JP7576003B2 - Webbing Winder - Google Patents

Description

The present invention relates to a webbing take-up device in which the spool is rotated in the take-up direction to take up the webbing on the spool.

For example, in a webbing take-up device as disclosed in the following Patent Document 1, a clutch gear (30) is formed on the axial side of the spool (20), and when the pinion of the pretensioner mechanism rotates in the webbing take-up direction, the clutch pawl connected to the pinion rotates in the take-up direction and protrudes outward in the rotational radius direction of the pinion. When the clutch pawl protrudes in this way, the clutch pawl engages with the clutch gear, which connects the pinion and the spool and rotates the spool in the webbing take-up direction.

When the clutch gear and the spool are separate, the clutch gear and the spool are connected by crimping the spool so that the clutch gear rotates integrally with the spool. However, this type of crimping process requires materials such as expensive, special aluminum die-cast material with high elongation or heavy zinc die-cast material.

JP 2011-73595 A

Taking the above facts into consideration, the present invention aims to obtain a webbing take-up device that can use inexpensive or lightweight materials.

The webbing take-up device in the first aspect of the present invention includes a spool having a bottomed hollow portion along the axial direction of rotation, and a seat belt device webbing is taken up by rotating in the take-up direction; an axle member disposed in the hollow portion, the relative rotation of which is restricted at one end of the spool with respect to the spool, and extending coaxially with the spool from the other end of the spool through the bottom of the hollow portion to the outside of the spool; a push stopper formed in a disk shape coaxial with the spool, provided on the opposite side of the bottom from the hollow portion, and held by the axle member with the axle member penetrating through it; and a ring clutch formed in a ring shape coaxial with the spool, with a component of the pretensioner mechanism engaging with the inner or outer periphery when the pretensioner mechanism is activated, and held in the axial direction of the spool by the push stopper and the bottom of the hollow portion of the spool.

The webbing take-up device of the first aspect of the present invention is equipped with a ring clutch. When the pretensioner mechanism is activated, a component of the pretensioner mechanism engages with the inner or outer periphery of the ring clutch. This allows the driving force of the pretensioner mechanism to be transmitted to the ring clutch. The ring clutch is clamped in the axial direction of the spool between the push stopper and the bottom of the hollow portion of the spool.

In this way, the ring clutch, which is clamped in the axial direction of the spool between the push stopper and the bottom of the hollow portion of the spool, can impart the transmitted driving force of the pretensioner mechanism to the spool. This causes the spool to rotate in the winding direction, allowing the webbing of the seat belt device to be wound onto the spool.

Here, the ring clutch is clamped in the axial direction of the spool between the push stopper and the bottom of the hollow part of the spool. This means that there is no need to create a crimped part on the spool to hold the ring clutch. This means that there is no need to use expensive special aluminum die-cast material with high elongation or heavy zinc die-cast material for the spool.

The webbing take-up device in the second aspect of the present invention is the webbing take-up device in the first aspect of the present invention, in which the push stopper is farther from the ring clutch toward the center than the curved end of the radially middle portion, and the diameter of the hole formed in the bottom of the spool through which the shaft member passes is smaller than the diameter of the curved end of the push stopper.

In the webbing take-up device of the second aspect of the present invention, the push stopper is farther from the ring clutch on the central side than the curved end of the radially middle portion, and the diameter of the hole formed in the bottom of the spool through which the shaft member passes is smaller than the diameter of the curved end of the push stopper. This allows it to bear the removal load of the push stopper.

A webbing take-up device according to a third aspect of the present invention is the webbing take-up device according to the first or second aspect of the present invention, further comprising: a hollow portion that opens toward one axial side of the spool and accommodates the ring clutch, the hollow portion being disposed on the one axial side of the spool;
The difference between the depth dimension at the radial center of the hollow portion and the depth dimension of the portion of the hollow portion where the ring clutch is disposed is smaller than the thickness dimension of the ring clutch at the center of the ring clutch.

In the webbing take-up device of the third aspect of the present invention, a hollow portion is provided on one axial side of the spool. The hollow portion opens toward one axial side of the spool and houses a ring clutch. The difference between the depth dimension of this hollow portion at the radial center and the depth dimension of the portion of the hollow portion where the ring clutch is disposed is smaller than the thickness dimension of the ring clutch at the center of the ring clutch. Therefore, the push stopper can effectively suppress floating of the ring clutch.

