Patent Description:
Electric power steering devices for assisting torque necessary for steering a vehicle by an electric motor have been known (for example, Patent Literature <NUM>).

In conventional electric power steering devices, when vibration and noise caused by an electric motor as well as vibration and noise caused by a joint and a gear that transmit driving power from the electric motor to an output shaft of a steering wheel create a synergistic effect, unignorable vibration and noise may occur.

It is an object of the present invention to provide an assist mechanism and an electric power steering device capable of suppressing vibration and noise. Solution to Problem.

To achieve the object, an assist mechanism according to the present invention is disclosed in claim <NUM>.

To achieve the object, an assist mechanism according to an aspect of the present invention includes: an electric motor in which a rotor has a magnet and a stator has a winding; a shaft-shaped member in which a worm engaged with a worm wheel is formed; and a coupling that couples an output shaft of the electric motor and the shaft-shaped member to each other, wherein a smallest rotation order among rotation orders of the electric motor and a rotation order of the worm are prime to each other, the smallest rotation order among the rotation orders of the electric motor and a rotation order of the coupling are prime to each other, and the rotation order of the worm and the rotation order of the coupling are prime to each other.

To achieve the object, an assist mechanism according to an aspect of the present invention includes: an electric motor; a shaft-shaped member in which a worm engaged with a worm wheel is formed; and a coupling that couples an output shaft of the electric motor and the shaft-shaped member to each other, wherein at least two rotation orders of a smallest rotation order among rotation orders of the electric motor, a rotation order of the worm, and a rotation order of the coupling are different from each other.

To achieve the object, an electric power steering device according to an aspect of the present invention includes the assist mechanism, wherein the worm wheel is provided to an output shaft of a steering wheel.

Thus, the rotation orders of rotating configurations can be made different to shift their timings of occurrence of vibration and noise. This can prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Consequently, vibration and noise can be reduced.

In the assist mechanism according to an aspect of the present invention, the rotation order of the electric motor is n times a larger natural number of two natural numbers other than <NUM> when a multiplication of the two natural numbers expresses the number of poles of a magnet in the rotor, n is a natural number of <NUM> or greater, and the rotation order of the worm is the number of teeth of the worm wheel that advance in response to one rotation of the shaft-shaped member.

In the assist mechanism according to the present invention, the rotation order of the worm is the number of teeth of the worm wheel that advance in response to one rotation of the shaft-shaped member, the coupling includes a first transmission member fixed to the output shaft, a second transmission member fixed to the shaft-shaped member, and a third transmission member engaged with a predetermined number of projections of each of the first transmission member and the second transmission member to couple the first transmission member and the second transmission member to each other, and the rotation order of the coupling is the predetermined number.

In the assist mechanism according to an aspect of the present invention, the rotation order of the electric motor is n times a larger natural number of two natural numbers other than <NUM> when a multiplication of the two natural numbers expresses the number of poles of a magnet or a commutator in the rotor, n is a natural number of <NUM> or greater, and the rotation order of the worm is the number of teeth of the worm wheel that advance in response to one rotation of the shaft-shaped member.

In the assist mechanism according to an aspect of the present invention, the rotation order of the electric motor is n times a larger natural number of two natural numbers other than <NUM> when a multiplication of the two natural numbers expresses the number of poles of a magnet or a commutator in the rotor, n is a natural number of <NUM> or greater, the coupling includes a first transmission member fixed to the output shaft, a second transmission member fixed to the shaft-shaped member, and a third transmission member engaged with a predetermined number of projections of each of the first transmission member and the second transmission member to couple the first transmission member and the second transmission member to each other, and the rotation order of the coupling is the predetermined number.

Thus, the rotation orders of the rotating configurations can be specified based on the number of configurations that move at positions away from the respective rotation axes along with the rotations of the rotating configurations, and the rotation orders can be made different. Therefore, their timings of occurrence of vibration and noise can be shifted. This can more reliably prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Consequently, vibration and noise can be more reliably reduced.

In the assist mechanism according to an aspect of the present invention, the rotation order of the coupling is <NUM> or <NUM>.

In the assist mechanism according to an aspect of the present invention, the coupling includes a first transmission member fixed to the output shaft, a second transmission member fixed to the shaft-shaped member, and a third transmission member engaged with five or seven projections of each of the first transmission member and the second transmission member to couple the first transmission member and the second transmission member to each other, and the rotation order of the coupling is the number of the projections.

Thus, the rotation order of the coupling can be made difficult to be equal to the rotation orders of other configurations. This can more reliably prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Consequently, vibration and noise can be more reliably reduced.

