 to the outer ring 16 with interference fit, bonding or the like. The guide member 18 is axially positioned relative to the outer ring 16 by bringing an axially inner surface of the coupling part 23 into contact with an axially outer surface of the outer ring 16. At this state, the center axes of the rocking shafts 26 are provided in a direction (the front and back sides of
In the meantime, when the rocking member 19 is moved by a wedge portion 33 (described later) and the worm shaft 5a is thus moved in a direction of coming close to the worm wheel 4a, the center axes of the rocking shafts 26 may deviate from the direction perpendicular to the meshing direction.
The rocking member 19 is formed to have a cylinder shape of which both axial sides are opened, is made of metal such as iron-based alloy, aluminum-based alloy or the like or a synthetic resin, and has a main body part 28 and a collar part 29. The main body part 28 is formed to have a cylinder shape having a substantially circular arc section as a whole (refer to
An outer peripheral surface of the main body part 28 is provided with an outer spherical convex surface part 32. A center of curvature Oout of the outer spherical convex surface part 32 is located at a position, which deviates axially outwards from the center of curvature OinB of the inner spherical concave surface part 30, on the center axis LB of the rocking member 19. Therefore, a radial thickness of a part of the main body part 28, which is interposed between the outer spherical convex surface part 32 and the inner spherical concave surface part 30, gradually increases from the axially inner side towards the axially outer side. In the first embodiment, a portion, which can be positioned in a further opposite direction (the upper side in
A pair of pivotal support concave parts 34, 34 (refer to
An end portion of the outer spherical convex surface part 32 in the opposite direction (the upper side in
The collar part 29 is a circle ring-shaped part protruding radially inwards from a part adjacent to an axially outer side of the inner spherical concave surface part 30 at an axially outer end portion of the inner peripheral surface of the main body part 28. One portion (one portion of a front side in the front and back sides of
In the rocking member 19 configured as described above, the fitting cylinder part 22 of the guide member 18 is inserted into the radially inner side of the main body part 28 through the axially inner end opening of the main body part 28 (while guiding the inner spherical convex surface part 25 by the guide surface part 31). Thereby, the inner spherical concave surface part 30 is spherically fitted to the inner spherical convex surface part 25 so as to be slidable (the centers of curvature OinA, OinB thereof coincide with each other). The rocking shafts 26 configuring the guide member 18 are inserted to the axially outer end portions of the inner sides of the pivotal support concave parts 34, 34 through the axially inner end openings of the pivotal support concave parts 34, 34. The rocking shafts 26 are rockingly supported by the axially outer end portions of the pivotal support concave parts 34, 34. Thereby, the rocking member 19 can rock relative to the guide member 18 about the rocking shafts 26 (along the inner spherical convex surface part 25) and is prevented from rotating about the center axis (the center axis of the worm shaft 5a) LA Of the guide member 18.
In the first embodiment, at this state, the inner peripheral surface of the collar part 29 configuring the rocking member 19 is made to face the outer peripheral surface of the axially outer end portion of the boss part 24 configuring the guide member 18. Based on the engagement between the inner peripheral surface of the collar part 29 and the outer peripheral surface of the axially outer end portion of the boss part 24, a rockable range of the rocking member 19 relative to the guide member 18 about the rocking shafts 26 is regulated. In the first embodiment, the rockable range is regulated to about ±10° to 15° with respect to a state where the center axis LB of the rocking member 19 and the center axis LA of the guide member 18 coincide with each other. Thereby, the interference between the rocking member 19 and the worm shaft 5a is prevented. Meanwhile, in the first embodiment, the inner peripheral surface of the collar part 29 is an example of the rocking-side engaging part in claims, and the outer peripheral surface of the axially outer end portion of the boss part 24 is an example of the guide-side engaging part in claims.
The plate spring 20 has a partially cylindrical shape as a whole and has a pair of claw portions 37, 37 bent radially inwards at both circumferential end portions. A spring constant of the plate spring 20 is set to be sufficiently small. The plate spring 20 is provided with a circle hole 39 at an axially center portion of a circumferentially center portion thereof, so that rigidity of the plate spring 20 in the circumferentially center portion is made to be lower than rigidity of the other portion. An inner diameter dimension of the plate spring 20 at a free state is made smaller than an outer diameter dimension of the outer spherical convex surface part 32.