As explained above, the webbing take-up device of the present invention has a spool with high elongation, does not require the use of expensive special aluminum material, and can be manufactured using inexpensive materials.

1 is an exploded perspective view of a main portion of a webbing take-up device according to an embodiment of the present invention; 1 is a cross-sectional view of a main portion of a webbing take-up device according to an embodiment of the present invention. 1 is an exploded perspective view of a webbing take-up device according to an embodiment of the present invention;

Next, an embodiment of the present invention will be described with reference to the drawings. In each drawing, the arrow FR indicates the front side of the vehicle to which the webbing take-up device 10 is applied, the arrow LF indicates the left side in the vehicle width direction, and the arrow UP indicates the upper side of the vehicle. In each drawing, the arrow A indicates the take-up direction, which is the rotation direction of the spool 14 when the spool 14 takes up the webbing, and the arrow B indicates the unwinding direction, which is opposite to the take-up direction. In addition, the same reference numerals are used for parts that are basically the same as those in the embodiment described above, and detailed description thereof will be omitted.

<Configuration of this embodiment>
3, the webbing take-up device 10 according to the present embodiment includes a frame 12. The frame 12 is fixed to a vehicle underside of a package tray trim (not shown) at the rear of a rear seat (not shown) which is a seat of the vehicle.

The frame 12 is also provided with a spool 14. The spool 14 is made of metal, for example, aluminum die-cast material, and is formed into a generally cylindrical shape and is rotatable around a central axis (in the directions of arrows A and B in FIG. 3). The base end of a long strip-shaped webbing (not shown) is engaged with the spool 14, and when the spool 14 is rotated in the winding direction (in the direction of arrow A in FIG. 3), the webbing is wound onto the spool 14 from the base end side in the longitudinal direction.

The longitudinal tip of the webbing extends from the spool 14 toward the upper front of the vehicle, and passes through a slit hole in the package tray trim to extend to the upper side of the rear seat. The webbing that extends to the upper side of the rear seat runs along the upper end of the rear seat back and the front side of the vehicle, before extending to the lower side of the vehicle. The webbing that extends to the lower side of the vehicle passes through the rear end of the rear seat cushion (not shown) and is fixed to the vehicle body via an anchor plate (not shown) on the underside of the seat cushion.

The seat belt device for a vehicle to which the webbing retractor 10 is applied is equipped with a buckle device (not shown). The buckle device is provided on the inside of the rear seat to which the webbing retractor 10 is applied, in the vehicle width direction. With the webbing wrapped around the body of an occupant seated in the rear seat, a tongue (not shown) provided on the webbing engages with the buckle device, thereby attaching the webbing to the occupant's body.

Furthermore, a torsion shaft 16 is provided inside the spool 14 as an axial member. A first engagement portion 18 is provided on the right side of the torsion shaft 16 in the vehicle width direction, and when the first engagement portion 18 is engaged inside the spool 14, the relative rotation of the torsion shaft 16 with respect to the spool 14 is restricted.

A second engagement portion 20 is formed on the torsion shaft 16 at a portion to the left of the first engagement portion 18 in the vehicle width direction. A lock base 22 is engaged with the second engagement portion 20 in a state in which relative rotation with respect to the torsion shaft 16 is restricted. A lock plate 24 is provided on the lock base 22.

The lock plate 24 is supported so as to be able to swing relative to the lock base 22 in the radial direction of the lock base 22. A ratchet wheel 26 of the leg plate 12A of the frame 12 faces the radially outward side of the lock base 22, and when the lock plate 24 rotates radially outward of the lock base 22, the ratchet portion at the tip of the lock plate 24 engages with the ratchet wheel 26 of the leg plate 12A of the frame 12. This limits the rotation of the lock base 22 in the pull-out direction. By limiting the rotation of the lock base 22 in the pull-out direction, the rotation of the torsion shaft 16, and therefore the spool 14, in the pull-out direction is limited.

A pin 28 is formed on the lock plate 24. The pin 28 of the lock plate 24 extends from the left side surface of the lock base 22 in the vehicle width direction to the left in the vehicle width direction. The pin 28 of the lock plate 24 penetrates the left leg plate 12A of the frame 12. A sensor holder 30 is attached to the left leg plate 12A of the frame 12, and a sensor cover 32 is attached to the outside of the sensor holder 30.

