JP2001214943A - Power transmission structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of abnormal sound by rattling during rotation and to elongate the life of the structure by preventing the abrasion of a contact face. SOLUTION: This power transmission structure 102 is formed by provided with a cylindrical member (spline coupling 116) with an axial engagement groove 116a recessed in the internal circumferential face and axial members (first axial member 118 and second axial member 120) engaged with the engagement groove 116a by a plurality of axial projections 118a, 120a projecting from its outer circumferential face, and transmits the rotation power with circumferential play allowed thereto. This structure is also provided with a magnetic field generation part (permanent magnet 150) generating a magnetic field on the contact face between the projections 118a, 120a and the engagement groove 116a so that a circumferential attractive force acts on the interval between the projections 118a, 120a and the engagement groove 116a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power transmission structure for transmitting rotational power while allowing play in a circumferential direction by a cylindrical member and a shaft member inserted into the cylindrical member. In particular, the present invention relates to a technique for preventing noise during rotation caused by the play.

[0002]

2. Description of the Related Art Hitherto, for example, in a gear-type shaft coupling, a spline shaft, a spline coupling, etc., a power transmission structure for transmitting rotational power in a state of allowing a small play in a circumferential direction has been widely adopted.

FIG. 6 shows a state in which this kind of power transmission structure 2 is applied to a geared motor 10.

[0004] The geared motor 10 includes an electric motor (motor) 12 and a speed reducer 14.
The above-described power transmission structure 2 is applied in order to transmit the rotational power to the speed reducer 14.

As shown in an enlarged view in FIG. 7, the power transmission structure 2 has a spline coupling 16 (a cylindrical member) having a plurality of axially engaging grooves 16a formed in its inner peripheral surface. And a motor shaft 18 (a first shaft member) and an input shaft 20 on the side of the speed reducer 14 (a second shaft member) which are coaxially inserted into the spline coupling 16 from both sides. . The motor shaft 18 and the input shaft 20 respectively have axial projections 18a, 20a on their outer peripheral surfaces.
Are formed in a plurality of projections, and are engaged with the engagement grooves 16a of the spline coupling 16 in the circumferential direction.

As shown in FIG. 8, the engagement groove 16a and the projections 18a and 20a have a small gap 22 in the engaged state, and as a result, there is some play in the circumferential direction. Rotation power is transmitted coaxially between the motor shaft 18 and the input shaft 20 via the spline coupling 16 in a state where the play is allowed.

The reason why the gap 22 (play) is necessary is as follows.
(1) it is difficult to form the engagement groove 16a and the projections 18a and 20a completely accurately, (2) the spline coupling 16 can be easily assembled, and (3)
Even when the motor shaft 18 or the input shaft 20 and the spline coupling 16 expand at different ratios in the circumferential direction or the radial direction due to a high-temperature environment or the like, an excessive amount of space between the engagement groove 16a and the projections 18a and 20a. To prevent contact stress from being generated, and (4) to allow a certain amount of misalignment of the shaft center between the motor shaft 18 and the input shaft 20 and to allow the shaft ends to swing to some extent during rotation.

Next, an example in which this kind of power transmission structure is applied to a spline shaft or the like will be described.

FIG. 9 shows a so-called sliding type spline shaft 30. The spline shaft 30 is coaxially inserted into a cylindrical spline 32 (which is a cylindrical member) having a plurality of axially engaging grooves 32a recessed in an inner peripheral surface of the spline shaft 30 and is a cylindrical member. The engagement groove 3 is formed by a plurality of axial projections 34a protruding from the outer peripheral surface of itself.
Spline 3 (which is a shaft member) circumferentially engaged with 2a
4 is provided. As shown in FIG. 8, the spline shaft 30 also has a gap between the engaging groove 32a and the projection 34a in the engaged state (not shown), and the spline shaft 30 is allowed to have a play in the circumferential direction by the gap. , The rotational power is transmitted between the spline 34 and the cylindrical spline 32. The cylindrical spline 32 and the spline 34 are relatively slidable in the axial direction.

[0010]

SUMMARY OF THE INVENTION As described above,
Peripheral play in this type of power transmission structure (play)
Is an indispensable element for sufficiently exhibiting its own functions, but also causes noise and the like.

