Electric power steering system

An electric power steering system includes a joint that connects a rotary shaft of an electric motor with a worm shaft of a speed reducer. The joint includes a first rotational element that has a boss into which a first end portion of the worm shaft is press-fitted, and a flange extending radially from the boss. The first bearing is held by a housing and supports the first end portion so that the first end portion is rotatable and movable in an axial direction. A first elastic body, which urges the worm shaft in the axial direction toward the electric motor through the first rotational element, is provided between an inner ring of the first bearing and the flange.

INCORPORATION BY REFERENCE/RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2012-167188 filed on Jul. 27, 2012 the disclosure of which, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electric power steering system.

2. Discussion of Background

In an electric power steering system, an output from an electric motor is transmitted to a steered mechanism after speed of the output is reduced through a worm shaft and a worm wheel, and thus, torque assistance for steering operation is provided. Although backlash is needed in engagement between the worm shaft and the worm wheel, rattle may be caused by the backlash while a vehicle is traveling.

Therefore, conventionally, an electric power steering system has been proposed in which backlash is removed by elastically urging a bearing, which supports one end of a worm shaft, toward a worm wheel. Another electric power steering system has been proposed in which paired elastic bodies are arranged on respective sides of an inner ring of a bearing in order to suppress backlash of the bearing (for example, US2005/0224278 A1 and US2012/0111657 A1).

In US2005/0224278 A1, an elastic body is supported between a nut that is screwed and fitted to an outer periphery of the other end of the worm shaft, and an inner ring of the bearing in a state in which the elastic body is compressed in an axial direction. In US2012/0111657 A1, the elastic body is supported between a circular plate, which is fixed by a retaining ring to an outer periphery of the other end of a worm shaft in an axial direction, and the inner ring of the bearing in a state in which the elastic body is compressed in the axial direction. Meanwhile, in addition to the structure that supports the elastic body in the axial direction, there is provided a joint mechanism that connects the other end of the worm shaft with a rotary shaft of an electric motor so that torque is transmittable between the other end of the worm shaft and the rotary shaft. Therefore, a structure is complicated.

SUMMARY OF THE INVENTION

The invention provides an electric power steering system having a simplified structure.

According to a feature of an example of the invention, there is provided an electric power steering system including: an electric motor having a rotary shaft; a speed reducer that includes a worm shaft having a first end portion and a second end portion, and a worm wheel that meshes with the worm shaft; a joint that includes a rotational element that has a boss into which the first end portion of the worm shaft is press-fitted and a flange extending radially from the boss, the joint connecting the first end portion of the worm shaft with the rotary shaft so that torque is transmittable between the first end portion of the worm shaft and the rotary shaft; a housing that houses the speed reducer; a bearing that is held by the housing and supports the first end portion of the worm shaft so that the first end portion of the worm shaft is rotatable and movable in an axial direction; and an elastic body that is provided between an inner ring of the bearing and the flange, and urges the worm shaft in the axial direction toward the electric motor through the rotational element.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1is a schematic structural view of an electric power steering system according to an embodiment of the present invention. An electric power steering system1includes a steering shaft3having one end to which a steering member2such as a steering wheel is connected, an intermediate shaft5that is connected to the steering shaft3through a universal joint4, a pinion shaft7that is connected to the intermediate shaft5through a universal joint6, and a rack bar8that serves as a steered shaft, and that includes a rack8athat meshes with a pinion7aprovided in the vicinity of an end portion of the pinion shaft7, the rack bar8extending in a right-left direction of a vehicle. The pinion shaft7and the rack bar8A constitute a steered mechanism A that includes a rack-and-pinion mechanism.

The rack bar8is supported within a housing9through a plurality of bearings (not shown) so that the rack bar8is able to be linearly reciprocated. The housing is fixed to a vehicle body. Both end portions of the rack bar8project toward respective sides of the housing9, and tie rods10are connected to the respective end portions. The tie rods10are connected to corresponding steered wheels11through corresponding knuckle arms (not shown). As the steering member2is operated and the steering shaft3is rotated, the rotation is converted by the pinion7aand the rack8ainto a linear motion of the rack bar8along the right-left direction of the vehicle. Thus, the steered wheels11are steered.

