JP2002303363A - Friction gear device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve relatively large friction torque from small pressing force and thereby sufficiently and safely reduce tooth hammering noise even when an object gear is relatively small. SOLUTION: A level difference is provided on a surface of a driven gear 4 on the sub gear 5 side, and the inner peripheral side is raised higher than the outer peripheral side. A level difference is provided on a surface of the sub gear 5 on the driven gear 4 side, and an outer peripheral side is raised higher than the inner peripheral side. Thus, the driven gear 4 and the sub gear 5 are abutted with each other only near an end of the respective rising parts. A dish spring 6 is actuated from just above the abutting surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a friction gear device, and more particularly, to a relatively small gear which is a transmission element constituting a valve train of an engine.
The present invention relates to a technique for sufficiently reducing the rattle sound.

[0002]

2. Description of the Related Art In a gear generally used as a power transmission element, in order to prevent collision between gears due to backlash, one of a driving gear and a driven gear driven and rotated by the driving gear is provided. It is known to add a friction gear device (Japanese Patent Laid-Open No. 6-66).
No. 111). The basic operation principle of the friction gear device is as follows.

[0003] A friction gear device generally includes an idler gear (sub gear) provided so as to be idle and coaxial with one of a drive gear and a driven gear constituting a main gear with respect to the main gear. It is configured by pressing elastically. here,
The number of teeth of the auxiliary gear is appropriately increased or decreased according to whether the main gear is a driving gear or a driven gear. For example, a case in which a sub gear is attached to a driven gear to form a friction gear device will be described.

In this case, the number of teeth of the auxiliary gear is increased (for example, only one) from the number of teeth of the driven gear (main gear). For this reason, when the driven gear and the auxiliary gear are compared, the teeth of the auxiliary gear are located closer to the contact point than the teeth of the driven gear as the distance from the contact point (meshing point) of both tooth surfaces increases. When the drive gear is rotated in this state, the auxiliary gear is pressed against the driven gear by the elastic member, and thus tries to rotate integrally with the driven gear. However, rotation progressed,
As described above, when the position where the teeth of the auxiliary gear and the teeth of the driven gear are largely shifted approaches, the driving gear bites into the narrow gap between the teeth of the auxiliary gear and the teeth of the immediately preceding driven gear. Will go out.

At this time, the front surface of the teeth of the auxiliary gear in the rotation direction is
The auxiliary gear, which abuts against the rear surface of the teeth of the drive gear in the rotation direction and is pressed only by the elastic member, is pushed back in the rotation direction opposite to the driven gear. Here, torque resistance (friction torque) due to friction between the driven gear and the auxiliary gear acts on the drive gear. When the torque of the driving gear becomes negative or the torque in the positive direction is added to the driven gear, the driven gear is likely to rotate relatively faster than the driving gear. Friction torque works. Therefore, the relative position between the driven gear and the driving gear can be maintained, or the relative acceleration of the driven gear can be suppressed. It can be prevented. In this way, the backlash between the driving gear and the driven gear is filled with the auxiliary gear, and the collision between the driving gear and the driven gear is prevented.

When the friction gear device is constructed by attaching the auxiliary gear to the drive gear, the same effect can be obtained by reducing the number of teeth of the auxiliary gear (for example, only one) from the number of teeth of the drive gear. be able to.

[0007]

Such a friction gear device is widely used in a vehicle engine. Among them, a direct injection diesel engine using a low-temperature premixed combustion system has the following particular features. There's a problem. In this type of diesel engine, since the shape of the combustion chamber is flat, the distance between the two camshafts constituting the valve train cannot be made large. Therefore, it is necessary to drive the camshaft with a relatively small gear.

On the other hand, in order to adopt a friction gear device and prevent a collision between a driving gear and a driven gear, the pressing force of the elastic member must be extremely large in order to obtain a sufficient friction torque. . However, when the camshaft is driven by a small gear as described above, the pressing force of the elastic member tends to be insufficient, so that a sufficient noise suppressing effect cannot be obtained or the pressing force is excessive. If so, it becomes difficult to sufficiently ensure the durability and strength of the friction gear device.

Therefore, the present invention is to generate a sufficient friction torque even with a relatively small pressing force, so that even if the target gear is relatively small as in the above-described direct injection diesel engine, It is an object of the present invention to provide a friction gear device capable of sufficiently and safely reducing the rattle sound.

