Method for adjusting steering system and adjustment apparatus for steering system

A method for adjusting a steering system and an adjustment apparatus for a steering system are provided which allow a clearance between a support yoke and a yoke plug to be appropriately set. When a yoke plug is loosened which has been temporarily fastened to a degree that a support yoke is subjected to elastic compressive deformation, the yoke plug is loosened to a position where a clearance C between the support yoke and the yoke plug reaches a target clearance using, as a reference, a position where a rate of a change in an axial position of the yoke plug with respect to an angular position of the yoke plug increases rapidly.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-260916 filed on Dec. 24, 2014 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for adjusting a steering system and an adjustment apparatus for a steering system.

2. Description of Related Art

A rack-and-pinion steering system has been known as described in, for example, Japanese Patent Application Publication No. 2008-018828 (JP 2008-018828 A). The steering system has a pinion shaft that rotates in conjunction with a steering wheel, a rack shaft that meshes with the pinion shaft, and a housing that houses both the pinion shaft and the rack shaft. The rack shaft moves in an axial direction thereof in conjunction with rotation of the pinion shaft to allow the direction of wheels to be changed.

The steering system includes a support mechanism for eliminating backlash between the rack shaft and the pinion shaft. The support mechanism has a support yoke provided in a holding hole of the housing so as to be able to move back and forth, a yoke plug screw-threaded into the holding hole, and a compression coil spring interposed between the yoke plug and the support yoke. The support yoke is constantly biased toward the rack shaft by an elastic force of the compression coil spring. The support yoke supports the rack shaft so that the rack shaft is slidable along the axial direction thereof while pressing the rack shaft toward a pinion.

To suppress hammering sound resulting from abutting contact between the support yoke and the yoke plug, a predetermined amount of clearance is provided between the support yoke and the yoke plug. For strict management, the clearance is adjusted, for example, as follows. First, the yoke plug is temporarily fastened to the holding hole to the degree that the yoke plug comes into abutting contact with the support yoke.

Subsequently, the yoke plug is loosened until the clearance reaches a target clearance.

However, this method for adjusting the clearance raises the following concerns. Although the clearance is preferably adjusted using, as a reference, a zero clearance position that is the position where the yoke plug just comes into abutting contact with the support yoke, whether the yoke plug is located at a true zero clearance position is not known. Furthermore, when the yoke plug is temporarily fastened, the zero clearance position may vary among products. The variation in zero clearance position may lead to a variation in adjusted clearance.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for adjusting a steering system and an adjustment apparatus for a steering system that allow a clearance between a support yoke and a yoke plug to be appropriately set.

An aspect of the present invention provides a method for adjusting a steering system including loosening a yoke plug, which has been temporarily fastened, to a degree that a support yoke is subjected to elastic compressive deformation. The steering system includes a rack shaft that makes linear motion inside a housing, a pinion shaft rotatably supported by the housing to mesh with the rack shaft, the support yoke housed in a guide hole provided in the housing so as to be able to move back and forth along the guide hole and supporting the rack shaft so as to allow the rack shaft to slide along an axial direction of the rack shaft, the yoke plug fixed with screw-threaded in the guide hole, and a bias member interposed between the yoke plug and the support yoke to bias the support yoke toward the rack shaft. In the loosening of the yoke plug, the yoke plug is loosened to a position where an axial position of the yoke plug reaches a target clearance between the support yoke and the yoke plug using, as a reference, a position where a rate of a change in the axial position of the yoke plug with reference to the support yoke increases rapidly with respect to a change in an angular position of the yoke plug

When the temporarily fastened yoke plug is loosened to the degree that the support yoke is subjected to elastic compressive deformation, the axial position of the support yoke initially increases slowly with respect to an increase in the angular position of the support yoke as the compressive deformation of the support yoke is released. After the compressive deformation of the support yoke is released, the axial position of the support yoke increases rapidly with respect to the increase in the angular position of the support yoke. This is because the axial position of the yoke plug increases with respect to the increase in angular position of the yoke plug according to a screw pitch.

