Steering system

A steering system includes a motor that rotates a power transmitting member, a rotating body to which rotative power of the motor is transmitted via the power transmitting member, a rack shaft that is driven in its axial direction according to rotation of the rotating body, a rack housing that houses the rack shaft and the rotating body, and a bearing interposed between the rack housing and the rotating body. A receiving portion of the rotating body which engages with the power transmitting member is provided at an opposed surface as a portion of an outer circumferential surface of the rotating body which is opposed to an output shaft of the motor, and the bearing is provided on a non-opposed surface other than the opposed surface.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2011-005026 filed on Jan. 13, 2011 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a steering system including a motor that rotates a power transmitting member, a rotating body to which rotative power of the motor is transmitted via the power transmitting member, a rack shaft that is driven in an axial direction in accordance with rotation of the rotating body, a rack housing that houses the rack shaft, and a bearing interposed between the rack housing and the rotating body.

2. Description of Related Art

A known example of the steering system as described above is described in, for example, Japanese Patent Application Publication No. 2005-343434 (JP-A-2005-343434). In the steering system, the motor is positioned such that the direction in which the rack shaft is driven and the direction in which an output shaft of the motor extends are generally in parallel with each other, and the motor is disposed outside a peripheral wall of the rack housing that houses the rack shaft. On the other hand, the rack shaft is supported by the rack housing, via a ball screw, nut, and a bearing.

In the meantime, there has been a demand to reduce space as measured in the direction in which the rack shaft and the output shaft of the motor are arranged, in the steering system. Namely, there has been a demand to reduce the center distance between the axis of the rack shaft and the axis of the output shaft of the motor.

SUMMARY OF THE INVENTION

The invention provides a steering system in which the distance between a rack shaft and an output shaft of a motor is small.

A steering system according to one aspect of the invention includes a power transmitting member, a motor that rotates the power transmitting member, a rotating body to which rotative power of the motor is transmitted via the power transmitting member, a rack shaft that is driven in an axial direction thereof in accordance with rotation of the rotating body, a rack housing that houses the rack shaft and the rotating body, and a bearing interposed between the rack housing and the rotating body. In the steering system, an outer circumferential surface of the rotating body includes an opposed surface that is opposed to an output shaft of the motor, and a non-opposed surface other than the opposed surface, and the rotating body has a receiving portion that is provided at the opposed surface and engages with the power transmitting member, while the bearing is provided on the non-opposed surface.

According to the above aspect of the invention, the bearing is provided on the non-opposed surface of the rotating body, namely, the bearing is not provided between the motor and the rack housing. Therefore, the space between the motor and the rack housing can be reduced, and the center distance between the axis of the output shaft of the motor and the axis of the rack shaft can be reduced.

In the steering system according to the above aspect of the invention, the rack housing may include a first housing that supports the rack shaft via the bearing, and houses a portion of the rack shaft, and a second housing that houses another portion of the rack shaft, and supports the motor.

In the steering system as described above, the rack shaft is supported by the first housing, and the motor is supported by the second housing. With this arrangement, each component of a power transmitting mechanism that transmits rotative power of the motor to the rack shaft is supported by the rack housing, so that the position of each component of the power transmitting mechanism relative to the motor is prevented from being shifted or changed.

In the steering system according to the above aspect of the invention, the rotating body may include a driven body to which the rotative power of the motor is transmitted via the power transmitting member, and a screw mechanism that rotates with the driven body and converts the rotative power into linear motion of the rack shaft, and the rack shaft may be driven in the axial direction in accordance with rotation of the screw mechanism, while the bearing may be interposed between the rack housing and the driven body. In this steering system, the rack housing may include a third housing that houses a portion of the rack shaft, and supports the motor, the power transmitting member, the driven body, and the bearing, which are mounted on the third housing, and a fourth housing that houses another portion of the rack shaft, and the fourth housing and the motor may not overlap each other as viewed in a radial direction of the rack shaft.

In the steering system as described above, the fourth housing does not support the motor, and is positioned so as not to overlap the motor as viewed in the radial direction of the rack shaft. Namely, the fourth housing is provided separately from the motor, which makes it possible to remove the fourth housing without removing the motor from the third housing. With this arrangement, the rack shaft and the screw mechanism can be removed from the third housing, in a condition where the motor remains on the third housing.

