US20060064872A1

US20060064872A1 - Camshaft and assembling method thereof

Info

Publication number: US20060064872A1
Application number: US11/234,825
Authority: US
Inventors: Tetsumi Shirai
Original assignee: Musashi Seimitsu Industry Co Ltd
Current assignee: Musashi Seimitsu Industry Co Ltd
Priority date: 2004-09-28
Filing date: 2005-09-23
Publication date: 2006-03-30
Also published as: JP2006097485A

Abstract

The present invention is an assembling type of camshaft including: a cam piece having a shaft hole; and a shaft to be press-fitted into the shaft hole of the cam piece; wherein a press-fitting groove and a penetration groove are respectively formed to extend in an axial direction in an inside circumferential surface of the shaft hole of the cam piece; a press-fitting tooth is formed to extend in the axial direction partially in an outside circumferential surface of the shaft; and the press-fitting tooth is adapted to be press-fitted into the press-fitting groove in the axial direction and to pass through the penetration groove in the axial direction.

Description

TECHNICAL FIELD

The present invention relates to a camshaft used in an internal combustion engine of an automobile or the like.

BACKGROUND ART

A camshaft used in an engine of an automobile is generally an integral-type camshaft formed by casting. However, recently, use of an assembling type of camshaft has been increased, in which one or more cam pieces are press-fitted to a shaft.
Every automobile part is required to weigh less. In the assembling type of camshaft, a hollow pipe may be used for the shaft, so that weight reduction and cost saving may be easily achieved. In addition, when the cam pieces need to have a predetermined hardness, only the cam pieces may be thermally processed, so that the cam pieces may be easily made harder than the shaft. Thus, the assembling type of camshaft can be manufactured at lower cost, compared with a case wherein the whole unitary camshaft undergoes a thermal process.
An example of the assembling type of camshaft is shown in FIG. 10. In the assembling type of camshaft 101 shown in FIG. 10, the inside diameter of a cam piece 106 is formed a little smaller than the outside diameter of a shaft 102. Then, the cam piece 106 is fixed to the shaft 102 by means of the difference (interference).
Herein, an assembling method of the camshaft 101 as shown in FIG. 10 is explained. The cam piece 106 is press-fitted to a predetermined position on the shaft 102 while a phase of the cam piece 106 is adjusted, from a one-side end of the shaft 102 that is hollow. Subsequently, the other cam pieces 106 are press-fitted in turn. Thus, the cam shaft 101 is assembled.
Herein, since the inside diameter of the cam piece 106 is a little smaller than the outside diameter of the shaft 102, the outside surface of the camshaft 102 and the inside surface of the cam piece 106 may be damaged while the cam piece 106 moves to LABEL the predetermined position on the shaft 102. If their surfaces are damaged, the press-fitting strength between the cam piece 106 and the shaft 102 may be deteriorated.
In addition, in order to enhance the press-fitting strength in a rotation direction, the interference has to be increased. However, if the interference is increased too much, residual stress of the cam piece 106 may be increased too much, and fatigue strength thereof may be deteriorated, that is, durability of the camshaft 101 may be deteriorated.
Furthermore, the cam piece 106 has to be press-fitted to the shaft 102 under a state wherein the phase of the cam piece 106 has been adjusted in advance. However, the phase of the cam piece 106 may be undesirably changed during the press-fitting step.
In another assembling type of camshaft 201 as shown in FIG. 11, spline engaging grooves 208 are formed in a cam piece 206, and spline engaging teeth 204 are formed in a shaft 202. The spline engaging grooves 208 and the spline engaging teeth 204 are engaged in a spline manner with each other. In the camshaft 201, since the spline engaging grooves 208 of the cam piece 206 are press-fitted to the spline engaging teeth 204 of the shaft 202, rotation of the cam piece 206 may be inhibited. In addition, phase relation between the cam piece 206 and the shaft 202 may be adjusted at the same time.
However, in the camshaft 201 as shown in FIG. 11 as well, since the spline engaging grooves 208 of the cam piece 206 are smaller than the spline engaging teeth 204 of the shaft 202, the spline engaging teeth 204 of the shaft 202 and the spline engaging grooves 208 of the cam piece 206 may be damaged while the cam piece 206 moves to a predetermined press-fitting position on the shaft 202.

