Patent Description:
An electric actuator includes an electric motor capable of rotating forward and backward and a motion conversion mechanism configured to convert rotation of the electric motor into a reciprocating linear motion of a screw shaft. When a nut of the motion conversion mechanism is rotated forward and backward by the electric motor, the screw shaft threadedly inserted through the nut in a back and forth movable manner moves in a reciprocating linear motion. Such an electric actuator is used in, for example, an electric parking brake device of a vehicle (see <CIT>).

In an electric parking brake device including such an electric actuator, when a parking brake lever provided in a drum brake moves from a release position to an operating position by a reciprocating linear motion of a screw shaft of the electric actuator, brake shoes are pressed against an inner peripheral surface of a brake drum that rotates together with a wheel so that a frictional resistance force generated therebetween brakes the rotation of the wheel, bringing the vehicle into a parking brake state. Further, when the electric motor of the electric actuator is rotated backward from the parking brake state, the parking brake lever moves from the operating position to the release position so that the brake shoes move away from the inner peripheral surface of the brake drum, releasing the vehicle from the parking brake state.

Incidentally, in a motion conversion mechanism of an electric actuator, a screw shaft screw threadedly inserted through a nut moves in a reciprocating linear motion relative to the nut. Thus, there is a problem in that sliding friction is generated at the threaded engagement portion between the screw shaft and the nut, which causes wear at the threaded engagement portion.

In view of this, in <CIT>, there is proposed a configuration in which an oil groove is formed at a female thread portion of a nut threadedly engaged with a male thread portion of a screw shaft and lubricating oil is collected in the oil groove. With such a configuration, the lubricating oil collected in the oil groove is drawn out from the oil groove by the relative rotation of the screw shaft and the nut, and the lubricating oil is supplied to the entire threaded engagement portion between the male thread portion of the screw shaft and the female thread portion of the nut, thereby lubricating the engagement portion.

Further, in <CIT>, there is proposed a configuration in which a solid lubricant is embedded in a screw sleeve (nut) that threadedly engages with a screw spindle (screw shaft), and a solid lubricant stop plate is provided at upper and lower ends of the screw sleeve. With such a configuration, the solid lubricant stop plate prevents the solid lubricant embedded in the screw sleeve escape from exiting the screw sleeve, enabling stable lubrication of the threaded engagement portion between the screw spindle and the screw sleeve by the solid lubricant.

<CIT> is related to the preamble of claim <NUM>.

Incidentally, with the electric actuator proposed in <CIT>, when a lubricant, for example, grease is applied to the screw shaft to lubricate the threaded engagement portion between the nut and the screw shaft, the rotation of the nut may cause the lubricant to enter the interior of the nut, lubricating the threaded engagement portion between the nut and the screw shaft. However, the following problems arise.

That is, as illustrated in <FIG>, when a screw shaft <NUM> moves in a reciprocating linear motion in the direction of the arrow, the lubricant applied to the screw shaft <NUM> is wiped off by a nut <NUM> and accumulates at both end portions (at or near the movement limit) of the screw shaft <NUM> in an axial direction. Further, when the lubricant accumulates at an end surface of the nut <NUM>, the lubricant is scattered out by the centrifugal force generated by the rotation of the nut <NUM>. Thus, there is a problem in that the lubricant at a central portion (portion A of <FIG>) of the screw shaft <NUM> in the axial direction becomes insufficient and the oil film disappears, which causes acceleration of wear at this portion.

Further, with the lubrication structure proposed in <CIT>, the oil groove is formed at the upper end of the female thread portion of the nut. Thus, there is a problem in that this lubrication structure can be applied only to a configuration in which the nut and the screw shaft are arranged in a vertical direction.

Further, with the lubrication structure proposed in <CIT>, the lubricant is limited to a solid lubricant. Thus, there is a problem in that the application range of this lubrication structure is narrow.

The present invention has been made in view of the problems described above, and an object of the present invention is to provide an electric actuator that can prevent wear at a threaded engagement portion between a nut and a screw shaft by sufficiently lubricating the threaded engagement portion, thereby being capable of improving the durability of the nut and the screw shaft.

