WO2007049444A1

WO2007049444A1 - Steering apparatus for vehicle

Info

Publication number: WO2007049444A1
Application number: PCT/JP2006/320025
Authority: WO
Inventors: Masaki Fujimoto; Takatoshi Saito; Masaharu Yamashita; Miwanori Oikawa
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2005-10-24
Filing date: 2006-09-29
Publication date: 2007-05-03
Also published as: JP2007112397A

Abstract

A speed reducer 40 of an electric power steering apparatus is configured as a 2-stage speed reducer equipped with a worm speed reducer 41 adapted to receive rotational torque from an electric motor 30 and output the rotational torque from an output element (worm wheel 412) while reducing rotational speed, and with a planetary-gear-type speed reducer 42 adapted to receive the rotational torque from the output element and output the rotational torque while reducing rotational speed. The rotational speed of the electric motor 30 is reduced at two stages by the two speed reducers, thereby achieving high output (high speed reduction). Speed-reducing elements (a sun gear, planetary gears, a carrier, etc.) of the planetary-gear-type speed reducer 42 are disposed in the interior of the worm wheel 412 of the worm speed reducer 41, thereby implementing compactness.

Description

DESCRIPTION

STEERING APPARATUS FOR VEHICLE

TECHNICAL FIELD

The present invention relates to a steering apparatus for a vehicle, and more particularly to a steering apparatus for a vehicle which includes an electric motor which generates a steering force on the basis of a steering operation of a steering wheel.

BACKGROUND ART

Conventionally, a steering apparatus for a vehicle which includes a so-called power steering apparatus has been known. The power steering apparatus has an actuator for assisting a steering operation of a steering wheel and assists the steering of steerable wheels which is effected according to a steering operation of the steering wheel. Known power steering apparatus include a hydraulic power steering apparatus in which a hydraulic actuator generates an assist torque, and an electric power steering (EPS) apparatus in which an electric motor generates an assist torque. Particularly in recent years, in view of mountability in a vehicle and improvement of fuel consumption rate, the electric power steering apparatus has been of particular interest.

Also, a so-called steer-by-wire system has been developed. In the steer-by-wire system, a steering wheel and steerable wheels to be steered are mechanically isolated from each other, and a drive control circuit drives and controls a steering motor mechanically connected to the steerable wheels and a steering-wheel motor mechanically connected to the steering wheel, whereby the steerable wheels can be steered according to an operation of the steering wheel. Even this steer-by-wire system uses an electric motor as a steering actuator.

In the electric power steering apparatus or the steer-by-wire system, the rotational speed of an output shaft of the electric motor is reduced by means of a speed reducer, and a drive force associated with the reduced speed is transmitted to an output destination member, such as a steering shaft, a rack bar, or steerable wheels. Implementation of high output (high speed reduction) is required of speed reducers for use in the electric power steering apparatus and the steer-by-wire system. For example, JP2004-217046A discloses a speed reducer of a 2-stage type in which a planetary-gear-type speed reducer is used at the first stage, and a ball-screw-mechanism speed reducer is used at the second stage. JP2004-338553A discloses a speed reducer of a 2-stage type in which a pulley-type speed reducer using a pulley and a belt is used at the first stage, and a worm speed reducer using a worm and a worm wheel is used at the second stage.

The speed reducers disclosed in Patent Documents mentioned above are of a 2-stage type and can achieve high output. However, since two speed reducers are combined, the overall speed-reducer size is increased. Such an increased size raises a problem of poor mountability in a vehicle.

The present invention has been achieved for solving the above problems, and an object of the invention is to provide a steering apparatus for a vehicle which includes a speed reducer which reduces an output rotational-speed of an electric motor adapted to generate a steering force, on the basis of a steering operation of a steering wheel, can achieve high output (high speed reduction), and can be configured compactly to thereby offer good mountability in a vehicle.

DISCLOSURE OF THE INVENTION

To achieve the above object, the present invention provides a steering apparatus for a vehicle which includes an electric motor for generating a steering force on the basis of a steering operation of a steering wheel. The steering apparatus comprises a first speed reducer adapted to receive rotational torque from the electric motor and output the rotational torque from an output element while reducing rotational speed; and a second speed reducer adapted to receive the rotational torque from the output element of the first speed reducer and output the rotational torque while reducing rotational speed by means of speed-reducing elements disposed in the interior of the output element of the first speed reducer.

The speed reducer according to the present invention is a multistage-type speed reducer comprising the first speed reducer and the second speed reducer and thus can achieve high output (high speed reduction). Since the speed-reducing elements of the second speed reducer are disposed in the interior of the output element of the first speed reducer, the speed reducer can be more compact as compared with conventional multistage-type speed reducers in which two speed reducers are merely connected together. The present invention can provide a speed reducer which achieves both high output and compactness. Herein, the term "output element" refers to a component element of a speed reducer for transmitting an output after reduction of speed to the exterior of the speed reducer. The term "speed-reducing elements" refers to all of or part of component elements of a speed reducer. Accordingly, in the present invention, the entire second speed reducer may be disposed in the interior of the output element of the first speed reducer; alternatively, a portion of the second speed reducer may be disposed in the interior of the output element of the first speed reducer.

The expression "disposed in the interior of the output element" means disposition within an outline of the output element. For example, when the output element has a cylindrical shape, the expression means disposition at the inner side of the cylinder. In this case, speed-reducing elements may be partially disposed in the interior of the output element, since even partial disposition in the interior of the output element contributes to implementation of compactness.

The second speed reducer may be a planetary-gear-type speed reducer having a ring gear which has internal gear teeth, a sun gear which is coaxially disposed in the ring gear, a planetary gear which is disposed between the ring gear and the sun gear, and a carrier which is attached to the planetary gear and rotates in association with revolution of the planetary gear. Furthermore, preferably, the output element of the first speed reducer and the ring gear are formed integrally with each other. The planetary-gear-type speed reducer is configured such that speed-reducing elements are accommodated in the interior of the ring gear. Accordingly, use of the planetary-gear-type speed reducer as the second speed reducer readily allows disposition of speed-reducing elements of the second speed reducer in the interior of the output element of the first speed reducer. Furthermore, integral formation of the output element of the first speed reducer and the ring gear of the second speed reducer allows the first and second speed reducers to share some components. Thus, a reduction in cost and implementation of compactness can be enhanced.

The first speed reducer may be a worm speed reducer having a worm and a worm wheel meshed with the worm, and the worm wheel serves as the output element of the first speed reducer and is formed integrally with the ring gear of the second speed reducer. By means of imparting a ring-like shape to the worm wheel, which is an output element of the worm speed reducer, and forming internal gear teeth on the inner circumferential surface of the worm wheel, the worm wheel can be used as the ring gear of the second speed reducer. Accordingly, other speed-reducing elements of the second speed reducer, such as the planetary gear and the sun gear, can be disposed within the ring gear, which also serves as the worm wheel. Thus, the speed reducer can be formed compactly.

According to another preferable configuration, the first speed reducer is a pulley-type speed reducer having a drive pulley, a driven pulley, and a drive belt looped around and extending between the drive pulley and the driven pulley for transmitting a driving force of the drive pulley to the driven pulley. The driven pulley serves as the output element of the first speed reducer. Further, the driven pulley is formed integrally with the ring gear which is a speed-reducing element of the second speed reducer. In this case, by means of forming gear teeth on the inner circumferential surface of the driven pulley, the driven pulley can be used as the ring gear. By means of disposing other speed-reducing elements of the second speed reducer within the ring gear, which also serves as the driven pulley, the speed reducer can be formed compactly.

According to still another preferable configuration, the first speed reducer is a gear-type speed reducer having a drive gear, and a driven gear meshed with the drive gear, and the driven gear serves as the output element of the first speed reducer and is formed integrally with the ring gear of the second speed reducer. In this case, by means of forming internal gear teeth on the inner circumferential surface of the driven gear, the driven gear can be used as the ring gear. By means of disposing other speed-reducing elements of the second speed reducer within the ring gear, which also serves as the driven gear, the speed reducer can be formed compactly.

Preferably, in the case where the first speed reducer and the second speed reducer are the worm speed reducer and the planetary-gear-type speed reducer, respectively, while the worm wheel of the worm speed reducer and the ring gear of the planetary-gear-type speed reducer are formed integrally with each other, the worm wheel has external gear teeth formed from resin on an outer circumferential surface thereof and internal gear teeth formed from metal on an inner circumferential surface thereof. Formation of the external gear teeth of the worm wheel from resin effectively prevents or suppresses generation of rattle in the worm speed reducer. By means of forming from metal the internal gear teeth of the worm wheel which are used as the internal gear teeth of the ring gear, rigidity of the worm wheel can be enhanced.