[0010]

For this reason, the friction gear device according to the first aspect of the present invention provides a driven gear that meshes with the driving gear, wherein the driven gear is a main gear that is a driven gear. An idler gear provided coaxially with the main gear and rotatably with respect to the main gear, an elastic member for elastically pressing the idler gear and the main gear, In the friction gear device comprising: at least one of the opposing surfaces of the main gear and the idler gear is raised including a region on or near the line of action of the pressing force by the elastic member, the main gear and The idle gear is pressed through the raised portion.

According to a second aspect of the present invention, there is provided a friction gear device comprising: a driving side gear meshing with a driven side gear; A friction gear device comprising: an idler gear coaxially provided with a gear and rotatably attached to the main gear; and an elastic member for elastically pressing the idler gear and the main gear. At least one of the opposing surfaces of the gear and the idle gear is raised, including a region on or near the line of action of the pressing force by the elastic member, so that the main gear and the idle gear are raised. It is characterized by being pressed through a part.

In such a configuration, between the main gear and the idle gear pressed through the raised portion on the opposing surface, a gap (the gap between the main gear and the idle gear) is provided adjacent to the raised portion. Non-contact area) is formed. It is preferable that the raised portion of the facing surface is annularly raised.

According to a fourth aspect of the present invention, as the raised portion, one of the main gear and the idler gear has a tip edge on a tip side with respect to the region. It is preferable to provide a first raised portion and, on the other hand, to provide a second raised portion forming an axial edge on the rotation axis side with respect to the region. Alternatively, as described in claim 5, in the main gear or the idler gear, a tip edge on a tip side with respect to the region, and a shaft edge on a rotating shaft side with respect to the region. May be provided.

According to a sixth aspect of the present invention, the tip of such a convex ridge is provided.
As described in above, the shape may be an arc. Further, it is preferable that the tooth tip side edge substantially coincides with the tooth bottom position. Further, the action line may be formed substantially in the middle between the tooth end side edge and the shaft side edge.

According to a ninth aspect of the present invention, there is provided a friction gear device comprising: a driven gear engaged with a driving gear; the main gear being a driven gear; the number of teeth being increased as compared with the main gear; A friction gear device comprising: an idler gear coaxially provided with a gear and rotatably attached to the main gear; and an elastic member for elastically pressing the idler gear and the main gear. A spacer is interposed between the gear and the idle gear on or near the line of action of the pressing force by the elastic member, and the main gear and the idle gear are pressed through the spacer. Features.

According to a tenth aspect of the present invention, there is provided a friction gear device comprising: a driving gear engaged with a driven gear; A friction gear device comprising: an idler gear coaxially provided with a gear and rotatably attached to the main gear; and an elastic member for elastically pressing the idler gear and the main gear. A spacer is interposed between the gear and the idle gear on or near the line of action of the pressing force by the elastic member, and the main gear and the idle gear are pressed through the spacer. Features.

In such a configuration, a gap (a non-contact area between the main gear and the idler gear) is formed between the main gear and the idler gear adjacent to the spacer. The spacer interposed in this manner is preferably annular, as described in claim 11. In addition, as described in claim 12, a tip end-side gap is formed between the main gear and the idle gear including the tip end side edge of the spacer, and an axial side of the spacer is formed. Preferably, a gap on the rotating shaft side is formed between the main gear and the idler gear including the edge (the edge of the spacer on the gear rotating shaft side).

It is preferable that at least one of the contact surfaces of the spacer with the main gear or the idle gear is arc-shaped. Further, it is preferable that the tooth tip side edge of the spacer is substantially coincident with the tooth bottom position. Further, the action line is defined by claim 1
As described in Item 5, it is preferable to form it approximately in the middle between the tip end side edge and the shaft side edge.

In the friction gear device according to the present invention, at least one of the opposing surfaces of the main gear and the idler gear has a tip end side edge closer to the shaft side than the shaft side edge. And the main gear or the idle gear is brought into contact with the corresponding raised portion. According to such a configuration, the main gear or the idle gear is supported by another raised portion.

[0020] In the friction gear device according to the invention described in claim 17, a spacer is interposed between the main gear and the idle gear on the shaft side from the shaft side edge, so that the main gear and the idler gear are separated from each other. The idle gear is brought into contact with the spacer. According to the present invention, a disc spring may be employed as the elastic member.