In order to achieve the object described above, the present invention has a first obligatory feature that there is provided an electric actuator including: an electric motor capable of rotating forward and backward; and a motion conversion mechanism configured to convert rotation of the electric motor into a reciprocating linear motion of a screw shaft, the motion conversion mechanism including: a nut configured to be rotationally driven by the electric motor; and the screw shaft capable of being threadedly inserted through the nut in a back and forth movable manner. A lubricant holding portion having an outer diameter larger than a diameter of a screw hole of the nut and being capable of housing a lubricant is formed on an inner periphery of an end of the nut in an axial direction. A lid member configured to cover an opening end of the lubricant holding portion in the axial direction is provided.

Further, in addition to the first feature, the present invention has a second obligatory feature that, the lid member integrally includes: a lid portion; and a tubular portion coaxial with the nut, and the lid member is mounted to the nut through fitting of a protrusion portion formed on an inner periphery of the tubular portion to a recess portion formed in an outer periphery of the nut or through fitting of a protrusion portion formed on the outer periphery of the nut to a recess portion formed in the inner periphery of the tubular portion.

In addition to the second feature, the present invention has a third feature that a scraper portion having a cylindrical shape is formed on an inner periphery of the lid portion, and an inner diameter of the scraper portion is slightly larger than an outer diameter of the screw shaft so that a small gap in a radial direction is formed between the scraper portion and the screw shaft.

In addition to the second or third feature, the present invention has a fourth feature that an outer diameter of the tubular portion is equal to or less than an outer diameter of the nut.

Further, in addition to any one of the first to fourth features, the present invention has a fifth feature that the lid member is made of a rubber material.

According to the first feature of the present invention, even when rotation of the nut causes the screw shaft to enter the nut, the lubricant applied to the screw shaft is held in the lubricant holding portion of the nut. Thus, extra lubricant does not accumulate at the end surface of the nut, and the accumulated lubricant is not scattered out by the centrifugal force generated by the rotation of the nut. Further, the lubricant collected in the lubricant holding portion of the nut is applied to the screw shaft when the screw shaft comes out from the nut. However, the application amount of the lubricant is restricted by the lid member. Thus, the necessary sufficient amount of lubricant is applied to the screw shaft in a broad range. As a result, partial insufficiency of lubricant (oil film disappearing) on the screw shaft does not occur, and the threaded engagement portion between the nut and the screw shaft is sufficiently lubricated, thereby preventing wear at the threaded engagement portion and improving the durability of the nut and the screw shaft.

According to the second feature of the present invention, the lid member can be easily mounted to the outer periphery of the nut by one action of fitting recess portions and protrusion portions.

According to the third feature of the present invention, the inner diameter of the scraper portion formed on the inner periphery of the lid portion of the lid member is slightly larger than the outer diameter of the screw shaft so that a small gap in the radial direction is formed therebetween. Thus, the lubricant collected in the lubricant holding portion of the nut is scraped by the scraper portion and is applied evenly at a predetermined thickness on the screw shaft. In this case, the scraper portion is in non-contact with the screw shaft. Thus, no frictional resistance force is generated therebetween, thereby allowing the screw shaft to slide smoothly without resistance. Further, the scraper portion does not receive an external force from the screw shaft. Thus, the scraper portion is prevented from being damaged, thereby improving the durability of the lid member.

According to the fourth feature of the present invention, the outer diameter of the tubular portion of the lid member is equal to or less than the outer diameter of the nut. Thus, when the lid member is installed, and the nut is installed on the radial bearings, the nut can be mounted without the lid member interfering with the radial bearings.

According to the fifth feature of the present invention, the lid member is made of a rubber material that is flexible and has good elasticity. Thus, even in a case in which the lid member comes into contact with the screw shaft, the lid member is prevented from being damaged, thereby improving the durability of the lid member.

An embodiment of the present invention is now described with reference to the accompanying drawings.

<FIG> is a perspective view of an electric actuator according to the present invention. <FIG> is a planar sectional view of the electric actuator.

An electric actuator <NUM> according to this embodiment is provided in an electric parking brake device (not shown). As illustrated in <FIG>, the electric actuator <NUM> includes an electric motor <NUM> which serves as a drive source and is capable of rotating forward and backward; a motion conversion mechanism <NUM> configured to convert rotation of the electric motor <NUM> into a reciprocating linear motion of a screw shaft <NUM>; and a transmission mechanism (gear mechanism) <NUM> configured to transmit rotation of the electric motor <NUM> to the motion conversion mechanism <NUM>. The electric motor <NUM>, the motion conversion mechanism <NUM>, and the transmission mechanism <NUM> are housed in an actuator case <NUM>. In the following description, the direction of the arrows illustrated in <FIG> and <FIG> is defined as a front-and-rear direction.