Various components that make up the VSIR mechanism or WSIR mechanism are provided inside the sensor cover 32, and the pin 28 of the lock base 22 engages with one of the various components. The VSIR mechanism is activated when the vehicle suddenly decelerates during a vehicle collision or other such situation. In contrast, the WSIR mechanism is activated when the rotational acceleration in the unwinding direction (arrow B direction in Figure 3), which is opposite to the winding direction of the spool 14, exceeds a predetermined magnitude. When the VSIR mechanism or WSIR mechanism is activated, a locking member (not shown) that makes up the locking mechanism is activated, restricting the rotation of the spool 14 in the unwinding direction.

On the other hand, as shown in FIG. 3, the right side of the torsion shaft 16 protrudes to the right in the vehicle width direction from the right end of the spool 14 in the vehicle width direction and the leg plate 12B on the right side of the frame 12 in the vehicle width direction. A case 34 is provided on the right side of the leg plate 12B in the vehicle width direction. A gear case 36 is provided on the right side of the case 34 in the vehicle width direction. A circular hole 38 is formed in the case 34 coaxially with the spool 14, and the right side of the torsion shaft 16 in the vehicle width direction passes through the circular hole 38 of the case 34. A pinion 40 constituting a pretensioner mechanism is arranged on the right side of the circular hole 38 in the vehicle width direction. The pinion 40 is supported on the right side of the torsion shaft 16 in the vehicle width direction so as to be rotatable relative to the right side of the torsion shaft 16 in the vehicle width direction.

In contrast, a clutch plate 42 is supported non-rotatably in the circular hole 38 of the case 34. A number of legs are formed on the inner periphery of the clutch plate 42, and the legs of the clutch plate 42 fit into a recess formed integrally with the pinion 40 on the left side of the pinion 40 in the vehicle width direction. A rack bar 44 is disposed radially to the side of the pinion 40, which constitutes a pretensioner mechanism together with the pinion 40. The rack bar 44 has teeth that can mesh with the pinion 40. The rear portion of the rack bar 44 is formed as a piston 46, and is housed in a base cartridge 48 that constitutes the pretensioner mechanism together with the piston 46 and the rack bar 44.

A micro gas generator 50 (hereinafter, "micro gas generator 50" will be abbreviated as "MGG50") is provided in the vehicle rear portion of the base cartridge 48. When the MGG50 is activated and gas is instantly ejected from the MGG50, the internal pressure of the base cartridge 48 increases, and the piston 46 and, in turn, the rack bar 44 move. When the rack bar 44 moves, the rack bar 44 meshes with the pinion 40, causing the pinion 40 to rotate in the winding direction. When the pinion 40 rotates in the winding direction, the leg of the clutch plate 42, which is inserted in a recess provided integrally with the pinion 40 on the left side in the vehicle width direction relative to the pinion 40, moves radially outward from the pinion 40.

A spring cover 52 is provided on the right side of the gear case 36 in the vehicle width direction, and a torsion coil spring 54 is further provided within the spring cover 52 as a biasing member. The torsion coil spring 54 is concealed by the spring cover 52 and a spring seat 56 provided on the left side of the spring cover 52 in the vehicle width direction. The inner end of the torsion coil spring 54 in the spiral direction is biased in the winding direction relative to the outer end of the torsion coil spring 54 in the spiral direction.

The outer end of the torsion coil spring 54 in the spiral direction is engaged with the spring cover 52. In contrast, the inner end of the torsion coil spring 54 in the spiral direction is engaged with the adapter 58. One axial end of the adapter 58 is rotatably supported by the spring cover 52, and the other axial end of the adapter 58 is rotatably supported by the spring seat 56. The left side of the adapter 58 in the vehicle width direction is connected to the right side of the torsion shaft 16 in the vehicle width direction, and the torsion shaft 16, and therefore the spool 14, are biased in the winding direction.

On the other hand, a non-circular (approximately polygonal) guide hole 60 is formed at the left end of the spool 14 in the vehicle width direction. A force limiter stopper 62 (hereinafter, "force limiter stopper 62" will be abbreviated to "FL stopper 62") is inserted into the guide hole 60. The outer peripheral shape of the FL stopper 62 is approximately equal to the inner peripheral shape of the guide hole 60, and therefore the relative rotation of the FL stopper 62 with respect to the spool 14 is limited.