Preferably, in the case where the second speed reducer is the planetary-gear-type speed reducer in which the ring gear is an input element, the sun gear is a stationary element, and the carrier is an output element, the steering apparatus for a vehicle may further comprises a torque limiter for allowing rotation of the sun gear when a rotational torque transmitted from the planetary gear to the sun gear attains a predetermined value or greater. In the planetary-gear-type speed reducer in which the sun gear is stationary, and the ring gear is an input element, the planetary gear rolls on the sun gear to thereby transmit torque to the carrier, which is an output element. In this configuration, for example, when a large torque is input from the carrier, which is an output element; i.e., when a so-called reverse input arises, this reverse input torque is transmitted to the planetary gear via the carrier. The reverse input torque attempts to rotate the planetary gear so as to be transmitted to the first speed reducer via the ring gear. However, when the first speed reducer has a considerably large resistance torque against the reverse input as in the case of the worm speed reducer, the first speed reducer does not rotate. As a result, although the planetary gear is fixed by the sun gear and the ring gear and is thus unable to rotate, the reverse input torque is input.

By contrast, according to the present invention, when a rotational torque transmitted from the planetary gear to the sun gear attains a predetermined value or greater, the torque limiter allows rotation of the sun gear. Thus, in the above-mentioned condition where a reverse input torque attains the predetermined value or greater, the planetary gear and the sun gear rotate. Rotation of the sun gear releases the reverse input torque, thereby preventing application of a torque equal to or greater than a predetermined value to the planetary-gear-type speed reducer, to the first speed reducer, and further to the electric motor and thus protecting these devices. Preferably, in the case where the first speed reducer and the second speed reducer are the worm speed reducer and the planetary-gear-type speed reducer, respectively, the planetary gear, which is a speed-reducing element of the second speed reducer, is in the form of a helical gear. Since the helical gear has tooth traces inclined in relation to an axial direction thereof, a thrust force is generated in the axial direction while the helical gear and a mating gear rotate. Accordingly, when the planetary gear is in the form of a helical gear, a thrust force is exerted on the ring gear. When the first speed reducer is the worm speed reducer, a thrust force is generated in an axial direction of the worm wheel meshed with the worm.

Accordingly, in the case where the ring gear and the worm wheel are formed integrally with each other, the above-mentioned two thrust forces are exerted on the ring gear (worm wheel). By means of adjusting the helix angle of the planetary gear in the form of a helical gear so as to cause mutual cancellation of the two thrust forces, the thrust forces exerted on the ring gear (worm wheel) can be weakened, or even the generation of the thrust forces can be prevented. Thus, unusual noise which could otherwise be generated from the above-mentioned thrust forces can be effectively suppressed, or the generation of the noise can be prevented.

Preferably, in the case where the first speed reducer is the worm speed reducer, the steering apparatus for a vehicle further comprises mesh adjustment means for adjusting a meshing condition between the worm and the worm wheel. Even when backlash varies because of dimensional changes or the like caused by wear stemming from operation of the worm speed reducer or by temperature variations, the mesh adjustment means can immediately adjust the meshing condition. This adjustment of the meshing condition can effectively prevent generation of unusual noise which could otherwise arise Jn the first speed reducer due to backlash variations or the like.

In this case, preferably, the mesh adjustment means comprises worm-wheel-urging means for urging the worm wheel in such a direction that the worm and the worm wheel approach each other, and a holder for supporting the worm-wheel-urging means. Preferably, the worm-wheel-urging means comprises an abutment member in abutment with an inner circumferential surface of the ring gear of the second speed reducer, a rod having a first end portion connected to the abutment member and extending in a direction toward the worm, and a rod-urging member for urging the rod toward a second end portion of the rod from the first end portion (connected to the abutment member) of the rod.

Preferably, in the case where the first speed reducer is the worm speed reducer, the steering apparatus for a vehicle further comprises holder means for maintaining a relative arrangement condition between the worm and the worm wheel. Generally, in a worm speed reducer, a worm is rotatably supported by, for example, a housing thereof. This housing may be formed from a material (e.g., aluminum) different from that for speed-reducing elements In order to reduce weight. When the housing is deformed because of thermal expansion or the like, the worm supported by the housing changes in position in relation to the worm wheel. Particularly, when the distance between the worm and the worm wheel increases, backlash increases, resulting in generation of unusual noise. By contrast, according to the present invention, the holder means maintains a relative arrangement condition between the worm and the worm wheel; thus, the distance therebetween does not increase, thereby effectively preventing generation of unusual noise which could otherwise be induced by deformation of the housing stemming from thermal expansion or the like.

Preferably, in the case where the second speed reducer is the planetary-gear-type speed reducer, the planetary gear has the shape of a truncated cone, whose axial section is trapezoidal, and the steering apparatus for a vehicle further comprises planetary-gear-urging means for urging the planetary gear in an axial direction directed from a large-diameter portion of the truncated cone shape toward a small-diameter portion of the truncated cone shape. Imparting the above-mentioned shape to the planetary gear and providing the planetary-gear-urging means result in generation of a pressing force which presses the planetary gear against mating gears (the ring gear and the sun gear) at meshing portions. This pressing force adjusts meshing between the planetary gear and each of the ring gear and the sun gear. This adjustment of meshing can effectively prevent generation of unusual noise which could otherwise arise in the second speed reducer due to backlash variations or the like.

In the case where the second speed reducer is the planetary-gear-type speed reducer, the carrier, which is a speed-reducing element of the second speed reducer, serves as an output element of the second speed reducer, and the carrier is connected to an output destination member (e.g., a steering shaft or a rack bar). In this case, preferably, the carrier is connected to an output destination member in such a manner as to be rotatable in relation to the output destination member within a predetermined range. Even when an initial rotational torque is input from the output destination member, the connection in this manner allows the carrier to rotate in relation to the output destination member, thereby absorbing the initial rotational torque.

An electric power steering apparatus generates an assist torque for assisting a steering operation of a steering wheel, on the basis of the steering operation. However, a certain time elapses until a controller such as the ECU recognizes a steering operation and drives an electric motor according to the steering operation; thus, response to the steering operation involves a certain delay. If the carrier, which is an output element of the second speed reducer, is fixedly connected to an output destination member, an assist torque will not be provided at the beginning of a steering operation, due to the delay, thus raising a problem of poor feel of steering.

In this respect, according to the present invention, the carrier is connected to an output destination member in such a manner as to be movable in relation to the output destination member within a predetermined range. Accordingly, even at the initial stage of steering, the steering wheel can be operated with a small torque regardless of whether an assist torque is generated, thereby providing an improved feel of steering.

In this case, preferably, the carrier and the output destination member are connected together via an elastic body. A restoration force associated with elastic deformation of the elastic body serves as a steering torque, thereby providing a certain steering torque at the initial stage of steering. Thus, stability of a steering operation is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, general view of a steering apparatus for a vehicle according to an embodiment of the present invention and having an assist function for assisting a steering operation of a driver.

FIG. 2 is a front view showing an assist mechanism according to a first embodiment of the present invention.

FIG. 3 is a schematic, sectional view taken along line A — A of FIG. 2.

FIG. 4 is a schematic, partial sectional view of a speed reducer according to a second embodiment of the present invention in which a torque limiter is attached to a sun gear.

FIG. 5 is a schematic, partial sectional view of a speed reducer according to a third embodiment of the present invention in which planetary gears each assume the form of a helical gear.

FIG. 6 is a front view of a worm wheel (ring gear) according to a fourth embodiment of the present invention.

FIG. 7 is a front view showing a schematic configuration of a speed reducer according to a fifth embodiment of the present invention in which a first speed reducer is a pulley-type speed reducer.

FIG. 8 is a front view showing a schematic configuration of a speed reducer according to a sixth embodiment of the present invention in which a first speed reducer is a gear-type speed reducer.

FIG. 9 is a schematic, partial sectional view of a speed reducer according to a seventh embodiment of the present invention in which a first speed reducer is provided with meshing-force adjustment mechanisms.

FIG. 10 is a view as viewed in the direction of arrow B of FIG. 9.

FIG. 11 is a schematic, partial sectional view of a modified speed reducer of the seventh embodiment.

FIG. 12 is a schematic, partial sectional view of another modified speed reducer of the seventh embodiment.

FIG. 13 is a schematic, partial sectional view of a speed reducer according to an eighth embodiment of the present invention in which a first speed reducer is provided with a holder mechanism.