Based on this, in the above-described adjustment method, the yoke plug is loosened to the position where the axial position of the yoke plug reaches the target clearance using, as a reference, a position where the compressive deformation of the support yoke is released (what is called a zero clearance position). Thus, the clearance between the support yoke and the yoke plug can be made closer to the target clearance. Therefore, a more appropriate clearance can be set.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described.

As depicted inFIG. 1representing a general configuration of a steering system to which the present invention is applied, a steering system10has a housing11fixed to a vehicle body. A rack shaft12passes through the housing11. The rack shaft12is supported so as to be slidable along an axial direction thereof (the direction orthogonal to the sheet ofFIG. 1) with respect to the housing11A pinion shaft13extending along a direction crossing the rack shaft12(an up-down direction inFIG. 1) passes through the housing11. The pinion shaft13is supported via a bearing14so as to be rotatable with respect to the housing11. The pinion shaft13rotates in conjunction with an operation of the steering wheel (not depicted in the drawings). The rack shaft12moves in the axial direction thereof in conjunction with the rotation of the pinion shaft13to change the direction of wheels (not depicted in the drawings).

In the housing11, in an area on the opposite side of the rack shaft12from the pinion shaft13, a cylindrical guide hole15is formed which extends along a direction orthogonal to the rack shaft12(a lateral direction inFIG. 1). The guide hole15is open toward the outside of the housing11.

In an inner peripheral surface of the guide hole15, an internal thread portion16is formed at an end of the guide hole15on the opening side. Inside the guide hole15, a support mechanism20is provided which allows the rack shaft12to be pressed against the pinion shaft13.

The support mechanism20has a support yoke21that supports a peripheral surface of the rack shaft12such that the rack shaft12is slidable, a yoke plug22screw-threaded in the internal thread portion16of the guide hole15, and a compression coil spring23interposed between the support yoke21and the yoke plug22to serve as a bias member.

The support yoke21is formed of a metal material such as aluminum and shaped to be cylindrical. The support yoke21moves back and forth along a depth direction of the guide hole15. An outer peripheral surface of the support yoke21is slidably guided on an inner peripheral surface of the guide hole15. Two annular grooves24and24are formed in the outer peripheral surface of the support yoke21. O rings25and25are fitted in the grooves24and24. The O rings25and25elastically contact the inner peripheral surface of the guide hole15to seal between the inner peripheral surface of the guide hole15and the outer peripheral surface of the support yoke21. The O rings25and25elastically support the support yoke21in a radial direction thereof. This suppresses possible backlash of the support yoke21.

In a side surface of the support yoke21, which is closer to the rack shaft12(a left side surface inFIG. 1), a recessed surface (circular arc surface)26is formed which is shaped along a peripheral surface of the rack shaft12. A sheet27that slidably contacts the rack shaft12is attached to the recess surface26. The sheet27is formed of a metal material, for example, bronze.

A spring housing hole28is formed in a side surface of the support yoke21, which is away from the rack shaft12(a right side surface inFIG. 1). One end (a left end inFIG. 1) of the compression coil spring23is inserted in the spring housing hole28. Thus, the compression coil spring23is restrained from moving in a radial direction thereof. The support yoke21is constantly biased toward the rack shaft12by an elastic force of the compression coil spring23interposed between an inner bottom surface of the spring housing hole28and a side surface of the yoke plug22, which is closer to the support yoke21(a left side surface inFIG. 1). The recessed surface26is pressed against the peripheral surface of the rack shaft12via the sheet27such that the rack shaft12is slidable.

The yoke plug22is formed of a metal material such as steel and shaped to be cylindrical. A cylindrical surface22aand an external thread portion22bare formed on an outer peripheral surface of the yoke plug22in this order from the support yoke21side. An annular groove29is formed in the cylindrical surface22a. An O ring30is fitted in the groove29.

