Steering system

In a steering system including a lock mechanism, at a time of locking, a first cam that is rotated together with a lock shaft generates a pressing force for pushing up an inner tube in an upward tilt direction. A second cam that is rotated together with the lock shaft presses an abutting portion of an urging member in a downward tilt direction. The abutting portion of the urging member applies a pressing reaction force in the upward tilt direction, to the second cam. The pressing reaction force serves as a force for pushing up the first cam in the upward tilt direction, through the second cam and the lock shaft.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2012-260174 filed on Nov. 28, 2012 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.

2. Description of Related Art

There has been proposed a steering system including a telescopic mechanism constituted by an outer tube and an inner tube that are fitted to each other so as to be slidable relative to each other, wherein the steering system has a telescopic lock function of restricting relative slide movement in the telescopic mechanism, and wherein a single eccentric cam, which is rotated together with a lock shaft in accordance with the operation of an operation lever, presses an outer periphery of the inner tube through an opening formed in the outer tube so as to achieve telescopic lock. For example, refer to Japanese Patent Application Publication No. 2008-132819 (JP 2008-132819 A) and Japanese Patent Application Publication No. 2010-30579 (JP 2010-30579 A). In the steering system described in JP 2010-30579 A, a balance spring is provided for supporting the weight of a steering column at the time of unlocking, in order to assist tilt operation. One end of the balance spring is locked to a fixing bracket that is secured to a vehicle body, and an abutting portion of the balance spring abuts on a ring fitted to an outer periphery of the eccentric cam. The balance spring urges the steering column in an upward tilt direction through the eccentric cam and the lock shaft that is rotated together with the eccentric cam.

However, there is a possibility that a force for maintaining the locked condition may be weakened at the time of locking, due to variation in dimensional accuracy among individual components. In more detail, the rotation angle position of the eccentric cam varies at the time of locking, due to the variation in dimensional accuracy among individual components. Accordingly, the direction of load due to the torque for rotating the operation lever varies, and thus, a push-up force applied by the eccentric cam varies. As a result, there is a possibility that a required push-up force may not be obtained.

A plurality of rings which are to be fitted to the eccentric cam, and which have different outer diameters in multiple specifications may be prepared, and a specific one among these rings in the multiple specifications may be selected for an individual steering system so that the eccentric cam can be held at an appropriate rotation angle position, and a required push-up force, that is, a required maintaining force can be obtained. In this case, however, a component cost and an assembly cost are increased, and as a result, a manufacturing cost is increased.

SUMMARY OF THE INVENTION

An object of the invention is to provide, at a low cost, a steering system in which a sufficient lock maintaining force is ensured, in spite of variation in dimensional accuracy.

According to an aspect of the invention, there is provided a steering system including: a steering column including an outer tube and an inner tube that are fitted to each other so as to be axially slidable relative to each other, and that support a steering shaft, the steering column being tiltable around a tilt center axis; a fixing bracket including a pair of side panels opposed to each other; a movable bracket including a pair of side panels fixed to the outer tube and opposed respectively to the side panels of the fixing bracket; a lock mechanism including a lock shaft inserted through insertion holes in the side panels of the fixing bracket and insertion holes in the side panels of the movable bracket, the lock mechanism achieving locking by bringing each of the side panels of the fixing bracket into pressure contact with a corresponding one of the side panels of the movable bracket in accordance with an operation of an operation lever that is rotated together with the lock shaft; and an urging member that urges the steering column in an upward tilt direction. The lock mechanism further includes a first cam that is rotatable together with the lock shaft, and a second cam that has a cam shape different from a cam shape of the first cam, and that is rotatable together with the lock shaft. The urging member includes a locked portion locked to a fixing member, and an abutting portion that abuts on the second cam. At a time of locking by the lock mechanism, the first cam pushes up the inner tube in the upward tilt direction through an opening in the outer tube, and the urging member pushes up the second cam in the upward tilt direction.