The actuator case <NUM> includes a case main body <NUM>, a motor cover <NUM>, and a gear cover <NUM>. The case main body <NUM> integrally includes a first case 11A and a second case 11B each having a cylindrical shape. The motor cover <NUM> is mounted to an opening end of the first case 11A of the case main body <NUM>. The gear cover <NUM> is mounted to an opening end of the case main body <NUM> on an opposite side (right side of <FIG>) to the motor cover <NUM>.

Further, as illustrated in <FIG>, the electric motor <NUM> is housed in the first case 11A of the case main body <NUM>, and a driving gear <NUM> having a small diameter is mounted at an end portion (rear end portion) of an output shaft (motor shaft) 2a extending rearward from the electric motor <NUM> (rightward in <FIG>) that faces the interior of the gear cover <NUM>. One end (front end) of the electric motor <NUM> in the axial direction is fitted to and held in a recess portion 12a formed in the motor cover <NUM>, and another end (rear end) of the electric motor <NUM> in the axial direction is fitted to and held in a circular hole 11b formed in an end wall 11a of the first case 11A. The output shaft (motor shaft) 2a of the electric motor <NUM> is rotatably supported in the first case 11A through intermediation of a bearing <NUM>.

Further, the second case 11B of the case main body <NUM> is integrally disposed on a lateral side of the first case 11A, and the motion conversion mechanism <NUM> is housed in the second case 11B. The motion conversion mechanism <NUM> includes a nut <NUM> and the screw shaft <NUM>. The nut <NUM> is rotatable. The screw shaft <NUM> is threadedly inserted through the nut <NUM> so as to be movable in a linear motion.

The nut <NUM> includes a sleeve portion 8A and a drum portion 8B. The drum portion 8B has a diameter larger than that of the sleeve portion 8A. The nut <NUM> is rotatably supported in the second case 11B through intermediation of a radial bearing (ball bearing) <NUM> mounted to an outer periphery of the sleeve portion 8A. A female thread 8a is formed in a helical pattern in an inner periphery of the sleeve portion 8A.

The screw shaft <NUM> is a slide member having a round rod-like shape elongated in the front-and-rear direction. A brake cable (not shown) is coupled to a circular hole 3a formed in a front end portion of the screw shaft <NUM>, and another end of the brake cable is coupled to a parking brake lever of the electric parking brake device (not shown). Further, a flange portion 3A is integrally formed with the rear end of the screw shaft <NUM>. An outer periphery of the flange portion 3A is engaged with a pair of guide grooves 14a formed along the axial direction in an inner periphery of a tubular guide member <NUM> housed in the gear cover <NUM>. Accordingly, rotation of the screw shaft <NUM> is prevented by the guide member <NUM>, and the screw shaft <NUM> moves in a reciprocating linear motion in the front-and-rear direction without rotation.

A male thread 3b is formed in a helical pattern in an outer periphery of the screw shaft <NUM> in a predetermined range, and the screw shaft <NUM> is threadedly inserted through the sleeve portion 8A of the nut <NUM>. Thus, when the screw shaft <NUM> is inserted through the nut <NUM>, the male thread 3b formed in the outer periphery of the screw shaft <NUM> and the female thread 8a formed in the inner periphery of the sleeve portion 8A of the nut <NUM> are threadedly engaged with one another.

Further, the portion of the screw shaft <NUM> extending to the exterior of the actuator case <NUM> is covered by a bellows boot <NUM>. The boot <NUM> is made of, for example, a rubber material that is flexible and has good elasticity, and is extendable and contractable. One end (front end) of the boot <NUM> is fitted on an outer periphery of the front end portion of the screw shaft <NUM>, and another end (rear end) of the boot <NUM> is fitted on an outer periphery of a tubular portion 11c that is disposed so as to project from the second case 11B of the actuator case <NUM>.

Incidentally, as illustrated in <FIG>, a plurality of disc springs <NUM> are housed under a state of being stacked in the axial direction in a space in the axial direction formed inside the drum portion 8B of the nut <NUM> between the drum portion 8B and the flange portion 3A of the screw shaft <NUM>. The position of the disc springs <NUM> in the axial direction is regulated by a retainer <NUM> that comes into abutment against the driven gear <NUM>. The disc springs <NUM> are urging members configured to urge the screw shaft <NUM> in the direction away from the nut <NUM> (rearward).