FIG. 14 is a schematic, partial sectional view of a speed reducer according to a ninth embodiment of the present invention in which planetary gears have the shape of a truncated cone.

FIG. 15 is a detailed view of connection between a carrier and a steering shaft in a tenth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment:

A first embodiment of the present invention will be described in detail with reference to FIGS. 1 to 3. FIG. 1 is a schematic, general view of a steering apparatus for a vehicle according to the present embodiment and the following embodiments and having an assist function for assisting a steering operation of a driver.

This steering apparatus has a steering shaft 12. A steering wheel 11 is connected to an upper end of the steering shaft 12 in a unitarily rotatable manner. A pinion gear 13 is connected to a lower end of the steering shaft 12 in a unitarily rotatable manner. The pinion gear 13 is meshed with rack teeth formed on a rack bar 14, thereby forming a rack-and-pinion mechanism. Left and right front wheels FW1 and FW2 are connected to opposite ends of the rack bar 14 in a steerable manner. The left and right front wheels FW1 and FW2 are steered leftward and rightward according to an axial displacement of the rack bar 14 associated with rotation of the steering shaft 12 about its axis. An assist mechanism 20 for assisting steering is connected to the steering shaft 12. The assist mechanism 20 includes an electric motor 30 and a speed reducer 40. A drive circuit 21 is electrically connected to the electric motor 30. The drive circuit 21 outputs drive current to the electric motor 30 under instructions of an electronic control unit 22.

The present embodiment will be described while mentioning a column-assist-type electric power steering apparatus in which the assist mechanism 20 is attached to the steering shaft 12 so as to apply a steering assist torque to the steering shaft 12. However, the present invention is not limited to this type of electric power steering apparatus, but is applicable to electric power steering apparatus of other types, such as a rack assist type in which torque is applied to the rack bar 14, and a pinion assist type in which torque is applied to the pinion gear 13. Also, the present invention is not limited to electric power steering apparatus, but is applicable to steer-by-wire apparatus which use an electric motor.

A main component of the electronic control unit 22 is a microcomputer which includes a CPU, ROM, and RAM. The electronic control unit 22 receives a steering torque, a steering angle, and a vehicle speed which are input from an unillustrated steering torque sensor, an unillustrated steering angle sensor, and an unillustrated vehicle speed sensor, respectively. On the basis of such input information, the electronic control unit 22 executes an assist control program for assisting an operation of the steering wheel 11 and drives and controls the electric motor 30 via the drive circuit 21.

FIG. 2 is a schematic view showing the assist mechanism 20, and FIG. 3 is a schematic, partial sectional view taken along line A— A of FIG. 2. The electric motor 30 functions as an actuator for assisting a steering operation of the steering wheel 11. As shown in FIGS. 2 and 3, the electric motor 30 has a body portion 31 and an output shaft 32. An output generated in the body portion 31 is transmitted to the output shaft 32, and the output shaft 32 outputs a rotational torque to the exterior of the electric motor 30.

The speed reducer 40 is adapted to reduce an output rotational speed of the electric motor 30 and assumes the form of a multistage-type speed reducer in which a worm speed reducer 41 serving as a first speed reducer and a planetary-gear-type speed reducer 42 serving as a second speed reducer are combined. The worm speed reducer 41 serving as the first speed reducer includes a worm shaft 410, a worm 411 , and a worm wheel 412. The worm shaft 410 is disposed coaxially with the output shaft 32 of the electric motor 30. Rotation of the output shaft 32 is transmitted to the worm shaft 410 via an unillustrated coupling or the like. The worm shaft 410 is supported rotatably at opposite ends thereof by an unillustrated inner wall of a housing of the speed reducer 40.

The worm 411 has a thread 411a formed on the outer circumferential surface thereof and is attached coaxially to the worm shaft 410 in a unitarily rotatable manner. The worm wheel 412 has a hollow, cylindrical shape. External gear teeth 412a of resin are formed on the outer circumferential surface of the worm wheel 412 along the circumference of the worm wheel 412. The worm wheel 412 is disposed such that the external gear teeth 412a are meshed with the thread 411a of the worm 411. As the worm 411 rotates, the worm wheel 412 rotates at a reduced speed. In the present embodiment, the worm wheel 412 is an output element of the worm speed reducer 41.

The planetary-gear-type speed reducer 42 serving as a second speed reducer includes a ring gear 421 , planetary gears 422, a sun gear 423, and a carrier 424. The ring gear 421 is formed integrally with the worm wheel 412 of the worm speed reducer 41. Specifically, internal gear teeth 421a of metal are formed on the inner circumferential surface of the worm wheel 412 along the circumference of the worm wheel 412. The internal gear teeth 421a function as the teeth of the ring gear 421 of the planetary-gear-type speed reducer 42. As shown in FIG. 2, speed-reducing elements of the planetary-gear-type speed reducer 42, such as the planetary gears 422, the sun gear 423, and the carrier 424, are disposed at the inner side of the worm wheel 412. The sun gear 423 is disposed at the center of the ring gear 421. The three planetary gears 422 are disposed between the ring gear 421 and the sun gear 423 while being spaced at equal intervals.

The sun gear 423 assumes a ring-like shape. External gear teeth 423a are formed on the outer circumferential surface of the sun gear 423. The steering shaft 12 extends through the sun gear 423. As shown in FIG. 3, a fixture bracket 425 is attached to one end surface of the sun gear 423. The fixture bracket 425 is fixed to the housing. Accordingly, the sun gear 423 is fixedly supported by the fixture bracket 425 and is nonrotatable.

As shown in FIG. 3, the carrier 424 is attached to the outer circumferential surface of the steering shaft 12. The carrier 424 includes a support portion 424a, a flange portion 424b, and pin portions 424c. The support portion 424a is fixedly attached to the outer circumferential surface of the steering shaft 12. The flange portion 424b extends from the support portion 424a in radial directions of the steering shaft 12. In the present embodiment, as shown in FIG. 2, the flange portion 424b has an approximately triangular shape. For convenience of description, in FIG. 2, the flange portion 424b is skeletally illustrated so that a structure behind the flange portion 424b is visible.

The pin portions 424c are provided on the triangular flange portion 424b in the vicinity of three corresponding vertexes of the flange portion 424b. The pin portions 424c extend from the flange portion 424b in an axial direction of the steering shaft 12.

As shown in FIG. 2, the planetary gears 422 are disposed in a ring-like space between the ring gear 421 and the sun gear 423 and each assume a ring-like shape. The pin portions 424c of the carrier 424 are inserted into the corresponding planetary gears 422. Accordingly, the planetary gears 422 are rotatably supported by the corresponding pin portions 424c. External gear teeth 422a are formed on the outer circumferential surface of each of the planetary gears 422. The external gear teeth 422a are meshed with the internal gear teeth 421a of the ring gear 421 and with the external gear teeth 423a of the sun gear 423 at diametrally opposite positions.

In the present embodiment, a vibration insulator, such as resin or rubber, is provided at a connection between the support portion 424a of the carrier 424 and the steering shaft 12. Provision of the vibration insulator suppresses or prevents transmission of unusual noise and vibration from the electric motor 30 and the speed reducer 40 to the steering shaft 12.

Next will be described operation of the thus-configured present embodiment. When an ignition switch is turned on, the electronic control unit 22 starts to execute an assist control program repeatedly every predetermined short period of time. An instruction value which is determined through execution of the assist control program is output to the drive circuit 21. In accordance with the determined instruction value, the drive circuit 21 outputs drive current to the electric motor 30.

The electric motor 30 outputs a rotational torque to its output shaft 32. The rotational torque is transmitted to the worm shaft 410 and is exerted on the worm 411 attached to the worm shaft 410. Meshing between the worm 411 and the worm wheel 412 effects first-stage speed reduction in the worm speed reducer 41. A reduction ratio in the worm speed reducer 41 is determined by the ratio Zw/Z between the number of thread ridges Zw of the thread 411a formed on the worm 411 and the number of teeth Z of the external gear teeth 412a formed on the worm wheel 412. Since the external gear teeth 412a of the worm wheel 412 is formed from resin, during speed-reducing operation in the worm speed reducer 41 , rattle generated by meshing between the worm wheel 412 and the worm 411 is lowered greatly, or even generation of rattle is prevented. Accordingly, generation of unusual noise is effectively prevented.