DISCLOSURE OF THE INVENTION

As described above, the problem of the conventional assembling type of camshaft wherein the shaft is press-fitted into a shaft hole of the cam piece by using an interference is that an outside surface of the shaft and an inside surface of the cam piece may be damaged while the cam piece moves to a predetermined press-fitting position on the shaft, so that press-fitting strength of the shaft and the cam piece may be deteriorated.
Accordingly, it is an object of the present invention to solve the aforesaid problems and to provide a camshaft and an assembling method thereof wherein press-fitting surfaces suffer no damage during the assembling step so that press-fitting strength is not deteriorated.
The present invention is an assembling type of camshaft comprising: a cam piece having a shaft hole; and a shaft to be press-fitted into the shaft hole of the cam piece; wherein a press-fitting groove and a penetration groove are respectively formed to extend in an axial direction in an inside circumferential surface of the shaft hole of the cam piece; a press-fitting tooth is formed to extend in the axial direction partially in an outside circumferential surface of the shaft; and the press-fitting tooth is adapted to be press-fitted into the press-fitting groove in the axial direction and to pass through the penetration groove in the axial direction.
According to the above feature, since the press-fitting groove of the cam piece and the press-fitting tooth of the shaft are press-fitted to each other, press-fitting strength in a rotation direction may be enhanced. Thus, it is unnecessary to increase the interference. In addition, residual stress can flee to the penetration groove. Thus, deterioration of fatigue strength may be inhibited.
Preferably, a plurality of press-fitting teeth is formed at regular or odd intervals in a circumferential direction of the outside circumferential surface of the shaft, a plurality of press-fitting grooves and a plurality of penetration grooves are respectively formed at regular or odd intervals in a circumferential direction of the inside circumferential surface of the shaft hole of the cam piece, correspondingly to an arrangement of the plurality of press-fitting teeth, and the plurality of press-fitting teeth is adapted to be press-fitted into the plurality of press-fitting grooves in the axial direction together and to pass through the plurality of penetration grooves in the axial direction together. In the case, the press-fitting strength in a rotation direction may be enhanced more. In addition, the residual stress can flee to the penetration groove more effectively.
For example, the press-fitting manner between the plurality of press-fitting teeth and the plurality of press-fitting grooves may be an involute spline manner wherein the press-fitted area (contact line) follows an involute curve. In the case, the contact surface of the cam piece and the shaft is stable. Thus, generation of looseness (backlash) may be remarkably inhibited.
Alternatively, the press-fitting manner between the plurality of press-fitting teeth and the plurality of press-fitting grooves may be a serration manner wherein the press-fitted area (contact area) forms a serration plane. In the case too, the contact surface of the cam piece and the shaft is stable. Thus, generation of looseness (backlash) may be remarkably inhibited.
Preferably, the number of press-fitting grooves is a multiple of twelve, the press-fitting grooves are formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of the cam piece, the number of penetration grooves is also a multiple of twelve, the penetration grooves are also formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of the cam piece. Such a cam piece may be widely used for respective engine shafts of a three-cylinder engine, a four-cylinder engine, a six-cylinder engine and an eight-cylinder engine.
In addition, the present invention is an assembling type of camshaft comprising: a plurality of cam pieces each of which has a shaft hole; and a shaft to be press-fitted into the shaft holes of the plurality of cam pieces; wherein a press-fitting groove and a penetration groove are respectively formed to extend in an axial direction in an inside circumferential surface of the shaft hole of each cam piece; a plurality of press-fitting-tooth parts is formed at regular or odd intervals in the axial direction in an outside circumferential surface of the shaft, each of the plurality of press-fitting-tooth parts having a press-fitting tooth extending partially in the axial direction; and the press-fitting tooth is adapted to be press-fitted into the press-fitting groove in the axial direction and to pass through the penetration groove in the axial direction.
