Electric power steering apparatus

An electric power steering apparatus in which steering assistance is achieved by transmitting a rotary motion of an electric motor for steering assistance to a steering shaft on which a worm wheel is mounted through a worm shaft on which a worm is mounted, wherein the worm shaft is deflectable toward the worm wheel and equipped with a biasing member biasing the worm shaft in the deflective direction thereof, or alternatively, the worm wheel is deflectable toward the worm shaft and equipped with a biasing member biasing the worm wheel in the deflective direction thereof.

BACKGROUND OF THE INVENTION

This invention relates to an electric power steering apparatus with an electric motor as a source generating a steering-assist force.

Steering of automotive vehicles is realized by transmitting a rotary operation of a steering wheel disposed in the vehicle's cabin to a steering mechanism provided outside the cabin for adjusting directions of the wheels (generally front wheels).

The electric power steering apparatus for use in an automotive vehicle, as shown inFIG. 1, includes: a first steering shaft102connected to a steering wheel101; a torque sensor105sensing a steering torque by a relative rotary displacement of a second steering shaft104to the first steering shaft102, both being linked via a torsion bar103; and a reduction mechanism109having a worm107and a worm wheel108, reducing a rotary motion of an electric motor106for steering assistance, which is driven based on a sense result of the torque sensor105, to transmit the reduced rotary motion to the second steering shaft104, wherein operations of the steering mechanism responsive to the rotary motion of the steering wheel101are assisted by a rotary motion of the electric motor106so as to alleviate a physical load imposed on a driver in steering.

A worm shaft on which the worm is mounted and the second steering shaft on which the worm wheel is mounted are each supported by bearings at opposed ends in the longitudinal direction and each blocked from displacement in the radial direction and the longitudinal direction.

In a case where a worm and a worm wheel are employed as described above, there arise backlash between opposed tooth flanks. Therefore, in the prior art, when a worm shaft and a worm wheel were assembled, it was performed within machining accuracies of parts such that no backlash is produced. However, with inevitable dimensional errors inherent in a worm, a worm shaft, bearings for supporting the shaft, a worm wheel, a second steering shaft for supporting the wheel and the like, backlash was produced after the assembly with a relatively high ratio.

Further, in a recent, advanced case applied with a high-power steering-assist force, wear of teeth of the worm and the worm wheel has been increased, thereby, problematically disabling avoidance of backlash. In this case, the worm shaft and the second steering shaft are blocked in displacement, disabling adjustment of backlash, and therefore unpleasant noise caused by backlash is leaked into the cabin of an automotive vehicle.

BRIEF SUMMARY OF THE INVENTION

It is accordingly an object of the invention is to provide an electric power steering apparatus capable of properly eliminating backlash without receiving any adverse influence of dimensional errors of parts such as a worm.

It is another object of the present invention to provide an electric power steering apparatus capable of easily eliminating backlash due to increase in wear of teeth of a worm and a worm wheel.

A first aspect of the present invention is an electric power steering apparatus in which steering assistance is achieved by transmitting a rotary motion of an electric motor for steering assistance to a steering shaft on which a worm wheel is disposed through a worm shaft on which a worm is disposed, wherein the worm shaft is deflectable toward the worm wheel and equipped with a biasing member biasing the worm shaft in the deflective direction thereof.

A second aspect of the present invention is an electric power steering apparatus in which steering assistance is achieved by transmitting a rotary motion of an electric motor for steering assistance to a steering shaft on which a worm wheel is disposed through a worm shaft on which a worm is disposed, wherein the worm wheel is deflectable toward the worm shaft and equipped with a biasing member biasing the worm wheel in the deflective direction thereof.

According to such electric power steering apparatuses, the worm shaft or the worm wheel can be moved toward the worm wheel or the worm shaft by the biasing member. Therefore, backlash can be properly eliminated without receiving any adverse influence of dimensional errors of parts such as the worm to increase a production yield. In addition, even in a case where wear of teeth of the worm and the worm wheel has been increased, backlash can be properly eliminated, thereby enabling prevention of leakage of backlash noise into the cabin of an automotive vehicle.