According to the above-described aspect, at the time of locking by the lock mechanism, the first cam that is rotated together with the lock shaft generates a pressing force for pushing up the inner tube in the upward tilt direction. Further, at the time of locking by the lock mechanism, the abutting portion of the urging member applies a pressing reaction force in the upward tilt direction, to the second cam. The pressing reaction force serves as a force for pushing up the first cam in the upward tilt direction, through the second cam and the lock shaft. This pressing reaction force is superposed on the pressing force with which the first cam itself pushes up the inner tube. Thus, even if there is variation in dimension accuracy among individual components, it is possible to ensure a sufficient lock maintaining force for the inner tube. Further, since the urging force of the urging member that is originally provided for supporting the weight of the steering column at the time of unlocking is applied to the second cam so as to enhance the force for maintaining the locked condition, the structure of the steering system can be simplified and a manufacturing cost can be reduced.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be hereinafter specifically described with reference to the accompanying drawings. Referring toFIG. 1, a steering system1includes a steering member2such as a steering wheel, and a steering mechanism3that steers steered wheels (not shown) in accordance with a steering operation of the steering member2. For example, a rack and pinion mechanism is used as the steering mechanism3.

The steering member2is mechanically connected to the steering mechanism3through a steering shaft4, an intermediate shaft5and the like. The rotation of the steering member2is transmitted to the steering mechanism3through the steering shaft4and the intermediate shaft5and the like. The rotation transmitted to the steering mechanism3is converted to axial movement of a rack shaft (not shown). Thus, the steered wheels are steered.

The steering shaft4includes a tubular upper shaft6and a tubular lower shaft7that are fitted to each other through spline fitting, serration fitting or the like so that the upper shaft6and the lower shaft7are slidable relative to each other. The steering shaft4can be extended and contracted in an axial direction X1 of the steering shaft4. The steering member2is connected to an upper end portion of the upper shaft6in the axial direction X1. Further, the steering shaft4is inserted through a tubular steering column8, and is rotatably supported by the steering column8through a plurality of bearings9,10.

The steering column8includes an outer tube11as an upper tube and an inner tube12as a lower tube. The outer tube11and the inner tube12are fitted to each other so as to be slidable relative to each other. The steering column8can be extended and contracted in the axial direction X1. The outer tube11supports the upper shaft6through the bearing9so that the upper shaft6is rotatable. The outer tube11is coupled to the upper shaft6through the bearing9so as to be movable in the axial direction X1 of the steering shaft4together with the upper shaft6.

A lower side movable bracket13is secured to an outer periphery of the inner tube12so that the lower side movable bracket13is movable together with the inner tube12. The lower side movable bracket13is supported by a lower side fixing bracket15secured to the vehicle body14through a tilt center shaft16so that the lower side movable bracket13is rotatable. Thus, the steering column8and the steering shaft4are rotatable (tiltable) around the tilt center shaft16as a fulcrum. By rotating (tilting) the steering shaft4and the steering column8around the tilt center shaft16as a fulcrum, adjustment of the position of the steering member2, that is, so-called tilt adjustment can be performed. Further, by extending or contracting the steering shaft4and the steering column8in the axial direction X1, adjustment of the position of the steering member2, that is, so-called telescopic adjustment can be performed.

An upper side movable bracket17that corresponds to a distance bracket is secured to the outer tube11. The upper side movable bracket17is movable together with the outer tube11. Further, an upper side fixing bracket18is secured to the vehicle body14. When the movable bracket17and the fixing bracket18are locked together by a lock mechanism20including a lock shaft19that is inserted through the both brackets17,18, the position of the steering column8is fixed with respect to the vehicle body14, and accordingly the position of the steering member2is fixed.

Further, the lock mechanism20includes a first cam21that is rotatable together with the lock shaft19, and a pair of second cams22(only one of which is shown inFIG. 1) that are rotatable together with the lock shaft19. Each of the second cams22has a cam shape that is different from a cam shape of the first cam21. When an operation lever23that is rotated together with the lock shaft19is operated to rotate the lock shaft19in a locking direction that corresponds to a rotating direction at the time of locking, the first cam21that is rotated together with the lock shaft19pushes up the inner tube12in an upward tilt direction Y1, through an opening24formed in the outer tube11, so as to press the inner tube12against the outer tube11. Thus, the inner tube12is fixed to the outer tube11.

The steering system1includes an urging member25that urges the steering column8in the upward tilt direction Y1 so as to support the weight of the steering column8at the time of unlocking by the lock mechanism20. The urging member25includes torsion coil springs. Further, the urging member25includes locked portions26fixed to the fixing bracket18and abutting portions27that abut on the second cams22. The urging member25urges the steering column8in the upward tilt direction Y1 through the second cams22, the lock shaft19and the movable bracket17.