According to the above feature, since the press-fitting groove of each cam piece and the press-fitting tooth of the shaft are press-fitted to each other, press-fitting strength in a rotation direction may be enhanced. Thus, it is unnecessary to increase the interference. In addition, residual stress can flee to the penetration groove. Thus, deterioration of fatigue strength may be inhibited.
Preferably, each of the plurality of press-fitting-tooth parts has a plurality of press-fitting teeth formed at regular or odd intervals in a circumferential direction of the outside circumferential surface of the shaft, a plurality of press-fitting grooves and a plurality of penetration grooves are respectively formed at regular or odd intervals in a circumferential direction of the inside circumferential surface of the shaft hole of each cam piece, correspondingly to an arrangement of the plurality of press-fitting teeth, and the plurality of press-fitting teeth is adapted to be press-fitted into the plurality of press-fitting grooves in the axial direction together and to pass through the plurality of penetration grooves in the axial direction together. In the case, the press-fitting strength in a rotation direction may be enhanced more. In addition, the residual stress can flee to the penetration groove more effectively.
For example, the press-fitting manner between the plurality of press-fitting teeth and the plurality of press-fitting grooves may be an involute spline manner wherein the press-fitted area (contact line) follows an involute curve. In the case, the contact surface of the cam piece and the shaft is stable. Thus, generation of looseness (backlash) may be remarkably inhibited.
Alternatively, the press-fitting manner between the plurality of press-fitting teeth and the plurality of press-fitting grooves may be a serration manner wherein the press-fitted area (contact area) forms a serration plane. In the case too, the contact surface of the cam piece and the shaft is stable. Thus, generation of looseness (backlash) may be remarkably inhibited.
Preferably, the number of press-fitting grooves is a multiple of twelve, the press-fitting grooves are formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of each cam piece, the number of penetration grooves is also a multiple of twelve, the penetration grooves are also formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of each cam piece. Such cam pieces may be widely used for respective engine shafts of a three-cylinder engine, a four-cylinder engine, a six-cylinder engine and an eight-cylinder engine.
In addition, the present invention is an assembling method of an assembling type of camshaft including: a plurality of cam pieces each of which has a shaft hole, and a shaft to be press-fitted into the shaft holes of the plurality of cam pieces, wherein a press-fitting groove and a penetration groove are respectively formed to extend in an axial direction in an inside circumferential surface of the shaft hole of each cam piece, a plurality of press-fitting-tooth parts is formed at regular or odd intervals in the axial direction in an outside circumferential surface of the shaft, each of the plurality of press-fitting-tooth parts having a press-fitting-tooth extending partially in the axial direction, and the press-fitting tooth is adapted to be press-fitted into the press-fitting groove in the axial direction and to pass through the penetration groove in the axial direction; the assembling method comprising: a step of locating each cam piece at a predetermined position by causing each cam piece to pass over one or more portions of the shaft wherein no press-fitting-tooth part is formed, if necessary, and by causing each cam piece to pass through one or more press-fitting-tooth parts of the shaft by using the penetration groove of each cam piece, if necessary; a step of adjusting a phase of each cam piece to the shaft by rotating each come piece at the predetermined position; and a step of press-fitting each cam piece to the press-fitting tooth of a press-fitting-tooth part of the shaft by using the press-fitting groove of each cam piece.
According to the above feature, the cam shaft can be assembled without damaging the inside surface of the shaft hole of each cam piece and the outside surface of the shaft. In addition, press-fitting strength of the assembled camshaft is high enough.
