JP2000280923A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the transmission of torque applying an excessive force by a simple structure in an electric power steering device. SOLUTION: This electric power steering device is provided with a rack shaft 5 freely moved according to a steering force, a ball screw mechanism 30 having a ball nut 31, a first gear 21 attached to the output shaft 20 of a electric motor 6, and a second gear 22 engaged with the first gear. The second gear 22 is attached to the outer periphery of the ball nut 31 via a torque limiter 36.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering apparatus for generating a steering assist force for reducing a steering force of a vehicle, and more particularly, to an output of an electric motor provided in the electric power steering apparatus. The present invention relates to a structure for transmitting torque from a shaft.

[0002]

2. Description of the Related Art An example of an electric power steering apparatus for reducing a steering force by a steering wheel of a vehicle is disclosed in Japanese Patent Application Laid-Open No. 8-207796. The electric power steering apparatus described in this publication includes a small bevel gear attached to an output shaft of an electric motor, a rack shaft to which a tie rod is connected, a spiral screw groove formed on an outer periphery, and an inner periphery of a ball nut. A ball screw mechanism having a number of balls filled in a helical annular space formed by a helical screw groove formed in a helical gear groove, and a large bevel gear meshed with the small bevel gear and attached to a ball nut And The rotation of the output shaft of the electric motor drives the ball nut to rotate via both bevel gears, and the rack shaft is moved in the axial direction by the ball screw mechanism, whereby steering assist force by the electric motor is applied. is there.

[0003]

By the way, in the conventional electric power steering apparatus, the rack shaft moves at a high speed to the rack shaft stopper due to, for example, the curb climbing of steered wheels when the rack shaft is near the stroke end. When struck, the rotor of the electric motor continues to rotate due to its inertia, although the movement of the rack shaft stops immediately. Therefore, the torque due to the inertial rotation of the rotor becomes excessive torque, and is transmitted to the secondary reduction mechanism including the ball screw mechanism via the primary reduction mechanism including the small bevel gear and the large bevel gear, and the torque of the ball screw mechanism is reduced. It acts on the thread groove of the rack shaft, the thread groove of the ball nut, and the ball, which are the components, as excessive force. Therefore, in the ball screw mechanism, it is necessary to take measures such as increasing the strength of the member so as to withstand the excessive force, which has resulted in an increase in cost and an increase in weight.

[0004] The present invention has been made in view of such circumstances, and it is an object of the present invention to prevent the transmission of torque for applying an excessive force to a member with a simple structure in an electric power steering device. And

[0005]

The invention according to claim 1 of the present application provides a rack shaft that is movable in the axial direction to steer steered wheels of a vehicle in accordance with an input steering force, and a rotating shaft. A conversion mechanism having a member and converting the rotation of the rotating member into axial movement of the rack shaft, an electric motor for generating an assisting force for assisting a steering force, and a second motor attached to an output shaft of the electric motor. In an electric power steering apparatus provided with a first gear and a second gear rotatably driven by meshing with the first gear, the second gear is an electric power steering device mounted on the outer periphery of the rotating member via a torque limiter. It is a power steering device.

According to the first aspect of the present invention,
With a simple structure in which a torque limiter is interposed between the inner periphery of the boss of the second gear to which the rotation of the electric motor is transmitted and the outer periphery of the rotating member, excessive torque can be prevented from being transmitted to the rotating member. In addition, it is possible to prevent an excessive force from acting on the conversion mechanism. Therefore, it is not necessary to design a conversion mechanism having a high strength capable of withstanding an excessive force generated when an excessive torque is transmitted, so that cost and weight can be reduced.

Further, as in the second aspect of the present invention, in the electric power steering apparatus according to the first aspect, since the torque limiter is an annular elastic member, the structure of the torque limiter itself is simple, Moreover, it can be easily mounted between the second gear and the rotating member.

Further, in the electric power steering apparatus according to the second aspect, since the elastic member is a rubber member, a torque limiter can be easily formed and slippage occurs. It is easy to set the elastic force of the rubber member that determines the magnitude of the torque acting on the second gear or the rotating member, in other words, to set the sliding load.