In the assist mechanism according to an aspect of the present invention, the smallest rotation order among the rotation orders of the electric motor is <NUM>, the rotation order of the worm is <NUM>, and the rotation order of the coupling is <NUM> or <NUM>.

Thus, all rotation orders of the electric motor, the worm, and the coupling can be made different. This can more reliably prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Consequently, vibration and noise can be more reliably reduced.

In the assist mechanism according to an aspect of the present invention, the smallest rotation orders among the rotation orders of the electric motor is <NUM>, the rotation order of the worm is <NUM>, and the rotation order of the coupling is <NUM>.

In the assist mechanism according to an aspect of the present invention, the smallest rotation order among the rotation orders of the electric motor is <NUM>, the rotation order of the worm is <NUM>, and the rotation order of the coupling is <NUM>.

According to the present invention, vibration and noise can be reduced.

Embodiments of the present invention are described below with reference to the drawings, but the present invention is defined in the appended claims. The requirements in the embodiments described below can be combined as appropriate. Some of components are not used in some cases.

<FIG> is a diagram illustrating a configuration example of an electric power steering device <NUM> including an assist mechanism <NUM> in a first embodiment. The electric power steering device <NUM> includes, in the order in which force applied by a steerer is transmitted, a steering wheel <NUM>, a steering shaft <NUM>, an assist mechanism <NUM>, a universal joint <NUM>, a lower shaft <NUM>, a universal joint <NUM>, a pinion shaft <NUM>, a steering gear <NUM>, and a tie rod <NUM>. The electric power steering device <NUM> further includes an electronic control unit (ECU) <NUM>, a torque sensor 91a, and a vehicle speed sensor 91b.

The steering shaft <NUM> includes an input shaft 82a and an output shaft 82b. One end portion of the input shaft 82a is coupled to the steering wheel <NUM>, and the other end portion is coupled to the assist mechanism <NUM> through the torque sensor 91a. One end portion of the output shaft 82b is coupled to the assist mechanism <NUM>, and the other end portion is coupled to the universal joint <NUM>. In the first embodiment, the input shaft 82a and the output shaft 82b are formed from magnetic material such as iron.

One end portion of the lower shaft <NUM> is coupled to the universal joint <NUM>, and the other end portion is coupled to the universal joint <NUM>. One end portion of the pinion shaft <NUM> is coupled to the universal joint <NUM>, and the other end portion is coupled to the steering gear <NUM>.

The steering gear <NUM> includes a pinion 88a and a rack 88b. The pinion 88a is coupled to the pinion shaft <NUM>. The rack 88b is engaged with the pinion 88a. The steering gear <NUM> is formed as a rack and pinion steering gear. The steering gear <NUM> converts rotational motion transmitted to the pinion 88a into linear motion by the rack 88b. The tie rod <NUM> is coupled to the rack 88b.

<FIG> is a diagram illustrating a configuration example of the assist mechanism <NUM>. <FIG> illustrates a diagram in which a part of a housing <NUM> is cut open for the purpose of illustrating an internal configuration of the housing <NUM>. The assist mechanism <NUM> includes an electric motor <NUM>, a housing <NUM>, a worm gear shaft <NUM>, a worm wheel <NUM>, and a coupling <NUM>.

The housing <NUM> houses the worm gear shaft <NUM>, the worm wheel <NUM>, and the coupling <NUM> therein. The electric motor <NUM> is fixed to the housing <NUM>. An output shaft <NUM> of the electric motor <NUM> extends inward of the housing <NUM>. The electric motor <NUM> rotationally drives the output shaft <NUM>. The output shaft <NUM> is coupled to the worm gear shaft <NUM> through the coupling <NUM>. When the output shaft <NUM> rotates, the worm gear shaft <NUM> rotates. The worm gear shaft <NUM> is engaged with the worm wheel <NUM>. When the worm gear shaft <NUM> rotates, the worm wheel <NUM> rotates. The worm wheel <NUM> is fixed such that its rotation axis coincides with a rotation axis of the output shaft 82b. When the worm wheel <NUM> rotates, rotational driving force of the worm wheel <NUM> is applied to the output shaft 82b. In this manner, the assist mechanism <NUM> assists the rotation of the output shaft 82b with rotational driving force of the electric motor <NUM>. In other words, the assist mechanism <NUM> applies, to the output shaft 82b, torque for assisting steering performed through the steering wheel <NUM>.

In the electric power steering device <NUM> in the first embodiment, a steering column is formed using the steering shaft <NUM>, the torque sensor 91a, and the assist mechanism <NUM>. In other words, the electric power steering device <NUM> in the first embodiment is, for example, a column assist power steering device.