The steering system as described above may further include a seal member that fills a clearance between the third housing and the fourth housing, and a clearance between the third housing and the motor.

When the steering system has a first structure in which the bearing is provided on the non-opposed surface as a part of the outer circumferential surface of the rotating body, and a second structure in which one housing (the third housing) supports the motor, power transmitting member, driven body, and the bearing, which are mounted thereon, the steering system naturally has the following structure.

Namely, since the rack shaft, power transmitting member, and the driven body are located in a connecting portion of the third housing, the connecting portion has an elliptical shape. On the other hand, a connecting portion of the other housing generally has a substantially circular shape, and a connecting portion of the motor has a circular shape. Therefore, if the fourth housing and the motor are connected to the connecting portion of the third housing, a clearance(s) may be formed in the connecting portion, and air-tightness cannot be ensured.

In the steering system as described above, on the other hand, the seal member fills the clearance between the third housing and the fourth housing, and fills the clearance between the third housing and the motor, thus assuring air-tightness.

In the steering system as described above, the seal member may be constructed so that no clearance is formed between the motor and the seal member when the motor is moved so as to change a distance between the rack shaft and the output shaft of the motor.

With the above arrangement, even if the motor is moved for adjustment of the position of the motor, for example, no clearance is formed between the motor and the seal member, and therefore, air-tightness of the steering system can be ensured,

According to the invention, the steering system in which the center distance between the rack shaft and the output shaft of the motor is small can be provided.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring toFIG. 1throughFIG. 5, a first embodiment of the invention will be described.FIG. 1shows the overall construction of a steering system1as the first embodiment of the invention.

The steering system1converts rotary motion of a steering shaft into linear motion by means of a rack-and-pinion mechanism10, and transmits the linear motion to a rack shaft20, so as to operate steered wheels connected to distal end portions of the rack shaft20. The linear motion of the rack shaft20is assisted by power of a motor30. The rotative power of the motor30is transmitted to the rack shaft20, via a gear mechanism40that converts the rotative power into linear motion.

A middle portion21of the rack shaft20is housed in a rack housing50made of aluminum. End portions of the rack shaft20protrude outward from the rack housing50. The rack-and-pinion mechanism10is provided at a location spaced from a first end22of the rack shaft20by a suitable distance. The gear mechanism40is mounted on the rack shaft20at a location spaced from a second end23of the rack shaft20by a suitable distance.

In the following description, the direction from the second end23to the first end22of the rack shaft20, namely, the direction from a portion of the rack shaft20in which the gear mechanism40is provided to a portion thereof in which the rack-and-pinion mechanism10is provided, will be referred to as “A direction”. The direction opposite to the A direction will be referred to as “B direction”.

The rack housing50is divided into two housings, i.e., a first housing51and a second housing60. Where PL denotes a position at which the first housing50and the second housing60are joined to each other inFIG. 1, the first housing51houses a portion of the rack shaft.20which extends from the position PL in the B direction, and the second hosing60houses a portion of the rack shaft20which extends from the position PL in the A direction. The motor30and the rack-and-pinion mechanism10are mounted on the second housing60.

Referring toFIG. 2, the structure of the gear mechanism40, and the positional relationship between the gear mechanism40and the rack housing50will be described.FIG. 2is an enlarged view of a cross-section of the gear mechanism40, which is rotated180degrees from the position shown inFIG. 1.

The gear mechanism40includes a first pulley41mounted on an output shaft31of the motor30, a belt42(a power transmitting member) that transmits rotative power of the first pulley41, a second pulley44to which the rotative power of the first pulley41is transmitted via the belt42, a nut case46mounted on to the inside of the second pulley44, and a ball screw45mounted on to the inside of the nut case46. A lock nut47for fixing the ball screw45in position is fitted in an end portion of the nut case46.

The second pulley44is formed with a receiving portion44A over which the belt42is looped. The second pulley44, nut case46, and the ball screw45are secured to each other, so as to rotate as a unit. The ball screw45is mounted on the rack shaft20. The rack shaft20moves in the axial direction in accordance with rotation of the ball screw45.