In addition, the present invention is a cam piece having a shaft hole to be press-fitted to a shaft; wherein a plurality of press-fitting grooves is formed at regular intervals in a circumferential direction of an inside circumferential surface of the shaft hole, each of the plurality of press-fitting grooves extending in an axial direction, the number of the plurality of press-fitting grooves being a multiple of twelve; a plurality of penetration grooves is formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole correspondingly to an arrangement of the plurality of press-fitting grooves, each of the plurality of penetration grooves extending in the axial direction, the number of the plurality of penetration grooves being a multiple of twelve; the plurality of press-fitting grooves are adapted to be press-fitted together in the axial direction to a plurality of press-fitting teeth formed on an outside circumferential surface of the shaft correspondingly to an arrangement of the plurality of press-fitting grooves; and the plurality of penetration grooves are adapted to pass through the plurality of press-fitting teeth together in the axial direction.
The cam piece having the above feature can be assembled with the shaft into a camshaft having satisfactory press-fitting strength without damaging the outside surface of the shaft. In addition, the cam piece having the above feature can be widely used for respective engine shafts of a three-cylinder engine, a four-cylinder engine, a six-cylinder engine and an eight-cylinder engine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an assembling type of camshaft according to an embodiment of the present invention;
FIG. 2 is a cross-sectional view of FIG. 1;
FIG. 3 is a plan view showing a first assembling step of the camshaft of FIG. 1;
FIG. 4 is a plan view showing a second assembling step of the camshaft of FIG. 1;
FIG. 5 is a plan view showing a third assembling step of the camshaft of FIG. 1;
FIG. 6 is a plan view showing a fourth assembling step of the camshaft of FIG. 1;
FIG. 7 is a cross-sectional view taken along the A-A line in FIG. 4;
FIG. 8 is a cross-sectional view taken along the B-B line in FIG. 5;
FIG. 9 is a cross-sectional view taken along the C-C line in FIG. 6;
FIG. 10 is a cross-sectional view of a conventional assembling type of camshaft; and
FIG. 11 is a cross-sectional view of another conventional assembling type of camshaft.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described hereinafter with reference to the attached drawings.
FIG. 1 is a plan view of an assembling type of camshaft according to an embodiment of the present invention. FIG. 2 is a cross-sectional view of FIG. 1. FIG. 3 is a plan view showing a first assembling step of the camshaft of FIG. 1. FIG. 4 is a plan view showing a second assembling step of the camshaft of FIG. 1. FIG. 5 is a plan view showing a third assembling step of the camshaft of FIG. 1. FIG. 6 is a plan view showing a fourth assembling step of the camshaft of FIG. 1. FIG. 7 is a cross-sectional view taken along the A-A line in FIG. 4. FIG. 8 is a cross-sectional view taken along the B-B line in FIG. 5. FIG. 9 is a cross-sectional view taken along the C-C line in FIG. 6.
As shown in FIGS. 1 and 2, the assembling type of camshaft 1 according to the embodiment includes a cylindrical hollow shaft 2, a plurality of cam pieces (cam roller) 6 mounted on the shaft 2, a plurality of cam journal 11 mounted on the shaft 2, and end members 12 mounted at both ends of the shaft 2.
Each cam piece 6 has a base circle portion having a section of a complete circle, and a top portion smoothly protruding from the base circle portion, and a through shaft hole 7 extending in the axial direction concentrically with the base circle portion. Each cam journal 11 is disk-like, and has a through shaft hole 7 extending in the axial direction in the same manner as each cam piece 6.
Each press-fitting position of each cam piece 6 and each cam journal 11 on the outside surface of the shaft 2 is a spline-tooth part (press-fitting-tooth part) 3 formed partially in the axial direction, as shown in FIG. 3. Each spline-tooth part 3 has twelve spline teeth 4 that are formed at regular intervals in a circumferential direction of the shaft 2, as shown in FIG. 9. Portions between the respective spline-tooth parts 3 of the shaft 2 and both end portions of the shaft 2 are flat portions 5, wherein no spline tooth is formed.
In each shaft hole 7 of each cam piece 6 or each cam journal 11, twelve press-fitting grooves 8 to be press-fitted to the spline teeth 4 of the shaft 2, and twelve not-press-fitting grooves (penetration grooves) 9 larger than the press-fitting grooves 8 and capable of passing through the spline teeth 4 of the shaft 2 are alternately formed at regular intervals in the circumferential direction thereof. Then, as shown in FIG. 9, the spline teeth 4 of the shaft 2 and the press-fitting grooves 8 of the cam piece 6 or the cam journal 11 are press-fitted to each other.