[0021]

According to the first and second aspects of the present invention,
Since the non-contact area between the main gear and the idle gear is formed, the friction torque is increased as compared with the case where the main gear and the free gear are brought into contact with each other. Also, a relatively large friction torque can be obtained with a small pressing force (the pressing force for obtaining the same friction torque is reduced).
Therefore, the reliability in design can be easily improved.

According to the third aspect of the present invention, the main gear and the idle gear can be stably pressed. According to the fourth aspect of the present invention, while the main gear and the idler gear are raised wide, the contact area between the first raised portion and the second raised portion can be easily reduced. Therefore, it is possible to greatly increase the friction torque while securing the strength of (the raised portion of) the gear. Further, the present invention can be implemented at low cost with a small number of processing steps.

According to the fifth aspect of the present invention, the friction torque can be greatly increased by using the existing main gear or idle gear. According to the sixth aspect of the present invention, since the main gear or the idle gear is pressed against the tip of the arc-shaped convex ridge, the working area of the pressing force can be extremely reduced. Therefore, a large friction torque can be obtained with a very simple configuration.

According to the seventh aspect of the present invention, since the pressing force by the elastic member acts to the maximum on the tooth tip side, a larger friction torque can be obtained. According to the eighth aspect of the present invention, it is possible to suppress the generation of a deformation moment with respect to the idler gear, and to prevent the occurrence of uneven contact and gear noise caused by the deformation of the idler gear.

According to the ninth and tenth aspects of the present invention,
Since the non-contact area between the main gear and the idle gear is formed, the friction torque is increased as compared with a case where the main gear and the idler gear are brought into contact with each other. Also, a relatively large friction torque can be obtained with a small pressing force (the pressing force for obtaining the same friction torque is reduced).
Therefore, the reliability in design can be easily improved.

According to the eleventh aspect, the main gear and the idle gear can be stably pressed. According to the twelfth aspect, it is possible to further reduce the working area of the pressing force and further increase the friction torque. According to the thirteenth aspect, since the working area of the pressing force can be made extremely small, a large friction torque can be obtained with a very simple configuration.

According to the fourteenth aspect of the present invention, the pressing force of the elastic member acts to the maximum on the tooth tip side, so that a larger friction torque can be obtained. According to the invention described in claim 15, it is possible to suppress the generation of a deformation moment with respect to the idle gear, and to prevent the occurrence of uneven contact and gear noise due to the deformation of the idle gear.

According to the sixteenth aspect of the present invention, the main gear and the idler gear are supported by another protruding portion on the shaft side, and the protruding portion (or the spacer) on the line of action of the pressing force is used as a fulcrum. Deformation of the rolling gear can be prevented, and deterioration of gear noise due to resonance of the idle gear can be prevented. According to the seventeenth aspect, the same effect can be easily obtained without increasing the number of processing steps.

According to the eighteenth aspect, an optimal member is provided as the elastic member.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention.
It is a top view which shows the outline of a structure of the friction gear apparatus which concerns on embodiment. Here, a case where the present invention is applied to a camshaft drive gear constituting a valve train of an engine will be described.

In this valve gear, two camshafts 1a
The (intake side) and 1b (exhaust side) are rotatably supported by the cylinder head 2 by cam caps 3a to 3c, respectively. Gears are attached to these camshafts at the tips protruding outside from the cam cap 3a. A main gear (hereinafter, referred to as "driven gear") 4, which is a driven gear, is attached to the camshaft 1a. The driven gear 4 is positioned and fixed to the camshaft 1a by a dowel pin (not shown).

The driven gear 4 has a sub gear 5 as an idle gear mounted coaxially with the driven gear 4. Here, the sub gear 5 is merely pressed against the driven gear 4 by the disc spring 6 as an elastic member, and is in a state in which the sub gear 5 can freely rotate with respect to the driven gear 4.
This pressing force is generated when the disc spring 6 is compressed between the cap 8 and the sub gear 5 by the tightening force of the bolt 7.
The number of teeth of the sub gear 5 is one (or more) than the number of teeth of the driven gear 4.

On the other hand, a main gear (hereinafter referred to as "drive gear") 9 as a drive gear for driving the driven gear 4 is attached to the camshaft 1b. Drive gear 9
Is positioned and fixed to the camshaft 1b by a dowel pin (not shown). The drive gear 9 has
A chain sprocket 10 is integrally mounted. Then, the drive gear 9 and the chain sprocket 1
0 is fixed to the camshaft 1b by a bolt 12 via a washer 11.