A rotational output whose speed is reduced by the worm speed reducer 41 is output from the worm wheel 412, which is an output element of the worm speed reducer 41. Since the worm wheel 412 is formed integrally with the ring gear 421 of the planetary-gear-type speed reducer 42, the rotational output from the worm wheel 412 becomes an input to the ring gear 421 as it is. The rotational torque which is thus-input to the ring gear 421 is transmitted to the planetary gears 422, which are meshed with the internal gear teeth 421a of the ring gear 421 ; thus, the planetary gears 422 rotate. The planetary gears 422 are also meshed with the sun gear 423. However, since the sun gear 423 is made nonrotatable (fixed) by the fixture bracket 425, the rotational torque from the ring gear 421 causes the planetary gears 422 to rotate on their own axes and to roll oη the sun gear 423; thus, the planetary gears 422 revolve around the sun gear 423.

Such a revolving motion of the planetary gears 422 effect speed reduction in the planetary-gear-type speed reducer 42. When Za represents the number of the external gear teeth 423a of the sun gear 423, and Zb represents the number of the internal gear teeth 421a of the ring gear 421 , a rotational speed is reduced at a ratio of (Za + Zb)/Za. The thus-speed-reduced rotation is transmitted to the carrier 424 via the pin portions 424c, which rotatably support the corresponding planetary gears 422. The carrier 424 serves as an output element of the planetary-gear-type speed reducer 42 and outputs speed-reduced rotation. Since the carrier 424 is connected to the steering shaft 12, which is an output destination member, the output is transmitted to the steering shaft 12. In this manner, the steering shaft 12 receives a rotational torque from the electric motor 30, thereby assisting a steering operation.

The present embodiment which operates as described above assumes the form of a 2-stage speed reducer. Specifically, the speed reducer 40 of the electric power steering apparatus has the worm speed reducer 41 , which serves as a first speed reducer, is adapted to reduce an output rotational-speed of the electric motor 30 and to output a reduced speed, and has an output element (worm wheel 412) for outputting the reduced speed, and the planetary-gear-type speed reducer 42, which serves as a second speed reducer and is adapted to reduce an output speed of the worm speed reducer 41 and to output a reduced speed. By means of reducing a rotational speed of the electric motor 30 at two stages by use of two speed reducers, high output (high speed reduction) is implemented. Speed-reducing elements of the planetary-gear-type speed reducer 42; specifically, the planetary gears 422, the sun gear 423, and the carrier 424, are disposed in the interior of the worm wheel 412, which is an output element of the worm speed reducer 41. Specifically, as shown in FIG. 3, the planetary gears 422, the sun gear 423, and portions of the pin portions 424c of the carrier 424 are disposed in an inner, cylindrical region of the cylindrical worm wheel 412. In this manner, the 2-stage speed reducer is configured compactly. Through employment of the configuration of the present embodiment, there can be provided a steering apparatus for a vehicle which includes a speed reducer which can achieve high output (high speed reduction) and can be configured compactly to thereby offer good mountability in a vehicle.

By means of forming the external gear teeth 412a on the outer circumferential surface of the worm wheel 412 of the worm speed reducer

41 and forming the internal gear teeth 421a on the inner circumferential surface of the worm wheel 412, the worm wheel 412 of the worm speed reducer 41 and the ring gear 421 of the planetary-gear-type speed reducer

42 are formed integrally with each other; i.e., a single member serves as both the worm wheel 412 and the ring gear 421. This implements compactness and reduces cost through sharing of components. Furthermore, forming the external gear teeth 412a of the worm wheel 412 from resin suppresses rattle which is generated by meshing between the worm 411 and the worm wheel 412. Also, forming the internal gear teeth 421a of the worm wheel 412 (ring gear 421) from metal imparts sufficient rigidity to the worm wheel 412 (ring gear 421) even if the external gear teeth 412a are formed of resin. Second Embodiment:

Next, a second embodiment of the present invention will be described. The present embodiment is characterized in that a torque limiter is attached to a sun gear of a planetary-gear-type speed reducer which serves as a second speed reducer. Other structural features of the present embodiment are similar to those of the above-described first embodiment. In the following description of embodiments, like structural features are denoted by like reference numerals, and repeated description thereof is omitted. The description will be centered on structural features different from those of embodiments which have already been described.

FIG. 4 is a schematic, partial sectional view of a speed reducer according to the present embodiment. As illustrated, in the present embodiment, a torque limiter 43 is attached to the sun gear 423 of the planetary-gear-type speed reducer 42, which serves as a second speed reducer. The torque limiter 43 is divided into a primary member 431 and a secondary member 432. As illustrated, the primary member 431 is formed into a cylindrical shape and surrounds an outer circumferential surface of the steering shaft 12. One end surface of the primary member 431 is connected to an end surface of the sun gear 423. The secondary member 432 is also formed into a cylindrical shape similar to that of the primary member 431. One end surface of the secondary member 432 is fixed to the housing or the like of the speed reducer. The primary member 431 is inserted into the secondary member 432 in such a manner that the outer circumferential surface of the primary member 431 is in contact with the inner circumferential surface of the secondary member 432.

A predetermined pressing fore is generated in an overlap region between the primary and secondary members 431 and 432. An example means for generating such a pressing force is a configuration in which a tolerance ring is used as the primary member 431 and/or the secondary member 432. The tolerance ring has a portion which is corrugated along the circumference thereof. When the corrugated portion is pressed, a predetermined reaction force is generated. This reaction force can be utilized as a pressing force. According to another means, the inside diameter of the secondary member 432 is made smaller than the outside diameter of the primary member 431 ; an axial slit is formed in the primary member 431; and the primary member 431 is inserted into the secondary member 432 while being elastically deformed, whereby a restoring force associated with the elastic deformation can serve as a pressing force. According to still another means, a friction material is attached to the outer circumferential surface of the primary member 431 and/or the inner circumferential surface of the secondary member 432.

The above-mentioned pressing force generates a frictional force when the sun gear 423 is to rotate. This frictional force acts as a resistance torque against a rotational torque. Accordingly, when a rotational torque input to the sun gear 423 is less than a resistance torque induced by the pressing force, the sun gear 423 does not rotate. When the rotational torque is in excess of the resistance torque, the sun gear 423 rotates against the pressing force. In the planetary-gear-type speed reducer 42 equipped with the torque limiter 43, rotation input from the ring gear 421 , which also serves as the worm wheel 412 of the worm speed reducer 41 , is transmitted to the planetary gears 422. The input rotational torque causes the planetary gears 422 to rotate. At this time, the planetary gears 422 transmit a rotational torque to the sun gear 423. Usually, since the transmitted rotational torque is smaller than a resistance torque generated in the torque limiter 43, the sun gear 423 does not rotate and remains stationary. Accordingly, the planetary gears 422 revolve on and around the SUIT gear 423. This revolution of the planetary gears 422 effect speed reduction. Speed-reduced rotation is transmitted to the carrier 424, which is an output element. Finally, the rotation is transmitted to the steering shaft 12 connected to the carrier 424, thereby assisting steering.

When a wheel hits against a curb, an associated impact force is transmitted to the rack bar or the steering shaft. Then the impact force is input to the speed reducer (such an input is called a reverse input). When a reverse input arises, a reverse input torque is transmitted to the planetary gears 422 via the carrier 424. The reverse input torque attempts to rotate the planetary gears 422 so as to be transmitted to the worm speed reducer 41 via the ring gear 421. However, since the worm speed reducer 41 has a considerably large resistance torque against the reverse input from the worm wheel 412, the worm wheel 412 cannot rotate the worm 411. As a result, although the planetary gears 422 are fixed by the sun gear 423 and the ring gear 421 and are thus unable to rotate, the reverse input torque is input to the planetary gears 422.

In such a condition, when a reverse input torque becomes greater than a resistance torque generated in the torque nmiter 43, the sun gear 423 becomes rotatable. Thus, the reverse input torque causes the planetary gears 422 to rotate together with the sun gear 423. The sun gear 423 is rotated in this manner, and the rotation of the sun gear 423 releases the reverse input torque, thereby preventing application of an excessive torque to the planetary-gear-type speed reducer 42, to the worm speed reducer 41 , and further to the electric motor 30. _v

As described above, according to the present embodiment, the planetary-gear-type speed reducer 42 serving as a second speed reducer is a speed-reducing mechanism having a ring gear serving as an input element, a sun gear serving as a stationary element, and a carrier serving as an output element. Also, the present embodiment is provided with the torque limiter 43 for allowing rotation of the sun gear 423 when a rotational torque equal to or greater than a predetermined torque (resistance torque) is transmitted to the sun gear 423. Accordingly, when a reverse input arises and causes a rotational torque equal to or greater than a resistance torque to be transmitted to the sun gear 423 via the planetary gears 422, the sun gear 423 rotates to thereby release the rotational torque. Therefore, the speed reducer 40 and the electric motor 30 can be protected from an excessive rotational torque. Third Embodiment:

Next, a third embodiment of the present invention will be described. The present embodiment is characterized in that planetary gears of a planetary-gear-type speed reducer serving as a second speed reducer each assume the form of a helical gear, and the helix angle of the helical gears is adjusted appropriately. Other structural features of the present embodiment are similar to those of the above-described first embodiment.

