Vehicle steering apparatus

In a vehicle steering apparatus, a rotating cylinder supported by a rack housing is threadingly engaged via rolling elements with a screw shaft integrated with a rack which meshes with a pinion which rotates by steering operation. The rotating cylinder is driven by a motor. All of the rolling elements are arranged between a rack support member which supports the rack in the position where the rack is meshed with the pinion, and a support body that is arranged in the movement range of the screw shaft and is capable of supporting the outer circumference of the screw shaft. Loads acting on the rack from the road surface are received by the support body and the rack support member.

FIELD OF THE INVENTION

The present invention relates to a steering apparatus which generates a force causing a rack to move by a motor rotatably driving a rotating cylinder threadingly engaged with a screw shaft integrated with the rack, and changes steering angle by transmitting the movement of the rack to vehicle wheels.

DESCRIPTION OF THE RELATED ART

The conventional electric power steering apparatus100shown inFIG. 7comprises a screw shaft102integrated with a rack101which engages with a pinion (not shown) which rotates by steering operation, a ball nut104threadingly engaged with the screw shaft102via balls103, a driven bevel gear105integrated with the ball nut104so as to rotate together, a motor107for generating a steering assist force to drive a driving bevel gear106engaged with the driven bevel gear105, and a rack housing108rotatably supporting the ball nut104and the driven bevel gear105. By transmitting the movement of the rack101to vehicle wheels the steering angle changes.

In the electric power steering apparatus100as mentioned above, loads acting on the vehicle wheels from the road surface generate flexure and vibration in the rack101. In order to absorb such vibrations, it has been proposed to support the rack101with a bearing composed of a material with strong vibration-dampening properties, in a position (on the right of the balls103inFIG. 7) between the above-mentioned balls103and a rack support member (not shown) which supports the rack101in the position where the rack101is meshed with the pinion (U.S. Pat. No. 6,041,885).

However, even if vibration could be dampened with a bearing composed of a material with strong vibration-dampening qualities, it was not possible to adequately reduce flexure of the rack101. An increased flexure of the rack101is an increased flexure of the screw shaft102, so that friction between the balls103and the ball nut104and between the balls103and the screw shaft102grows, and in some cases the transmitting efficiency of the power output of the motor107to the rack101declines by over 10%, efficient use of energy cannot be achieved, and the feeling of the steering declines.

Furthermore, arranging the screw shaft102in the vicinity of the end of the rack101has been desired because of necessities in the layout of preventing interference between the motor107and other components within the vehicle body. However, if the screw shaft102is arranged at the vicinity of the end of the rack101, flexure of the rack101grows because the distance between the screw shaft102and the rack support member increases. In a rack-and-pinion steering apparatus employing a rack in which the screw shaft102is not provided in the above-mentioned way, flexure of the rack is prevented by supporting the vicinity of the end of the rack with a bush. However, in a case where the screw shaft102is provided at the vicinity of the end of the rack101being supported by a bush, the bush comes into contact with the opening edges of a spiral race102aon the screw shaft102and becomes easily worn. As a result, transmitting efficiency deteriorates with play in the screw shaft.

An object of the present invention is to provide a vehicle steering apparatus which enables the above-mentioned problems to be solved.

SUMMARY OF THE INVENTION

The present invention provides a vehicle steering apparatus which comprises a pinion which rotates by steering operation, a rack which engages with the pinion, a screw shaft integrated with the rack, a rotating cylinder threadingly engaged with the screw shaft via rolling elements, a motor for driving the rotating cylinder, a rack housing rotatably supporting the rotating cylinder, and a rack support member which supports the rack in the position where the rack is meshed with the pinion, and which changes the steering angle by transmitting the movement of the rack to the vehicle wheels, wherein the vehicle steering apparatus further comprises a support body which is capable of supporting the outer circumference of the screw shaft and is arranged in the movement range of the above-mentioned screw shaft, all of the above-mentioned rolling elements are arranged between the support body and the above-mentioned rack support member, and loads acting on the above-mentioned rack from the road surface are received by the support body and the above-mentioned rack support member.