For example, in the case of the geared motor 10 shown in FIG. 6, it is considered that the change over time of the transmission torque acting on the power transmission mechanism 2 is in the state shown in FIG. That is, the transmission torque T1 is constant as a whole.
Is acting, but this transmitted torque is pulsating TS over time.
(Hereinafter referred to as torque pulsation), and a constant torque is not always applied. This torque pulsation may occur, for example, when the motor 12 is controlled by an inverter, when the generated torque of the motor 12 includes a torque pulsation due to harmonic components generated from the inverter, or when the motor 12 is driven by a single-phase induction transmission. In the case of (1), it is highly likely to occur when there is a large periodic variation in the strength of the internal rotating magnetic field.

Another factor is that the geared motor 10
This torque pulsation is also likely to occur when there is uneven rotation on the side of the speed reducer 14 in the above, or when a mating machine (external load) connected to the output shaft of the speed reducer 14 itself has a torsional vibration element.

In such a case, as shown in FIG. 11, when the transmission torque T2 acting on the power transmission mechanism 2 is small on average, the negative component (-TS) of the torque pulsation TS becomes As a result, the transmission torque exceeds the transmission torque T2, and the negative torque P is generated intermittently.

When the geared motor 10 is continuously operated in such a state, when the negative torque P is generated, the engagement groove 16a
The play in the circumferential direction due to the gap 22 between the protrusions 18a and 20a moves to the opposite side, and the engaging groove 16a and the protrusions 18a and 20a move.
There has been a problem that the side surfaces a collide with each other and generate abnormal noise and noise.

The problem of abnormal noise due to the movement of play (play) described above also occurs when the rotational speed of the rotational power transmitted by the power transmission mechanism 2 is controlled to give a predetermined change. That is, even when the acceleration / deceleration is precisely switched to control the rotation speed, play (play) occurs every time the acceleration changes from positive to negative or from negative to positive,
The engagement groove 16a and the projections 18a and 20a collide with each other and generate noise. Further, such a case is not only a problem of noise, but also leads to a delay in control response or an error, which is not preferable.

This collision is caused by the engagement groove 16a and the projection 18
In addition, there is a problem that the circumferential gap 22 is further increased and the noise is increased because abrasion occurs on the contact surface with the a and 20a. Further, the collision gives fatigue to each member, and the motor shaft 18 and the spline coupling 16
Was also a cause of shortening the life of the device. Note that these problems also apply to the spline shaft 30 shown in FIG.

The present invention has been made in view of the above-mentioned problems. For example, in the meshing type power transmission structure as described above, the generation of abnormal noise during the transmission of rotational power is suppressed, and the structure is further improved. It is intended to prolong the service life.

[0018]

According to the first aspect of the present invention, there is provided a cylindrical member having a plurality of axially engaging grooves formed in an inner peripheral surface of the cylindrical member, and a cylindrical member which is coaxially inserted into the cylindrical member and has A plurality of axial projections protrudingly provided on the outer peripheral surface of the outer peripheral surface. The engaging member includes a shaft member that circumferentially engages with the engagement groove. In a power transmission structure for transmitting rotational power between a shaft member and a cylindrical member in a permissible state, a magnetic field generating portion for generating a magnetic field on a contact surface of the projection and the engagement groove is provided, and the projection and the engagement are generated by the magnetic field. The above object is achieved by enabling transmission of rotational power in a state where a circumferential attractive force acts between the mating grooves.

With this arrangement, the projection and the engagement groove come into contact with each other at the start of rotation, and the projection and the engagement groove are attracted by the magnetic field acting on this contact surface. In addition, easy movement of play (play) due to separation of the projection and the engagement groove is suppressed, generation of abnormal noise due to collision between the engagement groove and the projection can be prevented, and power can be transmitted smoothly.

On the other hand, in this power transmission structure, the play is still "allowed", the manufacture and assembly are easy, and the shaft center between the shaft member and the cylindrical member is minute. Deviations and the like can also be absorbed. Further, when a negative torque exceeding the attractive force of the magnet acts during rotation, the play quickly moves to the opposite side, and a part of the negative torque overcomes the attraction by the magnet and the play moves. Since the energy is consumed as energy, it is possible to prevent the excessive fluctuation from being transmitted as it is, so that it is possible to reduce damage to the power transmission structure itself and external machines connected thereto.

