JP2016203851A - Worm reducer for electric power steering - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a worm reducer for an electric power steering comprising a mechanism which prevents enlargement of backlash caused by displacement of a worm accompanying change in a dimensional shape of a resin worm wheel due to change in atmospheric temperatures and abrasion of an engagement portion.SOLUTION: The worm reducer for an electric power steering comprises: a shaft end-side bearing 5 which can bear one end of a worm 2 and move the worm toward a worm wheel 3 in a shaft right-angle direction; a belt holding portion 9 provided integrally with a housing 1 in a radial direction of the shaft end-side bearing 5 and in a space 11 near the worm wheel 3; and an annular elastic belt 10 hung over an outer periphery of an outer wheel 6 of the shaft end-side bearing 5 and the belt holding portion 9, while being extended in a longitudinal direction, where the annular elastic belt 10 compresses with contractile force the worm 2 against the worm wheel 3 with predetermined pressure.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a worm reducer for electric power steering.

  The electric power steering uses an electric motor as a drive source of a steering assist force that assists a driver's steering force. The driving force of the electric motor is decelerated and increased via a reduction gear connected to the electric motor and transmitted to the steering shaft, and becomes an assist force for assisting the manual steering force by the driver. As this speed reducer, a worm speed reducer is generally used, and a metal worm and a resin worm wheel are often used.

  The worm and worm wheel are initially set to have almost no backlash at the meshing portion. Resin worm wheels may experience expansion and contraction or deformation due to changes in ambient temperature, or tooth surface wear due to meshing with metal worms. As a result, the backlash increases between the resin worm wheel tooth surface and the metal worm tooth surface. The enlargement of the backlash may cause a hitting sound between the tooth surfaces generated when the rotation direction of the worm wheel is changed, and may be transmitted as an unpleasant noise to the driver.

  In order to prevent this, Patent Document 1 discloses that the outer periphery of the support bearing at the tip of the worm shaft is pressurized from the anti-worm wheel side to the worm wheel side by a coil spring to maintain the preload between the worm and the worm wheel and increase or decrease the preload. The structure which prevents the expansion of a backlash by suppressing is disclosed.

  In Patent Document 2, as shown in FIGS. 4 and 5, an annular leaf spring 30 is arranged between the outer periphery of the outer ring 26 of the shaft end side bearing 25 of the worm 22 and the inner periphery of the housing 21. A structure is disclosed in which 25 outer rings 26 are pressurized from the opposite side of the worm wheel 23 to maintain the preload between the worm 22 and the worm wheel 23 and prevent the backlash from expanding.

JP 2006-27368 A JP 2012-56459 A

  However, the backlash expansion preventing structure of Patent Document 1 requires a space for accommodating a coil spring as a pressurizing means on the side opposite to the worm wheel in the radial direction of the support bearing, which is larger than a part of the current housing protruding outward. There was a problem that the housing had to be used. Further, in the backlash expansion preventing structure of Patent Document 2, a leaf spring that can be used in an existing housing space is used. However, in order to obtain a required pressure force with a small extension (operation range) of the leaf spring, the spring constant should be reduced. I can't. For this reason, the change in the applied pressure with respect to the displacement of the worm increases, it is difficult to obtain a stable applied pressure covering the entire range of the displacement of the worm, and the leaf spring cannot follow when the displacement of the worm is large. was there.

  It is an object of the present invention to reliably cover the entire range of worm displacement associated with a change in the size and shape of a resin worm wheel without requiring a large housing, and to obtain a stable pressurizing force. An object of the present invention is to provide a worm speed reducer for electric power steering equipped with a structure capable of preventing an increase in backlash between wheels.