In the electric power steering apparatus of the first aspect of the present invention, the worm shaft is deflectable in a side of the worm shaft, interlocked with an output shaft of the electric motor and an interlocking member interlocking the worm shaft and the output shaft is provided therebetween. Since the deflection of the worm shaft can be absorbed by the interlocking member, it is prevented for the deflection of the worm shaft to adversely affect a rotary characteristic of the worm shaft from occurring even if much of the deflection arises.

In the electric power steering apparatus of the first aspect of the present invention, the worm shaft is supported in a gear housing having a tapped hole, and the biasing member includes a screw body tightly fastened in the tapped hole and a spring body interposed between the screw body and the worm shaft. Even when backlash cannot be eliminated only by a force of the spring body, backlash can be properly eliminated by operating the screw body.

In the electric power steering apparatus of the first aspect of the present invention, the worm shaft is supported in a gear housing having a tapped hole, and the biasing member includes a screw body which is tightly fastened in the tapped hole and which is in contact with the worm shaft or a bearing fitted to the worm shaft. It is properly prevented for the worm shaft to move away from the worm wheel and in addition, a contact resistance between the worm and the worm wheel can be smaller than in the case where the spring body is employed, thereby, enabling the rotary characteristic of the worm wheel to improve.

In the electric power steering apparatus of the second aspect of the present invention, the worm wheel is fitted to the steering shaft, and the biasing member includes an elastic ring fitted to a fit portion of the worm wheel to the steering shaft. Since a simple structure is adopted in which the elastic ring is only fitted to the worm wheel, parts is few in number and an assembling property is good, thereby enabling reduction in cost.

DETAILED DESCRIPTION OF THE INVENTION

Detailed description will be given of the present invention with reference to the figures showing embodiments thereof.

First Embodiment

FIG. 2is a sectional view of the whole of an electric power steering apparatus relating to the present invention. The electric power steering apparatus, as shown inFIG. 2, includes: a first steering shaft2connected to a steering wheel1; a torque sensor5sensing a steering torque by a relative rotary displacement of a second steering shaft4to the first steering shaft2, the second steering shaft4being coupled with the first steering shaft2via a torsion bar3; a constant velocity joint transmitting a rotary motion of the second steering shaft4to a steering mechanism; a reduction mechanism7having a worm71and a worm wheel72, which reduces a rotary motion of an electric motor6for steering assistance driven based on a sense result of the torque sensor5and transmits the reduced rotary motion to the second steering shaft4; a first shaft housing9and a second shaft housing10to enclose and support the first steering shaft2; a sensor housing11accommodating the torque sensor5; a gear housing accommodating the reduction mechanism7; and a mounting bracket for mounting the first shaft housing9on a vehicle body. Herein, the electric motor6is attached to the gear housing8and the other end portion of the first shaft housing9is fitted to one end portion of the second shaft housing10so as to enable a relative movement in the longitudinal direction.

The steering wheel1is mounted to one end of the first steering shaft2. An intermediate portion of the first steering shaft2includes: a first shaft body2ahaving a cylindrical form supported by the first shaft housing9having a cylindrical form with a bearing13interposed therebetween; a second shaft body2bhaving a rod-like form, not capable of relative rotation, and fitted to the other end of the first shaft body2aso as to be movable in the longitudinal direction: and a third shaft body2dhaving a cylindrical form, connected to the second shaft body2bthrough a dowel pin2c. Between the first and second shaft bodies2aand2b, inserted is a shock energy absorber2erade of a synthetic resin, absorbing shock energy, which acts on a driver through the steering wheel1. Further, between the third shaft body2dand the second steering shaft4, disposed is the torque sensor5.