It is widely known that when a magnetic field is applied to a ferromagnetic material or the like, the wear of the surface of the member is greatly reduced as disclosed in Japanese Patent Application Laid-Open No. 10-14173.

Therefore, in the present invention, in addition to the reduction in the number of collisions between the engagement grooves and the projections, the above-described effect of reducing the wear by the magnetic field is obtained, so that the wear of each member is significantly reduced as compared with the prior art. Thus, the life of the structure is prolonged.

The magnetic field generating section may be a magnetic member which is capable of generating a magnetic field by magnetizing a cylindrical member or a shaft member itself, or a case where a ring-shaped magnet is provided on the outer periphery of the cylindrical member. Although any means are possible, it is particularly preferable to house the magnet inside the cylindrical member and near the shaft end of the shaft member.

In this way, the present invention can be easily applied and the effect can be sufficiently obtained without making a significant change to the existing power transmission structure. Furthermore, if the magnet is inserted inside the cylindrical member as described above, the magnetic poles are not exposed to the outside, so that the self-demagnetizing action of the magnet is reduced.
There is no need to replace magnets over time. The magnetic field generated by the magnet arranged as described above is formed, for example, so as to return from the shaft member to the self via the cylindrical member (or vice versa). The magnetic field hardly affects the bearings, other components, and the like arranged in the vehicle.

The power transmission structure described above can be applied to the second invention as described below.

Specifically, the second invention provides a cylindrical member having a plurality of axially engaging grooves formed in its inner peripheral surface, and coaxially inserted into the cylindrical member from both sides. A first shaft member and a second shaft member which are circumferentially engaged with the engagement groove by a plurality of axial protrusions provided on the outer peripheral surface of the own, and the engagement groove and the protrusion in the engaged state; In the power transmission structure for transmitting rotational power between the first and second shaft members via the cylindrical member in a state in which play in the circumferential direction is allowed by the gap of the first member and the second shaft, A magnet is housed between the shaft ends of the members, and a magnetic field is generated on the contact surface of the projection and the engagement groove by the magnet, and the magnetic field rotates with a circumferential attractive force acting between the projection and the engagement groove. The above object is achieved by transmitting power.

In this manner, the present invention can be easily applied to existing spline type couplings, gear type shaft couplings and the like, and substantially the same effects as those of the first aspect can be obtained. In particular, such a power transmission structure generally has a fixed gap between the first shaft member and the second shaft member in many cases, and if a magnet is accommodated in this gap, it is easy and inexpensive. Thus, the second invention can be realized. That is, one magnet (although the present invention is not limited to one) can effectively generate a magnetic field between the first shaft member and the cylindrical member and between the second shaft member and the cylindrical member. However, a sufficient effect can be obtained though the structure is simple. Further, since the magnet is completely housed inside, it is possible to sufficiently maintain the durability of the magnetic force.

In order to allow relative movement in the axial direction between the first shaft member and the second shaft member, it is preferable to leave a gap from the beginning in the internal space of the structure. In order to prevent the magnetic force from lowering to some extent with a gap, it is preferable to use a bonded magnet or the like having elasticity and to permit the above-mentioned movement by elastic deformation.

In the first and second aspects of the present invention,
The strength of the circumferential attractive force acting between the projection and the engagement groove,
It is preferable to set the projection torque and the engagement groove high enough to maintain the contact state of the projection and the engagement groove against the negative torque generated by the torque pulsation of the rotational power.

In this manner, when transmitting rotational power having torque pulsation, a stable contact state is always maintained with respect to the intermittent negative torque, so that quiet operation is possible. The magnitude of this negative torque depends on the type of power generation device connected to the power transmission structure,
Since it differs depending on the characteristics of the external load and the like, the magnetic force of the magnet may be appropriately adjusted in consideration of the environment actually applied.

Furthermore, in any of the above-described inventions, it is possible to have a structure that is not restricted to a certain direction of rotation as a feature thereof.

Specifically, the strength of the attractive force in the circumferential direction is
What is necessary is just to set it as low as the contact state of a projection and an engagement groove can be changed by the starting torque of rotational power. With this configuration, the torque (starting torque) of the rotating power input to the power transmission structure at the time of starting (starting torque) becomes greater than the above-mentioned attractive force, so that the contact state between the projection and the engagement groove at the time of starting is optimized. At the same time, they are mutually adsorbed, and thereafter this contact state can be maintained.