When the lock shaft19is rotated by operating the operation lever23, the second cams22that are rotated together with the lock shaft19press the abutting portions27of the urging member25in a downward tilt direction Y2 that is opposite to the direction of urging by the urging member25. On the other hand, the urging member25applies a pressing reaction force in the upward tilt direction Y1 to the second cams22through the abutting portions27. The pressing reaction force acts upon the inner tube12through the second cams22, the lock shaft19and the first cam21so that the inner tube12is pushed up and is pressed against the outer tube11.

FIG. 2is a sectional view taken along the line II-II inFIG. 1for illustrating the steering system1. Referring toFIG. 2, the movable bracket17is a groove-shaped member that is opened upward inFIG. 2. The movable bracket17has a bilaterally symmetric shape. Specifically, the movable bracket17includes a pair of side panels28opposed to each other, and a connecting panel29that connects respective ends of the side panels28(respective lower ends of the side panels28inFIG. 2).

A lock shaft insertion hole30is formed in each of the side panels28. The lock shaft insertion hole30is an elongated hole for telescopic operation that extends in the axial direction X1 (direction orthogonal to the paper surface ofFIG. 2). Thus, the steering shaft4and the steering column8can be extended and contracted within a range corresponding to a longitudinal length (length in the axial direction X1) of the lock shaft insertion hole30. The respective other ends (respective upper ends inFIG. 2) of the side panels28are secured to an outer peripheral surface of the outer tube11.

The fixing bracket18includes a pair of side panels31opposed to each other, a connecting panel32that connects respective ends (respective upper ends inFIG. 2) of the side panels31, a plate-shaped attachment stay33that is secured to an upper surface of the connecting panel32, and a pair of reinforcing extension panels34extending outward respectively from edge portions of the side panels31so as to be substantially orthogonal to the side panels31. The side panels31and the connecting panel32define a groove-shaped portion that is opened downward inFIG. 2. The steering shaft4, the steering column8and the movable bracket17are located between the side panels31of the fixing bracket18.

The fixing bracket18is secured to the vehicle body14through a pair of attachment members35connected to the attachment stay33. The attachment members35are connected to the attachment stay33by synthetic resin pins36that extend through the attachment stay33and that can be broken at the time of impact absorption. The attachment members35are secured to the vehicle body14by fixing bolts37. Locking holes38are respectively formed in the extension panels34of the fixing bracket18. The locking holes38serve as locking portions in which the locked portions26of the urging member25are hooked to be locked, respectively. An inner surface31bof each of the side panels31of the fixing bracket18is opposed to an outer surface28aof a corresponding one of the side panels28of the movable bracket17.

Referring toFIG. 3, the urging member25includes a pair of coil springs60symmetrically arranged, that is, the urging member25is formed of the so-called double torsion wire spring. The coil springs60have a common center axis C2. Each of the coil springs60has a first extension portion61that linearly extends from one end coil portion60a, that is, an outer end coil portion of the coil spring60, and a second extension portion62that extends from a distal end of the first extension portion61in a direction orthogonal to the first extension portion61. The locked portion26to be locked in the locking hole38in the corresponding extension panel34of the fixing bracket18is provided at the distal end of the second extension portion62. The locked portion26is formed so as to be bent into a crank shape.

Each of the coil springs60has a third extension portion63that linearly extends from the other end coil portion60b, that is, an inner end coil portion, and a fourth extension portion64that extends inward from the third extension portion63toward the other of the coil springs60. Fifth extension portions65as a pair of straight portions extend from opposed distal ends of the fourth extension portions64of the coil springs60. The fifth extension portions64extend in a direction orthogonal to the fourth extension portions64, and extend in parallel with each other. Distal ends of the fifth extension portions65are connected to each other through a connecting portion66. The abutting portions27are respectively provided in the fifth extension portions65(straight portions). Each of the abutting portions27abuts on a corresponding one of the second cams22.

Referring toFIG. 2, a lock shaft insertion hole39for tilt operation is formed in each of the side panels31. The lock shaft insertion hole39is an elongated hole that extends in the upward and downward tilt directions Y1, Y2 (up-down direction inFIG. 2). The lock shaft19of the lock mechanism20is inserted through the lock shaft insertion holes30that are the elongated holes for telescopic operation and through the lock shaft insertion holes39that are the elongated holes for tilting operation. The lock mechanism20holds the fixing bracket18so that the movable bracket17is locked by the fixing bracket18. In addition, the lock mechanism20presses the inner tube12so that the inner tube12is locked to the outer tube11.