The configuration of the transmission mechanism <NUM> is now described below with reference to <FIG>.

The transmission mechanism <NUM> is configured to transmit rotation of the output shaft 2a of the electric motor <NUM> to the nut <NUM> of the motion conversion mechanism <NUM>. The transmission mechanism <NUM> includes the driving gear <NUM> mounted to the output shaft 2a of the electric motor <NUM>, an intermediate gear <NUM> rotatably supported by a support shaft <NUM>, and the driven gear <NUM> mounted to an outer periphery of the drum portion 8B of the nut <NUM>.

The intermediate gear <NUM> is rotatably supported by the support shaft <NUM> disposed between the case main body <NUM> of the actuator case <NUM> and the guide member <NUM> and integrally includes a large-diameter intermediate gear 19a and a small-diameter intermediate gear 19b, which are different in diameter. The large-diameter intermediate gear 19a meshes with the driving gear <NUM> and has a diameter larger than the diameter of the driving gear <NUM>. Further, the small-diameter intermediate gear 19b meshes with the driven gear <NUM> having a diameter larger than that of the large-diameter intermediate gear 19a. The driven gear <NUM> is fitted on the outer periphery of the drum portion 8B of the nut <NUM>.

Next, the characteristic configuration of the present invention is now described on the basis of <FIG>.

<FIG> is a planar sectional view of a nut and screw shaft portion of an electric actuator according to the present invention. <FIG> is an enlarged detailed view of the portion <NUM> of <FIG>. <FIG> is a sectional view taken along the line <NUM>-<NUM> of <FIG>. <FIG> is a sectional view taken along the line <NUM>-<NUM> of <FIG>.

In this embodiment, as illustrated in <FIG>, a lubricant holding portion <NUM> having an outer diameter larger than the diameter of the screw hole of the nut <NUM> is formed on an inner periphery of one end (front end portion) 8A1 of the sleeve portion 8A of the nut <NUM> in the axial direction, and the lubricant holding portion <NUM> is capable of housing a lubricant, for example, grease. Further, according to the invention, a lid member <NUM> configured to cover an opening end of the lubricant holding portion <NUM> in the axial direction is mounted to the outer periphery of the one end portion (front end portion) 8A1 of the sleeve portion 8A of the nut <NUM> in the axial direction.

The lid member <NUM> is made of a rubber material and integrally formed. As illustrated in <FIG>, the lid member <NUM> integrally includes a lid portion 22A and a tubular portion 22B. The lid portion 22A has a ring plate-like shape that covers the opening end of the lubricant holding portion <NUM> in the axial direction from the axial direction. The tubular portion 22B has a cylindrical shape and is coaxial with the nut <NUM>. As illustrated in <FIG>, a protrusion portion 22b having a ring shape is integrally formed around the entire periphery of an inner periphery of the end portion of the tubular portion 22B of the lid member <NUM>, and a recess portion 8b having a ring groove-like shape is integrally formed around the entire periphery of the outer periphery of the end portion of the sleeve portion 8A of the nut <NUM>.

Further, through fitting of the protrusion portion 22b formed on the inner periphery of the end portion of the tubular portion 22B into the recess portion 8b formed in an outer periphery of the end portion of the sleeve portion 8A of the nut <NUM>, the lid member <NUM> can be easily mounted to the outer periphery of the end portion of the sleeve portion 8A of the nut <NUM> by one action. In this embodiment, the protrusion portion 22b is formed on the tubular portion 22B side of the lid member <NUM>, and the recess portion 8b is formed on the sleeve portion 8A side of the nut <NUM>. However, conversely, a recess portion may be formed on the tubular portion 22B side of the lid member <NUM>, and a protrusion portion may be formed on the sleeve portion 8A side of the nut <NUM>. Also through engagement of the recess portion and the protrusion portion together in this manner, the lid member <NUM> can be easily mounted to the outer periphery of the end portion of the sleeve portion 8A of the nut <NUM> by one action.