The plate spring 20 is resiliently supported between the outer spherical convex surface part 32 and the inner peripheral surface of a single cylindrical shape of the holding part 17 with the circumferentially center portion being positioned in the opposite direction (the upper side in
At this state, an axially inner end portion of the plate spring 20 is abutted to a step surface 43 of an axially inner end portion of the inner peripheral surface of the holding part 17, so that the plate spring is axially positioned relative to the holding part 17. The plate spring 20 is prevented from being circumferentially displaced (rotated) relative to the holding part 17 by a frictional force which is to be applied to a contact part between the inner peripheral surface of the holding part 17 and the step surface 43. The plate spring 20 is configured to prevent the rocking member 19 from rotating about the center axis LB relative to the plate spring 20 based on the engagement between the claw portions 37, 37 and the engaging concave portions 36, 36 and the engagement between the center engaging portion 40 and the guide groove 35. In the meantime, as described above, the circumferential width dimension of each of the engaging concave portions 36, 36 is regulated to the minimum dimension within the range in which even when the rocking member 19 rocks about the rocking shafts 26 or the plate spring 20 (the elastically deformable portions 41, 41) is radially squashed between the outer spherical convex surface part 32 and the inner peripheral surface of the holding part 17 and the circumferential length of the plate spring 20 is increased as much as that, the claw portions 37, 37 are not to strongly interfere with both circumferential inner surfaces of the engaging concave portions 36, 36. As described above, in the first embodiment, the plate spring 20 is mounted such that the rocking member 19 is internally fitted to the holding part 17 without the radial rattling by the elastic force of the plate spring 20.
The coil spring 21 is externally fitted to an axially inner half portion of the boss part 24 configuring the guide member 18. At this state, one end-side portion of the coil spring 21 is engaged to one of the guide-side spring engaging portions 27 provided at the boss part 24. The other end-side portion of the coil spring 21 is engaged to the rocking-side spring engaging portion 38 provided at the collar part 29 of the rocking member 19. At this state, the portion of the collar part 29, at which the rocking-side spring engaging portion 38 is provided, is pressed in the opposite direction (the upward direction in
In the meantime, one end-side portion of the coil spring 21 is applied with a reactive force to the force by which the coil spring 21 presses the collar part 29 in the opposite direction to the meshing direction. For this reason, one end-side portion of the coil spring 21 urges the guide member 18 in the meshing direction via the guide-side spring engaging portion 27 of the boss part 24.
The urging of the guide member 18 in the meshing direction is not limited to the configuration where the urging is made via the guide-side spring engaging portion 27 and may be made via a contact surface between an upper surface of the boss part 24 of the guide member 18 and the coil spring 21.
Here, in the first embodiment, the wedge portion 33 configuring the rocking member 19 has a radial thickness which increases from one side (a right side in
In the first embodiment, even when a meshing distance between the worm wheel 4a and the worm teeth 6a is changed due to the rotation of the worm wheel 4a, the rotation of the worm shaft 5a, a change in thermal expansion amount of each member resulting from a change in temperature, and the like, it is possible to reduce a variation in meshing force between the worm wheel 4a and the worm teeth 6a, based on the elastic deformation of the plate spring 20 (the elastically deformable portions 41, 41). That is, when the meshing distance is changed, the dimensions of the fine gaps 42, 42 between the inner peripheral surface of the plate spring 20 (the elastically deformable portions 41, 41) and the outer spherical convex surface part 32 are changed. However, in the first embodiment, the spring constant of the plate spring 20 is set to be sufficiently low. Therefore, even when the dimensions of the fine gaps 42, 42 are changed, it is possible to reduce the variation in meshing force. As a result, it is possible to reduce the variation in frictional force which is to be applied to the meshing portion between the worm wheel 4a and the worm teeth 6a.
Meanwhile, in the first embodiment, the dimensions of the fine gaps 42, 42 are determined by the dimension of the plate spring 20, the inner diameter of the holding part 17, and the center of curvature of the outer spherical convex surface part 32. Since the dimension, the inner diameter and the center of curvature are respectively finished with precision, it is possible to sufficiently suppress the inequality of the dimensions of the fine gaps 42, 42. When the backlash at the meshing portion between the worm wheel 4a and the worm teeth 6a and the fine gaps 42, 42 increase due to the wears of tooth surfaces of the worm wheel 4a and the worm teeth 6a, the rocking member 19 rocks to one side in the rocking direction, so that the backlash and the fine gaps 42, 42 are prevented from increasing. In addition, since the balanced state of the initial forces of the urging reactive force to be applied from the guide member 18 to the rocking member 19 and the reactive force based on the elastic deformation of the plate spring 20 is kept, it is possible to always keep the dimensions of the fine gaps 42, 42 to appropriate dimensions. Even when the guide member 18 and the rocking member 19 are applied with the load and are thus elastically deformed, the rocking member 19 rocks to one side in the rocking direction, so that the balanced state of the initial forces is kept. Therefore, it is possible to always keep the dimensions of the fine gaps 42, 42 to the appropriate dimensions.
