Patent Description:
<CIT> discloses a barrel unit that includes a barrel support shaft, a barrel and bearings. The barrel support shaft is supported by an outer periphery of a barrel holder which is rotationally driven. The barrel is supported by the barrel support shaft. The bearings allow the barrel support shaft to rotationally support the barrel. The barrel includes an entry restriction part. The entry restriction part restricts entry of foreign objects from the axial outer side of the barrel support shaft into the bearings. Mecanum wheels are known as wheels used for a transfer device. This wheel includes a plurality of barrels and a barrel holder. The barrels are barrel-shaped rotating bodies. The barrel holder supports the plurality of barrels rotatably on the circumference of the barrel holder. The barrel holder is rotationally driven by a drive device such as a motor (see, for example, <CIT>).

The plurality of barrels are held on the circumference of the barrel holder at an angle to the holder rotation axis of the barrel holder. Such wheels are disposed at around the four corners of the transfer device. Each wheel located at the four corners of the transfer device is independently controlled by the drive device. That is, the transfer device can move freely in various directions by controlling the direction of rotation and torque of each wheel individually. The wheels on the left side and the wheels on the right side of the transfer device are set differently such that their barrels have the opposite inclination held on the circumference of the barrel holder.

A pair of support flanges are provided at two axially outer ends of the barrel holder and extend outward in the radial direction of the barrel holder. A plurality of barrel support shafts extend between the pair of support flanges and are fixed thereon. The barrels are rotatably supported by the corresponding barrel support shaft and barrel holder via radial and thrust bearings.

The thrust bearing is disposed in a bearing receiving portion between a circular recess formed at the axial end of the barrel and the outer surface of the barrel support shaft. The thrust bearing includes a first race, a second race, and a plurality of rolling elements. The first race contacts a member disposed on the bottom side of the bearing receiving portion in the bearing receiving portion. The second race contacts a member disposed on the support flange side of the barrel holder. The plurality of rolling elements rotatably contact with the first race and the second race. The plurality of rolling elements are rotatably supported by a retainer. The rolling elements and the retainer are disposed between the first and second races.

In the above conventional wheel (mecanum wheel), once the barrel holder is rotationally driven by a drive device such as a motor, the outer surfaces of the barrels sequentially contact a traveling surface on which the wheel travels. The transfer device is propelled by receiving reaction force from the traveling surface. At this time, the contact position of each barrel where it contacts the ground surface moves continuously between one end and the other of the barrel in the axial direction as the barrel holder rotates. Thus, when the transfer device travels, a large thrust load is applied alternately to the thrust bearing disposed at axially one end of the barrel and the thrust bearing disposed at the other end of the barrel. When a large thrust load is applied to one thrust bearing, the thrust load on the other thrust bearing is reduced. This may displace the first and second races in a direction away from the plurality of rolling elements. If the separation of the first and second races from the rolling elements becomes large, the retainer may be displaced in the radial direction together with the rolling elements. In this case, there is a possibility that the retainer may interfere with surrounding components such as the barrel support shaft.

To address this, for example, the clearance between the thrust bearing and a member (e.g., the member on the barrel holder side) that contacts the first and second races in the axial direction may be strictly controlled. However, this approach increases the manufacturing cost of the components and requires the manufacturer to perform complicated work to control the clearance.

It is an object of the present invention to provide a wheel in which interference between a bearing retainer and surrounding members can be prevented without requiring strict clearance control between the members. According to the present invention said object is solved by a wheel having the features of the independent claim <NUM>. Preferred embodiments are laid down in the dependent claims.

In the wheel according to one aspect of the invention, the groove is provided in the region of the outer circumferential surface of the barrel support shaft. The region extends in the axial direction and includes the radially inner projected area of the retainer. With this configuration, when the direction of the thrust load applied to the bearing changes alternately as the wheel rotates, the bearing retainer is radially displaced, which prevents interference with the outer surface of the barrel support shaft. Therefore, by employing the wheel according to one aspect of the invention, interference between the bearing retainer and the surrounding members can be prevented without requiring strict clearance control between the members.

In the wheel according to another aspect of the invention, the groove is provided at least in the region of the inner circumferential surface of the recess. The region extends in the axial direction and includes a radially outer projected area of the retainer. With this configuration, when the direction of the thrust load applied to the bearing changes alternately as the wheel rotates, the bearing retainer is radially displaced, which prevents interference with the inner circumferential surface of the recess formed in the barrel. Therefore, by employing the wheel according to another aspect of the invention, interference between the bearing retainer and the surrounding members can be prevented without requiring strict clearance control between the members.