Further, in the electric power steering apparatus according to the third aspect, the rubber member is burned on the inner periphery of the boss portion of the second gear, so that the torque limiter can be simplified. It can be firmly fixed to the boss part by appropriate means.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a schematic overall view of an electric power steering apparatus according to an embodiment of the present invention, and FIG. 2 is an enlarged view of a main part thereof.

The electric power steering apparatus 1 includes a substantially cylindrical housing 2 extending in the left-right direction of the vehicle, and a rack shaft 5 is accommodated in the housing 2 so as to be movable in the axial direction. The housing 2 includes a housing body 3
And a gear housing 4 mounted on an expanded opening end which is one end of the housing body 3. The gear housing 4 has a mounting portion 4a for the electric motor 6 and an insertion portion 4b through which the rack shaft 5 is movably inserted in the axial direction. Further, both ends of the rack shaft 5 are connected to tie rods 7, 7 via ball joints, respectively. The movement of the rack shaft 5 causes the tie rods 7, 7 to move, and further via a steering mechanism,
The steered wheels of the vehicle are steered.

One ball joint has one end fitted to the end of the housing body 3 and the other end connected to the tie rod 7.
Is covered by the first boot 8 for dustproof fitting. The other ball joint has one end fitted to the insertion portion 4b of the gear housing 4 and the other end covered by a second dustproof boot 9 fitted to the tie rod 7.

In the vicinity of the end of the housing body 3, the first
A steering gear box 3a is arranged to extend from the housing main body 3 at a position closer to the center than the fitting portion of the boot 8. That is, the center line of the steering gear box 3a (in this embodiment, the center line coincides with the axis of the input shaft 10) on a plane that includes the axis of the rack shaft 5 and is parallel to the axis of the input shaft 10 described later. ing)
A half straight line extending from the intersection of the first straight line parallel to the axis of the rack shaft 5 toward the center position of the rack shaft 5 and a part of the axis of the rack shaft 5; A part of the input shaft 10 is inclined so that an angle formed between the input shaft 10 and a half line extending in a certain direction becomes an obtuse angle. The size of the obtuse angle is appropriately set as needed.

An input shaft 10 connected via a joint to a steering shaft on which a steering wheel is integrally mounted is connected to the steering gear box 3a.
Are rotatably supported via bearings. In the steering gear box 3a, an output shaft is rotatably supported by the gear box 3a via a bearing, and the input shaft 10 is connected to the output shaft via a torsion bar so as to be relatively twistable. Have been.

A pinion is formed at the lower end of the output shaft, and meshes with a rack portion formed on the rack shaft 5. Therefore, the steering shaft, the input shaft 10, the torsion bar with torsion, and the output shaft are rotated by the input of the steering force from the steering wheel, and the pinion and the rack are engaged according to the rotation of the output shaft. The rack shaft 5 moves in the axial direction.

On the other hand, in the gear housing 4 forming the other end of the housing 2, the mounting portion 4a
Further, an electric motor 6 for generating an assisting force for assisting the steering force from the steering wheel is attached by bolts. The mounting portion 4a is arranged to extend obliquely from the gear housing 4 at a position where the axial distance from the center position of the housing 2 is substantially equal to that of the steering gear box 3a.

That is, on a plane that includes the axis of the rack shaft 5 and is parallel to the axis of the electric motor 6 (the rotation center line of the rotor of the electric motor 6), or includes the axis of the rack shaft 5 and the axis of the electric motor 6. On a plane (in this embodiment, the axis of the rack shaft 5 and the axis of the electric motor 6 are on the same plane), the center line of the mounting portion 4a (in this embodiment, the center line and the axis of the electric motor 6) (When the axis of the electric motor 6 and the axis of the rack shaft 5 are in a twisted position, a second straight line parallel to the center line of the mounting portion 4a). A half line extending from the rack shaft 5 to the center of the rack shaft 5 and being part of the axis of the rack shaft 5 from the intersection of the rack shaft 5 and the center line of the mounting portion 4a from the intersection (that is, the electric motor). 6 axis) and there is an electric motor 6 Can angle the ray forms extending (or a part there is an electric motor 6 direction of the second straight line) is inclined so that an obtuse angle alpha. The magnitude of the obtuse angle α is appropriately set as necessary, but in this embodiment, the steering gear box 3a
Is substantially equal to the size of the obtuse angle.