The steering shaft3includes a first steering shaft portion3aon an input side that has one end to which the steering member2is connected, a second steering shaft portion3bon an output side that is continuous with the pinion shaft7, and a torsion bar12that connects the first steering shaft portion3awith the second steering shaft portion3bso that the first steering shaft portion3aand the second steering shaft portion3bare rotatable relative to each other on a common axis. There is provided a torque sensor13that detects steering torque based on the amount of relative rotational displacement between the first steering shaft portion3aand the second steering shaft portion3bthat are connected with one another through the torsion bar12. Results of torque detection by the torque sensor13are provided to an ECU14. The ECU14drives and controls a steering-assisting electric motor16via a drive circuit15, based on, for example, the results of torque detection and results of vehicle speed detection provided by a vehicle speed sensor (not shown).

Speed of output rotation of the electric motor16is reduced through a speed reducer17that serves as a transmission device, and then, the rotation whose speed has been reduced is transmitted to the pinion shaft7, and is converted into a linear motion of the rack bar8. Thus, steering is assisted. The speed reducer17includes a worm shaft18that serves as a driving gear and that is driven to rotate by the electric motor16, and a worm wheel19that serves as a driven gear and that meshes with the worm shaft18and is connected to the second steering shaft portion3bof the steering shaft3so as to rotate together with the second steering shaft portion3b.

As shown inFIG. 2, the worm shaft18is arranged coaxially with a rotary shaft20of the electric motor16. The worm shaft18includes a first end portion18aand a second end portion18b, and a teeth portion18cin an intermediate portion between the first end portion18aand the second end portion18b. The worm wheel19is connected to an axial intermediate portion of the second steering shaft portion3bof the steering shaft3so that the worm wheel19is rotatable together with the axial intermediate portion of the second steering shaft portion3band is unable to move in an axial direction. The worm wheel19includes a circular cored bar19athat is connected to the second steering shaft portion3bso as to rotate together with the second steering shaft portion3b, and a synthetic resin member19bthat surrounds a periphery of the cored bar19a. A teeth portion19cis formed on an outer periphery of the synthetic resin member19b.

The first end portion18aof the worm shaft18is connected with an end portion of the rotary shaft20(an output shaft) of the electric motor16, which faces the first end portion18a, through a joint21so that torque is transmittable between the first end portion18aof the worm shaft18and the end portion of the rotary shaft20, and the first end portion18aof the worm shaft18and the end portion of the rotary shaft20are able to oscillate with respect to each other. More specifically, the joint21includes a first rotational element22that is connected to the first end portion18aof the worm shaft18so that the first rotational element22is rotatable together with the first end portion18aof the worm shaft18and is unable to move in the axial direction, a second rotational element23that is connected to the rotary shaft20of the electric motor16so that the second rotational element23is rotatable together with the rotary shaft20of the electric motor16and is unable to move in the axial direction, and an elastic member24that is provided between the first rotational element22and the second rotational element23and transmits torque between both of the rotational elements22and23.

As shown inFIG. 3, the first rotational element22includes a boss25in which a fitting hole25ais formed, and a circular flange26that extends radially outward from the boss25. The first end portion18aof the worm shaft18(seeFIG. 2) is press-fitted into the fitting hole25a. The boss25is fitted to the first end portion18aof the worm shaft18so that the boss25is rotatable together with the first end portion18aof the worm shaft18and is unable to move in the axial direction. The second rotational element23includes a body27in which a fitting hole27ais formed, the body27facing the flange26of the first rotational element22in an axial direction X1. The rotary shaft20of the electric motor16(seeFIG. 2) is press-fitted into the fitting hole27a.

A plurality of engaging projections261, which project toward the body27of the second rotational element23, are provided in the flange26of the first rotational element22so that the engaging projections261are spaced from each other at equal intervals in a circumferential direction Z1. Also, a plurality of engaging projections271, which project toward the flange26of the first rotational element22, are provided in the body27of the second rotational element23so that the engaging projections271are spaced from each other at equal intervals in the circumferential direction Z1. The engaging projections261and the engaging projections271are arranged alternately in the circumferential direction Z1. The elastic member24includes a circular main body portion28, and a plurality of engaging arms29that extend radially from the main body portion28. The engaging arm29of the elastic member24is sandwiched between the corresponding engaging projections261and271of the rotational elements22and23, the engaging projections261and271being adjacent to each other in the circumferential direction Z1.

With reference toFIG. 2, the first end portion18aof the worm shaft18is supported by a housing17aof the speed reducer17through a first bearing30so that the first end portion18aof the worm shaft18is rotatable. The second end portion18bof the worm shaft18is supported by the housing17aof the speed reducer17through a second bearing31so that the second end portion18bof the worm shaft18is rotatable. As the elastic member24of the joint21is elastically deformed, the worm shaft18is allowed to oscillate about the center of the first bearing30, with respect to the rotary shaft20.