  In order to solve the above-mentioned problem, the structural feature of the invention according to claim 1 is that a metal worm, a pair of support bearings that support shafts on both sides of the worm, and a resin meshed with the worm. Worm wheel, and the worm, the support bearing, and a housing for housing the worm wheel, the worm speed reducer for electric power steering, wherein the housing is a shaft end side bearing that is one of the support bearings A belt holding portion integrally provided with the housing on the worm wheel side in the space, and having a space extending in the radial direction of the shaft end side bearing and on the worm wheel side, An annular belt that can be expanded and contracted in the direction, and is stretched from the natural length to the outer periphery of the shaft end side bearing and the belt holding portion and is hung with a contraction force. The shaft end side bearing is displaceable to the worm wheel side in the space, and the worm exerts a predetermined force on the worm wheel by the contraction force of the annular elastic belt. It is that it is pressurized with.

  According to the above configuration, since the accommodation space for the annular elastic belt used for the pressurizing means can be provided inside the housing on the worm wheel side, the housing is placed outside as in the case where the coil spring is used for the pressurizing means. In other words, it is not necessary to overhang the anti-worm wheel and increase the size. If an annular elastic belt is used, the elongation (operation range) can be increased and the spring constant can be reduced as compared with the case where a leaf spring is used as the pressing means. For this reason, even if the displacement of the worm wheel made of plastic increases as a result of deformation due to atmospheric temperature or wear on the tooth surface, the entire range of displacement of the worm can be achieved without greatly changing the applied pressure. Thus, the applied pressure can be stably maintained. Further, there is no loss of pressurization in which the elongation of the annular elastic belt cannot follow the displacement amount of the worm. Therefore, it is possible to prevent the backlash between the worm and the worm wheel from expanding.

  In order to solve the above problems, a structural feature of the invention according to claim 2 is that the annular elastic belt is made of a rubber material or a resin material having elasticity.

  According to the above configuration, by using an elastic rubber material or resin material for the annular elastic belt, the elongation can be increased and the spring constant can be made smaller. Even when the amount of displacement of the worm to be pressurized becomes large, the change in the applied pressure with respect to the change in the amount of displacement can be made smaller to stably hold the pressure, and the pressure that can not follow the amount of displacement of the worm. The escape is also avoided, and the backlash between the worm and the worm wheel can be prevented more effectively.

  According to the above invention, by using the annular elastic belt as the pressurizing means for the worm wheel, the housing space for the pressurizing means can be provided inside the housing on the worm wheel side of the support bearing. With a structure that can reliably cover the entire range of worm displacement accompanying the change in wheel size and shape and obtain stable applied pressure, and prevent the backlash between the worm and worm wheel from expanding. A worm speed reducer for electric power steering can be provided.

It is the schematic of the worm axial direction cross section of the worm reduction gear for electric power steering in one embodiment of the present invention. It is the schematic of the tooth surface clearance compensation mechanism seen from the worm shaft end side of the worm speed reducer for electric power steering in one embodiment of the present invention. It is the figure which showed the influence of the temperature with respect to the spring load in the cyclic | annular rubber belt in one Embodiment of this invention, and the metal leaf | plate spring in a prior art. It is the schematic of the worm axial direction cross section of the worm reduction gear for electric power steering in a prior art. It is the schematic which looked at the attachment state of the spring which pressurizes a worm in a prior art from the worm shaft end side.

  Embodiments of the present invention will be described below with reference to the drawings.

  First, the configuration of a worm reduction gear according to an embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic view of a cross section in the worm axis direction of a worm speed reducer for electric power steering according to an embodiment of the present invention. The worm speed reducer includes a housing 1, a worm 2, a worm wheel 3, a motor side bearing 4, a shaft end side bearing 5, an annular elastic belt 10, and a cover 16. FIG. 2 is a schematic view of the worm speed reducer according to the embodiment of the present invention viewed from the worm shaft end side with the cover 16 removed.