FIG. 5 is a schematic, partial sectional view of a speed reducer according to the present embodiment. In the present embodiment, the planetary gears 422 of the planetary-gear-type speed reducer 42 serving as a second speed reducer each assume the form of a helical gear. In the case where the planetary gears 422 each assume the form of a helical gear, an axial thrust force Fp1 is generated when the planetary gears 422 rotate. The ring gear 421 which engages with the planetary gears 422 receives a thrust force Fp2 which is a reaction force against the thrust force Fp1. Meshing between the worm 411 and the worm wheel 412 of the worm speed reducer 41 generates an axial thrust force Fw, in addition to a rotational torque, on the worm wheel 412.

In the case where the worm speed reducer 41 is used as a first speed reducer, the thrust force Fw is generated as mentioned above. This thrust force Fw acts in axial direction different from a rotational direction of the worm wheel 412 (ring gear 421). Thus, the thrust force Fw influences a condition of meshing and rotation of the worm wheel 412, thereby possibly inducing generation of unusual noise. By contrast, according to the present embodiment, in addition to the thrust force Fw, the thrust force Fp1 is generated by means of using the planetary gears 422 which each assume the form of a helical gear. The thrust force Fp2, which is a reaction force against the thrust force Fp1 , acts on the worm wheel 412 (ring gear 421).

Accordingly, by means of setting the helix angle of the planetary gears 422 appropriately, the thrust force Fp2 and the thrust force Fw can be directed in opposite directions and approximately equal to each other. Therefore, the thrust forces Fp2 and Fw cancel each other. This cancellation reduces the magnitude of an axially exerted force, thereby effectively preventing generation of unusual noise. Other structural features and operation of the steering apparatus and operation of the speed reducer are similar to those of the above-described first embodiment, and thus repeated description thereof is omitted. Fourth Embodiment:

Next, a fourth embodiment of the present invention will be described. The present embodiment is characterized in that a phase difference between external gear teeth and internal gear teeth of a worm wheel which is formed integrally with a ring gear is adjusted appropriately. Other structural features of the present embodiment are similar to those of the above-described first embodiment.

FIG. 6 is a view showing the worm wheel 412 (ring gear 421) according to the present embodiment. As illustrated, the worm wheel 412 (ring gear 421) has the external gear teeth 412a and the internal gear teeth 421a. The external gear teeth 412a and the internal gear teeth 421a differ in phase by a phase angle θ (see the angle θ shown in FIG. 6). The phase angle θ is set appropriately on the basis of the timing of meshing of the external gear teeth 412a with a worm and the timing of meshing of the internal gear teeth 421a with a planetary gear.

When the external gear teeth 412a mesh with the worm, vibration having a predetermined frequency is generated. Also, when the internal gear teeth 421a mesh with the planetary gear, vibration having a predetermined frequency is generated. As described above, rotation of the worm wheel 412 (ring gear 421) involves vibrations induced by a plurality of causes. When these vibrations resonate, a great vibration arises. In order to prevent such resonance, according to the present embodiment, the external gear teeth 412a and the internal gear teeth 421a differ in phase by the phase angle θ. The phase angle θ is set appropriately so as not to induce resonance, thereby preventing generation of a great vibration. Since the phase angle θ depends on the number of the external gear teeth 412a and the number of the internal gear teeth 421a, the phase angle θ cannot be unconditionally specified, but is designed from the viewpoint of prevention of the above-mentioned resonance. Other structural features, operation of the steering apparatus, and operation of the speed reducer are similar to those of the above-described first embodiment, and thus repeated description thereof is omitted. Fifth Embodiment:

Next, a fifth embodiment of the present invention will be described. The present embodiment is characterized in that a pulley-type speed reducer is used as a first speed reducer. Other structural features of the present embodiment are similar to those of the above-described first embodiment.

FIG. 7 schematically shows the configuration of a speed reducer according to the present embodiment. In FIG. 7, the speed reducer 40 assumes the form of a multistage-type speed reducer in which a pulley-type speed reducer 51 serving as a first speed reducer, and the planetary-gear-type speed reducer 42 serving as a second speed reducer are combined. The pulley-type speed reducer 51 serving as a first speed reducer includes a drive shaft 510, a drive pulley 511 , a driven pulley 512, and a drive belt 513.

The drive shaft 510 is connected to an unillustrated output shaft of an electric motor coaxially and in a unitarily rotatable manner-..-. The drive pulley 511 has external gear teeth 511a formed on an outer circumferential surface thereof and is attached to the drive shaft 510 coaxially and in a unitarily rotatable manner. As illustrated, the driven pulley 512 has an outside diameter greater than that of the drive pulley 511 and has external gear teeth 512a formed on an outer circumferential surface thereof. The number of teeth N1 of the external gear teeth 511a formed on the drive pulley 511 is fewer than the number of teeth N2 of the external gear teeth 512a formed on the driven pulley 512. The drive belt 513 is looped around and extending between the drive pulley 511 and the driven pulley 512, thereby connecting the pulleys 511 and 512 and enabling transmission of rotation of the drive pulley 511 to the driven pulley 512. Internal teeth 513a are formed on the inner surface of the drive belt 513. The internal teeth 513a are shaped so as to be engaged with the external gear teeth 511a and 512a formed on the pulleys 511 and 512.

As illustrated, the driven pulley 512 is hollow for allowing accommodation therein of the planetary gears 422, the sun gear 423, and the carrier 424 of the planetary-gear-type speed reducer 42 serving as a second speed reducer. The internal gear teeth 421a are formed on the inner circumferential surface of the driven pulley 512 and function as teeth of the ring gear 421 of the planetary-gear-type speed reducer 42. Accordingly, the driven pulley 512 is formed integrally with the ring gear 421 of the planetary-gear-type speed reducer 42.

In the above-described configuration, a rotational torque from the electric motor is transmitted to the output shaft, which, in turn, outputs the rotational torque. This rotational torque is transmitted to the drive shaft 510 connected to the output shaft of the electric motor, and then to the drive pulley 511 unitarily attached to the drive shaft 510, thereby rotating the drive pulley 511. This rotation of the drive pulley 511 drives the drive belt 513 engaged with the drive pulley 511. As a result of the drive belt 513 being driven, the driven pulley 512 engaged with the drive belt 513 rotates. Since the number of teeth N1 of the external gear teeth 511a formed on the drive pulley 511 is fewer than the number of teeth N2 of the external gear teeth 512a of the driven pulley 512, an output rotational-speed of the driven pulley 512 is reduced according to the gear ratio N1/N2 between the drive and driven pulleys 511 and 512. In this manner, the pulley-type speed reducer 51 serving as a first speed reducer effects first-stage speed reduction. The speed-reduced rotational output is output from the driven pulley 512 serving as an output element.

As described above, the driven pulley 512 and the ring gear 421 are formed integrally with each other, and the internal gear teeth 421a are formed on the inner circumferential surface of the driven pulley 512 and function as the teeth of the ring gear 421. Accordingly, the rotation of the driven pulley 512 is transmitted to the planetary-gear-type speed reducer 42 serving as a second speed reducer, whereby second-stage speed reduction is effected. Second-stage speed reduction (speed reduction effected by the planetary-gear-type speed reducer 42) in the present embodiment is identical with that in the first embodiment, and thus repeated description thereof is omitted.

According to the present embodiment which operates as described above, a speed reducer for use in an electric power steering apparatus employs, as a first speed reducer, the pulley-type speed reducer 51 , which includes the drive pulley 511 , the driven pulley 512, and the drive belt 513. Thus, a 2-stage speed reducer can be designed in a simple configuration. Also, speed-reducing elements of the planetary-gear-type speed reducer 42 serving as a second speed reducer are disposed within the driven pulley 512, which is an output element of the pulley-type speed reducer 51. Thus, the speed reducer can be formed compactly. Since the driven pulley 512 and the ring gear 421 are formed integrally with_veach other by means of forming the internal gear teeth 421a on the inner circumferential surface of the follower pulley 512, the number of components can be reduced, thereby reducing cost. Sixth Embodiment:

Next, a sixth embodiment of the present invention will be described. The present embodiment is characterized in that a gear-type speed reducer is used as a first speed reducer. Other structural features of the present embodiment are similar to those of the above-described first embodiment.