As described above, the axis of the electric motor 6 is inclined with respect to the axis of the rack shaft 5, and the electric motor 6 is arranged to be inclined with respect to the housing 2. Therefore, the degree to which the electric motor 6 projects radially outward from the housing 2 is smaller than that of the electric motor 6 arranged so that the axis of the electric motor 6 is orthogonal to the axis of the rack shaft 5.
The electric power steering device 1 becomes compact.

Further, the first straight line with respect to the steering gear box 3a and the axis (or the second
Therefore, the steering gear box 3a and the electric motor 6 are disposed so as to protrude substantially on the same side with respect to the housing 2. Therefore, with respect to the center position of the housing 2, the steering gear box 3 a, the mounting portion 4 a, and the electric motor 6 are arranged substantially symmetrically, and the distance between them is radially outward from the housing 2. It is getting bigger.

An output shaft 20 extends from the electric motor 6 into the gear housing 4, and a first gear 21 is attached to the end of the output shaft 20 so that the output shaft 20 rotates integrally with the output shaft 20. ing. Further, since the axis of the output shaft 20 coincides with the axis of the electric motor 6 in this embodiment, the axis of the rack shaft 5 is formed by the axis of the electric motor 6 and the axis of the rack shaft 5. At an angle equal to the obtuse angle α.

The housing body 3 has a ball screw mechanism housing portion 3b having a diameter larger than that of the other portions of the housing body 3 in the vicinity of the mounting portion of the gear housing 4. The opening end of the mechanism accommodating portion 3b has a larger diameter, and is the above-described enlarged opening end.

In the ball screw mechanism accommodating section 3b, a ball nut 31, which is a rotating member disposed radially outward of the rack shaft 5, is provided coaxially with the rack shaft 5. The ball nut 31 is provided via an angular ball bearing 32 and via a ball bearing 35 near the end of the rack shaft.
It is supported by the housing body 3 so as to be rotatable and immovable in the axial direction. Further, the rack shaft 5 has a screw groove portion 33 in which a spiral screw groove is formed on the outer periphery in the range from the vicinity of the center position to the end on the second boot 9 side. A large number of balls 34 are rotatably interposed in a space formed between the helical thread groove 31 formed in the inner periphery of the ball 31 and the helical thread groove. The ball nut 31 is provided with a passage for returning the ball 34 that has advanced in the axial direction as the ball nut 31 rotates.

Therefore, the ball nut mechanism 31, the screw groove 33 of the rack shaft 5 and the ball 34 constitute the ball screw mechanism 30. Since the ball screw mechanism 30 causes the rack shaft 5 to move in the axial direction by the rotation of the ball nut 31,
A conversion mechanism for converting the rotation of the ball nut 31 into the axial movement of the rack shaft 5 is configured.

Therefore, with the arrangement of the steering gear box 3a, the electric motor 6, and the housing 3b of the ball screw mechanism 30 in the housing 2, the mounting portion 4a, the electric motor 6, and the steering The gear box 3a is disposed substantially symmetrically, and the distance between the two increases gradually from the housing 2 toward the outside in the radial direction. 2, the electric motor 6 protruding largely outward in the radial direction from the housing 2 can be arranged as close to the end of the housing 2 as possible. A relatively large space can be formed.

As shown in more detail in FIG. 2, on the outer periphery of the ball nut 31, a bearing support for supporting the inner race of the angular contact ball bearing 32 and the second gear 22 are arranged in this order from the center of the rack shaft 5. And a gear attachment portion to which the is attached. The second gear 22 is a ball nut 3
1 is mounted coaxially with the rack shaft 5 so that one end surface of the boss abuts against a step formed on the outer periphery of the ball nut 31.
Axial movement is prevented.

The portion of the second gear 22 where the teeth are formed is moved from the ball screw mechanism housing portion 3b to the housing portion 3b.
b protrudes into the gear housing housing 4 having a diameter larger than b.
In the gear housing 4, a first gear 21 attached to an output shaft 20 of the electric motor 6 meshes with a second gear 22, and the second gear 22 is rotated by the electric motor 6. The gear 22 is driven to rotate. Then, the ball nut 3 is moved from the boss portion 22a of the second gear 22.
The ball nut 31 is rotated by the torque transmitted to 1.