In addition, the outside diameter of the flat portions 5 of the shaft 2 is smaller than the inside diameter of spline-tooth parts 10 formed between the respective grooves of the cam piece 6 or the cam journal 11. In addition, the spline teeth 4 of the shaft 2 are a little larger than the press-fitting grooves 8 of the cam piece 6 or the cam journal 11, and a little smaller than the not-press-fitting grooves 9.
Next, an assembling method of the camshaft 1 is explained with reference to FIGS. 3 to 9.
At first, as shown in FIG. 3, one end member 12 is mounted at one end portion of the shaft 2. The other end portion of the shaft 2 is inserted into the shaft hole 7 of the cam piece 6 (or the cam journal 11) while positions (phase) of the not-press-fitting grooves 9 of the cam piece 6 (or the cam journal 11) are adjusted to positions (phase) of the spline teeth 4 of the shaft 2.
The inside diameter of the cam piece 6 (the inside diameter of the spline-tooth parts 10) is larger than the outside diameter of the flat portions 5 of the shaft 2, as shown in FIGS. 4 and 7. Thus, while the cam piece 6 passes over the flat portions 5 of the shaft 2, it can be avoided that the shaft hole 7 of the cam piece 6 and the outside surface of the shaft 2 damage each other.
In addition, when the cam piece 6 passes through a spline-tooth part 3, the not-press-fitting grooves 9 of the cam piece 6 are positioned with respect to the spline teeth 4 of the shaft 2, as shown in FIGS. 5 and 8. Thus, while the cam piece 6 passes through the spline-tooth parts 3 of the shaft 2, it can be avoided that the shaft hole 7 of the cam piece 6 and the outside surface of the shaft 2 damage each other.
Then, when the cam piece 6 reach a predetermined position on a flat portion 5 the nearest to a spline-tooth part 3 as a press-fitting portion of the shaft 2, the cam piece 6 is rotated such that the phase relation is adjusted for an assembling state. Then, while the adjusted phase relation is maintained, the cam piece 6 is caused to move in the axial direction. Thus, the press-fitting grooves 8 of the cam piece 6 are press-fitted into the spline teeth 4 of the shaft 2 so as to form a spline press-fitting state (see FIGS. 6 and 9). The press-fitting strength depends on interference between the press-fitting grooves 8 of the cam piece 6 and the spline teeth 4 of the shaft 2. In addition, residual stress of the press-fitting operation can flee to the not-press-fitting grooves 9. Thus, deterioration of the fatigue strength may be inhibited.
Subsequently, in the same manner as the above steps, the other cam pieces 6 and cam journals 11 are inserted onto the shaft 2 in a predetermined order and press-fitted at respective predetermined press-fitting positions.
Finally, the other end-member 12 is mounted at the other end portion of the shaft 2. Then, the camshaft 1 as shown in FIG. 1 is completed.
The above camshaft 1 can be assembled without damaging the inside surface of the shaft hole 7 of each cam piece 6 or each cam journal 11 and without damaging the outside surface of the shaft 2.
In addition, since the press-fitting grooves 8 of each cam piece 6 or each cam journal 11 and the spline-tooth part 3 (spline teeth 4) of the shaft 2 are press-fitted to each other, the press-fitting strength in a rotation direction may be enhanced satisfactorily. Thus, it is unnecessary to increase the interference.
In addition, the residual stress of the press-fitting operation can flee to the not-press-fitting grooves 9. Thus, deterioration of the fatigue strength may be inhibited.
In addition, tooth surfaces of the spline teeth 4 of the shaft 2 and the press-fitting grooves 8 of the cam piece 6 are in contact with each other on an involute curve, so as to form an involute spline press-fitting state. Thus, the contact surface of the cam piece 6 and the shaft 2 is stable.
In addition, since the cam piece 6 has the twelve press-fitting grooves 8, the cam piece 6 can be positioned correspondingly to each of a three-cylinder engine, a four-cylinder engine and a six-cylinder engine and an eight-cylinder engine. (In a general eight-cylinder engine, two four-cylinder engines are arranged in parallel.) The shaft 2 of the above embodiment is hollow. However, the present invention is not limited thereto.
In addition, in the above embodiment, the spline teeth 4 of the shaft 2 and the respective grooves of the cam piece 6 are respectively formed at regular intervals. However, the present invention is not limited thereto.