The FL stopper 62 also has a female threaded hole. The female threaded hole of the FL stopper 62 is formed coaxially with the spool 14. A male threaded portion formed on the right side of the lock base 22 in the vehicle width direction is screwed into the female threaded hole of the FL stopper 62. The male threaded portion of the lock base 22 is screwed into the tip portion of the female threaded hole of the FL stopper 62, and the FL stopper 62 is rotated until the male thread of the lock base 22 bottoms out at the end of the lock plate 24 side.

In contrast, as shown in Figures 1 and 2, a ring clutch housing 64 is formed as a hollow portion at the right end of the spool 14 in the vehicle width direction. The ring clutch housing 64 has a main body 66 that is circular and coaxial with the spool 14. A plurality of load receiving portions 68 are formed in the main body 66. The multiple load receiving portions 68 of the ring clutch housing 64 protrude radially outward from the inner periphery of the main body 66 and are formed at predetermined angular intervals around the main body 66.

The outer portion of the bottom of the main body 66 is a mortar portion 66A. The mortar portion 66A has a depth that gradually becomes shallower toward the radial outside of the main body 66. On the other hand, the radially inner portion of the main body 66 is a flat portion 66B. The radially inner portion of the main body 66 is a flat portion 66B, and the depth of the flat portion 66B is equal to the dimension of the radially inner end of the mortar portion 66A. A hole 70 is formed in the approximate center of the bottom of the ring clutch housing 64. The right side portion of the torsion shaft 16 in the vehicle width direction from the first engagement portion 18 is disposed to pass through the hole 70. The periphery of the hole 70 is a thick portion 71. The thickness of the thick portion 71 is thicker than the thickness of the flat portion 66B.

A ring clutch 72 is housed inside this ring clutch housing 64. The ring clutch 72 is made of metal and formed into a ring shape coaxial with the spool 14. A plurality of rotation stoppers 74 are formed to protrude from the outer periphery of the ring clutch 72. With the ring clutch 72 housed inside the ring clutch housing 64, each of the plurality of rotation stoppers 74 is housed inside each of the plurality of load receiving portions 68. This limits the relative rotation of the ring clutch 72 with respect to the spool 14.

Furthermore, a flange portion 78 is formed on the radially inner side of the ring clutch 72. Furthermore, a hole portion 75 is formed in approximately the center of the flange portion 78. The diameter dimension of the hole portion 75 is larger than the diameter dimension of the thick portion 71 of the ring clutch accommodating portion 64, and when the ring clutch 72 is placed in the ring clutch accommodating portion 64, the thick portion 71 of the ring clutch accommodating portion 64 fits inside the hole portion 75 of the ring clutch 72.

The thickness dimension of the flange portion 78 of the ring clutch 72 is slightly larger than the thickness dimension of the thick portion 71 from the flat portion 66B.

A knurled portion 76 is formed on the inner periphery of the ring clutch 72. The tip of the leg of the clutch plate 42 that has moved radially outward of the pinion 40 is pressed against the knurled portion 76. This causes the tip of the leg of the clutch plate 42 to deform, and the tip of the leg of the clutch plate 42 is fixed to the knurled portion 76.

The push stopper 80 is provided to the right of the ring clutch 72 in the vehicle width direction. The push stopper 80 is disk-shaped. The portion from the outer periphery of the push stopper 80 to the curved end of the radial middle portion is a flat surface along a direction perpendicular to the central axis of the spool 14. The portion from the curved end of the radial middle portion of the push stopper 80 to the center is curved to the right in the vehicle width direction relative to the portion from the outer periphery to the curved end of the radial middle portion of the push stopper 80. In addition, the diameter dimension of the curved end of the radial middle portion of the push stopper 80 is larger than the diameter dimension of the hole portion 70 on the right side of the spool 14 in the vehicle width direction.

As described above, the thickness of the flange portion 78 is greater than the depth of the inner portion of the radially middle portion of the bottom of the body portion 66 of the ring clutch housing portion 64 relative to the radially central portion of the right end of the spool 14 in the vehicle width direction. It is preferable that this difference in dimensions be as small as possible from the viewpoint of ensuring the removal force of the push stopper 80.