In the first embodiment, upon generation of the auxiliary power of the electric motor 9, when an engaging reactive force is applied from the meshing portion to the worm shaft 5a and the tip end-side bearing 8a is thus applied with high radial load, at least one of the fine gaps 42, 42 is removed, so that it is possible to bear the high radial load by the inner peripheral surface of the holding part 17. In the first embodiment, as shown in
In the first embodiment, the rocking member 19 is internally fitted to the holding part 17 without the radial rattling by the elastic force of the plate spring 20. For this reason, when rotational vibrations are applied from the wheels to the worm wheel 4a, based on a component included in the engaging reactive force to be applied from the meshing portion to the worm shaft 5a in the direction (the front and back sides of
According to the worm reducer of the first embodiment configured as described above, the guide member 18 is configured to urge the worm shaft 5a in the meshing direction via the tip end-side bearing 8a. For this reason, it is not necessary to provide the tip end-side portion of the worm shaft 5a with a part further extending towards the tip end-side than a part which is to be supported by the tip end-side bearing 8a. Therefore, it is possible to reduce the axial dimension of the tip end-side portion of the worm shaft 5a for that amount.
In the meantime, when implementing the present invention, a part, which is positioned at an opposite end portion to the meshing direction, of the outer peripheral surface (the inner spherical convex surface part 25) of the fitting cylinder part 22 may be provided with an inner guide part 44, which is a groove or a convex part, in a direction perpendicular to the circumferential direction, and a part, which is to fit together with the inner guide part 44, of the inner peripheral surface (the inner spherical concave surface part 30) of the rocking member 19 may be provided with an outer guide part 45, which is a convex part or a groove capable of engaging with the inner guide part 44, so that it is possible to guide the rocking of the rocking member 19 about the rocking shafts 26 (to prevent the rocking member 19 from rotating relative to the guide member 18 about the center axis LA) by engagement between the inner and outer guide parts 44, 45, i.e., the convex part and the groove.
When implementing the present invention, the rocking member may be provided with a pair of rocking shafts, and the guide member may be provided with a pivotal support part such as the pivotal support concave parts configured to pivotally support the rocking shafts.
Meanwhile, in the first embodiment, both the end portions of the coil spring 21 are respectively engaged with the guide-side spring engaging portion 27 provided at the boss part 24 and the rocking-side spring engaging portion 38 provided at the collar part 29 of the rocking member 19, and the rocking member 19 is urged to one side in the rocking direction by the elastic force of the coil spring 21. However, as in a modified embodiment shown in
Therefore, in the modified embodiment of the first embodiment, the guide member 18 is urged in the meshing direction and the worm shaft 5a is urged in the meshing direction via the tip end-side bearing 8a by both the urging (rocking) of the rocking member 19 to one side in the rocking direction and the reactive force F to be applied to the contact portion 24a of the boss part 24.
A second embodiment of the present invention is described with reference to
In the second embodiment, the rocking direction of a rocking member 19a about the pair of rocking shafts 26 for urging the worm shaft 5a in the meshing direction (a downward direction in
To this end, in the second embodiment, a part, at which the rocking-side spring engaging portion 38 (refer to
The center of curvature Oout of an outer spherical convex surface part 32a is positioned at an axially inner side (a right side in
In the second embodiment, an inner diameter dimension of an axially inner opening of the inner spherical concave surface part 30a provided on an inner peripheral surface of the rocking member 19a is made smaller than an outer diameter dimension of an inner spherical convex surface part 25a provided on an outer peripheral surface of a fitting cylinder part 22a configuring the guide member 18a. To this end, in the second embodiment, the rocking member 19a is configured by coupling and fixing both circumferential end portions of a pair of division elements each of which has a semi-cylindrical shape, so that the inner spherical concave surface part 30a can be spherically fitted to the inner spherical convex surface part 25a.
The other configurations and operations are similar to the first embodiment.
When incorporating the worm reducer of the present invention into an electric power steering device, an assist type of the electric power steering device is not limited to the column assist type of the above embodiments, and may be a pinion assist type or a rack assist type.
The worm reducer of the present invention can be incorporated into various mechanical apparatuses, without being limited to the electric power steering device.
When implementing the present invention, the inner spherical convex surface part, the inner spherical concave surface part, the outer spherical convex surface part and the wedge portion are not necessarily provided over the entire circumference, and may be provided at any parts which can be positioned in a further opposite direction to the meshing direction than the center axis of the rocking shaft.
The present invention is based on a Japanese Patent Application No. 2015-98471 filed on May 13, 2015, which is herein incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-098471 | May 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/063487 | 4/28/2016 | WO | 00 |