A first elastic body32and a second elastic body33, which urge the worm shaft18to a neutral position in the axial direction, are arranged in the first end portion18aof the worm shaft18. The first bearing30includes an inner ring34that is fitted to the first end portion18aof the worm shaft18so that the inner ring34is rotatable together with the first end portion18aof the worm shaft18, and an outer ring37that is held in a bearing hole35through a bush36, the bearing hole35being provided in the housing17aof the speed reducer17.

FIG. 4is an enlarged view of a part ofFIG. 2. The outer ring37and a circular flange36ain an end portion of the bush36are sandwiched in the axial direction between a positioning step portion38that is provided in an end portion of the bearing hole35, and a fixing member39that is screwed and fitted into the bearing hole35. Thus, an axial movement of the outer ring37is restricted. The inner ring34of the first bearing30is fitted to the outer periphery of the first end portion18aof the worm shaft18so that the inner ring34is rotatable together with the first end portion18aof the worm shaft18. The first elastic body32and the second elastic body33are arranged on respective sides of the inner ring34in the axial direction X1, and thus the inner ring34is sandwiched between the first elastic body32and the second elastic body33in the axial direction X1. The first elastic body32and the second elastic body33elastically urge the worm shaft18to the neutral position in the axial direction X1. Each of the elastic bodies32and33is, for example, a bush that is formed of an elastic material such as rubber and thermoplastic elastomer.

The second elastic body33is provided between a circular receiving plate41that abuts on a positioning step portion40on an outer periphery of the worm shaft18, and a circular receiving plate42that abuts on a second end face34bof the inner ring34, and the second elastic body33is compressed in the axial direction X1. The first elastic body32is provided between a circular receiving plate43that abuts on a first end face34aof the inner ring34, and a receiving plate44that abuts on the flange26of the first rotational element22of the joint21, and the first elastic body32is compressed in the axial direction X1. Each of the receiving plates41to44is made of, for example, metal.

The outer periphery of the first end portion18aincludes a first portion45, a second portion46, and a third portion47. The receiving plate41, the second elastic body33, and the receiving plate42are fitted to and held on the first portion45. The second portion46has a smaller diameter than a diameter of the first portion45, and serves as an inner ring fitting portion to which the inner ring34is fitted. The third portion47has a smaller diameter than the diameter of the second portion46, and serves as a boss fitting portion to which the boss25of the first rotational element22is fitted to the third portion47. The receiving plate43, the first elastic body32, and the receiving plate44are fitted to and held on an outer periphery25bof the boss25. A positioning step portion48is formed between the first portion45and the second portion46(the inner ring fitting portion). The positioning step portion48faces the second end face34bof the inner ring34.

A positioning step portion49is formed between the second portion46(the inner ring fitting portion) and the third portion (the boss fitting portion). An end face25cof the boss25abuts on the positioning step portion49, and thus, the boss25is positioned with respect to the worm shaft18in the axial direction X1. A distance L1between the positioning step portion48and the end face25cof the boss25is set so as to be longer than a distance L2between both of the end faces34aand34bof the inner ring34. A difference between the distances L1and L2(L1−L2) corresponds to a range of movement of the worm shaft18in the axial direction X1(for example, 0.3 mm on each of the right and left sides from the neutral position, thus 0.6 mm in total).

As shown inFIG. 2andFIG. 4, an inner ring50of the second bearing31is fitted to a fitting recessed portion51provided on the outer periphery of the second end portion18bof the worm shaft18so that the inner ring50is rotatable together with the second end portion18bof the worm shaft18. One end face of the inner ring50abuts on a positioning step portion52provided on the outer periphery of the second end portion18b. Thus, an axial movement of the inner ring50with respect to the worm shaft18is restricted. The housing17ais provided with a bearing hole53for holding the second bearing31. The bearing hole53is formed so as to hold the second bearing31such that the second bearing31is able to be biased in directions Y1and Y2in which a center-to-center distance D1between the worm shaft18and the worm wheel19(corresponding to a distance between a center C1of rotation of the worm shaft18and a center C2of rotation of the worm wheel19) is increased and decreased (a direction Y1in which the center-to-center distance D1is increased and a direction Y2in which the center-to-center distance D1is decreased).