Incidentally, in this embodiment, as illustrated in <FIG>, the tubular portion 22B of the lid member <NUM> and the sleeve portion 8A of the nut <NUM> have the same outer diameter. Further, a cylindrical scraper portion (scraping portion) 22a integrally projecting forward (leftward in <FIG>) is integrally formed on an inner periphery of the lid portion 22A of the lid member <NUM>, and a tapered surface 22b1 that increases in diameter forward (leftward in <FIG>) is formed on an inner periphery of the front end of the scraper portion 22a. As illustrated in <FIG>, an inner diameter φD of the scraper portion 22a is slightly larger than an outer diameter φd of the screw shaft <NUM> (φD>φd). Thus, a ring-shaped small gap δ (= (D-d)/<NUM>) in the radial direction is formed between the scraper portion 22a and the screw shaft <NUM>.

Further, the electric actuator <NUM> having the configuration described above is used as an actuator of an electric parking brake device (not shown). The actions of the electric actuator <NUM> is now described with reference to <FIG>. The electric actuator <NUM> is positioned and fixed in the electric parking brake device using a snap ring <NUM> (see <FIG> and <FIG>) fitted around an outer periphery of the actuator case <NUM>.

In the state illustrated in <FIG>, the screw shaft <NUM> is advanced forward, a brake cable (not shown) mounted to the distal end of the screw shaft <NUM> is slack. In this state, the brake shoes of the drum brake are separated from the inner peripheral surface of the brake drum. Thus, no frictional resistance force is generated between the brake shoes and the brake drum, and the electric parking brake device is in a state with the parking brake released. Accordingly, the brake drum and the wheel can freely rotate, and the vehicle can thus travel.

In this state, when a parking brake switch (not shown) is turned ON, the electric motor <NUM> of the electric actuator <NUM> is energized so that the electric motor <NUM> starts up. Then, rotation of the output shaft 2a of the electric motor <NUM> is reduced in speed by the transmission mechanism <NUM> and transmitted to the motion conversion mechanism <NUM>, and rotation of the output shaft 2a of the electric motor <NUM> is converted into a linear motion (backward movement) of the screw shaft <NUM>.

That is, rotation of the output shaft 2a of the electric motor <NUM> is reduced in speed and transmitted from the driving gear <NUM> to the intermediate gear <NUM>, and the intermediate gear <NUM> rotates at a predetermined speed. Further, rotation of the intermediate gear <NUM> is reduced in speed via the small-diameter intermediate gear 19b and the driven gear <NUM> in mesh with each other and transmitted to the nut <NUM> of the motion conversion mechanism <NUM>, and the nut <NUM> rotates at a predetermined speed.

As described above, when the nut <NUM> rotates, the screw shaft <NUM> threadedly inserted through the nut <NUM> moves backward in a linear motion as illustrated in <FIG> so that the brake cable (not shown) mounted to the distal end of the screw shaft <NUM> is pulled. As a result, the parking brake lever of the electric parking brake device is operated so that the brake shoes open out, and the brake shoes are pressed against the inner peripheral surface of the brake drum, causing a frictional resistance force therebetween. The rotation of the brake drum and the wheel is braked by the frictional resistance force, and the vehicle is thus brought into a parking brake state.

When the parking brake switch (not shown) is turned OFF to release the parking brake state, the electric motor <NUM> rotates backward, with rotation of the output shaft 2a being transmitted to the nut <NUM> of the motion conversion mechanism <NUM> via a transmission path similar to that described above. This causes the nut <NUM> to rotate backward. When the nut <NUM> rotates backward in this manner, the screw shaft <NUM> threadedly inserted through the nut <NUM> moves forward in a linear motion in the direction of the arrow of <FIG>, and the electric actuator <NUM> returns to the state illustrated in <FIG>. Thus, the electric parking brake device is brought into a released state.

In this embodiment, the lubricant holding portion <NUM> having an outer diameter larger than the diameter of the screw hole of the nut <NUM> and being capable of housing a lubricant is formed on an inner periphery of an end of the nut <NUM> in the axial direction, and the lid member <NUM> configured to cover the opening end of the lubricant holding portion <NUM> in the axial direction is provided. Thus, even when rotation of the nut <NUM> causes the screw shaft <NUM> to enter the nut <NUM>, the lubricant applied to the screw shaft <NUM> is held in the lubricant holding portion <NUM> of the nut <NUM>. Thus, as illustrated in <FIG>, extra lubricant does not accumulate at the end surface of the nut <NUM>, and the accumulated lubricant is not scattered out by the centrifugal force generated by the rotation of the nut <NUM>.