An engagement hole31is formed in a side surface of the yoke plug22, which is away from the support yoke21(a right side surface inFIG. 1). The engagement hole31engages with a tool (not depicted in the drawings) used to rotationally operate the yoke plug22. An appropriate shape such as a hexagon or a dodecagon may be adopted for the engagement hole31depending on the shape of the tool. A through-hole32is formed in a bottom wall of the yoke plug22. The through-hole32allows the inside and the outside of the engagement hole31to communicate with each other. The yoke plug22is fixed in the housing11by inserting, into the guide hole15, an end of the yoke plug22at which the cylindrical surface22ais formed, while fastening the external thread portion22bto the internal thread portion16.

A clearance C is provided between the side surface of the support yoke21, which is closer to the yoke plug22(the right side surface inFIG. 1) and the side surface of the yoke plug22, which is closer to the support yoke21(the left side surface inFIG. 1). The amount of the clearance C is strictly managed through adjustment of a position to which the yoke plug22is screw-threaded.

Now, an adjustment apparatus for the clearance will be described. The clearance C is adjusted using, for example, an adjustment apparatus described below. The adjustment apparatus is used in an adjustment step for the clearance C included in assembly steps for the steering system10.

As depicted inFIG. 2, an adjustment apparatus40has a base41, two support legs42and42provided on the base to support the steering system10, and a driving unit43that rotates the yoke plug22.

The two support legs42and42support both ends of the housing11of the pre-assembled steering system10. In this regard, the steering system10is kept in an orientation in which, for example, the guide hole15faces opposite to the base41(upward inFIG. 1).

The driving unit43is provided, for example, in a frame (not depicted in the drawings) that forms a framework of the adjustment apparatus40. As depicted by a blank arrow inFIG. 2, the driving unit43moves along a lifting direction D that is a direction in which the driving unit43moves toward and away from the base41through operation of a lifting mechanism (not depicted in the drawings).

The driving unit43has a motor44, a driving gear46fixed to an output shaft45of the motor44, a driven gear47that meshes with the driving gear46, and a rotating socket48coupled to the driven gear47. The motor44is provided with a rotation sensor44a. The driven gear47is provided at an end of the rotating socket48, which is on the opposite side of the driven gear47from the base41(an upper end inFIG. 2).

At an end of the rotating socket48, which is on the base41side (a lower end inFIG. 2), an engaging protruding portion49is provided which functions as a tool that engages with the engagement hole31in the yoke plug22. For the engaging protruding portion49, an appropriate shape such as a hexagonal prism or a dodecagonal prism may be adopted which corresponds to the shape of the engagement hole31. When the clearance C is adjusted, the driving unit43is lowered to insert the engaging protruding portion49into the engagement hole31in the yoke plug22. Thus, the yoke plug22can rotate integrally with the rotating socket48. This is because the engaging protruding portion49engages with the engagement hole31in a rotating direction of the engaging protruding portion49.

Now, the rotating socket48will be described in detail.

As depicted inFIG. 3, the rotating socket48has a cylindrical socket member51, a cylindrical reference member52, and a rod-like gauge head53.

The socket member51has a through-hole54extending along an axial direction of the socket member51. The reference member52is inserted into the through-hole54. The reference member52is movable along an axial direction thereof and relative to the socket member51. Although not depicted in the drawings, a retaining structure is preferably provided between the reference member52and the socket member51to regulate falling of the reference member52. The length of the reference member52in the axial direction is set larger than the length of the socket member51in the axial direction. Thus, when a tip portion of the engaging protruding portion49is in abutting contact with the inner bottom surface of the yoke plug22, a leading end (a lower end inFIG. 3) of the reference member52is in abutting contact with the inner bottom surface of the yoke plug22while a trailing end (an upper end inFIG. 3) of the reference member52protrudes from a trailing end surface (an end surface away from the yoke plug22).

When a screw-thread position of the yoke plug22is changed, the reference member52moves along the axial direction together with the yoke plug22. During the movement, the leading end of the reference member52is also kept in abutting contact with the inner bottom surface of the yoke plug22. For example, when the yoke plug22is fastened, the yoke plug22moves closer to the support yoke21and the reference member52moves closer to the support yoke21along with the yoke plug22by gravity. When the yoke plug22is loosened, the yoke plug22moves away from the support yoke21and the reference member52is pushed up by the yoke plug22to move away from the support yoke21along with the yoke plug22.