In addition, as long as the rotation direction at the time of the previous stop and the rotation direction at the time of the current start do not change, no impact occurs at the time of the start. Therefore, an extremely rational power transmission structure that is not always restricted in the rotation direction can be obtained.

The above-mentioned cylindrical member is not limited to a completely cylindrical member, but may be any cylindrical member as long as the shaft member can be housed inside. On the other hand, the shaft member is not limited to a solid structure. And a hollow structure into which a wiring or the like can be inserted. The case where the magnet is housed near the shaft end of the shaft member includes the case where the magnet is housed inside the shaft member or the shaft end itself is magnetized near the shaft end, and further includes a cylindrical member. Also includes one that fixes a magnet on the side.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a power transmission structure 102 according to a first embodiment of the present invention.

The power transmission structure 102 includes a spline coupling 116 which is a cylindrical member, and first and second shaft members 118 and 120 which are coaxially inserted into the spline coupling 116 from both sides. .

The first shaft member 118 and the second shaft member 12
The 0 and spline couplings 116 are both made of a ferromagnetic material.

The spline coupling 116 has a plurality of axially engaging grooves 116a formed in the inner peripheral surface thereof at equal intervals in the circumferential direction. On the other hand, the first and second shaft members 118,
120 have axial projections 118a,
A plurality of Oa are provided at equal intervals in the circumferential direction. These projections 118a and 120a are circumferentially engaged with the engagement grooves 116a, and a gap 122 (see FIG. 2) is formed between the engagement grooves 116a and the projections 118a and 120a in this engaged state. ing. Therefore, the spline coupling 116 is provided with the circumferential play allowed by the gap 122.
The rotational power is transmitted between the first and second shaft members 118 and 120 via the shaft.

In this power transmission structure 102, rotational power is input from the first shaft member 118 and transmitted (output) to the second shaft member 120 via the spline coupling 116.

As shown specifically in FIG.
In the protrusion 118a of the first shaft member 118 on the input side, a gap 122 is formed in the side wall 119b on the opposite side to the rotation direction by the contact of the side wall 119a on the side of rotation R with the engagement groove 116a. . This is because the first shaft member 118 is "driving" the spline coupling 116.

On the other hand, in the second shaft member 120 on the output side, as shown in FIG.
Is in contact with the engagement groove 116a, and a gap 122 is formed in the side wall 121a on the rotation direction R side. This is because the second shaft member 120 is "driven" by the spline coupling 116.

Up to this point, it is almost the same as the conventional shaft coupling structure 2 already shown in FIG.

The difference from the prior art is that the inside of the spline coupling 116 and the first and second shaft members 118 and 120 are provided.
Between the shaft ends 118b and 120b of the permanent magnet 15 having the N pole on the first shaft member 118 side and the S pole on the second shaft member 120 side.
0 is stored (see FIG. 1). Further, the entire magnetic field generated by the permanent magnet 150 is emitted from its own N pole, returns to the S pole of the permanent magnet 150 via the first shaft member 118, the spline coupling 116, and the second shaft member 120 in this order. Has become. That is, the power transmission structure 1
The magnetic field lines P shown in FIG.

Accordingly, as shown in FIG. 2, the projection 118a and the engagement groove 116a are formed by the permanent magnet 150.
A magnetic field B is generated on the contact surface 152 between the projection 118a and the engaging groove 116a, and this magnetic field B exerts an (at least) circumferential attractive force Q between the projection 118a and the engaging groove 116a.

As shown in FIG. 3, a magnetic field B is also generated from the spline coupling 116 toward the second shaft member 120 on the contact surface 154 on the second shaft member 120 side. A circumferential attractive force Q acts between the projection 120a and the engaging groove 116a.

According to the power transmission structure 102, the attractive force Q caused by the magnetic field B generated by the permanent magnet 150 causes
Since the projections 118a and 120a and the engagement groove 116a rotate in a contact state (adsorption state), rattling during rotation caused by the circumferential gap 122 is prevented, and the projections 118a and 120a are rotated.
Noise (abnormal noise) due to collision between 120a and engagement groove 116a
Is greatly reduced.