The embodiments will be hereinafter described with reference to the drawings. In the following embodiments, like elements may be labeled similarly and redundant descriptions will be omitted. As shown in <FIG>, a wheel <NUM> in the embodiments is used for a transfer device <NUM>. <FIG> illustrates the transfer device <NUM> viewed from above. The wheels <NUM> of each embodiment are disposed on the front left and right sides of a vehicle body <NUM> of the transfer device <NUM> and on the rear left and right sides of the vehicle body <NUM>. Each wheel <NUM> is driven independently by a drive device <NUM> supported by the vehicle body <NUM>. The drive device <NUM> includes a motor <NUM> and a speed reducer <NUM> that reduces the rotation of the motor <NUM> and transmits the reduced rotations to the corresponding wheel <NUM>. In this embodiment, a drive unit includes the wheel <NUM> and the drive device <NUM> that rotationally rotates the wheel <NUM>. The wheels on the left side and the wheels on the right side of the transfer device <NUM> are set in opposite direction such that their barrels <NUM> have the opposite inclination held on the circumference of a barrel holder <NUM>, which will be later described.

<FIG> is a perspective view of the wheel <NUM> relating to the first embodiment. <FIG> is a sectional view along line III-III of <FIG>. <FIG> is a sectional view along line IV-IV of <FIG>. <FIG> schematically illustrates the internal structure of the barrel holder <NUM> in a simplified form to avoid complex illustration of the internal structure of the barrel <NUM>, which is described later. As shown in <FIG>, the wheel <NUM> includes the cylindrical barrel holder <NUM> rotationally driven by the drive device <NUM> (see <FIG>), a plurality of barrel support shafts <NUM> supported on the circumference of the barrel holder <NUM>, and the barrels <NUM>, which are barrel-shaped rotating bodies, supported on the respective barrel support shafts <NUM>. Only one set of the barrel <NUM> and the barrel mounting parts such as the barrel support shaft <NUM> is shown in <FIG>, and the remaining sets including other barrels <NUM> are omitted. Only the central axis lines L2 of the barrel support shafts <NUM> for the remaining sets of the barrels <NUM> are shown in <FIG>.

The barrel holder <NUM> includes a pair of support flanges <NUM> disposed spaced apart in the axial direction (in the width direction of the vehicle body <NUM>) from each other, and a connecting cylinder <NUM> that connects the pair of support flanges <NUM>. The pair of support flanges <NUM> are formed in the shape of a disk with a center hole. Accordingly, the pair of support flanges <NUM> are formed in a substantially cylindrical shape continuous in the circumferential direction. Inside the barrel holder <NUM>, which has a substantially cylindrical shape, the drive device <NUM> (speed reducer <NUM> and a part of the motor <NUM>) shown in <FIG> is disposed. The output portion of the speed reducer <NUM> is connected, for example, to the support flange <NUM>. The output portion of the speed reducer <NUM> reduces the rotation of the motor <NUM> and transmits the reduced rotation to the barrel holder <NUM>. The axis of rotation of the barrel holder <NUM> when the barrel holder <NUM> is rotated by the rotation of the motor <NUM> is referred to as a holder rotation axis L1.

The outer diameter of the connecting cylinder <NUM> is smaller than that of the support flange <NUM>. Fixing grooves <NUM> are formed on the outer periphery of each support flange <NUM>. Both ends of the barrel support shaft <NUM> in its axial direction, which supports the corresponding barrel <NUM>, are fixed in the fixing groove <NUM>, for example, by a bolt <NUM>. The barrel support shafts <NUM> are fixed to the pair of support flanges <NUM> at a predetermined angle to the holder rotation axis L1. Barrel receiving recesses <NUM> are formed in the side edge of each of the support flanges <NUM> closer to the connecting cylinder <NUM>. The axial end of the barrel <NUM> supported by the barrel support shaft <NUM> is accommodated in the barrel receiving recess <NUM>. Each barrel receiving recess <NUM> has an opposing wall <NUM> that faces an axial end surface of the barrel <NUM>. The opposing wall <NUM> is orthogonal to a central axis L2 of the barrel <NUM> housed in the barrel receiving recess <NUM>.