According to the present invention, loads acting on the rack from the road surface via the vehicle wheels are received by the rack support member and the support body which supports the rack, so that flexure of the rack between the support body and the rack support member can be prevented. All of the rolling elements are arranged between the support body and the rack support member, so that flexure in the screw shaft can be reduced, and increases in friction between the rolling elements and the screw shaft and between the rolling elements and the rotating cylinder can be prevented. Furthermore, the support body is arranged in the movement range of the screw shaft and is capable of supporting the outer circumference of the screw shaft, so that the screw shaft can be arranged in the vicinity of the end of the rack, further away from the rack support member than all of the rolling elements. As a result, the degree of freedom in the layout of the motor in the steering apparatus can be enhanced. A bush or bearing can be employed as the support body.

The above-mentioned support body can be supported by the above-mentioned rack housing. In this manner, even if the distance of the screw shaft from the rack support member increases, the outer circumference of the screw shaft can be supported by the support body and flexure of the screw shaft can be reduced.

Alternatively, the above-mentioned support body can be supported by the above-mentioned rotating cylinder. As a result, uneven wearing of the support body is prevented because the support body rotates relatively to the screw shaft, and moreover uneven wear through contact with the opening edges of the spiral race on the screw shaft is also inhibited, enhancing longevity. In this case, it is preferable to use a rolling bearing as the support body. By using a rolling bearing, the screw shaft and the rotating cylinder are able to smoothly rotate relatively, and the transmitting efficiency can be improved.

It is preferable that opening edges of the spiral race on the outer circumference of the above-mentioned screw shaft are chamfered. In this manner, wear of the support body caused by the contact with the opening edges of the race can be inhibited, and deterioration in transmitting efficiency resulting from play in the screw shaft can be prevented. The chamfered edge can be a convex curve or a level surface.

It is preferable that the dimension in the screw shaft axial direction of the spiral race on the outer circumference of the above-mentioned screw shaft is made smaller than the dimension in the screw shaft axial direction of the interval between sections of the spiral race. As a result, pressure acting on the inner circumferential surface of the support body from the outer circumferential surface of the screw shaft can be reduced, wear of the support body can be inhibited, and deterioration in transmitting efficiency resulting from play in the screw shaft can be prevented.

According to the present invention, in a steering apparatus which gives an axial force to a rack by a motor rotatably driving a rotating cylinder threadingly engaged with a screw shaft integrated with the rack, and changes steering angle by transmitting the movement of the rack to vehicle wheels, deterioration in the transmitting efficiency of the motor power output to the rack can be prevented.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A rack-and-pinion type electric power steering apparatus1shown inFIG. 1andFIG. 2comprises a pinion3which rotates by steering operation with a steering wheel H linked to an input shaft2, and a rack4which meshes with the pinion3, wherein vehicle wheels (not shown) are linked to both ends of the rack4. The steering angle changes by transmitting the movement of the rack4by the rotation of the pinion3to the vehicle wheels.

A rack housing30which encloses the pinion3and the rack4is provided. The rack housing30is constructed by joining a first housing31to a second housing32by screws and the like, and is installed on the vehicle body. A torque sensor7is provided within the rack housing30for detecting the steering torque being transmitted to the pinion3by the steering operation with the steering wheel H. The torque sensor7can employ a known art. The rack4is supported by a rack support member9in the position where the rack4is meshed with the pinion3. The rack support member9has a known structure in this embodiment, that is, it is inserted in the rack housing30so as to be moveable in the radial direction of the rack4, and is pressed on the rack4by a spring8so as to support the opposite side of the rack4from the teeth of the rack4via its surface, which has an arc shape as viewed in the axial direction of the rack4.

As shown inFIG. 3, a screw shaft11is integrated with the rack4. In this embodiment, a region closer to one end side (the left side inFIG. 3) from the center in the rack4is taken as the screw shaft11. A ball nut (rotating cylinder)13is threadingly engaged with the screw shaft11via balls (rolling elements)12. The ball nut13is rotatably supported by the rack housing30via double row ball bearings33on one end side and a ball bearing34on the other end side.

A driven gear21is integrated with the ball nut13so as to rotate together. In this embodiment, the driven gear21is a bevel gear, that is fitted on the outer circumference of the ball nut13and integrated therewith by being sandwiched between a step on the outer circumference of the ball nut13and a nut14threadingly engaged with the ball nut13.