In the following description, a structure constituted by the nut case46and the second pulley44will be called “driven body43A”, and a structure constituted by the driven body43A and the ball screw45will be called “nut43(rotating body)”. A portion of the outer circumferential surface of the nut43, which is opposed to the output shaft31of the motor30, will be called “opposed surface MA”, and the remaining portion of the outer circumferential surface of the nut43, other than the opposed face MA, will be called “non-opposed surface MB”.

A bearing48has an inner ring provided on the non-opposed surface MB as a part of the outer circumferential surface of the nut43, and an outer ring provided on the inner circumferential surface of the first housing51. Namely, the bearing48is disposed between the nut ease46and the first housing,51. On the other hand, the receiving portion44A is located at the opposed surface MA as a part of the outer circumferential surface of the nut43.

The gear mechanism40operates in the following manner. The motor30rotates the first pulley41. The rotative power of the first pulley41is transmitted to the second pulley44via the belt42, so that the second pulley44, nut case46, and the ball screw45rotate as a unit. With the ball screw45thus rotated, the rack shaft20moves in the axial direction. Namely, the amount of movement of the rack shaft20is controlled, by controlling the rotational speed and the amount of rotation of the motor30.

The first housing51includes a first housing portion52that houses a portion of the rack shaft20, and a second housing portion53that extends from an end portion of the first housing portion52and houses a portion of the nut43. The first housing51further includes a third housing portion54that is located adjacent to the second housing portion53and houses a distal end portion of the first pulley41.

The second housing60includes a fourth housing portion61that houses a portion of the rack shaft20, and a fifth housing portion62that extends from an end portion of the fourth housing portion61and houses a portion of the nut43. The second housing60further includes a sixth housing portion63that is formed adjacent to the fifth housing portion62and houses a portion of the first pulley41of the motor30. The sixth housing portion63consists of a support wail63A that protrudes outward from a peripheral wall of the fourth housing portion61and supports the motor30, and a peripheral wall63B that is provided on the support wall63A and surrounds the periphery of the first pulley41. The support wall63A is formed with an insertion hole64through which the output shaft31is inserted.

The second housing portion53and third housing portion54of the first housing51provide a connecting portion (which will be called “first connecting portion55”) connected to the second housing60, The fifth housing portion62and sixth housing portion63of the second housing60provide a connecting portion (which will be called “second connecting portion65”) connected to the first housing51. With the first connecting portion55and the second connecting portion56abutting on each other, the first housing51and the second housing60are connected to each other, so that the gear mechanism40is housed in the rack housing50.

Referring toFIG. 3andFIG. 4, the gear mechanism40and a gear mechanism140having a known structure will be compared with each other. In the gear mechanism140having the known structure, as shown inFIG. 3, the bearing48is provided on the opposed surface MA of the outer circumferential surface of the nut43, and the receiving portion44A that engages with the belt42is provided on the Opposed surface MA of the outer circumferential surface of the nut43. Also, the motor30and the bearing48are disposed on the same side of the belt42as viewed in the axial direction of the rack shaft20. Thus, the bearing48is interposed between the nut43and the output shaft31of the motor30. Therefore, the center distance DS1between the axis of the rack shaft20and that of the output shaft31is determined in view of the dimensions of the bearing48.

On the other hand, the gear mechanism40as described above has the following structure. As shown inFIG. 4, in the gear mechanism40, the bearing48is provided on the non-opposed surface MB of the outer circumferential surface of the nut43, and the receiving portion44A that engages with the belt42is provided on the opposed surface MA of the outer circumferential surface of the nut43. Also, the motor30is disposed on one side of the belt42as viewed in the axial direction of the rack shaft20, and the bearing48is disposed on the other side of the belt42as viewed in the axial direction. Thus, the bearing48is not interposed between the nut43and the output shaft31of the motor30; therefore, the center distance DS2between the axis of the rack shaft20and that of the output shaft31is smaller than the center distance DS1of the known structure.