Specifically, the lock mechanism20includes the lock shaft19that is rotatable together with the operation lever23; a nut40screwed to a thread portion formed in one end portion of the lock shaft19; an annular cam41and an annular cam follower42that are fitted to an outer periphery of a shaft portion19bof the lock shaft19so as to fasten the side panels28,31; the first cam21that presses the inner tube12in the upward tilt direction Y1; and the second cams22that press the abutting portions27of the urging member25in the downward tilt direction Y2.

The cam41and the cam follower42constitute a cam mechanism43for fastening the side panels28,31. A plurality of cam protrusions (which are not shown) are formed on each of opposed surfaces of the cam41and the cam follower42, and the cam protrusions on the opposed surfaces are meshed with each other. The cam41and the cam follower42are arranged in the vicinity of a head portion19aof the lock shaft19. The cam41and the operation lever23are coupled with the head portion19aof the lock shaft19so as to be rotatable together with the head portion19a.

The cam follower42is fitted to an outer periphery of the shaft portion19bof the lock shaft19so as to be rotatable relative to the lock shaft19. The cam follower42has a first portion421and a second portion422. The first portion421of the cam follower42is arranged along an outer surface31aof one of the side panels31of the fixing bracket18. The second portion422of the cam follower42is fitted in the lock shaft insertion hole39in the one of the side panels31of the fixing bracket18and the lock shaft insertion hole30in one of the side panels28of the movable bracket17so as to be movable in directions in which the lock shaft insertion holes30,39extend. The rotation of the second portion422is restricted by the lock shaft insertion hole39since width across flats or the like are formed in a portion of the second portion422, the portion of the second portion422being fitted in the elongated lock shaft insertion hole39in the one of the side panels31.

A first intervening member44and a second intervening member45are interposed between the nut40screwed to one end portion of the lock shaft19, and the other side panel31of the fixing bracket18. The first intervening member44has a first portion441and a second portion442. The first portion441of the first intervening member44is arranged along the outer surface31aof the other side panel31of the fixing bracket18. The second portion442of the first intervening member44is fitted in the lock shaft insertion hole39in the other side panel31of the fixing bracket18and the lock shaft insertion hole30in the other side panel28of the movable bracket17so as to be movable along the directions in which the lock shaft insertion hole30and the lock shaft insertion hole39extend. The rotation of the second portion442is restricted by the lock shaft insertion hole39since width across flats or the like are formed in a portion of the second portion442, the portion of the second portion442being fitted in the elongated lock shaft insertion hole39in the other side panel31.

The second intervening member45includes a thrust washer46interposed between the first portion441of the first intervening member44and the nut40, and a needle roller thrust bearing47interposed between the thrust washer46and the first portion441of the first intervening member44. The nut40is smoothly rotated together with the lock shaft19due to the function of the second intervening member45including the needle roller thrust bearing47.

When the lock shaft19is rotated by the rotating operation of the operation lever23, the cam41moves the cam follower42toward the one of the side panels31of the fixing bracket18. Thus, the first portion421of the cam follower42and the second portion442of the first intervening member44hold the side panels31of the fixing bracket18from outside the side panels31so as to bring each of the side panels31of the fixing bracket18into pressure contact with a corresponding one of the side panels28of the movable bracket17. Accordingly, the movable bracket17is locked by the cam follower42and the first intervening member44in cooperation with the fixing bracket18.

The inner tube12includes a metal tube48and a resin tube49fitted to an outer periphery of the metal tube48. A plurality of protruding portions50are formed at the resin tube49such that the protruding portions50are spaced from each other in a circumferential direction Z1 of the resin tube49. Further, the protruding portions50are formed at plural positions that are spaced from each other in the axial direction of the resin tube49, though not shown in the figure. The resin tube49may be eliminated, and the protruding portions50may be formed at the outer periphery of the metal tube48.

As shown inFIG. 2andFIG. 4that is an enlarged view, the first cam21is an annular member that is fitted to and secured to the outer periphery of the shaft portion19bof the lock shaft19at a substantially center portion of the shaft portion19bso as to be rotatable together with the lock shaft19. As shown inFIG. 2, a portion of the first cam21enters the outer tube11through an opening24formed in the outer tube11. As shown inFIG. 5, a cam surface21athat is a portion of an outer periphery of the first cam21has such an arc cam shape that a distance D1 from the center axis C1 of the lock shaft19to the cam surface21ais continuously changed, with respect to rotating directions R1, R2 of the lock shaft19. The cam shape of the cam surface21ais such that the distance D1 from a first contact point P1, at which the cam surface21acontacts the metal tube48of the inner tube12, to the center axis C1 is continuously increased as the lock shaft19is rotated in the rotating direction R1 at the time of locking (corresponding to the locking direction). Accordingly, when the lock shaft19is rotated in the rotating direction R1 at the time of locking, a pressing force F1, with which the cam surface21apushes up the metal tube48of the inner tube12, is generated.