  The worm 2 is formed of a metal material (for example, alloy steel), and is engaged with the worm wheel 3 by a motor side bearing 4 and a shaft end side bearing 5 mounted on the shafts on both sides of the worm 2. 1 is rotatably supported. A coupling 17 is attached to the shaft end of the worm 2 on the electric motor side, and is coupled to the shaft of the electric motor (not shown). There is a gap between the outer periphery of the worm 2 and the inner periphery of the housing 1, and grease that lubricates the tooth surface of the worm 2 is enclosed.

  The worm wheel 3 includes a worm wheel outer peripheral portion 13 formed of a resin material (for example, MC nylon resin) and a worm wheel core portion 14 formed of a metal material, and the outermost outer periphery of the worm wheel outer peripheral portion 13 is a tooth portion. Is formed. A steering shaft 15 is press-fitted into the central portion of the worm wheel 3 and integrated. The worm wheel 3 is rotatably supported by the housing 1 by support bearings (not shown) disposed on both sides of the worm wheel 3 and mounted on the steering shaft 15. There is a gap between the outer periphery of the worm wheel 3 and the inner periphery of the housing 1, and grease that lubricates the tooth surfaces of the worm wheel 3 is enclosed.

  The housing 1 is an aluminum alloy member formed by die casting, and a space 11 is provided inside the shaft end side of the worm 2. The space 11 has a cross-sectional shape perpendicular to the worm axis, an arc-shaped portion having a radius slightly larger than the outer circumference of the shaft end side bearing 5, one straight line having a length shorter than the bearing outer diameter at a position opposite thereto, and A substantially front and rear circular shape composed of two straight lines connecting the arc-shaped portion and one straight line is formed. The space 11 is closed in the axial direction of the worm 2 by the space bottom surface 12 and the cover 16, and has a depth slightly larger than the width of the shaft end side bearing 5.

  The belt holding portion 9 is fixed to the space bottom surface 12 and is provided to protrude from the space bottom surface 12 into the space 11 parallel to the axis of the worm 2. The belt holding portion 9 has a protruding length substantially equal to or slightly shorter than the depth of the space 11 and has a constant width in the axial direction of the worm wheel 3. The outer periphery on the non-worm side of the belt holding part 9 has a curved surface or a smooth shape including a flat surface. The belt holding part 9 may be a part of the housing 1.

  The motor-side bearing 4 of the worm 2 has an inner ring that is press-fitted into the shaft of the worm 2 from the motor side, and an outer ring that is mounted at a predetermined position on the motor side in the housing 1, so that the radial movement of the motor-side bearing 4 is restricted. ing. In the shaft end side bearing 5 of the worm 2, the inner ring 7 is press-fitted into the worm shaft end 8, and the outer ring 6 has a gap between the inner periphery of the housing 1 in the space 11 and is displaced toward the worm wheel side in the radial direction. The worm 2 is displaceable in a direction perpendicular to the axis and meshing with the worm wheel 3 because it is accommodated.

  The annular elastic belt 10 is an annular belt made of an elastic rubber material (for example, AEM: ethylene acrylate rubber), and has a belt width close to the width of the outer ring 6 of the shaft end side bearing 5 of the worm 2. The annular elastic belt 10 is slightly stretched in the length direction and given tension, and the outer periphery of the outer ring 6 of the shaft end side bearing 5 of the worm 2 and the worm wheel side of the belt holding portion 9 protruding into the space 11. It is hung between the outer periphery.

  The cover 16 is fixed to the housing 1 by covering the space 11 that houses the shaft end side bearing 5, the annular elastic belt 10, and the belt holding portion 9 from the outside. The space 11 is blocked from the outside by the cover 16. A sealing member may be installed on the contact surface between the cover 16 and the housing 1 in order to improve the sealing performance.

  Next, the operation and effect of the worm speed reducer according to the embodiment of the present invention will be described with reference to FIGS. 1 and 2.