The barrel <NUM> supported by the corresponding barrel support shaft <NUM> includes a barrel base body <NUM> made of metal and a skin material <NUM> made of urethane. The barrel base body <NUM> has a barrel shape, bulging in the axial center region of the barrel base body <NUM>. The skin material <NUM> is bonded to the outer periphery of the barrel base body <NUM> by adhesive or other means. As shown in <FIG>, a shaft hole <NUM> extending in the axial direction is formed in the barrel base body <NUM> at the axial center of the barrel base body. The barrel support shaft <NUM> is inserted in the shaft hole <NUM>. Between the shaft hole <NUM> of the barrel base body <NUM> and the barrel support shaft <NUM>, a radial bearing <NUM> and thrust bearing <NUM> are inserted to freely support the barrel <NUM> on the barrel support shaft <NUM>. The radial bearing <NUM> and thrust bearing <NUM> are disposed in the shaft hole <NUM> and at each end of the shaft hole <NUM> in the axial direction. In the shaft hole <NUM>, the thrust bearing <NUM> is situated axially more outward than the radial bearing <NUM> and adjacent to the radial bearing <NUM>. The thrust bearing <NUM> encloses the barrel support shaft <NUM> and is located near the axially outer end of the barrel <NUM>. The thrust bearing <NUM> receives a thrust loads acting on the barrel <NUM>.

At the axially outer end of the barrel base material <NUM> is provided with a recess <NUM> that extends inwardly in the axial direction. The recess <NUM> is formed continuously with the shaft hole <NUM> in the barrel base body <NUM> and has a lager inner diameter than the shaft hole. The axial end of the barrel <NUM> is opposed to the opposing wall <NUM> of the barrel holder <NUM> in the barrel receiving recess <NUM> of the barrel holder <NUM>. The thrust bearing <NUM> is disposed in a bearing housing portion <NUM> defined by the recess <NUM> formed in the barrel <NUM> and the outer surface of the barrel support shaft <NUM>.

The thrust bearing <NUM> includes a first race 24a, a second race 24b, a plurality of rolling elements 24c (e.g., spherical bodies), and a retainer 24d. The first race 24a and the second race 24b are formed in a disk-shape with a center hole. The first race 24a and the second race 24b are arranged to face each other in the axial direction in the recess <NUM> formed in the barrel <NUM>. The first race 24a is disposed on the bottom wall side in the recess <NUM>. The outer end surface of the first race 24a abuts on the axially outward facing end surface <NUM> (first member) among the walls defining the recess <NUM>. The barrel support shaft <NUM> is fitted into the respective center holes of the first race 24a and the second race 24b. The thrust bearing <NUM> including the first race 24a and the second race 24b is disposed near the axially outer end of the barrel <NUM> and surrounds around the barrel support shaft <NUM>.

The plurality of rolling elements 24c are arranged annularly between the first race 24a and the second race 24b. Furthermore, the plurality of rolling elements 24c rollably contact an inner end surface of the first race 24a and an inner end surface of the second race 24b. The inner end surfaces of the first race 24a and the second race 24b are opposed to each other. The retainer 24d is formed in an annular shape and disposed between the first race 24a and the second race 24b. The retainer 24d rotatably holds the plurality of rolling elements 24c.

A metallic cylindrical spacer block <NUM> (second member) is fitted on the outer circumference of the barrel support shaft <NUM>. The spacer block <NUM> is fitted on the outer circumference of a part of the barrel support shaft <NUM> situated outside of the thrust bearing <NUM> (second race 24b) in the axial direction. The outer diameter of the spacer block <NUM> is smaller than the inner diameter of the recess <NUM> formed in the barrel <NUM>. A part of the spacer block <NUM> is inserted into the recess <NUM> from the outside in the axial direction. A first end surface of the spacer block <NUM> situated on one side in the axial direction abuts against the opposing wall <NUM> of the barrel holder <NUM>. A second end surface of the spacer block <NUM> situated on the other side in the axial direction abuts against the outer end surface of the second race 24b of the thrust bearing <NUM> (end surface facing away from the rolling element 24c).