A torque limiter 36 is provided between the inner periphery of the boss 22a of the second gear 22 and the outer periphery of the ball nut 31. That is, a part of the inner periphery of the boss portion 22a is a concave portion formed in an annular shape,
The torque limiter 36 is accommodated in the recess. The torque limiter 36 is made of a rubber member that is an annular elastic member, and is fixed by being burned to the boss portion 22a in the concave portion.

When the torque is transmitted from the second gear 22 to the ball nut 31, the elastic force acting on the ball nut 31 of the rubber member as the torque limiter 36 is set between the second gear 22 and the ball nut 31. Since the magnitude of the torque that causes the slip is determined, the load is hereinafter referred to as a slip load. In the state where the second gear 22 is fitted to the ball nut 31 by the slip load, the torque limiter 36 is set. The magnitude of the maximum static friction force generated on the contact surface between the ball and the ball nut 31 is set, and the maximum value of the torque at which the torque is transmitted without slipping between the two is set.

The set slip load of the torque limiter 36 is set so that the maximum static friction force having the following magnitude is generated. That is, when the ball nut 31 is rotated by the operation of the electric motor 6 in order to generate a steering assist force in a normal state, the torque limiter is determined by the magnitude of the torque acting between the second gear 22 and the ball nut 31. The force acting on the ball nut 31 from 36 is not more than the maximum static friction force. for that reason,
The rotation of the electric motor 6 is controlled by the first and second gears 21 and 22.
Transmitted to the ball nut 31 via the second gear 2
2 and the ball nut 31 rotate integrally without slippage.

When the rack shaft 5 is in the vicinity of the stroke end, the steered wheels ride on the curb,
When the second gear 22 tries to rotate the ball nut 31 by the rotation of the electric motor 6 due to the inertia of the rotor of the electric motor 6 when the rack shaft 5 stops at a high speed, When an excessive torque generated when the second gear 22 driven to rotate by the rotation of the electric motor 6 tries to rotate the ball nut 31 acts between the second gear 22 and the ball nut 31, a torque limiter 36 is provided. As a result, the force acting on the ball nut 31 becomes larger than the maximum static friction force, a slippage occurs between the two, and the torque transmitted from the second gear 22 to the ball nut 31 is reduced.

The first and second gears 21, 22
Since the axes of the output shaft 20 and the axis of the ball nut 31 which are the axes of the shafts intersect at an angle, they are each formed of a cross-shaft gear, for example, a bevel gear. The number of teeth of the second gear 22 is the same as that of the conventional electric power steering device.
1, the two gears 21 and
2 constitutes a speed reduction mechanism.

The teeth of the second gear 22 are formed so that the normal to the reference pitch plane, which extends from the reference pitch plane toward the tooth tip, is directed to the rack shaft 5. I have. Therefore, the surface of the second gear 22 on which the teeth are formed faces the rack shaft 5. The meshing portion between the first gear 21 and the second gear 22 is on the obtuse angle α side of the angle formed by the axis of the output shaft 20 and the axis of the rack shaft 5.

As described above, the first gear 21 and the second gear 22
Is on the obtuse angle α side, the second gear 22
Depends on the position of the inclined output shaft 20 and the first position.
A gear having a relatively large diameter is used as the second gear 22 despite the fact that there is little restriction based on the gear 21 and the axis of the electric motor 6 and the axis of the output shaft 20 are arranged inclined. It becomes possible. As a result, only the first and second gears 21 and 22 can increase the reduction ratio.

Further, since the normal to the reference pitch plane of the second gear 22 is formed so as to be directed to the rack shaft 5,
Since the meshing ratio of the first and second gears 21 and 22 can be increased, a large torque from the electric motor 6 which is a source of the steering assist force can be transmitted to the second gear 22 reliably. Further, the transmission load acting on the individual teeth of the meshing portion can be dispersed and reduced. Accordingly, since the strength and durability of the gears can be improved, it is also possible to employ resin gears as the first and second gears 21 and 22.