  The driving force of the electric motor is transmitted to the worm 2 through the coupling 17 and transmitted to the worm wheel 3 fixed to the steering shaft 15 by being decelerated and increased, and becomes an assist force that assists the manual steering force by the driver. Since the steering shaft 15 rotates in both the left and right directions according to the turning direction of the vehicle, the electric motor and the worm 2 connected thereto also rotate in both directions. If the backlash on the meshing tooth surface between the worm 2 and the worm wheel 3 is large, a hitting sound between the tooth surfaces is generated when the rotation direction is changed.

  As described above, the motor side bearing 4 of the worm 2 is mounted at a predetermined position of the housing 1, the movement in the radial direction is restricted, and the shaft end side bearing 5 is accommodated in the space 11 so as to be displaceable. An annular elastic belt 10 is hung between the belt holding portion 9 and a predetermined tension. Therefore, the worm 2 is pressed against the worm wheel 3 and pressurized by the tension of the annular elastic belt 10 applied to the shaft end side bearing 5 with the motor side bearing 4 as a fulcrum. For this reason, backlash does not occur on the meshing surface of the worm 2 and the worm wheel 3 during normal driving force transmission or rotation direction change, and no sound is generated between the tooth surfaces.

  If the structure using the annular elastic belt 10 as the pressurizing means as in the embodiment of the present invention, a space 11 for accommodating the annular elastic belt 10 may be installed inside the housing 1. There is no need to project outward, and the mechanism for preventing the backlash from expanding can be made compact based on the existing housing.

  The worm wheel outer peripheral part 13 including the tooth part is formed of resin. If the resin expands and contracts or deforms due to the influence of the ambient temperature due to the operating region of the vehicle where the electric power steering is installed or the speed reducer operates, or if tooth surface wear occurs due to deterioration of the lubrication state, the resin The shape and shape of the outer peripheral portion 13 of the worm wheel may be changed. At this time, if the displacement of the worm 2 cannot follow the worm wheel 3, the backlash of the meshing tooth surface expands, and a hitting sound between the tooth surfaces is generated when the rotation direction is changed.

  The annular elastic belt 10 which is a pressurizing unit according to an embodiment of the present invention is formed of a rubber material having elasticity, and this rubber material has a smaller elastic coefficient than that of a metal material. For this reason, even when the cross-sectional area of the annular elastic belt 10 is increased, the spring constant can be set low, and the elongation of the entire belt is also large. Therefore, even if the displacement amount of the worm 2 increases due to the change in the size and shape of the worm wheel 3, the annular elastic belt 10 can cope with the displacement of the worm 2 with a stable applied pressure. Can prevent the backlash from spreading.

  The rubber material forming the annular elastic belt 10 has a linear expansion coefficient close to that of the resin material forming the outer peripheral portion 13 of the worm wheel including the tooth portion as compared with the metal material. Since the length of the annular elastic belt 10 is similarly expanded with respect to the displacement of the worm 2 due to the expansion of the wheel 3 (the direction away from the worm wheel 3), the influence of the temperature change on the pressure applied to the worm wheel 3 by the worm 2 The pressure can be reduced and a stable pressure can be maintained. Further, the length of the annular elastic belt 10 is similarly contracted with respect to the displacement of the worm 2 (the direction approaching the worm wheel 3) due to the contraction of the resin worm wheel 3 due to the decrease in the ambient temperature. The influence of the temperature change with respect to the applied pressure to the wheel 3 can be reduced, and a stable applied pressure can be maintained.

  In addition, since the rubber material has a characteristic that the elastic modulus is lowered due to softening with respect to the temperature rise, the worm 2 is displaced (in the direction away from the worm wheel 3) due to the expansion of the resin worm wheel 3 due to the rise in the ambient temperature. On the other hand, it has the effect of reducing the spring constant of the annular elastic belt 10 and suppressing the increase of the spring load against the increase of the displacement of the worm 2. On the contrary, the spring constant of the annular elastic belt 10 is increased for the displacement of the worm 2 (the direction approaching the worm wheel 3) due to the shrinkage of the resin worm wheel 3 due to the decrease in the ambient temperature, and the displacement of the worm 2 is increased. It has the effect which suppresses that a spring load falls with respect to the increase in.