An annular recess <NUM> dents in a stepped manner and away from the second race 24b is formed on the inner circumference of the second end surface of the spacer block <NUM>. Therefore, the annular recess <NUM> opens radially inward and toward the second race 24b. The bottom surface of the annular recess <NUM> is a flat surface perpendicular to the axial direction of the spacer block <NUM>. An annular elastic member <NUM> is provided in the annular recess <NUM>. The elastic member <NUM> is, for example, an O-ring made of rubber, elastic resin, or the like. The elastic member <NUM> is attached on the outer surface of the barrel support shaft <NUM>. The elastic member <NUM> basically has the outer diameter larger than the axial depth of the annular recess <NUM>. When the barrel <NUM> and barrel support shaft <NUM> together with the thrust bearing <NUM> and spacer block <NUM> are assembled on the barrel holder <NUM>, the elastic member <NUM> is disposed between the bottom surface of the annular recess <NUM> (the surface facing the second race 24b) and the outer end surface of the second race 24b. At the same time, the elastic member <NUM> elastically contacts the outer surface of the barrel support shaft <NUM>. The second race 24b is pressed toward the first race 24a by the elastic force (elastic restoring force) of the elastic member <NUM>.

A groove <NUM> denting radially inward is formed in a portion of the outer circumferential surface of the barrel support shaft <NUM> that is exposed inside the bearing housing portion <NUM>. The groove <NUM> is formed in the outer surface of the barrel support shaft <NUM> with a predetermined width and a predetermined depth. The groove <NUM> is formed at least in a region of the outer circumferential surface of the barrel support shaft <NUM> situated on radially inner side of the retainer 24d and is formed along the axial direction. That is, the groove <NUM> extends in the axial direction and at least in a region of the outer surface of the barrel support shaft <NUM> that includes a radially inner projected area of the retainer 24d. More specifically, the groove <NUM> extends in the axial direction in the outer circumferential surface of the barrel support shaft <NUM> beyond an end portion 24ai of the inner end surface (surface situated closer to the rolling element 24c) of the first race 24a toward an end portion 24ao of the outer end surface (surface situated away from the rolling element 24c) of the first race 24a. The groove <NUM> further extends in the axial direction in the outer circumferential surface of the barrel support shaft <NUM> beyond an end portion 24bi of the inner end surface (surface situated closer to the rolling element 24c) of the second race 24b toward an end portion 24bo of the outer end surface (surface situated away from the rolling elements 24c) of the second race 24b. Note that the groove <NUM> is formed such that it does not cross over (extend beyond) the end portion 24ao of the first race 24a and the end portion 24bo of the second race 24b. The depth d1 of the groove <NUM> formed in the barrel support shaft <NUM> is preferably <NUM> ≤ d1 < <NUM> (that is, d1 is equal to or more than <NUM> and less than <NUM>).

The outer surface of the axially central region of the barrel support shaft <NUM> serves as a contact surface with which the radial bearing <NUM> contacts. The outer surfaces of the axially end regions of the barrel support shaft <NUM> are accurately fastened to the barrel holder <NUM> (the fixing grooves <NUM> of the support flanges <NUM>). The required outer diameter of the outer surface of the central region of the barrel support shaft <NUM> is different from the required outer surface of the end regions of the barrel support shaft <NUM>. Thus, a step of grinding the outer surface of the central region of the barrel support shaft <NUM> is performed separately from a step of grinding the outer surface of the end regions of the barrel support shaft <NUM>. The groove <NUM> formed in the barrel support shaft <NUM> also serves as a grinding groove for preventing the outer surface of the central region of the barrel support shaft <NUM> from being ground when the step of grinding the outer surface of the end regions of the barrel support shaft <NUM> is performed and preventing the outer surface of the end regions of the barrel support shaft <NUM> from being ground when the step of grinding the outer surface of the central region of the barrel support shaft <NUM> is performed.

The inner circumferential surface of the recess <NUM> further has a groove <NUM> that dents toward the radially outside. The groove <NUM> is formed in the inner circumferential surface of the recess <NUM> with a predetermined width and a predetermined depth. The groove <NUM> is formed at least in a region of the inner circumferential surface of the recess <NUM> situated radially outer side of the retainer 24d and is formed along the axial direction. That is, the groove <NUM> is provided at least in the region of the inner circumferential surface of the recess <NUM> that extends in the axial direction and includes a radially outer projected area of the retainer 24d. Similarly to the depth d1 of the groove <NUM>, the depth d2 of the groove <NUM> formed in the inner circumferential surface of the recess <NUM> is preferably <NUM> ≤ d2 < <NUM> (that is, d2 is equal to or more than <NUM> and less than <NUM>).