The center (not shown) of the arc of the cam shape is located offset from the center axis C1 of the lock shaft19. Accordingly, the first cam21can be referred to as “eccentric cam” as long as the cam shape of the first cam21is an arc cam shape. When the cam shape of the first cam21is the arc cam shape, the first cam21has an advantage that the first cam21can be easily produced. However, the cam shape need not necessarily be the arc cam shape in the present invention, if the distance D1 between the first contact point P1 and the center axis C1 is continuously increased as the lock shaft19is rotated in the rotating direction R1 at the time of locking.

As shown inFIG. 2andFIG. 4, the cam surface21aof the first cam21has a concave surface that matches the shape of an outer peripheral surface of the metal tube48in an axial direction K1 of the lock shaft19. The first cam21comes into contact with or moves away from the outer peripheral surface of the metal tube48in accordance with the operation of the operation lever23. As shown inFIG. 2andFIG. 4, the second cams22are arranged on respective sides of the first cam21so that the first cam21is interposed between the second cams22in the axial direction K1 of the lock shaft19. The second cams22have the same shape and are arranged to have the same phase as viewed in the axial direction K1 of the lock shaft19. As shown inFIG. 5, each of the second cams22is an annular member that is fitted to the outer periphery of the lock shaft19so as to be rotatable together with the lock shaft19, and a cam surface22aat an outer periphery of the second cam22has a cam shape that is different from the cam shape of the cam surface21aof the first cam21, as viewed in the axial direction of the lock shaft19(direction orthogonal to the paper surface ofFIG. 5).

As shown inFIG. 4, each of the second cams22has a pair of flanges22bformed at axially opposite ends of the second cam22so as to circumferentially extend in at least a portion of the outer periphery of the second cam22, and to project radially outward in order to prevent the abutting portion27of the urging member25from coming off from the cam surface22aof the second cam22. The cam surface22ahas a concave surface shape between the flanges22b. At the time of locking by the lock mechanism20as shown inFIG. 5, a pressing reaction force G1 applied onto the second cam22from the abutting portion27of the urging member25generates a moment for rotating the lock shaft19in the rotating direction R1 at the time of locking.

Specifically, at the time of locking by the lock mechanism20, as shown inFIG. 5, the position of a second contact point P2, at which the second cam22contacts the abutting portion27of the urging member25, is located ahead of a crossing point Q1 at which a plane PP crosses the urging member25, in the rotating direction R1 of the lock shaft19at the time of locking, as viewed in the axial direction K1 (direction orthogonal to the paper surface ofFIG. 5) of the lock shaft19, the plane PP including the center axis C1 of the lock shaft19and the first contact point P1 at which the cam surface21aof the cam21contacts the metal tube48of the inner tube12.

The pressing reaction force G1 has a first component G11 that is directed toward the center axis C1 of the lock shaft19, and a second component G12 that is orthogonal to the first component G11. The first component G11 does not generate the moment for rotating the lock shaft19. The second component G12 generates a moment M1 having the value that is obtained by multiplying the distance between the center axis C1 of the lock shaft19and the second contact point P2 by the value of the second component G12, and acting in the rotating direction R1 at the time of locking.

According to the embodiment, at the time of locking by the lock mechanism20, the first cam21rotating together with the lock shaft19generates the pressing force F1 that pushes up the inner tube12in the upward tilt direction Y1. Further, at the time of locking by the lock mechanism20, the second cams22rotating together with the lock shaft19press the abutting portions27of the urging member25in the downward tilt direction Y2 (corresponding to a direction in which the elastic repelling force of the torsion coil springs60in the urging member25is increased), and in reaction to this, the abutting portions27of the urging member25apply the pressing reaction force G1 in the upward tilt direction Y1, to the second cams22.

The pressing reaction force G1 serves as a force for pushing up the first cam21in the upward tilt direction Y1 through the second cams22and the lock shaft19, and this pressing reaction force G1 is superposed on the pressing force F1 with which the first cam21itself pushes up the inner tube12. Thus, even if there is variation in dimension accuracy among individual components, it is possible to ensure a sufficient lock maintaining force for the inner tube12. Since the urging force of the urging member25that is originally provided for supporting the weight of the steering column8at the time of unlocking is applied to the second cams22so as to enhance the force for maintaining the locked condition, the structure of the steering system can be simplified and a manufacturing cost can be reduced.