FIG. 8 schematically shows the configuration of a speed reducer according to the present embodiment. In FIG. 8, the speed reducer 40 assumes the form of a multistage-type speed reducer in which a gear-type speed reducer 61 serving as a first speed reducer and the planetary-gear-type speed reducer 42 serving as a second speed reducer are combined. The gear-type speed reducer 61 serving as a first speed reducer includes a drive shaft 610, a drive gear 611 , and a driven gear 612.

The drive shaft 610 is connected to an unillustrated output shaft of an electric motor coaxially and in a unitarily rotatable manner. External gear teeth 611a are formed on an outer circumferential surface of the drive gear 611. The drive gear 611 is attached to the drive shaft 610 coaxially and in a unitarily rotatable manner. As illustrated, the driven gear 612 has an outside diameter greater than that of the drive gear 611 and has external gear teeth 612a formed on an outer circumferential surface thereof. The number of teeth M1 of the external gear teeth 611a formed on the drive gear

611 is fewer than the number of teeth M2 of the external gear teeth 612a formed on the driven gear 612. The drive gear 611 and the driven gear

612 are arranged such that the external gear teeth 611a of the drive gear 611 and the external gear teeth 612a of the driven gear 612 are meshed with each other.

As illustrated, the driven gear 612 is hollow for allowing accommodation therein of the planetary gears 422, the sun gear 423, and the carrier 424 of the planetary-gear-type speed reducer 42 serving as a second speed reducer. The internal gear teeth 421a are formed on the inner circumferential surface of the driven gear 612 and function as teeth of the ring gear 421 of the planetary-gear-type speed reducer 42. Accordingly, the driven gear 612 is formed integrally with the ring gear 421 of the planetary-gear-type speed reducer 42.

In the above-described configuration, a rotational torque from the electric motor is transmitted to the output shaft, which, in turn, outputs the rotational torque. This rotational torque is transmitted to the drive shaft 610 connected to the output shaft of the electric motor and then to the drive gear 611 unitarily attached to the drive shaft 610, thereby rotating the drive gear 611. This rotation of the drive gear 611 rotates the driven gear 612 meshed with the drive gear 611. Since the number of teeth M1 of the external gear teeth 611a formed on the drive gear 611 is fewer than the number of teeth M2 of the external gear teeth 612a of the driven gear 612, an output rotational-speed of the driven gear 612 is reduced'aiccording to the gear ratio M1/M2 between the drive and driven gears 611" and 612. In this manner, first-stage speed reduction is effected. The speed-reduced rotational output is output from the follower gear 612 serving as an output element.

As described above, the driven gear 612 and the ring gear 421 are formed integrally with each other, and the internal gear teeth 421a are formed on the inner circumferential surface of the driven gear 612 and function as the teeth of the ring gear 421. Accordingly, the rotation of the driven gear 612 is transmitted to the planetary-gear-type speed reducer 42 serving as a second speed reducer, whereby second-stage speed reduction is effected. Second-stage speed reduction in the present embodiment is identical with that in the first embodiment, and thus repeated description thereof is omitted.

According to the present embodiment which operates as described above, a speed reducer for use in an electric power steering apparatus employs, as a first speed reducer, the gear-type speed reducer 61, which includes the drive gear 611 and the driven gear 612. Thus, a 2-stage speed reducer can be designed in a simple configuration. Also, speed-reducing elements of the planetary-gear-type speed reducer 42 serving as a second speed reducer are disposed within the driven gear 612, which is an output element of the gear-type speed reducer 61. Thus, the speed reducer can be formed compactly. Since the driven gear 612 and the ring gear 421 are formed integrally with each other by means of forming the internal gear teeth 421a on the inner circumferential surface of the follower gear 612, the number of components can be reduced, thereby reducing cost. Seventh Embodiment: >

Next, a seventh embodiment of the present invention will be described. The present embodiment is characterized in that a worm speed reducer is used as a first speed reducer and that means for adjusting a meshing force between a worm and a worm wheel of the worm speed reducer is provided. Other structural features of the present embodiment are similar to those of the above-described first embodiment.

FIG. 9 is a schematic partial sectional view of a speed reducer according to the present embodiment, and FIG. 10 is a view as viewed in the direction of arrow B of FIG. 9. In FIGS. 9 and 10, the speed reducer 40 assumes the form of a multistage-type speed reducer in which the worm speed reducer 41 serving as a first speed reducer and the planetary-gear-type speed reducer 42 serving as a second speed reducer are combined. Specific configurations of the worm speed reducer 41 and the planetary-gear-type speed reducer 42 are similar to those of the above-described first embodiment, and thus repeated description thereof is omitted.

As illustrated, two meshing-force adjustment mechanisms 70 are attached to the worm speed reducer 41. The meshing-force adjustment mechanisms 70 apply a constant load to the worm speed reducer 41 in such a direction as to press the thread 411a of the worm 411 and the external gear teeth 412a of the worm wheel 412 against each other, thereby adjusting a meshing condition between the worm 411 and the worm wheel 412. The meshing-force adjustment mechanisms 70 include a support ring 71 , respective holders 72, respective rods 73, respective coil springs 74, respective adjustment gears 75, and respective stoppers 76.

As shown in FIG. 9, the two meshing-force adjustment mechanisms 70 in the present embodiment are configured symmetrically on the corresponding opposite sides of the support ring 71. Accordingly, the configuration of one meshing-force adjustment mechanism 70 (located at the right in FIG. 9) will be described, and description of the configuration of the other one is omitted.

As shown in FIG. 10, the support ring 71 consists of a first support ring 71a and a second support ring 71b. The first and second support rings 71a and 71b are attached to the worm shaft 410. As illustrated, the first and second support rings 71a and 71b are attached to the worm shaft 410 at the outside of corresponding opposite ends of the worm 411 which is unitarily attached to the worm shaft 410. The first and second support rings 71a and 71b are rotatable in relation to the worm shaft 410 and are fixed to a housing or the like by means of an unillustrated bracket. Accordingly, even when the worm shaft 410 rotates, the first and second support rings 71a and 71b do not rotate.

As shown in FIG. 9, the holder 72 is attached to the support ring 71 in such a manner as to extend rightward. As shown in FIG. 10, one end of the holder 72 is connected to the first support ring 71a, and the other end of the holder 72 is connected to the second support ring 71b.

A through hole 72a is formed in the holder 72 in such a manner as to extend vertically in FIG. 9. The rod 73 is inserted through the hole 72a in a vertically movable manner. One end portion (an illustrated lower end portion) of the rod 73 supports the adjustment gear 75, and the other end portion (an illustrated upper end portion) of the rod 73 is formed into a threaded portion 73a. A nut 76a is screw-engaged with the threaded portion 73a. A washer 76b is disposed at the illustrated lower side of the nut 76a in such a manner that the rod 73 extends therethrough. A coil spring 74 is disposed at the illustrated lower side of the washer 76b and serves as a rod-urging member.

The illustrated lower end of the coil spring 74 abuts the holder 72, and the illustrated upper end of the coil spring 74 abuts the washer 76b. Accordingly, the rod 73 and the adjustment gear 75 connected to the rod 73 are supported by the holder 72 via the coil spring 74. The nut 76a and the washer 76b collectively function as a stopper 76 for restraining the position of the other end of the coil spring 74. In the present embodiment, since the nut 76a and the washer 76b constitute the stopper 76, the position of the nut 76a in relation to the rod 73 can be adjusted by rotating the nut 76a on the threaded portion 73a. This adjustment can also adjust an elastic force which the coil spring 74 generates.

The adjustment gear 75 connected to a lower end portion of the rod 73 includes a shaft portion 75a and a gear portion 75b. The shaft portion 75a is rotatably attached, at one end, to a lower end portion of the rod 73 and extends in an axial direction of the worm wheel 412. The gear portion 75b is coaxially attached to the other end portion of the shaft portion 75a. The rod 73, the coil spring 74, and the adjustment gear 75 constitute the worm-wheel-urging means in the present invention.

As illustrated, the gear portion 75b of the adjustment gear 75 is meshed with the internal gear teeth 421a formed on the inner circumferential surface of the ring gear 421 integrally formed with the worm wheel 412. Accordingly, as the ring gear 421 rotates, the adjustment gear 75 rotates. In the illustrated condition, the length of the coil spring 74 is made shorter than a natural length so as to generate a stretching force. Accordingly, the stretching force of the coil spring 74 urges all the time the rod 73 upward in FIG. 9. Thus, the adjustment gear 75 connected to the rod 73 urges the worm wheel 412 (ring gear 421) upward in FIG. 9. As a result, the worm wheel 412 is pulled toward the worm 411 such that the thread 411a of the worm 411 and the external gear teeth 412a of the worm wheel 412 are meshed with each other under a constant load all the time. As described above, the adjustment gear 75 abuts an inner circumferential surface of the worm wheel 412 (ring gear 421) all the time and thus serves as the abutment member in the present invention.