As is apparent from comparison with the power transmission structure 2 shown in the conventional example in FIG. 6, the structure is almost the same as the conventional one except for the permanent magnet 150. Therefore, the present embodiment can be realized with little improvement of existing equipment (parts), so that it is very inexpensive. In addition, since the power transmission structure allows the play in the circumferential direction as in the related art, the basic function of the spline coupling 116 is not lost.

Further, by the action of the magnetic field of the permanent magnet 150, the abrasion of the contact surfaces of the engaging groove 116a and the projections 118a, 120a is greatly reduced. Since this theory is clearly disclosed in Japanese Patent Application Laid-Open No. H10-14173 and the like, detailed description will be omitted.

Since both the S pole and the N pole of the permanent magnet 150 are completely in contact with the shaft ends of the first shaft member 118 and the second shaft member 120, respectively, a magnetic field is efficiently formed. Further, according to this structure, the self-demagnetizing effect which tends to occur when the magnetic pole is exposed is greatly reduced, so that the magnetic force can be maintained high for a long period of time.

In the first embodiment, the permanent magnet 15 is formed by the first shaft member 118 and the second shaft member 120.
However, the present invention is not limited to this, and the permanent magnet 150 may be housed inside the shaft member, or the permanent magnet 150 may be provided on the spline coupling 116 side. It can be fixed. Further, the shaft member or the shaft joint itself may be magnetically attached.

Next, a power transmission structure 202 according to a second embodiment of the present invention will be described in detail with reference to FIG.

The power transmission structure 202 includes an input shaft 204
And the output shaft 206 are connected by a so-called gear type (flexible) shaft joint 208.

The gear type shaft coupling 208 is connected to the input shaft 204
Are inserted into the inside and key-coupled, and a cylindrical coupling center 210 having an axial projection 218a formed on its outer periphery;
Is inserted inside, and a cylindrical coupling case 212 having a plurality of axially engaging grooves 216a protruding on its inner peripheral surface, and is flange-coaxially coupled to the coupling case 212, and And a cylindrical resett 214 into which the output shaft 206 is inserted and keyed.

When this structure is adopted in the same manner as in the first embodiment, the input shaft 204 and the coupling center 210 are integrated (without play) to constitute a "shaft member".
The coupling case 212, the rosette 214, and the output shaft 206 are integrated (without play) to form a “cylindrical member”. Therefore, the engagement groove 216a and the protrusion 21
The rotational power is transmitted between the "shaft member" and the "cylindrical member" in a state where "play" in the circumferential direction is allowed by the gap of the shaft member 8a.

This cylindrical member (coupling case 212)
), And near the shaft end of the shaft member (coupling center 210), a permanent magnet 250 is housed. The permanent magnet 250 generates a magnetic field on the contact surface between the protrusion 218a and the engaging groove 216a. I have. Therefore, the rotational power is transmitted in a state where a circumferential attractive force acts between the protrusion 218a and the engagement groove 216a by this magnetic field.

The power transmission structure 20 according to the second embodiment
In the second embodiment, the basic idea is not different from the power transmission structure 101 shown in the first embodiment.
Almost the same effects as those described in the embodiments can be obtained. Therefore, the detailed description of the operation state of the magnetic field and the like will be omitted because they are redundant.

Next, a power transmission structure 302 according to a third embodiment of the present invention will be described with reference to FIG. This power transmission structure 302 also has the input shaft 30 similarly to the second embodiment.
4 and the output shaft 306 are connected by a gear type shaft coupling 308.

The difference from the second embodiment is that the gear type shaft coupling 308 has a symmetrical structure, and two coupling centers 318 and 320 arranged coaxially, and these coupling centers 318 and 320 are different from each other. And a pair of coupling cases 312a and 312b which are inserted into their own parts and are flanged to each other by bolts (constituting a cylindrical member as a whole). Therefore, each coupling center 318, 320
Has a plurality of axial projections 318a, 320a protruding from the outer peripheral surface thereof, while the coupling case 312a,
312b has an axial engagement groove 31 on each inner peripheral surface.
6a and 316b are provided in a plurality.

The input shaft 304 and the output shaft 306 are inserted into a pair of coupling centers 318 and 320, respectively, and are keyed. As a result, the rotational power is applied between the input shaft 304 and the output shaft 306 in a state where play in the circumferential direction is allowed by the gaps of the engagement grooves 316a and 316b and the protrusions 318a and 320a (not shown). Is transmitted.