The pressing reaction force G1 from the urging member25generates the moment M1 for rotating the lock shaft19in the rotating direction R1 at the time of locking, through the second cams22. Therefore, the lock is hardly loosened. That is, the force for maintaining the locked condition can be enhanced. Specifically, at the time of locking by the lock mechanism20, the second contact point P2 between the second cam22and the urging member25is located ahead of the crossing point Q1 at which the plane PP crosses the urging member25, in the rotating direction R1 of the lock shaft19at the time of locking, the plane PP including the first contact point P1 between the first cam21and the inner tube12, and the center axis C1 of the lock shaft19. Accordingly, the moment M1 for rotating the lock shaft19in the rotating direction R1 at the time of locking can be generated by the pressing reaction force G1 from the urging member25.

Since the crossing point Q1 is separated from the second contact point P2 by a predetermined distance, the moment M1 for rotating the lock shaft19in the rotating direction R1 at the time of locking can be generated even if the second contact point P2 is shifted to a certain extent toward the crossing point Q1 as compared to the condition shown inFIG. 5, due to variation in dimensional accuracy. It is preferable that a plane including the center axis C1 and the second contact point P2 should extend at an angle in a range of, for example, 20 to 50 degrees with respect to the above-described plane PP.

The second cams22are located on respective sides of the first cam21, abut respectively on the abutting portions27of the fifth extension portions65that are the straight portions of the urging member25formed by the wire spring. Since the variation in the spring characteristics of the fifth extension portions65that are the straight portions of the wire spring is smaller than the variation in the spring characteristics of the connecting portion66connecting the fifth extension portions65, it is possible to suppress the variation in the force for maintaining the locked condition and the variation in the operating force for the operation lever23.

FIG. 6shows another embodiment of the present invention. This embodiment is different from the embodiment shown inFIG. 4in the following points. In the embodiment shown inFIG. 4, the first cam21is formed as the member that is separate from the second cams22, and is secured to the lock shaft19. In contrast, in this embodiment, a first cam21U and a pair of second cams22U constitute a cam unit U1 that is formed of a single material as an integrated body, in order to simplify the configuration. In this embodiment, the same reference numerals as in the embodiment shown inFIG. 4are assigned to the same constituent elements as in the embodiment shown inFIG. 4.

FIG. 7shows yet another embodiment of the present invention, in which a modified example of the second cams is used. Referring toFIG. 7, the difference between the this embodiment and the embodiment shown inFIG. 4is that the second cams22are directly supported by the lock shaft19in the embodiment shown inFIG. 4while annular second cams70are supported by bearings71that are held at the outer periphery of the lock shaft19in this embodiment.

A cam surface70ahaving a circular sectional shape is formed at the outer periphery of each of the second cams70. Each of the bearings71includes an inner ring72that is fitted to the outer periphery of the shaft portion19bof the lock shaft19so as to be rotatable together with the lock shaft19, an outer ring73that is fitted at an inner periphery70bof the second cam70so as to be rotatable together with the second cam70, and rolling elements74provided between the inner ring72and the outer ring73. A first direction A1 and a second direction A2 are defined as two directions that are orthogonal to each other. A center C3 of an outer periphery72aof the inner ring72is offset from the center axis C1 of the lock shaft19by an offset amount e1 in the first direction A1. A center C4 of a circle defined by the cam surface70aat the outer periphery of the second cam70is offset from the center C3 of a circle defined by the outer periphery72aof the inner ring72by an offset amount e2 in the second direction A2. The center of the inner periphery72bof the inner ring72is coincident with the center axis C1 of the lock shaft19. Both the outer periphery73aand the inner periphery73bof the outer ring73are concentric with each other.

According to this embodiment, even if the outer ring73fitted to the second cam70receives a force, the force is hardly transmitted to the inner ring72fitted to the outer periphery of the lock shaft19, and accordingly, the rotating resistance of the second cam70can be decreased. Thus, it is possible to suppress an increase in the operating force for the operation lever23while enhancing the force for maintaining the locked condition. Although not shown, the second cam70and the outer ring73may be formed of a single member, as an integrated body.

The present invention is be limited to the above-described embodiments, and various modifications can be made to the above-described embodiments.