As described above, the wheel <NUM> of this embodiment has the groove <NUM> formed at least in the outer surface of the portion of the barrel support shaft <NUM> situated on radially inner side of the retainer 24d of the thrust bearing <NUM>. In particular, the groove <NUM> is formed along the axial direction in the region of the outer surface of the barrel support shaft <NUM> including the projected area of the retainer 24d. Thus, even if a separation between the first race 24a and the second race 24b of the thrust bearing <NUM> increases and this allows the retainer 24d of the thrust bearing <NUM> to be displaced radially inward, interference of the retainer 24d with the outer surface of the barrel support shaft <NUM> can be avoided by the groove <NUM>. Therefore, by employing the wheel <NUM> of the embodiment, interference between the retainer 24d of the thrust bearing <NUM> and the surrounding members can be prevented without requiring strict gap control between the members. This makes it possible to reduce wear powder and like generated due to the interference of the retainer 24d with the surrounding members while simplifying the manufacturing process.

The wheel <NUM> of the embodiment has the groove <NUM> that is formed in the outer circumferential surface of the barrel support shaft <NUM> to extend along the axial direction. The groove <NUM> extends such that it crosses over the end portions 24ai and 24bi of the inner opposing end surfaces of the first race 24a and the second race 24b. Thus, even if the separation between the first race 24a and the second race 24b of the thrust bearing <NUM> increases and the retainer 24d of the thrust bearing <NUM> is displaced a little in the axial direction, the interference of the retainer 24d with the outer surface of the barrel support shaft <NUM> can be reliably avoided.

Further, in the wheel <NUM> of the embodiment, the groove <NUM> is formed in the outer circumferential surface of the barrel support shaft <NUM> along the axial direction and the groove <NUM> does not cross over (extend beyond) the end portions 24ao and 24bo of the outer end surfaces of the first race 24a and the second race 24b facing away from the rolling elements 24c. Thus, the groove <NUM> does not interfere with the stable support of the first race 24a and the second race 24b with respect to the barrel support shaft <NUM>. Therefore, with this configuration, rattling of the thrust bearing <NUM> can be suppressed and the wheel can move quietly.

Moreover, the wheel <NUM> of the embodiment has the groove <NUM> formed in the inner circumferential surface of the portion of the recess <NUM> situated at least radially outer side of the retainer 24d of the thrust bearing <NUM>. In particular, the groove <NUM> is formed along the axial direction in the region including the projected area of the retainer 24d. Thus, even if the separation between the first race 24a and the second race 24b of the thrust bearing <NUM> increases and this allows the retainer 24d of the thrust bearing <NUM> to be displaced radially outward, interference of the retainer 24d with the inner circumferential surface of the recess <NUM> can be avoided by the groove <NUM>.

The wheel <NUM> of the embodiment has the groove <NUM> formed in the outer circumferential surface of the barrel support shaft <NUM> and the groove <NUM> formed in the inner circumferential surface of the recess <NUM> of the barrel <NUM>. However, the present invention is not limited to this case. For example, it is also possible to form a relief groove only in either the outer circumferential surface of the barrel support shaft <NUM> or the inner circumferential surface of the recess <NUM>. When the relief groove (groove <NUM>) is formed only in the inner circumferential surface of the recess <NUM>, it is also possible to avoid the interference between the retainer 24d of the thrust bearing <NUM> and the inner circumferential surface of the recess <NUM>.

The wheel <NUM> of the embodiment has the elastic member <NUM> provided between the spacer block <NUM> and the second race 24b disposed around the barrel support shaft <NUM>. The elastic member <NUM> contacts the spacer block <NUM>, the end surface of the second race 24b, and the outer surface of the barrel support shaft <NUM>. Thus, the elastic restoring force of the elastic member <NUM> can be utilized to constantly press the second race 24b against the first race 24a. In this way, it is possible to prevent increase of the separation between the first race 24a and the second race 24b. As described above, with the above configuration, the displacement of the retainer 24d in the radial direction is prevented and thus it is possible to prevent the interference of the retainer 24d with the outer surface of the barrel support shaft <NUM> and the inner circumferential surface of the recess <NUM>.