In the above-mentioned condition, as the output shaft 32 of the electric motor 30 rotates, the worm shaft 410 and the worm 411 rotate, and the rotation of the worm 411 causes the worm wheel 412 to rotate at a reduced speed. At this time, by virtue of the meshing-force adjustment mechanisms 70, the external gear teeth 412a of the worm wheel 412 and the thread 411a of the worm 411 are meshed with each other under a constant load all the time. Accordingly, even when backlash between the worm 411 and the worm wheel 412 varies because of wear, machining errors, or atmospheric conditions or even when the worm 411 and the worm wheel 412 are machined with backlash different from a set value, the meshing-force adjustment mechanisms 70 adjust a clearance between gear teeth. This adjustment can effectively prevent generation of unusual noise (rattle, sliding noise, etc.) which could otherwise arise from backlash variations or the like. Particularly, in the case where the external gear teeth 412a of the worm wheel 412 are formed from resin, backlash may vary because of dimensional changes caused by thermal shrinkage or thermal expansion. Even in such a case, the meshing-force adjustment mechanisms 70 can adjust backlash. Other structural features, operation of the steering apparatus, and operation of the speed reducer are similar to those of the above-described first embodiment, and thus repeated description thereof is omitted.

The present embodiment may be modified as shown in FIG. 11. In contrast to the configuration of FIG. 9 in which the coil springs 74 are provided at the corresponding left and right sides of the support ring 71 , the coil spring 74 is provided at only one side of the support ring 71. In the case of FIG. 11 , an urging force of the coil spring 74 urges upward an adjustment gear 751 which is supported by a rightward-illustrated rod 731. Also, the coil spring 74 urges downward in FIG. 11 a right end portion of a rightward-illustrated holder 721. This urging force acts in such a manner as to rotate the support ring 71 clockwise in FIG. 11. As a result, a left end of a leftward-illustrated holder 722 is subjected to a force which acts upward in FIG. 11. This upward urging force also acts on a leftward-illustrated rod 732 fixedly connected to the holder 722 and also acts on an adjustment gear 752 supported by the rod 732, in such a manner as to urge the adjustment gear 752 upward. In this manner, the right and left adjustment gears 751 and 752 are subjected to an urging force directed upward in FIG. 11 , thereby adjusting a meshing condition between the worm 411 and the worm wheel 412.

Alternatively, the configuration of FIG. 12 may be employed. According to this configuration, in place of the adjustment gears 75 in FIG. 10, adjustment rollers 77 are provided as abutment members. Each of the adjustment rollers 77 has a shaft portion 77a and a roller portion 77b. The adjustment rollers 77 roll or slide on an inner circumferential surface of the ring gear 421. In this case, as illustrated, the internal gear teeth 421a of the ring gear 421 are not formed in those regions of the inner circumferential surface of the ring gear 421 where the adjustment roller 77 roll or slide. Even in these modified configurations, the meshing-force adjustment mechanism(s) 70 sufficiently yields the above-described function. Eighth Embodiment:

Next, an eighth embodiment of the present invention will be described. The present embodiment is characterized in that a worm speed reducer is used as a first speed reducer and that a holder means for maintaining a distance between a worm and a worm wheel of this worm speed reducer is provided. Other structural features of the present embodiment are similar to those of the above-described first embodiment.

FIG. 13 is a schematic, partial sectional view of a speed reducer according to the present embodiment. In FIG. 13, the speed reducer 40 assumes the form of a multistage-type speed reducer in which the worm speed reducer 41 serving as a first speed reducer and the planetary-gear-type speed reducer 42 serving as a second speed reducer are combined. Specific configurations of the worm speed reducer 41 and the planetary-gear-type speed reducer 42 are similar to those of the above-described first embodiment, and thus repeated description thereof is omitted.

As illustrated, a holder mechanism 80 is attached to the worm speed reducer 41. The holder mechanism 80 maintains a distance between the worm 411 and the worm wheel 412 so as to maintain a positional relationship therebetween. The holder mechanism 80 includes a support ring 81, a holder 82, a fixture rod 83, and a holder gear 84. The support ring 81 and the holder 82 are similar, in structure and their connection, to the support ring 71 and the holder 72 of the above-described seventh embodiment, and thus description thereof is omitted.

The fixture rod 83 is connected to the holder 82. The fixture rod 83 is fixed at one end to the holder 82 and extends downward in FIG. 13 from the holder 82. The holder gear 84 is connected to the other end portion of the fixture rod 83. The holder gear 84 includes a shaft portion 84a and a gear portion 84b. The shaft portion 84a is rotatably attached, at one end, to a lower end portion of the fixture rod 83 and extends in an axial direction of the worm wheel 412. The gear portion 84b is coaxially attached to the other end portion of the shaft portion 84a.

As illustrated, the gear portion 84b of the holder gear 84 is meshed with the internal gear teeth 421a formed on the inner circumferential surface of the ring gear 421 (worm wheel 412). Accordingly, as the ring gear 421 rotates, the holder gear 84 rotates. Since the holder gear 84 is connected to the fixture rod 83, which, in turn, is fixedly attached to the holder 82, the holder gear 84 is vertically immovable. Accordingly, even when the worm 411 attempts to move upward in FIG. 13, the holder gear 84 interferingly restricts the attempted movement.

With the above-described configuration, as the output shaft 32 of the electric motor rotates, the worm shaft 410 and the worm 411 rotate, and the rotation of the worm 411 causes the worm wheel 412 to rotate. At this time, the holder mechanism 80 maintains a positional relationship between the worm 411 and the worm wheel 412, thereby restricting a movement of the worm 411 in an upward direction in FfG. 13 or a movement βftf§ worm wheel 412 in a downward direction in FIG. 13. Thus, a relative arrangement between the worm 411 and the worm wheel 412 is maintained.

In the present embodiment, the worm shaft 410 is rotatably supported by a housing (not shown) formed from aluminum. Accordingly, in the case where the holder mechanism 80 is not provided, when the housing is deformed because of thermal expansion or the like, the worm shaft 410 and the worm 411 supported by the housing change in position in relation to the worm wheel 412. Particularly, when the worm 411 and the worm wheel 412 move away from each other, backlash increases, potentially resulting in generation of unusual noise. By contrast, according to the present embodiment, the holder mechanism 80 maintains a relative arrangement condition between the worm 411 and the worm wheel 412; thus, the distance therebetween does not increase, thereby effectively preventing generation of unusual noise which could otherwise be induced by deformation of the housing stemming from thermal expansion or the like. Ninth Embodiment:

Next, a ninth embodiment of the present invention will be described. The present embodiment is characterized in the following: planetary gears of a planetary-gear-type speed reducer serving as a second speed reducer each have the shape of a truncated cone; a sun gear and a ring gear are shaped so as to coincide with the shape of the planetary gears; and the planetary gears are urged from a large-diameter portion of the truncated cone shape toward a small-diameter portion. Other structural features of the present embodiment are similar to those of the above-described first embodiment. FIG. 14 is a schematic, partial sectional view of a speed reducer according to the present embodiment. As illustrated, the planetary gears 422 of the planetary-gear-type speed reducer 42 as a second speed reducer each assume a truncated cone shape and include a large-diameter portion (end surface having a large area) 422b, a small-diameter portion (end surface having a small area) 422c, and a taper surface 422d whose diameter reduces from the large-diameter portion 422b toward the small-diameter portion 422c. The external gear teeth 422a are formed on the taper surfaces 422d of the planetary gears 422. Accordingly, the inner circumferential surface of the ring gear 421 , which is meshed with the planetary gears 422, is also tapered. Similarly, the outer circumferential surface of the sun gear 423, which is meshed with the planetary gears 422, is also tapered.

Gear-urging means 85 are provided on a side toward the corresponding large-diameter portions 422b of the planetary gears 422. The gear-urging means 85 are supported by the corresponding pin portions 424c of the carrier 424. The gear-urging means 85 each include a restriction plate 86 and a coil spring 87. The restriction plate 86 assumes a ring-like shape like a plain washer and allows insertion of the pin portion 424c through a center hole 86a thereof.