The operation of the embodiment configured as described above will be described. When the driver turns the steering wheel to steer, the steering shaft integrated with the steering wheel rotates, and the rotation is performed via the joint by the input shaft 1.
0 is transmitted. The input shaft 10 rotates the output shaft while causing the torsion bar to twist, and the rotation causes the rack shaft 5 to move in the axial direction due to the engagement between the pinion and the rack portion of the rack shaft 5. . Then, the movement is transmitted to the steered wheels via the tie rods 7, 7, and the steered wheels are steered according to the steering force.

On the other hand, the steering force is detected from the magnitude of the twist of the torsion bar, and the electric motor 6 is driven based on a control signal corresponding to the detected steering force. And
Output shaft 2 that is rotationally driven by driven electric motor 6
Due to the rotation of the first gear 21 and the second gear 22, the second gear 22 and the ball nut 31 are driven to rotate around the thread groove of the rack shaft 5 without sliding. The rotation of the ball nut 31 causes the rack shaft 5 having the ball groove portion that meshes with the ball to move in the axial direction, and the movement is transmitted to the steered wheels via the tie rods 7, 7 to assist the steering force from the steering wheel. I do.

Further, when the rack shaft 5 is in the vicinity of the stroke end, the steered wheels ride on the curb,
Abuts the rack shaft stopper at high speed, and when an excessive torque acts between the second gear 22 driven by rotation of the electric motor 6 and the ball nut 31, when the torque limiter 36 and the ball nut 31 , A reduced torque is transmitted from the second gear 22 to the ball nut 31.

This embodiment has the following advantages because it is configured as described above. An annular elastic member which is burned and fixed to the inner periphery of the boss 22a between the inner periphery of the boss 22a of the second gear 22 to which the rotation of the electric motor 6 is transmitted and the outer periphery of the ball nut 31. A torque limiter 36 made of a rubber member is provided. As a result, with a simple structure in which the torque limiter 36 is interposed between the second gear 22 and the ball nut 31, an excessive torque can be prevented from acting on the ball nut 31,
Excessive force acting on the ball screw mechanism 30 can be prevented. Therefore, it is not necessary to design the ball screw mechanism 30 having a high strength capable of withstanding an excessive force generated when an excessive torque is transmitted, so that cost and weight can be reduced.

Since the torque limiter 36 is an annular elastic member, the structure of the torque limiter 36 itself is simple, and the torque limiter 36 can be easily mounted between the second gear 22 and the ball nut 31.

Further, since the elastic member is a rubber member, it is easy to form the torque limiter, and it is easy to set the sliding load of the torque limiter 36 which determines the magnitude of the torque at which the slip occurs. Then, by baking the rubber member on the inner periphery of the boss portion 22a, the torque limiter 36 can be firmly fixed to the boss portion 22a by simple means.

The axis of the electric motor 6 is inclined with respect to the axis of the rack shaft 5, and the electric motor 6 is arranged to be inclined with respect to the housing 2. Therefore, the degree to which the electric motor 6 projects radially outward from the housing 2 is
The axis of the electric motor 6 is smaller than that of the electric motor 6 arranged so as to be orthogonal to the axis of the rack shaft 5. As a result, the electric power steering device 1 becomes compact, and the degree of freedom of its layout can be increased.

Since the meshing portion between the first gear 21 and the second gear 22 is on the obtuse angle α side of the angle between the axis of the output shaft 20 and the axis of the rack shaft 5, the size of the second gear 22 is
The position of the inclined output shaft 20 and the restriction based on the first gear 21 are less likely to be imposed, and the second gear is formed despite the fact that the axis of the electric motor 6 and the axis of the output shaft 20 are inclined. As 22, a gear having a relatively large diameter can be adopted. As a result, the reduction ratio can be increased, and the degree of freedom in designing the reduction ratio is increased. Also, since a desired reduction ratio can be obtained only with the first and second gears 21 and 22, a complicated reduction mechanism composed of three or more gears is not required to obtain a desired reduction ratio. The number of points can be reduced, and the speed reduction mechanism can be made compact.

The teeth of the second gear 22 are formed so that the normal to the reference pitch plane thereof and the normal to the reference pitch plane extending toward the tooth tip side is directed to the rack shaft 5. Since the meshing ratio of the first and second gears 21 and 22 can be increased, a large torque from the electric motor 6 which is a source of the steering assist force can be transmitted to the second gear 22 reliably. Further, since the transmission load acting on each tooth of the meshing portion can be dispersed and reduced, the strength and durability of the gear are improved.