Referring toFIG. 5, an assembly operation for the steering system1of the first embodiment will be described. Initially, the bearing48, nut case46and the second pulley44are mounted on to the first housing51. Then, an assembly of the rack shaft20and the ball screw45is inserted through the nut43. The resulting structure is shown inFIG. 5. Then, the belt42is looped over the second pulley44, and the second housing60is attached to the first housing51. Finally, the motor30is mounted on the second housing60.

The first embodiment yields the following effects.

(1) In this embodiment, where the opposed surface MA denotes the portion of the outer circumferential surface of the nut43which is opposed to the output shaft31of the motor30, and the non-opposed surface MB denotes the portion of the outer circumferential surface of the nut43other than the opposed surface MA, the receiving portion44A that engages with the belt42is provided on the opposed surface MA, and the bearing48is provided on the non-opposed surface MB.

With the above arrangement, the bearing48is provided on the non-opposed surface MB, namely, the bearing48is not provided between the motor30and the rack housing50. Therefore, the space between the motor30and the rack housing50can be reduced, and the center distance DS2between the output shaft31of the motor30and the rack shaft20can be reduced to be smaller than the center distance

DS1of the known structure.

(2) In this embodiment, the rack housing50supports the rack shaft20via the bearing48, and includes the first housing51that houses a portion of the rack shaft20, and the second housing60that houses another portion of the rack shaft20and supports the motor30.

With the above arrangement, the rack shaft20is supported by the first housing51, and the motor30is supported by the second housing60. Thus, each structure of a power transmitting mechanism (gear mechanism40) that transmits rotative power of the motor30to the rack shaft20and the motor30are supported by the rack housing50, so that the position of each structure of the gear mechanism40relative to the motor30is prevented from being shifted or changed.

Referring toFIG. 6throughFIG. 9, a second embodiment of the invention will be described. A steering system2of this embodiment is provided by adding the following change to the arrangement of the first embodiment. Namely, in the first embodiment, the motor30is supported by the second housing60, while the belt42, second pulley44, nut case46and the bearing48are supported by the first housing51. In the second embodiment, on the other hand, the motor30, belt42, second pulley44, nut case46and the bearing48are supported by a single housing. In the following, differences from the arrangement of the first embodiment, which arise from the above change, will be described in detail. In the description of the second embodiment, the same reference numerals are assigned to the same or corresponding constituent elements as those of the first embodiment, and further explanation of these elements will not be provided.

Referring toFIG. 6throughFIG. 8, the structure of the rack housing50of the steering system2will be described. As shown inFIG. 6, the rack housing50includes a third housing70that houses a portion of the rack shaft20which extends from a first position PLA in the B direction, a fourth housing80that houses a portion of the rack shaft20which extends from a second position PLB in the A direction, and a seal member90interposed between the third housing70and the fourth housing80. As shown inFIG. 6, the first position PLA is provided at one side of the gear mechanism40which faces in the A direction. The second position PLB is provided at a location shifted further in the A direction from the first position PLA. The seal member90and the third housing70are joined to each other at the first position PLA. The seal member90and the fourth housing80are joined to each other at the second position. PLB. The spacing or distance between the first position PLA and the second position PLB is equal to the dimension of the seal member90as measured in the width direction.

Referring toFIG. 7, the structure of the gear mechanism40, and the positional relationship between the gear mechanism40and the rack housing50will be described.FIG. 7is an enlarged view of a cross-section of the gear mechanism40, which is rotated 180 degrees from the position shown inFIG. 6.

The third housing70includes an eleventh housing portion71that houses a portion of the rack shaft20, and a twelfth housing portion72that extends from an end portion of the eleventh housing portion71and houses the nut43. The third housing70further includes a thirteenth housing portion73that is formed adjacent to the twelfth housing portion72, and houses the first pulley41of the motor30. A fourth connecting portion81connected to the third housing70is formed on a peripheral wall of an end portion of the fourth housing80, so as to extend outward from the peripheral wall.