The torque sensor 91a illustrated in <FIG> detects steering force of a driver transmitted to the input shaft 82a through the steering wheel <NUM> as steering torque. The vehicle speed sensor 91b detects traveling speed of a vehicle having the electric power steering device <NUM> mounted thereon. The ECU <NUM> is electrically connected to the electric motor <NUM>, the torque sensor 91a, and the vehicle speed sensor 91b.

The ECU <NUM> controls operation of the electric motor <NUM>. The ECU <NUM> acquires signals from the torque sensor 91a and the vehicle speed sensor 91b. In other words, the ECU <NUM> acquires steering torque T from the torque sensor 91a, and acquires traveling speed V of the vehicle from the vehicle speed sensor 91b. The ECU <NUM> is supplied with power from a power supply device (for example, on-vehicle battery) <NUM> in the state in which an ignition switch <NUM> is turned on. The ECU <NUM> calculates an assist steering command value for assist command based on the steering torque T and the traveling speed V. The ECU <NUM> adjusts a power value X to be supplied to the electric motor <NUM> based on the calculated assist steering command value. The ECU <NUM> acquires, as operation information Y, information on induced voltage from the electric motor <NUM> or information on rotation of a rotor from a resolver described later.

Steering force input to the steering wheel <NUM> by a steerer (driver) is transmitted to the assist mechanism <NUM> through the input shaft 82a. In this case, the ECU <NUM> acquires steering torque T input to the input shaft 82a from the torque sensor 91a, and acquires traveling speed V from the vehicle speed sensor 91b. The ECU <NUM> controls the operation of the electric motor <NUM>. Assist steering torque produced by the electric motor <NUM> is transmitted to the assist mechanism <NUM>.

The steering torque T (including assist steering torque) output through the output shaft 82b is transmitted to the lower shaft <NUM> through the universal joint <NUM>, and further transmitted to the pinion shaft <NUM> through the universal joint <NUM>. The steering force transmitted to the pinion shaft <NUM> is transmitted to the tie rod <NUM> through the steering gear <NUM> to turn steered wheels.

Each configuration in the assist mechanism <NUM> is described below. The worm gear shaft <NUM> is a shaft-shaped member including a worm 94a and a rotation shaft portion 94b. The rotation shaft portion 94b in the worm gear shaft <NUM> is pivotally supported by bearings 95a and 95b fixed to the housing <NUM>. The bearings 95a and 95b pivotally support the worm gear shaft <NUM> such that the worm gear shaft <NUM> is rotatable. The worm 94a in the worm gear shaft <NUM> is formed between the bearing 95a and the bearing 95b.

<FIG> is a diagram illustrating a configuration example of the worm 94a and the worm wheel <NUM> in the first embodiment. The worm 94a in the first embodiment is a double thread worm. In other words, as indicated by a half rotation pitch P1 in <FIG>, gears of the worm wheel <NUM> are formed so as to advance by one in response to half rotation of the worm 94a. Thus, the gears of the worm wheel <NUM> advance by two in response to one rotation of the worm 94a.

<FIG> is a diagram illustrating an example of a configuration near the coupling <NUM>. The coupling <NUM> includes a first transmission member <NUM>, a second transmission member <NUM>, and a third transmission member <NUM>. The first transmission member <NUM> is fixed to the output shaft <NUM>. Specifically, the first transmission member <NUM> draws a circular ring that houses therein an end portion of the output shaft <NUM> extending from the electric motor <NUM>. Ideally, the center of the circular ring of the first transmission member <NUM> coincides with a rotation axis center 11a (see <FIG>) of the output shaft <NUM>. The second transmission member <NUM> is fixed to the rotation shaft portion 94b. Specifically, the second transmission member <NUM> draws a circular ring that houses therein an end portion of the rotation shaft portion 94b extending from the bearing 95a, which is located closer to the output shaft <NUM> and pivotally supports the worm gear shaft <NUM>, toward the output shaft <NUM>. Ideally, the center of the circular ring of the second transmission member <NUM> coincides with a rotation axis center of the rotation shaft portion 94b.