Further, the lubricant collected in the lubricant holding portion <NUM> of the nut <NUM> is applied to the screw shaft <NUM> when the screw shaft <NUM> comes out from the nut <NUM>. However, the lubricant application amount is restricted by the lid member <NUM>, and hence the necessary sufficient amount of lubricant is applied to the screw shaft <NUM> in a broad range. As a result, partial insufficiency of lubricant (oil film disappearing) on the screw shaft <NUM> does not occur, and the threaded engagement portion between the nut <NUM> and the screw shaft <NUM> is sufficiently lubricated, thereby preventing wear at the threaded engagement portion and improving the durability of the nut <NUM> and the screw shaft <NUM>.

Further, in this embodiment, the inner diameter φD of the scraper portion 22a formed on an inner periphery of the lid portion 22A of the lid member <NUM> is slightly larger than the outer diameter φd of the screw shaft <NUM> (φD > φd), and a small gap δ in the radial direction is formed therebetween. Thus, the lubricant collected in the lubricant holding portion <NUM> of the nut <NUM> is scraped by the scraper portion 22a, and, as illustrated in <FIG>, is applied evenly at a predetermined thickness δ on the screw shaft <NUM>. In this case, the scraper portion 22a is in non-contact with the screw shaft <NUM>. Thus, no frictional resistance force is generated therebetween, thereby allowing the screw shaft <NUM> to slide smoothly without resistance. Further, the scraper portion 22a does not receive an external force from the screw shaft <NUM>. Thus, the scraper portion 22a is prevented from being damaged, thereby improving the durability of the lid member <NUM>.

Incidentally, even though the nut <NUM> is inserted through and installed in the radial bearing <NUM>, the outer diameter of the tubular portion 22B of the lid member <NUM> is equal to or less than the outer diameter of the sleeve portion 8A of the nut <NUM>. Thus, even when the nut <NUM> is inserted through the radial bearing <NUM> after the lid member <NUM> is installed on the nut <NUM>, the nut <NUM> can be mounted without the lid member <NUM> interfering with the radial bearing <NUM>.

Further, in this embodiment, the lid member <NUM> is made of a rubber material that is flexible and has good elasticity. Thus, even in a case in which the lid member <NUM> comes into contact with the screw shaft <NUM>, the effects that the lid member <NUM> is prevented from being damaged and the durability of the lid member <NUM> is improved can still be obtained.

Claim 1:
An electric actuator (<NUM>) comprising:
an electric motor (<NUM>) capable of rotating forward and backward; and
a motion conversion mechanism (<NUM>) configured to convert rotation of the electric motor (<NUM>) into a reciprocating linear motion of a screw shaft (<NUM>), the motion conversion mechanism (<NUM>) including:
a nut (<NUM>) configured to be rotationally driven by the electric motor (<NUM>); and
the screw shaft (<NUM>) capable of being threadedly inserted through the nut (<NUM>) in a back and forth movable manner,
wherein the nut (<NUM>) includes a sleeve portion (8A) and a drum portion (8B) that has a diameter larger than that of the sleeve portion (8A),
wherein the screw shaft (<NUM>) is threadedly inserted through the sleeve portion (8A) of the nut (<NUM>),
wherein a lubricant holding portion (<NUM>) having an outer diameter larger than a diameter of a screw hole of the nut (<NUM>) and being capable of housing a lubricant is formed on an inner periphery of an end portion (8A1) of the sleeve portion (8A) of the nut (<NUM>) in an axial direction,
wherein a lid member (<NUM>) configured to cover an opening end of the lubricant holding portion (<NUM>) in the axial direction is provided to an outer periphery of the end portion (8A1) of the sleeve portion (8A) of the nut (<NUM>) in the axial direction,
characterized in that
the lid member (<NUM>) integrally includes:
a lid portion (22A); and
a tubular portion (22B) coaxial with the nut (<NUM>), and
that the lid member (<NUM>) is mounted to the outer periphery of the end portion of the sleeve portion 8A of the nut (<NUM>) through fitting of a protrusion portion (22b) formed on an inner periphery of the tubular portion (22B) to a recess portion (8b) formed in an outer periphery of the end portion of the sleeve portion (8A) of the nut (<NUM>) or through fitting of a protrusion portion formed on the outer periphery of the sleeve portion (8A) of the nut (<NUM>) to a recess portion formed in the inner periphery of the tubular portion (22B).