The reference member52is shaped like a bottomed cylinder that is occluded at a trailing end portion thereof (an upper end portion inFIG. 3). The reference member52has an insertion hole55extending in the axial direction of the reference member52. The gauge head53passes through the insertion hole55. The gauge head53is movable along an axial direction thereof and relative to reference member52. Although not depicted in the drawings, a retaining structure is preferably provided between the gauge head53and the reference member52to regulate falling of the gauge head53. The length of the gauge head53in the axial direction is set slightly larger than the length of the reference member52in the axial direction. Thus, with the tip portion of the engaging protruding portion49in abutting contact with the inner bottom surface of the yoke plug22, a leading end of the gauge head53extends through the through-hole32in the yoke plug22and is in abutting contact with an inner bottom surface of the spring housing hole28. Regardless of the screw-thread position of the yoke plug22, the gauge head53is kept in abutting contact with the inner bottom surface of the spring housing hole28by gravity.

A linear gauge56serving as a displacement sensor is provided at the trailing end (the upper end inFIG. 3) of the reference member52. The linear gauge56has a case61fixed to the reference member52, a spindle62protruding from an end of the case61on the reference member52side, and a detector63provided inside the case61.

The spindle62is movable along an axial direction thereof and relative to the case61. A tip portion (a lower end inFIG. 3) of the spindle62penetrates a trailing end wall (an upper end wall inFIG. 3) of the reference member52with play around the spindle62and is inserted into the reference member52(into the insertion hole55). Inside the reference member52, the tip portion of the spindle62is in abutting contact with the upper end of the gauge head53.

With the tip portion of the engaging protruding portion49in abutting contact with the inner bottom surface of the yoke plug22, the spindle62basically does not move. For example, when the reference member52moves closer to the support yoke21(downward inFIG. 3), the case61moves closer to the support yoke21along with the reference member52. In contrast, the spindle62is in abutting contact with the inner bottom surface of the yoke plug22via the gauge head53. Thus, the spindle62is prevented from moving closer to the support yoke21and the spindle62enters the case61relative to the case61.

When the reference member52moves away from the support yoke21(upward inFIG. 3), the case61moves away from the support yoke21along with the reference member52. In contrast, the spindle62is movable relative to the case61and is kept in abutting contact with the upper end of the gauge head53by gravity. Thus, the spindle62is prevented from moving away from the support yoke21. The spindle62extends relative to the case61. The tip portion of the spindle62may be coupled to the upper end of the gauge head53.

The detector63generates an electric signal corresponding to the position of the spindle62with respect to the reference member52. The position of the spindle62with respect to the reference member52is equal to the position of the reference member52with reference to the inner bottom surface of the spring housing hole28and therefore to the axial position of the yoke plug22with reference to the support yoke21. The axial position of the yoke plug22is represented by a gap ΔG between a leading end surface (a lower end surface inFIG. 3) of the reference member52and a leading end surface (a lower end surface inFIG. 3) of the gauge head53.

Now, an electrical configuration of the adjustment apparatus40will be described. As depicted inFIG. 4, the adjustment apparatus40has a control apparatus71and a monitor72. The control apparatus71integrally controls various portions of the adjustment apparatus40. The control apparatus71is connected to the linear gauge56and to the rotation sensor44a. The control apparatus71is also connected to the motor44and to the monitor72. The monitor72displays various pieces of information through display control.

The control apparatus71controls driving of the motor44. The control apparatus71calculates the rotation amount of the motor44and thus the angular position θ of the yoke plug22with respect to the housing11based on an electric signal generated by the rotation sensor44a. Moreover, based on an electric signal generated by the linear gauge56(to be exact, the detector63), the control apparatus71calculates the gap ΔG between the leading end surface of the reference member52and the leading end surface of the gauge head53, in other words, the axial position G of the yoke plug22with reference to the support yoke21. The control apparatus71executes an adjustment process for the clearance C based on the relationship between the angular position θ of the yoke plug22and the axial position G of the yoke plug22. In executing the adjustment process for the clearance C, the control apparatus71allows the monitor72to display a graph representing the relationship between the angular position θ of the yoke plug22and the axial position G of the yoke plug22.