A circular hole 82 is formed in the center of the push stopper 80, and the inner diameter dimension of the circular hole 82 is approximately equal to the outer diameter dimension of the right side portion of the torsion shaft 16 in the vehicle width direction. Therefore, when the right side portion of the torsion shaft 16 in the vehicle width direction passes through the circular hole 82 of the push stopper 80, the push stopper 80 engages with the torsion shaft 16, preventing the push stopper 80 from coming off the torsion shaft 16.

Also, the circle at the curved end of the radially intermediate portion of the push stopper 80 has a larger diameter than the hole 70 of the spool 14. Furthermore, the outer diameter dimension of the push stopper 80 is larger than the inner diameter dimension of the flange portion 78 of the ring clutch 72, and the portion of the push stopper 80 outside the radially intermediate portion overlaps with the flange portion 78 of the ring clutch 72 in the axial direction of the spool 14. Also, the radial difference between the inner diameter dimension of the flange portion 78 and the outer diameter dimension of the push stopper 80 is set to be 15% or more of the outer diameter dimension of the push stopper 80.

<Actions and Effects of the Present Embodiment>
Next, the operation and effects of this embodiment will be described.

When the occupant operates the brakes while the vehicle is moving, the VSIR mechanism in the sensor cover 32 is activated in response to the deceleration (acceleration) of the vehicle. In addition, when the occupant's body moves toward the front of the vehicle due to the deceleration of the vehicle, and the webbing wrapped around the occupant's body is pulled out, the WSIR mechanism in the sensor cover 32 is activated. When at least one of the VSIR mechanism and the WSIR mechanism is activated, the pin 28 of the lock base 22 is moved radially outward of the lock base 22 by the components of the VSIR mechanism and the WSIR mechanism. The ratchet portion at the tip of the lock plate 24 that has been moved radially outward of the lock plate 24 engages with the ratchet wheel 26 of the leg plate 12A of the frame 12. This restricts the rotation of the lock base 22 in the pull-out direction.

In this state, if the tensile force of the webbing exerted by the occupant's body is greater than the torsional rigidity of the torsion shaft 16, the first engagement portion 18 side of the torsion shaft 16 will twist around the central axis of the torsion shaft 16 relative to the second engagement portion 20 side. The spool 14 will rotate in the pull-out direction by the amount of this twisting of the torsion shaft 16. The occupant will move toward the front of the vehicle by the amount of the webbing pulled out from the spool 14. Also, part of the force exerted by the occupant to move toward the front of the vehicle will be used to twist the torsion shaft 16.

This twisting of the torsion shaft 16 is also the rotation of the spool 14 relative to the lock base 22. When such rotation occurs between the lock base 22 and the spool 14, the FL stopper 62 is moved toward the plate 24 side of the guide hole 60 and bottoms out. This limits further rotation of the lock base 22 relative to the spool 14.

Around the time of this torsional deformation of the torsion shaft 16, the MGG 50 of the base cartridge 48 is activated, and gas is instantly ejected from the MGG 50. This increases the internal pressure of the base cartridge 48, and the piston 46, and therefore the rack bar 44, moves. When the rack bar 44 moves, it meshes with the pinion 40, and the pinion 40 rotates in the winding direction. When the pinion 40 rotates in the winding direction, the leg of the clutch plate 42, which is inserted into a recess integrally formed with the pinion 40 on the left side in the vehicle width direction relative to the pinion 40, moves radially outward from the pinion 40.

As a result, the tip of the leg of the clutch plate 42 is pressed against the knurled portion 76 on the inner circumference of the ring clutch 72. This connects the pinion 40 and the spool 14 via the ring clutch 72, and the rotation of the pinion 40 in the winding direction is transmitted to the spool 14. When the rotation in the winding direction is transmitted to the spool 14 and the spool 14 rotates in the winding direction, the webbing is wound onto the spool 14 from the base end side in the longitudinal direction.

The flange portion 78 of the ring clutch 72 is inserted into the ring clutch housing portion 64 of the spool 14 and is clamped in the axial direction of the spool 14 between the push stopper 80 and the bottom of the ring clutch housing portion 64 of the spool 14. For this reason, it is not necessary to create a crimped portion in the spool 14 to hold the ring clutch 72. Therefore, it is not necessary to use a highly extensible, expensive, and special aluminum die-cast material or a high-mass zinc die-cast material for the spool 14.