The second steering shaft4having a cylindrical form is fitted to the other end portion of the torsion bar3and coupled with it through a dowel pin4a, wherein one end of the torsion bar3is coupled with the other end of the second shaft body2bthrough the dowel pin2c. An intermediate portion in the longitudinal direction of the second steering shaft4is rotatably supported by the gear hosing8with a pair of bearings14and15interposed therebetween, and the worm wheel72is mounted, in fit, on a fitting portion4bof the second steering shaft4between the bearings14and15.

The second shaft housing10, as shown inFIG. 2, is fitted to the first shaft hosing9at one end portion thereof and further fitted to the sensor housing11at the other end portion thereof. In absorption of the above described shock energy, the second shaft housing10allows the first shaft housing9to move in the longitudinal direction toward or away from the second shaft housing10.

FIG. 3is a sectional view of the reduction mechanism and the electric motor combined andFIG. 4is a sectional view taken on line of IV to IV ofFIG. 3. The reduction mechanism7includes: the worm71formed integratedly in an intermediate portion of the worm shaft70in the longitudinal direction, coupled with an output shaft60of the electric motor6; and the worm wheel72fixedly held at the fit portion4bof the second steering shaft4. A rotary motion of the electric motor6is reduced in velocity by the engagement between the worm71and the worm wheel72to be transmitted to the second steering shaft4and further, the rotary motion reduced in velocity is transmitted to the steering mechanism through the constant velocity joint.

The worm shaft70is disposed so as to be perpendicular to the axial line of the second steering shaft4and both end portions of the worm shaft70in the longitudinal direction are rotatably supported in first and second bearing holes81and82, respectively, of the gear housing8with first and second bearings16and17, both being ball bearings, interposed therebetween. The worm71is integratedly disposed in the worm shaft70between the first and second bearings16and17. Further, one end portion of the worm shaft70is inserted into a connecting cylinder18in spline fit to couple with the output shaft60.

The worm shaft70in the gear housing8described above is supported such that the second bearing17located opposite the output shaft60is deflectable toward the worm wheel72. A biasing member30including a spring body biasing the second bearing17in the deflective direction is provided to the gear housing8.

A concave portion83of a circular arc in section accepting movement of the second bearing17is formed in a second bearing hole82and the second bearing17is deflected into the concave portion83. Further, a tapped hole84is formed through the gear housing8opposite the concave portion83in the second bearing hole82so as to open outside the gear housing8and the biasing member30is mounted in the tapped hole84. It should be appreciated that since a deflection of the second shaft17toward the worm wheel72is about 30 μm, the deflection can be sufficiently absorbed by a backlash amount of a connecting portion between the worm shaft70and the output shaft60and a backlash amount of the first bearing16.

The biasing member30has a curved surface of a circular arc in section contacting an outer circumferential surface of the second bearing17and a spring supporting section, and, to be detailed, includes: a spring supporter31inserted so as to be movable toward the second bearing hole82; a spring body32of a coil spring supported by the spring supporter31; a screw body33adjusting a flexure amount of the spring body32contracted in the tapped hole84; and a lock nut34screwed on the screw body33. The biasing member30deflects the second bearing17toward the concave portion83by operating the screw body33, in a state where the lock nut34is loose, to move the spring supporter31.

In the gear housing8, provided are a first regulatory portion8aregulating movement of the first bearing16in the acting direction of a thrust load thereon and a second regulatory portion8bregulating movement of the second bearing17in the acting direction of a thrust load thereon. A screw body19in contact with the outer ring of the first bearing16is tightly screwed in the end portion closer to the electric motor6of the first bearing hole81and thereby, an operating force of the screw body19is transmitted to the worm shaft70through the first bearing16to prevent backlash of the worm shaft70in the longitudinal direction. A lock nut20in loose screwing with the screw body19in this state is then tightly screwed to lock a rotary motion of the screw body19.