The chain sprocket 10 rotates by transmitting the rotational power of a crankshaft (not shown) via a chain or the like. Then, the drive gear 9 rotates in accordance with this, and the driven gear 4 is driven.
The camshafts 1a and 1b rotate. FIG. 2 is a partial sectional view of the driven gear 4 and its peripheral structure. Next, the configuration of the friction gear device according to the present embodiment will be described in detail with reference to FIG.

In the present friction gear device, a step is formed on the surface of the driven gear 4 facing the sub gear 5, and the inner peripheral surface on the axis A side with respect to the radius r2 protrudes more than the outer peripheral surface on the tooth tip 4a side. (1st raised part), the recess of the said opposing surface is formed in the tooth tip 4a side. On the other hand, the sub gear 5
In this case, a step is formed on the surface facing the driven gear 4, and here, the outer peripheral surface on the tooth tip 5a side with respect to the radius r1 (r1 <r2) rises more than the inner peripheral surface on the axis A side (the 2 ridges), and the depression on the opposite surface is formed on the axis A side.

For this reason, the driven gear 4 and the sub gear 5 are in contact with each other only at a narrow annular portion having a radius r from r1 to r2. Further, the radius of the contact position between the disc spring 6 and the sub-gear 5 is set substantially at the middle between the radii r1 and r2. For this reason, the pressing force P of the disc spring 6 is generated from right above the contact portion between the driven gear 4 and the sub gear 5, thereby preventing the generation of an unnecessary moment that causes the deformation of the sub gear 5.

Next, with reference to FIG. 3, a description will be given of the friction torque T by the friction gear device according to the present embodiment. FIG. 3 is an enlarged view of a contact portion between the driven gear 4 and the sub gear 5, and a dashed line i is an action line of the pressing force P by the disc spring 6. Friction torque T when pressing force P is applied by disc spring 6 in the state of FIG.
Can be estimated as follows.

First, the pressure p generated at the contact portion is given by the following equation (1) which divides the pressing force P by the contact area.

[0039]

(Equation 1)

Next, the friction torque dT generated in the minute section dr having the radius r (r1 ≦ r ≦ r2) is given by the following equation (2) as a value obtained by multiplying the frictional force by the radius r. Here, μ is a coefficient of friction between the driven gear 4 and the sub gear 5.

[0041]

(Equation 2)

Accordingly, the friction torque T is estimated by integrating the above equation (2) in the range from the radius r1 to the radius r2. Here, if x = r1 / r2, the friction torque T is given by the following equation (3).

[0043]

(Equation 3)

FIG. 4 shows that the friction torque T is estimated based on the above equation (3), and x is changed in the range of 0 to 1 with the friction torque T0 at x = 0 as a reference (T0 = 1). Friction torque ratio T / T0
Are plotted respectively. FIG. 4 shows that the friction torque T increases as x increases, that is, as the inner diameter r1 of the contact portion approaches the outer diameter r2.

Although theoretically, an increase in friction torque of up to 50% is expected, the inner diameter r1 of the contact portion is actually limited to at least the outer diameter of the bolt 7 or the cap 8. Considering that the pressure p becomes infinite at r1 = r2, etc., a practical increase in friction torque is a little over 30% (for example, 0.
4 ≦ x ≦ 0.99).

Since the friction torque T is calculated by the product of the frictional force and the radius, the friction torque T can be further increased by increasing the contact portion outer diameter r2 as much as possible. For this purpose, for example, the outer diameter r2 may be approximately equal to the root diameter rf of the driven gear 4. Next, a second embodiment of the present invention will be described.

The friction gear device according to this embodiment is generally configured in the same manner as that of the first embodiment. Therefore, only the difference between the two will be described with reference to FIG. FIG. 5 is a partial sectional view of the driven gear 4 and its peripheral structure in the present friction gear device, and the same components as those of the first embodiment are denoted by the same reference numerals.