In addition, the torsion shaft 16 and ring clutch 72 can be fixed to the spool 14 with a single push stopper 80.

Moreover, the difference between the depth dimension at the flat portion 66B in the ring clutch housing portion 64 of the spool 14 and the depth dimension at the thick portion 71 in the ring clutch housing portion 64 of the spool 14 is smaller than the thickness of the flange portion 78 of the ring clutch 72. This allows the push stopper 80 to reliably abut against the ring clutch 72. This prevents a gap from being formed between the ring clutch 72 and the push stopper 80 or the bottom of the ring clutch housing portion 64 of the spool 14. This effectively prevents the ring clutch 72 from floating above the push stopper 80 or the bottom of the ring clutch housing portion 64 of the spool 14.

On the other hand, the push stopper 80 is radially farther from the ring clutch 72 on the central side than the radial middle part. Also, the diameter of the hole 70 formed in the bottom of the ring clutch housing part 64 of the spool 14 is smaller than the diameter of the curved end of the ring clutch 72. Therefore, it bears the removal load of the push stopper 80.

The difference between the inner diameter of the flange 78 of the ring clutch 72 and the outer diameter of the push stopper 80 is set to 15% or more of the outer diameter of the push stopper 80. Therefore, the push stopper 80 abuts against the flange 78 of the ring clutch 72 with a sufficient width. This effectively prevents the flange 78 of the ring clutch 72 from coming off.

In this embodiment, the tip of the leg of the clutch plate 42 is pressed against the roulette portion 76 on the inner circumference of the ring clutch 72 when it is moved outward in the direction of the rotation radius of the spool 14. However, even if the movement direction of the leg of the clutch plate 42 is not toward the outside of the rotation radius of the spool 14, it is sufficient that it is pressed against the roulette portion 76 of the ring clutch 72. For example, the leg of the clutch plate 42 may be configured to be pressed against the roulette portion 76 by being moved inward in the direction of the rotation radius of the spool 14.

In addition, in this embodiment, the FL stopper 62 is provided, but the configuration may be such that the FL stopper 62 is not provided.

Furthermore, in this embodiment, the webbing take-up device 10 is provided on the rear side of the rear seat. However, the location of the webbing take-up device 10 is not limited to the rear side of the rear seat. For example, the webbing take-up device 10 may be disposed on a pillar of the vehicle.

10: Webbing winding device, 14: Spool, 16: Torsion shaft (shaft member), 64: Ring clutch housing (hollow portion), 72: Ring clutch, 80: Push stopper

Claims (3)

a spool that is formed in a substantially cylindrical shape and is rotatable about a central axis, has a hollow portion at one end on an axial direction side thereof that is bottomed and opens toward the one axial direction side , and rotates in a winding direction to wind up the webbing of the seat belt device;
a shaft member provided inside the spool , the shaft member restricting relative rotation with respect to the spool at one end of the spool, and extending coaxially with the spool from the other end of the spool through the bottom of the hollow portion to the outside of the spool;
a push stopper formed in a disk shape coaxial with the spool, provided inside the hollow portion with respect to the bottom portion, and held by the shaft member in a state in which the shaft member passes through;
a ring clutch formed in a ring shape coaxial with the spool, housed in the hollow portion , with an inner circumferential portion or an outer circumferential portion of which a component of the pretensioner mechanism engages when the pretensioner mechanism is activated, and which is sandwiched in the axial direction of the spool between the push stopper and the bottom of the hollow portion of the spool;
Equipped with
A webbing take-up device in which a plurality of anti-rotation portions are protrudingly formed on the outer periphery of the ring clutch, and a concave load receiving portion is formed in the hollow portion corresponding to each of the plurality of anti-rotation portions, and the anti-rotation portions are housed inside the load receiving portions, thereby limiting the relative rotation of the ring clutch with respect to the spool . The webbing take-up device according to claim 1, wherein the push stopper is located farther from the ring clutch on the central side than the curved end of the radially middle portion, and the diameter of the hole formed in the bottom of the spool through which the shaft member passes is smaller than the diameter of the curved end of the push stopper. 3. The webbing take-up device according to claim 1, wherein a difference between a depth dimension at a radial center side of the hollow portion and a depth dimension of a portion of the hollow portion where the ring clutch is disposed is smaller than a thickness dimension of the ring clutch at the center side of the ring clutch.