With respect to the center position of the housing 2, the mounting portion 4 a and the electric motor 6 and the steering gear box 3 a are arranged substantially symmetrically, and the interval between them is radially outward from the housing 2. And the ball screw mechanism 30 is located closer to the center of the housing 2 than the mounting portion and the electric motor 6. Can be arranged as close to the end of the housing 2 as possible. for that reason,
Since a relatively large space is formed in the center of the housing 2, peripheral devices can be arranged in this space, and a compact layout of the peripheral devices is possible.

In the above-described embodiment, the elastic member is a rubber member. However, the elastic member is an annular metal spring that comes into contact with the inner periphery of the boss portion 22a of the second gear 22 and the outer periphery of the ball nut 31. There may be. Further, in the above embodiment, the concave portion is formed on the inner periphery of the boss portion 22a of the second gear 22, but may be formed on the outer periphery of the ball nut 31.

In the above embodiment, the axis of the electric motor 6 is inclined with respect to the axis of the rack shaft 5, but may be arranged to be orthogonal to the axis of the rack shaft 5.

In the above embodiment, the second gear 22 is formed so that the normal to the reference pitch plane is directed to the rack shaft 5, but the meshing portion between the first gear 21 and the second gear 22 is formed. As long as the angle between the axis of the output shaft 20 and the axis of the rack shaft 5 is on the obtuse angle α side, a relatively large-diameter gear can be adopted as the second gear 22, and the reduction gear ratio can be reduced. Can be increased. Therefore, for example, the normal of the reference pitch plane of the second gear 22 is
May be oriented in a direction parallel to the axis of the rack shaft or away from the axis of the rack shaft 5 radially outward.

In the above embodiment, the mounting portion 4a of the electric motor 6 and the electric motor 6 and the steering gear box 3a are arranged substantially symmetrically with respect to the center position of the housing 2, but they are arranged in the same inclination direction. They can also be placed. That is, the mounting portion 4 is attached to the housing 2 such that the center line of the mounting portion 4a and the axis of the electric motor 6 are parallel to the axis of the input shaft 10 in the embodiment.
a and the electric motor 6 can be arranged. In this case, the ball screw mechanism 30 is provided closer to the second boot 9 than the mounting portion 4a, and the position where the gear mounting portion of the ball nut 31 is formed is shifted from the position where the bearing support portion is formed.
Near the center position of

Also in this case, a gear having a relatively large diameter can be adopted as the second gear 22, so that the reduction ratio can be increased, and the second gear 22 can be used.
If the normal to the reference pitch plane is directed to the rack shaft 5, the meshing ratio can be increased.

[Brief description of the drawings]

FIG. 1 is a schematic overall view of an electric power steering device according to an embodiment of the present invention.

FIG. 2 is an enlarged view of a main part of FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electric power steering device, 2 ... Housing, 3
... Housing body, 4 ... Gear accommodating housing, 5 ... Rack shaft, 6 ... Electric motor, 7 ... Tie rod, 8,9 ... Boot, 10 ... Input shaft, 20 ... Output shaft, 21 ... First gear, 2
2 ... second gear, 30 ... ball screw mechanism, 31 ... ball nut, 32 ... angular ball bearing, 33 ... thread groove, 34
... ball, 35 ... ball bearing, 36 ... torque limiter.

Claims (4)

[Claims] 1. A rack shaft which is movable in an axial direction to steer a steered wheel of a vehicle in accordance with an input steering force, and a rotating member which rotates the rack member in an axial direction of the rack shaft. A conversion mechanism for converting the movement, an electric motor for generating an assisting force for assisting the steering force, a first gear attached to an output shaft of the electric motor, and a second gear driven by meshing with the first gear. An electric power steering device comprising two gears, wherein the second gear is attached to an outer periphery of the rotating member via a torque limiter. 2. The electric power steering apparatus according to claim 1, wherein the torque limiter is an annular elastic member. 3. The electric power steering apparatus according to claim 2, wherein said elastic member is a rubber member. 4. The electric power steering apparatus according to claim 3, wherein the rubber member is baked on an inner periphery of a boss portion of the second gear.