The coil spring 87 is disposed between the restriction plate 86 and the large-diameter portion 422b of the planetary gear 422. One end of the coil spring 87 abuts the restriction plate 86, and the other end of the coil spring 87 abuts the large-diameter portion 422b of the planetary gear 422. The coil spring 87 in the illustrated condition generates a stretching force. Accordingly, the stretching force of the coil spring 87 presses the restriction plate 86 up to a base end portion of the pin portion 424c, and further movement of the restriction plate 86 is blocked there. Also, the stretching force of the coil spring 87 urges the planetary gear 422 rightward in FIG. 14. In this manner, the gear-urging means 85 urges the planetary gear 422 axially from the large-diameter portion 422b of the planetary gear 422 toward the small-diameter portion 422c. The present embodiment is described while mentioning a coil spring serving as urging means. However, the gear-urging means 85 may be a spring of another type or be hydraulically or pneumatically configured.

When the gear-urging means 85 urges the planetary gear 422 rightward in FIG. 14, this urging force is resolved on the taper surface 422d into an axial force and a radial force. The radial force acts as a pressing force to press the planetary gear 422 against the ring gear 421 and the sun gear 423. This pressing force ensures meshing between the planetary gear 422 and the ring gear 421 and meshing between the planetary gear 422 and the sun gear 423, thereby preventing unusual rattle, and backlash variations.

Particularly, in the case where a planetary-gear-type speed reducer employs a plurality of planetary gears, a difference in machining error among the planetary gears may cause difference in terms of backlash between each planetary gear and a mating gear. Even in such a case, by virtue of the above-mentioned pressing force, the present embodiment can achieve uniformity in terms of backlash between each planetary gear and a mating gear, thereby effectively preventing generation of unusual noise which could otherwise result from backlash variations. Other structural features and operations are similar to those of the above-described first embodiment, and thus repeated description thereof is omitted. Tenth Embodiment: >

Next, a tenth embodiment of the present invention will be described. The present embodiment is characterized in that a carrier of a planetary-gear-type speed reducer serving as a second speed reducer is connected to a steering wheel in such a manner as to be movable by a predetermined amount and that an elastic body, intervenes between the carrier and the steering wheel. Other structural features of the present embodiment are similar to those of the above-described first embodiment.

FIG. 15 is a view showing a detailed structure of connection between the carrier 424 of the planetary-gear-type speed reducer 42 serving as a second speed reducer and the steering shaft 12, which is an output destination member. As illustrated, three support portions 424a of the carrier 424 are connected to the steering shaft 12 while being equally spaced along the circumference of the steering shaft 12. Projections 12a are provided on the steering shaft 12 in opposition to the corresponding support portions 424a of the carrier 424. A recess 424d is formed in each of the support portions 424a while opening toward the corresponding projection 12a. The projections 12a project into the corresponding recesses 424d. Accordingly, the steering shaft 12 is rotatable in relation to the support portions 424a within a range of an opening length L of the recesses 424d. Rubber members 88 serving as elastic bodies are disposed within the corresponding recesses 424d. The projections 12a are partially embedded in the corresponding rubber members 88, thereby connecting the steering shaft 12 with the carrier 424.

The rubber members 88 disposed in the corresponding recesses 424d are elastically deformable. Accordingly, when the steering shaft 12 rotates, an associated /otational torque is transmitted from the projections 12a to the corresponding rubber members 88. The rubber members 88 are elastically deformed, thereby absorbing the rotational torque. So long as the rubber members 88 absorb the rotational torque, the rotational torque is not transmitted to the carrier, and the steering shaft 12 rotates within a range of openings of the recesses 424d. When the steering shaft 12 continues rotating while elastically deforming the rubber members 88, and then the projections 12a hit against corresponding opening ends of the recesses 424d, the opening ends act as stoppers, thereby restricting rotation of the steering shaft 12.

In actuality, the above-mentioned operation is effective at the initial stage of steering of a steering wheel by a driver. At the initial stage of steering, a steering assist delays for the steering operation by the driver due to a delay in response. Accordingly, at the beginning of steering of the steering wheel, a steering assist is not available, thus raising poor feel of steering. In this respect, according to the present embodiment, the steering shaft 12 and the carrier 424 are rotatable in relation to each other within a range of the opening length L of the recesses 424d. Thus, even though a steering assist torque is not input from the carrier 424, the steering shaft 12 can be rotated with a light torque. Therefore, the above-mentioned poor feel of steering does not arise.

Furthermore, according to the present embodiment, the rubber members 88 serving as elastic bodies are disposed within the corresponding recesses 424d, and the projections 12a of the steering shaft 12 are partially embedded in the corresponding rubber members 88, thereby connecting the steering shaft 12 and the carrier 424. Thus, when, at the beginning of steering, the steering shaft 12 rotates in relation to the carrier 424, the steering shaft 12 receives as a reaction torque a restoration force associated with elastic deformation of the rubber members 88 caused by the projections 12a. This reaction torque generates an appropriate steering torque for operating a steering wheel, thereby providing an improved feel of steering. Other structural features and operations are similar to those of the above-described first embodiment, and thus repeated description thereof is omitted.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a steering apparatus for a vehicle which includes an electric motor which generates a steering force on the basis of a steering operation of a steering wheel.

Claims

1. A steering apparatus for a vehicle which includes an electric motor for generating a steering force on the basis of a steering operation of a steering wheel, characterized by comprising: a first speed reducer adapted to receive rotational torque from the electric motor and output the rotational torque from an output element while reducing rotational speed; and a second speed reducer adapted to receive the rotational torque from the output element and output the rotational torque while reducing rotational speed by means of speed-reducing elements disposed in the interior of the output element of the first speed reducer.

2. A steering apparatus for a vehicle according to claim 1 , wherein the second speed reducer is a planetary-gear-type speed reducer having a ring gear, a sun gear, a planetary gear, and a carrier, and the output element and the ring gear are formed integrally with each other.

3. A steering apparatus for a vehicle according to claim 2, wherein the first speed reducer is a worm speed reducer having a worm and a worm wheel, and the worm wheel is the output element.

4. A steering apparatus for a vehicle according to claim 2, wherein the first speed reducer is a pulley-type speed reducer having a drive pulley, a driven pulley, and a drive belt for transmitting a drive force of the drive pulley to the driven pulley, and the driven pulley is the output element.

5. A steering apparatus for a vehicle according to claim 2, wherein the first speed reducer is a gear-type speed reducer having a drive gear, and a driven gear meshed with the drive gear, and the driven gear is the output element.

6. A steering apparatus for a vehicle according to claim 3, wherein the worm wheel has external gear teeth formed from resin on an outer circumferential surface thereof and internal gear teeth formed from metal on an inner circumferential surface thereof.

7. A steering apparatus for a vehicle according to any one of claims 2 to 6, wherein the planetary-gear-type speed reducer is such that the ring gear is an input element, the sun gear is a stationary element, and the carrier is an output element, the steering apparatus further comprising a torque limiter for allowing rotation of the sun gear when a rotational torque transmitted from the planetary gear to the sun gear attains a predetermined value or greater.

8. A steering apparatus for a vehicle according to any one of claims 3 to 7, wherein the planetary gear is in the form of a helical gear.

9. A steering apparatus for a vehicle according to claim 3, further comprising mesh adjustment means for adjusting a meshing condition between the worm and the worm wheel.

10. A steering apparatus for a vehicle according to claim 9, wherein the mesh adjustment means comprises worm-wheel-urging means for urging the worm wheel in such a direction that the worm and the worm wheel approach each other, and a holder for supporting the worm-wheel-urging means.

11. A steering apparatus for a vehicle according to claim 10, wherein the worm-wheel-urging means comprises an abutment member in abutment with an inner circumferential surface of the ring gear, a rod having a first end portion connected to the abutment member and extending in a direction toward the worm, and a rod-urging member for urging the rod toward a second end portion of the rod from the first end portion of the rod.

12. A steering apparatus for a vehicle according to claim 9- further comprising holder means for maintaining a relative arrangement condition between the worm and the worm wheel.

13. A steering apparatus for a vehicle according to claim 3, wherein the planetary gear has the shape of a truncated cone, whose axial section is trapezoidal, the steering apparatus further comprising planetary-gear-urging means for urging the planetary gear in an axial direction directed from a large-diameter portion of the truncated cone shape toward a small-diameter portion of the truncated cone shape.

14. A steering apparatus for a vehicle according to claim 3, wherein the carrier is connected to an output destination member to which an output of the second speed reducer is transmitted, in such a manner as to be rotatable in relation to the output destination member within a predetermined range.

15. A steering apparatus for a, vehicle according to claim 14, wherein the carrier and the output destination member are connected together via an elastically deformable elastic body.