<FIG> is a perspective view of the coupling <NUM> in the first embodiment. <FIG> is an exploded perspective view of the coupling <NUM> in the first embodiment. The third transmission member <NUM> is engaged with a predetermined number of projections <NUM> of the first transmission member <NUM> and the predetermined number of projections <NUM> of the second transmission member <NUM>, thereby coupling the first transmission member <NUM> and the second transmission member <NUM> to each other. Specifically, for example, the third transmission member <NUM> has an outer peripheral portion <NUM> having a diameter greater than the outer diameters of the circular rings of the first transmission member <NUM> and the second transmission member <NUM>. The outer peripheral portion <NUM> has an outer peripheral surface having a columnar or cylindrical shape. On the inner side of the outer peripheral portion <NUM>, an inner peripheral portion <NUM> having a diameter less than that of the outer peripheral portion <NUM> is formed. The center of a circular ring drawn by the outer peripheral portion <NUM> coincides with the center of a circular ring drawn by the inner peripheral portion <NUM>. The third transmission member <NUM> has a cylindrical shape having a thickness of a wall surface corresponding to the difference between the outer peripheral portion <NUM> and the inner peripheral portion <NUM> or a columnar shape having a hole corresponding to the inner diameter of the inner peripheral portion <NUM>. The center axis of the third transmission member <NUM> described below refers to the center axis of the cylinder or the column.

The third transmission member <NUM> has a plurality of recesses <NUM> formed radially from the inner peripheral portion <NUM> toward the outer peripheral portion <NUM>. The recesses <NUM> are recesses bored in the inner peripheral portion <NUM> so as to spread radially from the center axis of the third transmission member <NUM>. The position (depth) of an end portion of the recess <NUM> in the radial direction with respect to the inner peripheral portion <NUM> is less than the thickness of the wall surface of the third transmission member <NUM>. The end portions of the recesses <NUM> are arranged circularly about the center axis of the third transmission member <NUM>. The end portion of each of the recesses <NUM> in the radial direction has an arc shape, for example, but a corner may be formed.

The first transmission member <NUM> has a plurality of projections <NUM> formed radially from the center of the circular ring of the first transmission member <NUM> to the outer periphery. More specifically, the first transmission member <NUM> has a cylindrical portion <NUM> that surrounds the output shaft <NUM> fixed on the inner side thereof. The projections <NUM> are formed so as to extend radially outward from the outer peripheral surface of the cylindrical portion <NUM>. In other words, the projections <NUM> are gear-shaped structures formed on the outer peripheral surface of the cylindrical portion <NUM>. Each of the projections <NUM> is a tooth thereof. For example, end portions of the extending projections <NUM> are arranged along the circular ring of the first transmission member <NUM>. The diameter of a circular ring drawn by the end portions of the projections <NUM> may be less than the diameter of the circular ring of the first transmission member <NUM>. The thicknesses of the cylindrical portion <NUM> and the projections <NUM> in a rotation axis direction of the output shaft <NUM> are equal to or less than a half of the thickness of the third transmission member <NUM> in a center axis direction.

The first transmission member <NUM> and the second transmission member <NUM> have shapes mirror-symmetric with respect to the third transmission member <NUM>. Specifically, the second transmission member <NUM> has a plurality of projections <NUM> formed radially from the center of the circular ring of the second transmission member <NUM> to the outer periphery. More specifically, the second transmission member <NUM> has a cylindrical portion <NUM> that surrounds the rotation shaft portion 94b fixed on the inner side thereof. The projections <NUM> are formed so as to extend radially outward from the outer peripheral surface of the cylindrical portion <NUM>. For example, end portions of the extending projections <NUM> are arranged along the circular ring of the second transmission member <NUM>. The diameter of a circular ring drawn by end portions of the projections <NUM> may be less than the diameter of the circular ring of the second transmission member <NUM>. The thicknesses of the cylindrical portion <NUM> and the projections <NUM> in a rotation axis direction of the rotation shaft portion 94b are equal to or less than a half of the thickness of the third transmission member <NUM> in the center axis direction.

In the first embodiment, the number of the projections <NUM>, the number of the projections <NUM>, and the number of the recesses <NUM> are <NUM>. The diameter of the circular ring drawn by the end portions of the projections <NUM> and the diameter of the circular ring drawn by the end portions of the projections <NUM> are equal to or less than the diameter drawn by the end portions of the recesses <NUM>. The shapes of the end portions of the projections <NUM> and the projections <NUM> are shapes housed inside the recesses <NUM>. Specifically, the shapes of the end portions of the projections <NUM> and the projections <NUM> are arc, for example. As illustrated in <FIG>, the projections <NUM> and the projections <NUM> are fitted so as to be housed inside the recesses <NUM>, so that the third transmission member <NUM> couples the first transmission member <NUM> and the second transmission member <NUM> to each other.