In the present friction gear, a step is formed on the surface of the sub gear 5 facing the driven gear 4, and the tooth tip 5a is raised more than the shaft A, so that a gap (clearance) is formed on the shaft A. A washer (spacer) 21 having the same thickness as the step is inserted into C.
If the washer 21 is not provided, the driven gear 4
Since the contact area between the sub gear 5 and the sub gear 5 is very small, the sub gear 5 is forcibly deformed in the vertical direction in the figure with the contact portion as a fulcrum as shown in FIG. For this reason,
The meshing with the drive gear 9 is deteriorated, and the gear noise is deteriorated.

Therefore, if the washer 21 is inserted as described above, the sub gear 5 is supported on the inner peripheral portion other than the contact portion with the driven gear 4. For this reason,
The above-described forced deformation can be prevented to prevent gear noise. Further, such another fulcrum can be obtained by providing an annular projection (flange portion) 51 having the same height as the step on the sub gear 5 at a similar place other than inserting another member such as a washer. , Can be formed. This is shown in FIG. The annular projection 51 is not limited to the sub gear 5 and may be provided on the driven gear 4 as the main gear.

Next, a third embodiment of the present invention will be described. The friction gear device according to the present embodiment is generally configured similarly to that of the first embodiment.
Therefore, only the difference between the two will be described with reference to FIG. FIG. 8 is a partial sectional view of the driven gear 4 and its peripheral structure in the present friction gear device.
The same components as those of the embodiment are denoted by the same reference numerals.

In the present friction gear, on the surface of the driven gear 4 on the side of the sub gear 5, the outer peripheral surface on the tooth tip 4a side is lowered by one step from the inner peripheral surface on the shaft A side by a predetermined radius. The spacer 61 is fitted. By making the thickness of the spacer 61 larger than this step, a raised portion is formed in a convex shape with respect to the sub-gear 5, and the opposing surface toward the sub-gear 5 adjacent to the spacer 61 is depressed. 5 is pressed against the upper surface of the spacer 61.

FIG. 9 is an enlarged sectional view of the spacer 61 and its peripheral structure. Referring to FIG. 9, it can be seen that the surface of the spacer 61 on the driven gear 4 side is flat, while the surface on the sub gear 5 side is a smooth arc. Thus, the sub gear 5 is supported by the driven gear 4 by an annular line contact.

According to such a configuration, the contact portion outer diameter (r
The ratio (x) of the contact portion inner diameter (r1) to 2) can be easily set to 1
Can be approached. For this reason, as shown in FIG. 4, it is possible to obtain an extremely large friction torque T by maximizing the friction torque increase margin. A similar effect can be obtained by protruding the surface of the driven gear 4 (or the sub gear 5) in a convex shape. In this case, there is no need to prepare a separate member, so that the number of components does not increase. If the tip of the convex ridge is formed in an arc shape, the increase in friction torque can be maximized.

The arc surface of the spacer 61 may be formed on the driven gear 4, which is the main gear. In this case, the step portion is formed on the sub gear 5. As described above, according to the present invention, the friction torque T generated under a constant pressing force P increases, so that the pressing force P is not excessively increased even if the gear is small, A sufficient friction torque T can be obtained.

In a relatively large gear, the pressing force P required to obtain the same friction torque T is reduced, so that the stress of the disc spring 6 is reduced and the durability and friction as a friction gear device are improved. Strength can be improved.

[Brief description of the drawings]

FIG. 1 is a plan view showing an overall configuration of a friction gear device according to a first embodiment of the present invention.

FIG. 2 is a partial cross-sectional view of the friction gear device.

FIG. 3 is an enlarged view of a contact portion between a driven gear and a sub gear.

FIG. 4 is a view showing a change in a friction torque ratio according to a ratio between a contact portion inner diameter and a contact portion outer diameter.

FIG. 5 is a partial sectional view of a friction gear device according to a second embodiment of the present invention.

FIG. 6 is a view showing forced deformation of a sub gear.

FIG. 7 is a cross-sectional view of a sub-gear having an annular protrusion at an inner peripheral edge of a clearance and a peripheral structure thereof

FIG. 8 is a partial sectional view of a friction gear device according to a third embodiment of the present invention.

FIG. 9 is an enlarged view of an annular spacer and its periphery.