As an urging member, a circular plate spring60, for example, is provided between an inner periphery of the bearing hole53and an outer ring54of the second bearing31. The plate spring60urges the second bearing31in the direction Y2in which the center-to-center distance D1is decreased. The plate spring60as the urging member is, for example, a thin plate-shaped member that is formed of a metal plate.FIG. 5is a sectional view taken along a line V-V inFIG. 2.FIG. 6is a perspective view of the plate spring60. The plate spring60includes a main body portion61having an ended circular shape that surrounds an outer periphery54aof the outer ring54of the second bearing31, paired rotation restricting portions62that are bent and extended respectively from a first end portion61aand a second end portion61bthat are end portions of the main body portion61in the circumferential direction, and paired cantilever-shaped elastic tongue pieces63bent and extended from the rotation restricting portions62, respectively.

A width of each of the rotation restricting portions62is smaller than a width of the main body portion61. The main body portion61is held on the inner periphery of the bearing hole53of the housing17athrough frictional engagement. As shown inFIG. 6, one of the pair of elastic tongue pieces63is arranged on a side of a first edge61c, and the other elastic tongue piece63is arranged on a side of a second edge61d. The elastic tongue pieces63cross each other.

With reference toFIG. 5, in a part of the inner periphery of the bearing hole53of the housing17a, a receiving recessed portion64is formed. The receiving recessed portion64is recessed in a direction (the direction Y1in which the center-to-center distance D1is increased) that is opposite to a direction toward the worm wheel19(the direction Y2in which the center-to-center distance D1is decreased) with respect to the second bearing31. A distal end of each of the elastic tongue pieces63of the plate spring60is received by a bottom64cof the receiving recessed portion64of the bearing hole53, and an urging force of each of the elastic tongue pieces63urges the second end portion18bof the worm shaft18through the second bearing31in the direction Y2in which the center-to-center distance D1is decreased. The receiving recessed portion64has paired inner walls64aand64bthat face each other in the circumferential direction Z1of the bearing hole53. The rotation restricting portions62of the plate spring60abut on the corresponding inner walls64aand64b, respectively, thereby restricting rotation of the plate spring60in the circumferential direction Z1of the bearing hole53.

According to this embodiment, the elastic body (the first elastic body32), which urges the worm shaft18in the axial direction X1(toward the electric motor16), is supported by the element (the first rotational element22) of the joint21in the axial direction X1. Therefore, the structure is simplified compared to a case where a support member is provided in addition to the joint21.

Since the elastic body (the first elastic body32) has a circular shape that surrounds the boss25, the elastic body (the first elastic body32) and a part of the joint21are able to be arranged so as to overlap each other in the axial direction X1. As a result, it is possible to reduce an axial space.

The worm shaft18is press-fitted into the boss25to a position where the boss25and the positioning step portion49between the second portion46(the inner ring fitting portion) and the third portion47(the boss fitting portion) abut on each other. Thus, the boss25is accurately positioned in the axial direction X1of the worm shaft18. Hence, it is possible to set an axial load applied by the elastic body (the first elastic body32) with high accuracy.

The worm shaft18is urged to the neutral position in the axial direction X1by the first elastic body32and the second elastic body33that are arranged on respective sides of the first bearing30. Therefore, the worm shaft18is slightly moved in the axial direction X1during minute steering, and thus, steering feeling is improved.

There is provided the urging member (the plate spring60) that urges the second end portion18bof the worm shaft18toward the worm wheel19. The joint21, which connects the first end portion18aof the worm shaft18with the rotary shaft20of the electric motor16, includes the first rotational element22that is connected to the worm shaft18so as to rotate together with the worm shaft18, the second rotational element23that is connected to the rotary shaft20so as to rotate together with the rotary shaft20, and the elastic member24that connects both of the rotational elements22and23with each other so that torque is transmittable between the rotational elements22and23. Therefore, as the rotational elements22and23are allowed to be inclined with respect to each other due to the elastic member24in the joint21, the worm shaft18that is urged by the urging member60is oscillated smoothly, and thus, backlash between the worm shaft18and the worm wheel19is reliably maintained at zero.

The present invention is not limited to the foregoing embodiment, and for example, a compression coil spring may be used as the urging member instead of the plate spring60. In the foregoing embodiment, steering assist force of the electric motor is applied to the steering shaft. However, the steering assist force of the electric motor may be applied to a pinion shaft instead.