A drive gear22which meshes with the driven gear21is integrally provided on a drive shaft23. The drive gear22is rotatably supported by a gear housing26via ball bearings24,25which support the drive shaft23. The gear housing26is formed integrally with the second housing32of the rack housing30. An output shaft of a steering assist force generating motor27attached to the gear housing26is connected to the drive shaft23via a coupling27a.As a result, the ball nut13is rotationally driven by the motor27via the drive gear22and the driven gear21which are components of a reduction gear mechanism.

An in-vehicle control device (not shown) connected to the above-mentioned torque sensor7and the motor27, drives the motor27in accordance with the steering torque detected by the torque sensor7. As a result, the power output of the motor27is transmitted to the ball nut13via the drive gear22and the driven gear21, and the rotational force of the ball nut13is transmitted to the rack4as a steering assist force via the balls12and the screw shaft11. The control method of the motor27is not limited specifically, and altering the steering assist force in accordance with a variable other than the steering torque such as the vehicle speed or the like is also acceptable.

In the movement range of the screw shaft11, a support body capable of supporting the outer circumference of the screw shaft11is provided. In this embodiment, the support body is a cylindrical bush40, and is supported by the rack housing30by being fitted into the inner circumference of a cylinder member42threadingly engaged into the inner circumference of the first housing31via threads41. The bush40is formed by coating the surface of a sheet steel with polytetrafluoroethylene or a copper compound or similar for example, and is provided with a split. The outer circumference of the screw shaft11is supported by the inner circumference of the bush40. In this embodiment, the bush40is arranged in the vicinity of the border region of the rack4and one end of the screw shaft11when the vehicle is travelling straight. Alternatively, the bush40can be arranged in a position constantly supporting the screw shaft11, or arranged in a position supporting the screw shaft11only when turning either right or left. All of the above-mentioned balls12are arranged between the bush40and the rack support member9. As a result, loads acting on the rack4from the road surface via the vehicle wheels are received by the bush40and the rack support member9, and flexure of the screw shaft11can be inhibited. The difference between the internal diameter of the bush40and the external diameter of the screw shaft11is made about 0.05 mm for example.

As shown inFIG. 4, opening edges11a′of a spiral race11aon the screw shaft11are chamfered so that wear of the bush40due to contact with the opening edges11a′of the race11ais inhibited. In this embodiment, each of the chamfered edges11a′is made a flat surface. It is desirable that the angle α made by each of the chamfered edges11a′against the axial direction of the screw shaft11is 30 degrees or less.

According to the above-mentioned embodiment, loads acting on the rack4from the road surface via the vehicle wheels are received by the rack support member9and the bush40which support the rack4, so that flexure of the rack4between the bush40and the rack support member9can be prevented. All of the balls12are arranged between the bush40and the rack support member9, so that flexure in the screw shaft11can be reduced, and increases in friction between the balls12and the screw shaft11and between the balls12and the ball nut13can be prevented. Furthermore, the bush40is arranged in the movement range of the screw shaft11and is capable of supporting the outer circumference of the screw shaft11, so that the screw shaft11can be arranged at the vicinity of the end of the rack4, further away from the rack support member9than all of the balls12. As a result, the degree of freedom in the layout of the motor27in the steering apparatus1can be enhanced. By supporting the bush40by the rack housing30, even if the distance between the rack support member9and the screw shaft11increases, the outer circumference of the screw shaft11can be supported by the bush40and flexure of the screw shaft11can be reduced. Furthermore, the opening edges11a′of the race11aon the outer circumference of screw shaft11are chamfered, so that wear of the bush40due to contacting with the opening edges11a′can be inhibited.