Now, a method for adjusting the clearance C will be described. The control apparatus71executes the adjustment process for the clearance C in accordance with a control program stored in a storage apparatus not depicted in the drawings. The adjustment process corresponds to one of the assembly steps for the steering system.

With the yoke plug22temporarily fastened in the guide hole15in advance, the steering system10is set between the two support legs42and42of the adjustment apparatus40. At this time, the yoke plug22is kept in abutting contact with the support yoke21. In this case, the clearance C between the yoke plug22and the support yoke21is zero, but the yoke plug22is expected to be fastened beyond a zero clearance position (zero touch position) to a zero clearance excess position to elastically compress the support yoke21. In this case, the yoke plug22is assumed to be temporarily fastened to the degree that the support yoke21is subjected to elastic compressive deformation.

The zero clearance position refers to the position where the yoke plug22just comes into contact with the support yoke21and where the axial force (pressing force) of the yoke plug22is prevented from acting on the support yoke21, which is thus subjected to no compressive deformation. The zero clearance excess position refers to a position where the yoke plug22is continuously fastened after coming into contact with the support yoke21to exert the axial force of the yoke plug22on the support yoke21, which is thus subjected to compressive deformation.

As illustrated in a flowchart inFIG. 5, the control apparatus71drives the motor44in a direction in which the temporarily fastened yoke plug22is loosened (step S101). The rotation speed of the motor44, that is, the rotation speed of the yoke plug22, is constant.

FIG. 6is a graph representing the relationship between the angular position θ of the yoke plug22to the housing11and the axial position G of the yoke plug22with reference to the support yoke21. In the graph, for the angular position θ of the yoke plug22, a change in a negative direction indicates that the yoke plug22is fastened, and a change in a positive direction indicates that the yoke plug22is loosened.

As depicted by a dashed line in the graph inFIG. 6, as the angular position θ of the yoke plug22increases from an angular position θ0obtained during the temporary fastening, the axial position G of the yoke plug22generally tends to increase linearly with a slight fluctuation. However, this increase in axial position G results from gradual release of compressive deformation (distortion) of the support yoke21. The rate of the change in axial position G with respect to the change in angular position θ has a smaller value than the rate varying according to the screw pitch of the external thread portion22bin a normal state. That is, the change gradient of the axial position G with respect to the increase in angular position θ is gentler than the change gradient in the normal state.

When the yoke plug22is further loosened, the change gradient of the axial position G with respect to the increase in angular position θ increases sharply in a circular arc pattern and eventually increases linearly with respect to the increase in angular position θ. At this time, the change gradient of the axial position G with respect to the increase in angular position θ varies according to the screw pitch of the external thread position22bof the yoke plug22. This indicates that the compressive deformation of the support yoke21is released.

As depicted in the flowchart inFIG. 5, the control apparatus71then calculates an inflection point P1(first time) (step S102). The inflection point P1is a boundary point where the change gradient of the axial position G. The control apparatus71determines two regression lines (approximate lines) L1and L2depicted by a long dashed short dashed line in the graph inFIG. 6, for example, using the least-square method based on the angular position θ and the axial position G. The regression line L1represents the changing trend of the axial position G in a compression area where the axial force of the yoke plug22acts on the support yoke21. The regression line L2represents the changing trend of the axial position G in a non-compression area where the compressive deformation of the yoke plug22is released to allow the axial position G to change according to the screw pitch of the external thread portion22b. The control apparatus71temporarily stores the intersection point between the regression lines L1and L2as the inflection point P1.

When the angular position θ of the yoke plug22reaches an angular position θ1corresponding to the inflection point P1, it can be assumed that the yoke plug22is located at the zero clearance position. Thus, the clearance C may be adjusted with reference to this position. Compared to the conventional technique that adjusts the clearance C using the screw-thread position of the yoke plug22during the temporary fastening as the zero clearance position, the present embodiment enables a more appropriate clearance C to be set. This is because the amount of compressive deformation of the support yoke21(in this case, an axial position G1corresponding to the inflection point P1with reference to an axial position G0obtained in the temporarily fastened state) is excluded from the adjusted clearance C.