In the first embodiment, in assembly of the worm shaft70into the electric power steering apparatus, the second bearing17is moved by operating the screw body33to force the worm71to be put in contact with the worm wheel72, then the worm wheel72is turned to confirm no backlash therebetween and thereafter, the lock nut34is tightly screwed to retain such an adjusted state. Therefore, backlash is properly eliminated without receiving any influence of dimensional errors of the parts such as the worm71, thereby enabling a production yield to increase.

When backlash is produced due to increase in wear of teeth of the worm71and the worm wheel72, the lock nut20and the screw body19are loosened to release the second bearing17from a constrained condition, such that the second bearing17and the worm shaft70are deflected toward the concave portion83by a force of the spring body32, with the result that no backlash becomes present. Further, in this case, if backlash cannot be eliminated only by the force of the spring body32, the lock nut34is loosened and the screw body33is operated to adjust a flexure amount of the spring body32, such that the second bearing17is deflected toward the concave portion83, thereby enabling backlash to be properly eliminated.

Second Embodiment

FIG. 5is a sectional view of the reduction mechanism and the electric motor combined in a second embodiment andFIG. 6is a sectional view taken on line of VI to VI ofFIG. 5. The electric power steering apparatus of the second embodiment has a structure in which comparing with the first embodiment, neither of the spring body32and the spring supporter31of the above-mentioned biasing member30is present, a screw body35is screwed in a tapped hole84of the gear housing8, the screw body35being put in contact with the outer circumferential surface of the second bearing17; the second bearing17is deflected toward the concave portion83by operating the screw body35and further, no first regulatory portion8ais present. The other parts of the structure and workings thereof are the same as those in the first embodiment, and therefore the same reference marks are attached to parts in common use with both embodiments and descriptions thereof are omitted.

In the second embodiment, when the worm shaft70is assembled into the apparatus, the procedure goes as follows: The second bearing17is moved by operating the screw body35in a loose condition of the lock nut20and the screw body19to force the worm71to be put in contact with the worm wheel72. After confirmation that no backlash is present between the worm71and the worm wheel72by turning the worm wheel72, the screw body19is operated to transmit an operating force thereof to the outer ring of the second bearing17through the outer ring of the first bearing16, balls thereof, the inner ring thereof, the worm shaft70, the inner ring of the second bearing17and balls thereof. The outer ring is then pressed to the second regulatory portion8bto constrain free deflection of the second bearing17and retain such an adjusted state by the screw body35. Therefore, backlash can be properly eliminated without receiving any influence of dimensional errors of parts such as the worm71, thereby enabling a production yield to increase.

When backlash is produced due to increase in wear of teeth of the worm71and the worm wheel72, the lock nut20and the screw body19are loosened to release the second bearing17from a constrained condition and further, the screw body35is operated to deflect the second bearing17toward the concave portion83, thereby enabling backlash to be properly eliminated.

Third Embodiment

FIG. 7is a sectional view of the reduction mechanism and the electric motor combined in a third embodiment. The electric power steering apparatus of the third embodiment has a structure in which comparing with the first embodiment, the first bearing16is deflectable instead of the second bearing17, the biasing member30biasing the first bearing16is provided and furthermore the output shaft60and the worm shaft70are coupled by a constant velocity joint21with neither of the screw body19and the lock nut20. The other parts of the structure and workings thereof are the same as those in the first embodiment, and therefore the same reference marks are attached to parts in common use with both embodiments and descriptions thereof are omitted.

The constant velocity joint21is coupled with the output shaft60and the worm shaft70, respectively, via yokes21band21b, and joint crosses21cand21cprovided at the both end portions of a shaft body21ato transmit a rotary motion of the output shaft60to the worm shaft70at a constant velocity.

In the third embodiment, when the worm shaft70is assembled into the apparatus, the first bearing16is moved by operating the screw body33to force the worm71to be put in contact with the worm wheel72and after confirmation that no backlash is present between the worm71and the worm wheel72by turning the worm wheel72, the lock nut34is tightly fastened to retain the adjusted state. Therefore, backlash can be properly eliminated without receiving any influence of dimensional errors of parts such as the worm71, thereby enabling a production yield to increase.