  FIG. 3 shows the effect of temperature on the spring load (index) when ethylene acrylate rubber (AEM) is used for the annular elastic belt 10 (that is, the applied pressure corresponding to the displacement amount of the worm 2 due to the dimensional shape change of the worm wheel 3). ) Is compared with the case of using a metal leaf spring in the prior art. The horizontal axis represents temperature, and the vertical axis represents spring load (index when the load at a temperature of 20 ° C. is 1.0). In the case of an annular rubber belt, the change in spring load is about 10% with respect to the change in temperature from 0 ° C to 100 ° C. On the other hand, in the case of a metal leaf spring, the change in spring load is as large as about 50% with respect to the change in temperature from 0 ° C to 100 ° C.

  The structure of the present invention in which ethylene acrylate rubber is used for the annular elastic belt 10 has a smaller change in spring load with respect to the ambient temperature than the structure of the prior art that uses a metal leaf spring. That is, according to the present invention, it is possible to maintain the meshing of the worm 2 with the worm wheel 3 with a stable pressure against the change in the dimensional shape of the worm wheel 3 and to prevent the backlash from expanding.

  As described above, without requiring a larger housing for the present invention, it is possible to reliably cover the entire range of the displacement of the worm accompanying the change in the size and shape of the worm wheel, and to maintain a stable pressure. It is possible to provide a worm speed reducer for electric power steering equipped with a structure capable of preventing an increase in backlash between the worm and the worm wheel.

  The annular elastic belt 10 is not limited to the rubber material used in the embodiment of the present invention, and even if it is made of a resin material or a metal material, it has elasticity in the length direction in the shape of the annular elastic belt 10. Any structure that can reduce the spring constant may be used. For example, even when the both ends of a flat, substantially elliptical coil spring are connected to form an annular belt, the effects of the present invention are sufficiently exhibited. The annular elastic belt 10 may be an elastic ring having a substantially circular shape or a substantially square shape whose cross-sectional shape is substantially equal in thickness and width.

  The present invention is not limited to the above-described embodiment, and can of course be implemented in various modes without departing from the gist of the present invention.

1: Housing, 2: Worm, 3: Worm wheel, 4: Motor side bearing,
5: shaft end side bearing, 6: outer ring, 7: inner ring, 8: worm shaft end, 9: belt holding portion, 10: annular elastic belt, 11: space, 12: bottom surface of space,
13: Worm wheel outer peripheral part, 14: Worm wheel core part,
15: Steering shaft, 16: Cover, 17: Coupling,
21: Housing, 22: Worm, 23: Worm wheel,
25: shaft end side bearing, 26: outer ring, 30: leaf spring

Claims (2)

A metal worm,
A pair of support bearings for supporting shafts on both sides of the worm;
A resin worm wheel meshing with the worm;
A housing for housing the worm, the support bearing and the worm wheel;
A worm speed reducer for electric power steering equipped with
The housing has a space that extends in the radial direction of the shaft end side bearing and on the worm wheel side in a portion that accommodates the shaft end side bearing that is one of the support bearings,
A belt holding portion provided integrally with the housing on the worm wheel side in the space;
An annular belt that can be expanded and contracted in the length direction, and has an annular elastic belt that is stretched from the natural length to the outer periphery of the shaft end side bearing and the belt holding portion and is hung in a contracted state.
The shaft end side bearing is displaceable to the worm wheel side in the space;
A worm speed reducer for electric power steering, wherein the worm is pressed against the worm wheel with a predetermined force by a contraction force of the annular elastic belt.   The worm reducer for electric power steering according to claim 1, wherein the annular elastic belt is made of a rubber material or a resin material having elasticity.

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