However, for example, a temporary fastening toque varies according to product specifications or the like, and thus, the fluctuation range of the change in axial position G with respect to the change in angular position θ in the compression area is expected to increase with an increase in the temporary fastening torque. Furthermore, even among the same products, the state (the change gradient or the like) of the change in axial position G with respect to the change in angular position θ varies with an increase in the temporary fastening torque. Thus, a slight variation may occur in the inflection point P1(zero clearance position) and therefore the amount of the clearance C calculated by the control apparatus71.

Therefore, to further reduce variation in clearance C, the control apparatus71continues to execute the following process. As illustrated in the flowchart inFIG. 5, the control apparatus71calculates the inflection point P1in the above-described step S102, and then drives the motor44in the direction in which the yoke plug22is fastened again (step S103).

As depicted in the graph inFIG. 6, fastening of the yoke plug22is started, for example, at a timing when the yoke plug22is loosened to an angular position θ2with a value larger than the value of the angular position θ1corresponding to the inflection point P1. As depicted by blank arrows in the graph inFIG. 6, the axial position G decreases with a decrease in the angular position θ.

As illustrated in the flowchart inFIG. 5, the control apparatus71then detects that the yoke plug22has been fastened by a set angle δθ (approximately 20°) with reference to the angular position θ1corresponding to the inflection point P1(S104). At this time, the angular position θ of the yoke plug22exhibits an angular position θ3that is larger than the angular position θ0obtained during the temporary fastening. The axial force acting on the support yoke21(the axial force of the yoke plug22) is weaker than the axial force exerted during the temporary fastening. According to the weak axial force, the support yoke21is subjected to slight compressive deformation. Subsequently, the control apparatus71drives the motor44in the direction in which the yoke plug22is loosened again (S105).

As depicted by filled-in arrows in the graph inFIG. 6, the axial position G again increases linearly like a gentle slope with a slight fluctuation as the angular position θ of the yoke plug22increases. The increasing gradient of the axial position G in this case is slightly larger than the increasing gradient of the axial position G obtained when the temporarily fastened yoke plug22is loosened as depicted by a solid arrow in the graph inFIG. 6. This is because the amount of compressive deformation of the support yoke21is smaller than the amount of compressive deformation occurring during the temporary fastening. Subsequently, the axial position G increases in a circular arc pattern with the increase in the angular position θ and eventually increases linearly with the increase in the angular position θ.

As illustrated in the flowchart inFIG. 5, the control apparatus71then calculates the inflection point (second time) (S106). The control apparatus71determines two regression lines L3and L4depicted by a long dashed short dashed line in the graph inFIG. 6using the least-square method as is the case with the above-described step S102. The regression line L3represents the changing trend of the axial position G in the compression area similarly to the above-described regression line L1. However, the slope of the regression line L3is slightly steeper than the slope of the above-described regression line L1. The regression line L4represents the changing trend of the axial position G in the non-compression area similarly to the above-described regression line L2. The regression line L4substantially matches the above-described regression line L2, and thus in the graph inFIG. 6, is denoted by a parenthesized reference numeral added to the regression line L2. The control apparatus71temporarily stores the intersection point between the regression lines L3and L4as an inflection point P2.

As illustrated in the flowchart inFIG. 5, the control apparatus71then recognizes that the yoke plug22is located at the true zero clearance position when the angular position θ of the yoke plug22reaches an angular position θ4corresponding to the second inflection point P2. The control apparatus71then detects that the axial position G has increased by an amount equal to the target clearance C* with reference to an axial position G2corresponding to the inflection point P2(step S107). Specifically, as illustrated in the graph inFIG. 6, an axial position G3is detected which reflects a value resulting from addition of the target clearance C* to the axial position G2corresponding to the inflection point P2.