As shown inFIG. 8, the seal member90is formed from a plate made of iron or aluminum, and the outer periphery of the seal member90is shaped in accordance with the shape of the outer periphery of an end portion of the third housing70. Also, the seal member90is formed with a first through-hole91through which the rack shaft20extends, and a second through-hole92through which the output shaft31of the motor30extends. The dimensions of the first through-hole91are determined so that an assembly (which will be called “Tint assembly100”) of the rack shaft20and the ball screw45can be inserted through the through-hole91. Namely, the diameter of the first through-hole91is larger than the outside diameter of the ball screw45. The dimensions of the second through-hole92are determined so that the output shaft31and the first pulley41can be inserted through the through-hole92.

The seal member90is mounted on a third connecting portion74of the third housing70, so as to close the connecting position74except for the first through-hole91and the second through-hole92. The first through-hole91is closed by the fourth housing80mounted on a corresponding portion of the through-hole91. The second through-hole92of the seal member90is closed by the motor30mounted on a corresponding portion of the through-hole92.

When the motor30is mounted on to the seal member90, the position of the motor30is adjusted by increasing or reducing the distance between the rack shaft20and the output shaft31of the motor30, so as to adjust the tension of the belt42looped over the first pulley41and the second pulley44. Namely, the position at which the motor30is fixed moves relative to the seal member90. In order to maintain the air-tightness of the steering system2, the connecting portion between the motor30and the seal member90is constructed so that the motor30and the seal member90are not spaced from each other, and no clearance is formed between the motor30and the seal member90, even if the motor30is moved within a predetermined adjustment range SA. More specifically, a peripheral portion of the second through-hole92and a front face32of the motor30are in surface contact with each other, as shown inFIG. 7.

In the process of producing the gear mechanism40, there is a demand to divide the gear mechanism40into two assemblies, namely, the first assembly100and a second assembly200, and assemble the first and second assemblies100,200together to complete the steering system2, after individually conducting performance evaluations on the first assembly100and the second assembly200. There is also a demand to individually conduct performance evaluations on the first assembly100and the second assembly200, after fabricating the steering system2.

In the second embodiment, the first assembly100consists of the rack shaft20and the ball screw45, as described above. The second assembly200consists of the motor30, belt42, driven body43A (the nut case46and the second pulley44), and the bearing48. With the gear mechanism40thus divided into the first and second assemblies100,200, an evaluation of at least a contact portion between the rack shaft20and the ball screw45can be conducted independently of evaluations of the other components. Thus, in the case of this embodiment, the first assembly100and the second assembly200are fabricated independently of each other.

Referring toFIG. 9, an assembly operation for the steering system2of the second embodiment will be described. Initially, the bearing48, nut case46and the second pulley44are mounted on the third housing70. Then, the belt42is looped over the second pulley44, while the seal member90is mounted on the third housing70, and the motor30is mounted on the seal member90. In this manner, the second assembly200is completed. Then, the first assembly100(the rack shaft20and the ball screw45) is inserted through the nut43, and, finally, the fourth housing80is mounted on the third housing70.

Namely, the second assembly200can be fabricated without mounting the first assembly100in the third housing70. Also, the first assembly100and the second assembly200can be separated from each other, by a reverse procedure to the above-described assembly operation.

The steering system2of this embodiment further yields the following effects, in addition to the above-described effects (1) and (2) according to the first embodiment. (3) In the second embodiment the third housing70supports the motor30, belt42, driven body43A and the bearing48mounted thereon. On the other hand, the fourth housing80is positioned so as not to overlap the motor30in radial directions of the rack shaft20.

With this arrangement, since the fourth housing80is provided separately from or independently of the motor30, the fourth housing80can be removed without removing the motor30from the third housing70. Thus, the first assembly (of the rack shaft20and the ball screw45) can be removed from the third housing70in a condition where the Motor30remains on the third housing70. Therefore, even after the steering system2is fabricated, performance evaluations can be individually conducted on the first assembly100and the second assembly200, respectively.

(4) in the second embodiment, the third housing70supports the motor30, belt42, second pulley44, nut case46and the bearing48such that these components are mounted on or engaged with each other. As described above, a linear movement mechanism that linearly moves the rack shaft20with the rotative power of the motor30is constituted by the first assembly100and the second assembly200. Meanwhile, from the viewpoint of the production quality control, it is desirable to assemble the first assembly100and the second assembly200together, after individually conducting performance evaluations on the first assembly100and the second assembly200. However, if the third housing70, the fourth housing80, and the motor30are assembled or put together in the following manner, the step of mounting the first and second assemblies on each other cannot be taken after the first and second assemblies are individually fabricated.