When backlash is produced due to increase in wear of teeth of the worm71and the worm wheel72, the first bearing16and the worm shaft70are forcibly deflected toward the concave portion83by a force of the spring body32to enable the backlash to be eliminated. In this case, if the backlash cannot be eliminated only by the force of the spring body32, the lock nut34is loosened to adjust a flexure amount of the spring body32to deflect the first bearing16toward the concave portion83, thereby enabling the backlash to be properly eliminated.

Further, in the third embodiment, since the worm shaft70is coupled with the output shaft60by the constant velocity joint21, the worm shaft70can be smoothly rotated without receiving any influence of a magnitude of defection of the worm shaft70.

It should be appreciated that the biasing member30may adopt a configuration of the second embodiment.

Fourth Embodiment

FIG. 8is a sectional view of the reduction mechanism and the electric motor combined in a fourth embodiment andFIG. 9is a sectional view of the biasing member. The electric power steering apparatus of the fourth embodiment has a structure in which comparing with the first embodiment, the worm wheel72is deflectable toward the worm shaft70instead of deflection of the worm shaft70, a biasing member22biasing the worm wheel72toward a deflecting direction thereof is provided with no biasing member30for the worm shaft70. The other parts of the structure and workings thereof are the same as those in the first embodiment, and therefore the same reference marks are attached to parts in common use with both embodiments and descriptions thereof are omitted.

In the worm wheel72, an inner diameter of a fit hole72aformed at the inner periphery thereof is set larger than an outer diameter of the second steering shaft4in the fit portion4bso as to allow the second steering shaft4to deflect toward the worm shaft70. Further, the biasing member22is an elastic ring23fitted between the fit portion4bof the second steering shaft4and the inner periphery, forming the fit hole72a, of the worm wheel72so as to enable a rotational force to be transmitted therebetween.

The elastic ring23is constructed from an inner ring23aand an outer ring23bhaving a rigidity such as made of a metal or a synthetic resin, combined in one body using binding means such as vulcanizing bonding. Irregular streaks such as serration are formed on the inner circumferential surface, forming a fit hole, of the inner ring23aand the outer circumferential surface of the outer ring23b, and the inner ring23ais fixed, by pressing, to the fit portion4bwhile the outer ring23bis fixed, by pressing, into the fit hole72aof the worm wheel72. With such a structure and an operation adopted, the worm wheel72becomes deflectable toward the worm shaft70via the second steering shaft4. It should be appreciated that the worm wheel72becomes to be one body with the second steering shaft4through the elastic ring23to rotate in synchronism with the second steering shaft4.

Fixing the elastic ring23may be realized by using a relative-rotation preventive member such as a pin or a key instead of fixing, by pressing, using irregular steaks such as serration provided on the inner ring23aand the outer ring23bas described above.

In the fourth embodiment, when the worm shaft70and the worm wheel72are assembled in the apparatus, for example, after the second steering shaft4is supported in the gear housing8by the bearings14and15(seeFIG. 2), not only is the worm71put in contact with the worm wheel72, but the worm71side portion of the elastic ring23is compressed in the radial direction as shown inFIG. 8to deflect the worm wheel72toward the opposite side of the worm71. Therefore, backlash can be properly eliminated without receiving any influence of dimensional errors of parts such as the worm71, thereby enabling a production yield to increase.

When backlash is produced due to increase in wear of teeth of the worm71and the worm wheel72, the worm wheel72is deflected toward the worm71by a force of the elastic ring23, thereby enabling the backlash to be eliminated.

As the invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiments are therefore illustrative and not restrictive. Since the scope of the invention is defined by the appended claims rather than by the description preceding them, all changes that fall within metes and bounds of the claims, or equivalence of such meters and bounds thereof are therefore intended to be embraced by the claims.