Furthermore, in this configuration, the elastic member <NUM> contacts the spacer block <NUM>, the end surface of the second race 24b, and the outer surface of the barrel support shaft <NUM>, so that it is possible to suppress rattle or clatter between the second race 24b and the spacer block <NUM> by the elastic member <NUM>. Moreover, it is possible to hold the spacer block <NUM> around the outer circumference of the barrel support shaft <NUM> by the elastic member <NUM> without using a fastening member or the like. With this configuration, it is not necessary to provide a dedicated part for fastening the spacer block <NUM> to prevent rotate around the outer circumference of the barrel support shaft <NUM>.

<FIG> is a sectional view similar to <FIG> of the wheel <NUM> of the first embodiment. As shown in <FIG>, the basic configuration of a wheel <NUM> in this embodiment is almost the same as that of the first embodiment. However, the configuration of the spacer block <NUM>, which is disposed adjacent to the thrust bearing <NUM> and around the barrel support shaft <NUM>, differs from the first embodiment. The spacer block <NUM> of the first embodiment has the annular recess <NUM> that has a rectangular cross section along the inner edge of the end surface facing the second race 24b. Whereas in the second embodiment, as shown in <FIG>, an annular recess <NUM> of a spacer block <NUM> includes a tapered surface 136a whose inner diameter increases gradually toward the second race 24b. When the barrel <NUM> and barrel support shaft <NUM> together with the thrust bearing <NUM> and the spacer block <NUM> are assembled on the barrel holder <NUM>, the elastic member <NUM> is disposed between the tapered surface 136a of the annular recess <NUM> and the outer end surface of the second race 24b. At the same time, the elastic member <NUM> elastically contacts the outer surface of the barrel support shaft <NUM>.

The wheel <NUM> of this embodiment is basically configured in the same manner as the first embodiment so that the wheel <NUM> can obtain the same advantageous effects as the first embodiment. In the wheel <NUM> of this embodiment, the annular recess <NUM> of the spacer block <NUM> has the tapered surface 136a. Thus, when the spacer block <NUM> is placed in contact with the barrel support shaft <NUM>, the barrel <NUM>, and the barrel holder <NUM>, together with the thrust bearing <NUM>, the elastic member <NUM> can be efficiently pressed against the outer surface of the barrel support shaft <NUM> and the second race 24b. With this configuration, it is possible to more securely control the separation between the second race 24b and the first race 24a and prevent the spacer block <NUM> from rotating around.

Claim 1:
A wheel (<NUM>), comprising:
a barrel holder (<NUM>) rotationally driven;
a plurality of barrel support shafts (<NUM>) arranged on a circumference of the barrel holder (<NUM>) at an angle to a rotation axis of the barrel holder (<NUM>);
barrels (<NUM>) rotatably supported on the corresponding barrel support shafts (<NUM>), each of the barrels (<NUM>) being a barrel-shaped rotating body, outer surfaces of the barrels (<NUM>) sequentially contacting a surface on which the wheel (<NUM>) travels as the barrel holder (<NUM>) rotates; and
bearings (<NUM>) disposed between the respective barrel support shafts (<NUM>) and the respective barrels (<NUM>), each of the bearings (<NUM>) rotatably supporting the corresponding barrel (<NUM>) and having a retainer (24d),
wherein the bearing (<NUM>) is disposed at an axially outer end of the barrel and encloses the barrel support shaft (<NUM>),
wherein a groove (<NUM>) is provided at least in a region of an outer circumferential surface of the barrel support shaft (<NUM>), the region extends in an axial direction and includes a radially inner projected area of the retainer (24d),
wherein the bearing (<NUM>) includes:
a first race (24a) abutting a first member in the axial direction, the first member being disposed closer to the barrel (<NUM>) than the bearing (<NUM>);
a second race (24b) abutting a second member in the axial direction, the second member being disposed closer to the barrel holder (<NUM>) than the bearing (<NUM>); and
a plurality of rolling elements disposed between the first race (24a) and the second race (24b), the plurality of rolling elements (24c) rollably contacting the first race (24a) and the second race (24b);
characterized in that
the retainer (24d) is disposed between the first race (24a) and the second race (24b) and supports the plurality of rolling elements (24c),
the groove (<NUM>) is provided in the outer circumferential surface of the barrel support shaft (<NUM>), the groove (<NUM>) extends in the axial direction and crosses over an end portion (24ai) of an inner end surface of the first race (24a) and an end portion (24bi) of an inner end surface of the second race (24b), and
wherein the inner end surface of the first race (24a) and the inner end surface of the second race (24b) oppose to each other.