[Explanation of symbols]

 1a, 1b camshaft 2 cylinder head 3a, 3b, 3c cam cap 4 driven gear 5 sub gear 6 disc spring 9 drive gear 21 washer 51 annular projection 61 spacer

Claims (18)

[Claims] In a driven side gear meshing with a driving side gear, a main gear which is a driven side gear has a larger number of teeth than the main gear, and has a freewheel with respect to the main gear coaxially with the main gear. A friction gear device comprising: a rotatable idler gear; and an elastic member that elastically presses the idler gear and the main gear, wherein the opposing surface of the main gear and the idler gear includes: Friction characterized in that at least one of the main gears and the idler gears is raised by including at least a region on or near the line of action of the pressing force by the elastic member, and the free gears are pressed through the raised portions. Gear device. 2. A drive-side gear meshing with a driven-side gear, comprising: a main gear, which is a drive-side gear; A friction gear device comprising: a rotatable idler gear; and an elastic member that elastically presses the idler gear and the main gear, wherein the opposing surface of the main gear and the idler gear includes: Friction characterized in that at least one of the main gears and the idler gears is raised by including at least a region on or near the line of action of the pressing force by the elastic member, and the free gears are pressed through the raised portions. Gear device. 3. The friction gear device according to claim 1, wherein the raised portion is annularly raised. 4. A method according to claim 1, wherein one of said main gear and said idler gear is provided with a first raised portion forming a tip side edge on the tip side with respect to said region, and the other with respect to said region. The friction gear device according to any one of claims 1 to 3, wherein a second raised portion that forms an axial side edge is provided on the rotating shaft side. 5. A protruding ridge in the main gear or the idler gear, the ridge having a tip edge on a tip side with respect to the region and an axial edge on a rotation axis side with respect to the region. The friction gear device according to any one of claims 1 to 3, wherein a portion is provided. 6. The friction gear device according to claim 5, wherein a tip of said convex ridge is formed in an arc shape. 7. The friction gear device according to claim 4, wherein said tooth tip side edge substantially coincides with a tooth bottom position. 8. The apparatus according to claim 4, wherein said action line is formed substantially at an intermediate point between said tooth tip side edge and said shaft side edge.
8. The friction gear device according to any one of 7 above. 9. A driven gear that meshes with a driving gear, comprising: a main gear, which is a driven gear, having a greater number of teeth than the main gear, and having a free shaft coaxial with the main gear and with respect to the main gear. A friction gear device comprising: a rotatable idler gear; and an elastic member that elastically presses the idler gear and the main gear, wherein the elasticity is provided between the main gear and the idler gear. By interposing a spacer on or near the line of action of the pressing force by the member,
A friction gear device wherein the main gear and the idle gear are pressed through the spacer. 10. A driving side gear meshing with a driven side gear, wherein the number of teeth of the main gear as the driving side gear is reduced as compared with the main gear, and the driving gear is coaxial with the main gear and with respect to the main gear. A friction gear device comprising: a rotatable idler gear; and an elastic member that elastically presses the idler gear and the main gear, wherein the elasticity is provided between the main gear and the idler gear. By interposing a spacer on or near the line of action of the pressing force by the member,
A friction gear device wherein the main gear and the idle gear are pressed through the spacer. 11. The friction gear device according to claim 9, wherein the spacer is annular. 12. A tip end side gap is formed between the main gear and the idle gear including the tip end side edge of the spacer, and the tip end side gap is formed including the axial end edge of the spacer. The friction gear device according to any one of claims 9 to 11, wherein a gap on the rotation shaft side is formed between the main gear and the idler gear. 13. The friction gear device according to claim 12, wherein at least one of the contact surfaces of the spacer with the main gear or the idle gear is formed in an arc shape. 14. The friction gear device according to claim 12, wherein the tip end side edge substantially coincides with a tooth bottom position. 15. The apparatus according to claim 1, wherein the action line is formed substantially at an intermediate point between the tip end side edge and the shaft side end edge.
15. The friction gear device according to any one of 2 to 14. 16. The main gear or the idler according to claim 16, wherein at least one of the opposing surfaces of the main gear and the idler gear is protruded with the tooth end side closer to the shaft side than the shaft side edge. The friction gear device according to any one of claims 4 to 8, and 12 to 15, wherein a gear is brought into contact with the corresponding raised portion. 17. A spacer is interposed between the main gear and the idle gear on the side closer to the axis than the shaft-side edge, and the main gear and the idle gear are brought into contact with the spacer. The friction gear device according to any one of claims 4 to 8, and 12 to 15, wherein: 18. The friction gear device according to claim 1, wherein the elastic member is a disc spring.