Claims

1. An assembling type of camshaft comprising

a cam piece having a shaft hole, and

a shaft to be press-fitted into the shaft hole of the cam piece,

wherein

a press-fitting groove and a penetration groove are respectively formed to extend in an axial direction in an inside circumferential surface of the shaft hole of the cam piece,

a press-fitting tooth is formed to extend in the axial direction partially in an outside circumferential surface of the shaft, and

the press-fitting tooth is adapted to be press-fitted into the press-fitting groove in the axial direction and to pass through the penetration groove in the axial direction.

2. An assembling type of camshaft according to claim 1, wherein

a plurality of press-fitting teeth is formed at regular or odd intervals in a circumferential direction of the outside circumferential surface of the shaft,

a plurality of press-fitting grooves and a plurality of penetration grooves are respectively formed at regular or odd intervals in a circumferential direction of the inside circumferential surface of the shaft hole of the cam piece, correspondingly to an arrangement of the plurality of press-fitting teeth, and

the plurality of press-fitting teeth is adapted to be press-fitted into the plurality of press-fitting grooves in the axial direction together and to pass through the plurality of penetration grooves in the axial direction together.

3. An assembling type of camshaft according to claim 2, wherein

the press-fitting manner between the plurality of press-fitting teeth and the plurality of press-fitting grooves is an EXPRESS MAIL LABEL involute spline manner.

4. An assembling type of camshaft according to claim 2, wherein

the press-fitting manner between the plurality of press-fitting teeth and the plurality of press-fitting grooves is a serration manner.

5. An assembling type of camshaft according to claim 2, wherein

the number of press-fitting grooves is a multiple of twelve,

the press-fitting grooves are formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of the cam piece,

the number of penetration grooves is also a multiple of twelve,

the penetration grooves are also formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of the cam piece.

6. An assembling type of camshaft comprising

a plurality of cam pieces each of which has a shaft hole, and

a shaft to be press-fitted into the shaft holes of the plurality of cam pieces,

wherein

a press-fitting groove and a penetration groove are respectively formed to extend in an axial direction in an inside circumferential surface of the shaft hole of each cam piece,

a plurality of press-fitting-tooth parts is formed at regular or odd intervals in the axial direction in an outside circumferential surface of the shaft, each of the plurality of press-fitting-tooth parts having a press-fitting tooth extending partially in the axial direction, and

7. An assembling type of camshaft according to claim 6, wherein

each of the plurality of press-fitting-tooth parts has a plurality of press-fitting teeth formed at regular or odd intervals in a circumferential direction of the outside circumferential surface of the shaft,

a plurality of press-fitting grooves and a plurality of penetration grooves are respectively formed at regular or odd intervals in a circumferential direction of the inside circumferential surface of the shaft hole of each cam piece, correspondingly to an arrangement of the plurality of press-fitting teeth, and

8. An assembling type of camshaft according to claim 7, wherein

the press-fitting manner between the plurality of press-fitting teeth and the plurality of press-fitting grooves is an involute spline manner.

9. An assembling type of camshaft according to claim 7, wherein

10. An assembling type of camshaft according to claim 7, wherein

the number of press-fitting grooves is a multiple of twelve,

the press-fitting grooves are formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of each cam piece,

the number of penetration grooves is also a multiple of twelve,

the penetration grooves are also formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole of each cam piece.

11. An assembling method of an assembling type of camshaft including:

a plurality of cam pieces each of which has a shaft hole, and

a shaft to be press-fitted into the shaft holes of the plurality of cam pieces,

wherein

the press-fitting tooth is adapted to be press-fitted into the press-fitting groove in the axial direction and to pass through the penetration groove in the axial direction,

the assembling method comprising:

a step of locating each cam piece at a predetermined position by causing each cam piece to pass over one or more portions of the shaft wherein no press-fitting-tooth part is formed, if necessary, and by causing each cam piece to pass through one or more press-fitting-tooth parts of the shaft by using the penetration groove of each cam piece, if necessary,

a step of adjusting a phase of each cam piece to the shaft by rotating each come piece at the predetermined position, and

a step of press-fitting each cam piece to the press-fitting tooth of a press-fitting-tooth part of the shaft by using the press-fitting groove of each cam piece.

12. A cam piece having a shaft hole to be press-fitted to a shaft, wherein

a plurality of press-fitting grooves is formed at regular intervals in a circumferential direction of an inside circumferential surface of the shaft hole, each of the plurality of press-fitting grooves extending in an axial direction, the number of the plurality of press-fitting grooves being a multiple of twelve,

a plurality of penetration grooves is formed at regular intervals in the circumferential direction of the inside circumferential surface of the shaft hole correspondingly to an arrangement of the plurality of press-fitting grooves, each of the plurality of penetration grooves extending in the axial direction, the number of the plurality of penetration grooves being a multiple of twelve,

the plurality of press-fitting grooves are adapted to be press-fitted together in the axial direction to a plurality of press-fitting teeth formed on an outside circumferential surface of the shaft correspondingly to an arrangement of the plurality of press-fitting grooves, and

the plurality of penetration grooves are adapted to pass through the plurality of press-fitting teeth together in the axial direction.