Thus, a more accurate inflection point P2is calculated by temporarily releasing the axial force on the support yoke21, fastening the yoke plug22more loosely than during the temporary fastening, and loosening the yoke plug22again. When the angular position θ corresponding to the second inflection point P2is reached, the yoke plug22is positioned even closer to the true zero clearance position. This is because the amount of compressive deformation of the support yoke21(the difference between the axial position G0in the temporarily fastened state and the axial position G2corresponding to the inflection point P2) is excluded from the adjusted clearance C. The yoke plug22is loosened by the target clearance C* with reference to the position more approximate to the true zero clearance position so that the clearance C can be made even closer to the target clearance C*. Thus, a more appropriate clearance C can be achieved.

Finally, the yoke plug22is fixed to the housing11. If the yoke plug22used is of a self-lock type, the fixation to the housing11is completed when the adjustment of the clearance C is completed. The self-lock type refers to a type in which the yoke plug22is shaped like an imperfectly circular cylinder and a threaded portion formed on an outer peripheral surface of the yoke plug22is fixed by friction caused by fastening the threaded portion. If the yoke plug22without a self-lock function is used, the yoke plug22is fixed to the housing11by caulking the yoke plug22or applying an adhesive to the yoke plug22after the adjustment operation for the clearance C is completed. The assembly operation for the steering system10is thus completed.

Therefore, the present embodiment can produce the following effects.

(1) When the yoke plug22is loosened which has been temporarily fastened to the degree that the support yoke21is subjected to elastic compressive deformation, the yoke plug22is loosened to the position where the axial position G of the yoke plug22reaches the target clearance C* using, as a reference, the position where the rate of change in the axial position G of the yoke plug22with respect to the change in the angular position θ of the yoke plug22increases rapidly. Thus, the clearance C can be made even closer to the target clearance C*. Therefore, a more appropriate clearance C can be set. A variation in clearance C among the products can be suppressed.

(2) Depending on the magnitude of the temporary fastening torque, the clearance C can be adjusted using the first-time inflection point P. A higher adjustment accuracy for the clearance C can be achieved when the temporary fastening torque has a smaller value. This is because the axial force of the yoke plug22exerts a smaller effect (a smaller amount of compressive deformation) on the adjustment accuracy for the clearance C.

(3) When the temporarily fastened yoke plug22is loosened, behavior exhibited by the support yoke21when the elastic compressive deformation of the support yoke21is released varies even among the same products. Consequently, the first-time inflection point P may vary. Thus, the yoke plug22is fastened more loosely than during the temporary fastening using the inflection point P as a reference. With the axial force on the support yoke21being lower than the axial force exerted during the temporary fastening, the yoke plug22is loosened again, and the second-time inflection point P2is calculated. The second-time inflection point P2is more accurate than the first-time inflection point P1. Thus, the clearance C is adjusted using the second-time inflection point P2as a reference so that a more appropriate clearance C can be set.

(4) Since a more accurate zero clearance position (inflection point P2) is determined, the clearance C can be set to as large a value as possible within an acceptable range in a pinpoint manner. A small clearance C may increase a needed steering torque. In this regard, by increasing the clearance to the vicinity of the limit of the acceptable range, it is possible to suppress an increase in steering torque.

(5) A more accurate zero clearance position is used as a reference to enable the clearance C to be more easily and more quickly adjusted.

(6) The rotating socket48is divided into three pieces. That is, the rotating socket48has the socket member51that allows the yoke plug22to be rotated, the reference member52that serves as a reference for measurement of the axial position G of the yoke plug22, and the gauge head53. The socket member51, the reference member52, and the gauge head53are independent of one another. The reference member52can move independently of the socket member51without being affected by movement of the socket member51. This prevents the state of contact between the reference member52and the yoke plug22from being fluctuated under the effect of the movement of the socket member51. Therefore, the axial position G of the yoke plug22can be more accurately detected.

(7) When the graph depicted inFIG. 6is displayed on the monitor72, an operator can visually check changes in axial position G with respect to the angular position θ, the zero clearance position (inflection points P1and P2), and the like.

The above-described embodiment may be modified into another embodiment. That is, an electric power steering system (EPS) can be constructed by providing the steering system10with an assist motor that is a source of a steering assist force.