FIG. 5shows a first modification of the present invention. A difference with the above-mentioned embodiment is firstly that a needle bearing240which is a rolling bearing in place of the bush40is employed as a support body. Furthermore, in place of the balls12and the ball nut13, a plurality (in this embodiment four) of ball bearings212arranged along the axial direction of the screw shaft11, and a cylinder body213covering these ball bearings212are employed. The cylinder body213is made by connecting a first member213ato a second member213bvia threads213c,and is supported rotatably by the rack housing30via a ball bearing233on one end side and a ball bearing234on the other end side. The driven gear21is fitted on the outer circumference of the first member213aof the cylinder body213, and is integrated with the cylinder body213by being sandwiched between the second member213band a step on the outer circumference of the first member213a.The inner ring of the ball bearing233on one end side is integrated with the outer circumference of the cylinder body213. The needle bearing240is supported by the cylinder body213by being fitted into the inner circumference of the cylinder body213. Balls212aof the ball bearings212are the rolling elements of the present invention. The cylinder body213is threadingly engaged with the screw shaft11via these balls212a.In other words, an outer ring212bof each of the ball bearings212is integrated with the cylinder body213so as to rotate together. An annular protruding portion212c′is formed on the inner circumference of an inner ring212cof each of the ball bearings212, so as to be capable of coming into contact with the spiral race11aon the screw shaft11. The rotational axes of the ball bearings212are inclined in relation to the center axis of the screw shaft11by an angle equal to the lead angle of the spiral race11a,with these directions of incline made mutually reversed between the two ball bearings212on the central side and the two ball bearings212on the opposite end sides. Furthermore, the rotational axis of each of the ball bearings212is eccentric to the center axis of the screw shaft11. Each of the ball bearings212is in contact at one position with the inner surface of the spiral race11avia the annular protruding portion212c′,and the positions of contact for the two ball bearings212on the central side are separated from the positions of contact for the two ball bearings212on the opposite end sides in the rotational direction by 180 degrees. As a result, when the cylinder body213is rotationally driven by the motor27, an axial force acts on the screw shaft11via the ball bearings212. This sort of rack feeding mechanism utilizing ball bearings can employ a known art disclosed in Japanese Patent Laid-Open No. 2000-352450 for example. According to the first modification, uneven wear is prevented because the needle bearing240rotates relatively to the screw shaft11, and wear through contact with the opening edges11a′of the race11aon the screw shaft11is also inhibited, enhancing longevity. Additionally, by using the needle bearing240which is a rolling bearing as a support body, the screw shaft11and the cylinder body213are able to smoothly rotate relatively, and the transmitting efficiency can be improved. The remainder is the same as the above-mentioned embodiment, and corresponding sections are indicated by identical symbols.

FIG. 6shows a second modification of the present invention. Differences to the above-mentioned embodiment and the first modification reside in that each of the chamfered edges11a′of the opening of the spiral race11ain the outer circumference of the screw shaft11is a convex curve, and a dimension L1in the screw shaft axial direction of the spiral race11ais made smaller than a dimension L2in the screw shaft axial direction of the interval between the sections of the spiral race11a.In this case where the opening edges11a′of the spiral race11aare chamfered, the dimensions L1, L2are determined as though the chamfer11a′had not been formed as illustrated by the broken line in the figure. In this manner, pressure acting on the inner circumferential surface of the bush40or the needle bearing240from the outer circumferential surface of the screw shaft11can be reduced, wear of the bush40or the needle bearing240can be inhibited, and deterioration in transmitting efficiency resulting from play in the screw shaft11can be prevented. The remainder is the same as the above-mentioned embodiment, and corresponding sections are indicated by identical symbols.

The present invention is not limited to the above-mentioned embodiments. For example, a rolling bearing can be used in place of the bush40in this embodiment, or the bush40can be supported by the ball nut13in place of the rack housing30. In the first modification, a bush can be used in place of the needle bearing240, or the needle bearing240can be supported by the rack housing30. Furthermore, the type of the drive gear22and the driven gear21is not limited to a bevel gear. Further, a motor having a rotor integrated with a rotating cylinder can be used, so that the rotating cylinder is driven without employing a gear mechanism. Furthermore, a steering apparatus pertaining to the present invention is not limited to a power steering apparatus, for example the present invention can be applied to an automatic steering apparatus which drives a rack only by motor output power in order to drive an unmanned vehicle along guidance markers on the road surface, or can be applied to a steering apparatus employing a steer-by-wire system which drives a rack only by motor output power driven according to the steering operation with a steering wheel which is not connected mechanically to the vehicle wheels.