Namely, when constituent components of the second assembly200other than the motor30are supported by one of the third housing70and the fourth housing80, and the motor30is supported by the other housing, the components of the second assembly200other than the motor do not engage with the motor in a condition where the housings70,80are spaced apart from each other. Therefore, the second assembly200cannot be fabricated unless the housings70,80are connected to each other. Accordingly, the second assembly200cannot be evaluated alone.

On the other hand, with the above arrangement of the second embodiment, the third housing70supports all of the components of the second assembly200; therefore, the second assembly200can be fabricated in a condition in which the third housing70and the fourth housing80are not connected to each other. Therefore, the first and second assemblies100,200can be mounted on to each other after these assemblies100,200are individually fabricated.

(5) In the second embodiment, the seal member90is provided for filling a clearance between the third housing70and the fourth housing80, and a clearance between the third housing70and the motor30. The steering system2naturally has the following structure when it is provided with a first structure in which the bearing48is provided on the non-opposed surface MB as a part of the outer circumferential surface of the nut43, and a second structure in which one housing (the third housing70) supports the motor30, belt42, second pulley44, nut case46, and the bearing48. Namely, since the rack shaft20, belt42, first pulley41and the second pulley44are located in the third connecting portion74of the third housing70, the third connecting portion74has an elliptical shape. On the other hand, the fourth connecting portion81of the fourth housing80has a generally circular shape, and the connecting portion of the motor30has a circular shape. Therefore, if the fourth housing80and the motor30are connected to the third housing70, clearances are formed between the third housing70, and the fourth housing80and the motor30, and air-tightness cannot be ensured.

On the other hand, with the above-described arrangement of the second embodiment, the seal member90fills the clearance between the third housing70and the fourth housing80, and fills the clearance between the third housing70and the motor30, thus assuring air-tightness.

(6) In the second embodiment, no clearance is formed between the motor30and the seal member90, due to the movement of the motor30for changing the center distance between the rack shaft20and the output shaft31of the motor30.

With this arrangement, even if the motor30is moved for adjustment of the position of the motor30, for example, no clearance is formed between the motor30and the seal member90, and therefore, the air-tightness of the steering system2can be maintained.

It is to be understood that the present invention is not limited to the embodiments as illustrated above, but may be embodied with changes or modifications as described below, for example. It is also to be understood that each of the following modified examples is not only applied to the corresponding embodiment, but different ones of the modified examples may be combined and implemented.

While the plate-like seal member90is provided between the third housing70and the fourth housing80in the second embodiment, the seal member90may take another form provided that it can fill a clearance(s) formed in a connecting portion of the third housing70and the second housing80when they are connected to each other. For example, a flange may be provided at an end portion of the third housing70, and a flange may be provided at an end portion of the motor30, so that these flanges fill a clearance between the fourth housing80and the third housing70.

In each of the illustrate embodiments, the ball screw45converts rotation of the ball screw45into linear motion of the rack shaft20in the axial direction. Namely, the ball screw45may be replaced by another structure or component provided it can convert its rotation into linear motion of the rack shaft20. For example, a planetary roller screw structure may be employed, in place of the ball screw45.

While the nut case46and the second pulley44are separate or non-integral bodies in each of the illustrated embodiments, the nut ease46and the second pulley44may be formed as an integral body or unit. With this arrangement, the number of components of the nut43can be reduced.

While a speed reducing mechanism that consists of the first pulley41, second pulley44and the belt42is provided in each of the illustrated embodiments, the belt may be replaced by a chain. Also a speed reducing mechanism consisting of gears may be employed in place of the speed reducing mechanism as described above.

In each, of the illustrated embodiments, the invention is applied to the steering system1,2in which the motor30is positioned such that the rack shaft20and the output shaft31of the motor30extend in parallel with each other. However, the invention may also be applied to a steering system in which the motor30is positioned such that a certain angle is formed between the rack shaft20and the output shaft31of the motor30. In this case, too, effects similar to the effects of each of the illustrated embodiments can be obtained.