A pair of coupling centers 318, 320
Between them, a permanent magnet 350 is inserted, and the permanent magnet 350 generates a magnetic field between the contact surfaces of the protrusions 318a, 320a and the engaging groove 316a.
Due to this magnetic field, the projections 318a, 320a and the engagement groove 3
A circumferential attractive force acts on 16a (detailed description is omitted).

Therefore, the basic concept of the power transmission structure 302 of the third embodiment is the same as that of the first embodiment, and the same effects as those of the first embodiment can be obtained.

In the above embodiment, the case where the power transmission structure according to the present invention is applied to the coupling (shaft coupling) is shown. However, the application of the present invention is not limited to this. The present invention can be applied to the spline shaft shown and other parts and members.

[0064]

According to the present invention, it is possible to obtain a power transmission structure in which abnormal noise due to backlash in the circumferential direction during rotation is suppressed, wear of the contact surface is suppressed, and the life is extended.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a power transmission structure according to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view of a part II-II in FIG.

FIG. 3 is an enlarged sectional view of a part III-III of FIG. 1;

FIG. 4 is a sectional view showing a power transmission structure according to a second embodiment of the present invention.

FIG. 5 is a sectional view showing a power transmission structure according to a third embodiment of the present invention.

FIG. 6 is a partial sectional view showing an example in which a conventional power transmission structure is applied to a geared motor.

FIG. 7 is an enlarged sectional view showing the power transmission structure.

FIG. 8 is an enlarged sectional view of VIII-VIII in FIG. 7;

FIG. 9 is a sectional view showing an example in which a conventional power transmission structure is applied to a spline shaft.

FIG. 10 is a diagram showing a time-dependent state of a transmission torque acting on a power transmission structure.

FIG. 11 is a diagram showing a state over time when the transmission torque is relatively small.

[Explanation of symbols]

 102, 202, 302 Power transmission structure 116 Spline coupling 116a Engagement groove 118 First shaft member 118a, 120a Projection 120 Second shaft member 122 Gap 150, 250, 350 Permanent magnet 208, 308 … Gear type shaft coupling

Claims (5)

[Claims] 1. A cylindrical member having a plurality of axially engaging grooves formed in its inner peripheral surface, and a plurality of shafts coaxially inserted into the cylindrical member and projecting from its own outer peripheral surface. A shaft member that is circumferentially engaged with the engagement groove by a projection in a direction, and the shaft member is configured to allow circumferential play by a gap of the engagement groove and the projection in an engaged state. And a power transmission structure for transmitting rotational power between the cylindrical members, wherein a magnetic field generating portion for generating a magnetic field is provided on a contact surface between the projection and the engagement groove, and the magnetic field is provided between the projection and the engagement groove. A power transmission structure characterized in that rotational power can be transmitted while a circumferential attractive force acts on the power transmission device. 2. The magnetic field generating section according to claim 1,
A power transmission structure, wherein a magnet is housed inside the cylindrical member and near a shaft end of the shaft member. 3. A cylindrical member having a plurality of axially engaging grooves formed in an inner peripheral surface thereof, and coaxially inserted into the cylindrical member from both sides, and a plurality of convexes are formed on each outer peripheral surface of the cylindrical member. A first shaft member and a second shaft member which are circumferentially engaged with the engagement groove by an axial projection provided, and are provided in a circumferential direction by a gap of the engagement groove and the projection in an engaged state. A power transmission structure for transmitting rotational power between the first and second shaft members via the cylindrical member in a state in which play of the first and second shaft members is allowed. A state in which a magnet is housed between the shaft ends and a magnetic field is generated by the magnet on the contact surface between the projection and the engagement groove, and a circumferential attractive force acts between the projection and the engagement groove by the magnetic field. A power transmission structure characterized in that rotational power can be transmitted through the power transmission. 4. The method according to claim 1, wherein the strength of the attractive force acting between the projection and the engagement groove in the circumferential direction is reduced with respect to a negative torque generated by a torque fluctuation of the rotational power. A power transmission structure, wherein the power transmission structure is set high enough to maintain a contact state between the projection and the engagement groove. 5. The method according to claim 1, wherein the strength of the attractive force acting between the projection and the engagement groove in the circumferential direction is controlled by the projection and the rotation torque. A power transmission structure wherein the contact state of the engagement groove is set low enough to be changed.