More specifically, in the first embodiment, margins are provided between the diameter of the circular ring drawn by the end portions of the projections <NUM> and the diameter drawn by the end portions of the recesses <NUM> and between the diameter of the circular ring drawn by the end portions of the projections <NUM> and the diameter drawn by the end portions of the recesses <NUM>. This allows the first transmission member <NUM> and the second transmission member <NUM> to be coupled to each other with the third transmission member <NUM> therebetween in a positional relation in which the first transmission member <NUM> and the second transmission member <NUM> are not limited to be coaxial to each other. Thus, warpage of the output shaft <NUM> and the rotation shaft portion 94b due to misalignment of axes caused between the output shaft <NUM> and the rotation shaft portion 94b can be suppressed.

In the examples illustrated in <FIG> and <FIG>, elastic members <NUM> and <NUM> are respectively attached to end surfaces of the cylinder (or column) of the third transmission member <NUM>. For example, the elastic member <NUM> has a circular ring-shaped protruding portion <NUM> extending toward the third transmission member <NUM>. The protruding portion <NUM> is fitted to a circular ring-shaped groove member <NUM> formed so as to be located between the outer peripheral portion <NUM> and the recesses <NUM> of the third transmission member <NUM>. The elastic member <NUM> fitted to the third transmission member <NUM> is located between the second transmission member <NUM> and the third transmission member <NUM>. In the elastic member <NUM>, an inner peripheral portion <NUM> having a diameter equal to that of the inner peripheral portion <NUM> is formed. The elastic member <NUM> has a plurality of recesses <NUM> formed radially outward from the inner peripheral portion <NUM> in a manner similar to the recesses <NUM> of the third transmission member <NUM>. The third transmission member <NUM> is provided with projections <NUM> provided so as to be located between the recesses <NUM> arranged circularly. The projections <NUM> extend from the inner peripheral portion <NUM> toward the rotation shaft portion 94b along the center axis direction of the third transmission member <NUM>. In a plate surface <NUM> of the elastic member <NUM>, holes <NUM> for fitting the projections <NUM> therein are formed. The positional relation between the recesses <NUM> and the projections <NUM> corresponds to the positional relation between the recesses <NUM> and the holes <NUM>. In this manner, as illustrated in <FIG>, the projections <NUM> are fitted so as to be housed inside the recesses <NUM>.

The first transmission member <NUM> and the second transmission member <NUM> have shapes mirror-symmetric with respect to the third transmission member <NUM>. The end surfaces of the cylinder (or column) of the third transmission member <NUM> have mirror-symmetric shapes. Specifically, the elastic member <NUM> has a circular ring-shaped protruding portion <NUM> extending toward the third transmission member <NUM>. In the elastic member <NUM>, an inner peripheral portion <NUM> having a diameter equal to that of the inner peripheral portion <NUM> is formed. The elastic member <NUM> has a plurality of recesses <NUM> formed radially outward from the inner peripheral portion <NUM> in a manner similar to the recesses <NUM> of the third transmission member <NUM>. In a plate surface <NUM> of the elastic member <NUM>, holes <NUM> for fitting therein projections <NUM> extending from the inner peripheral portion <NUM> toward the output shaft <NUM> are formed. The elastic member <NUM> fitted to the third transmission member <NUM> is located between the first transmission member <NUM> and the third transmission member <NUM>, and the projections <NUM> are fitted so as to be housed inside the recesses <NUM>.

The first transmission member <NUM> and the second transmission member <NUM> are formed from synthetic resin mixed with reinforced fibers as needed, or metal such as an iron alloy, a copper alloy, or an aluminum alloy. The first transmission member <NUM> and the second transmission member <NUM> are formed into a circular ring shape as a whole by a method such as injection molding, casting, forging, sintering, or cutting. The first transmission member <NUM> is externally fitted and fixed to the output shaft <NUM> by interference fitting, spline fitting, or swaging in the state in which relative rotation and axial relative displacement are prevented. The first transmission member <NUM> may be formed integrally with the output shaft <NUM>. The second transmission member <NUM> is externally fitted and fixed to the rotation shaft portion 94b by interference fitting, spline fitting, or swaging in the state in which relative rotation and axial relative displacement are prevented. The second transmission member <NUM> may be formed integrally with the rotation shaft portion 94b.

The third transmission member <NUM> is formed from material having rigidity higher than that of elastic material forming the elastic member <NUM> and the elastic member <NUM>. For example, the third transmission member <NUM> is formed from belt material obtained by reinforcing rubber by cloth, synthetic resin (such as PPS, PEEK, and polyamide) mixed with reinforced fibers as needed, or metal such as an iron alloy, a copper alloy, or an aluminum alloy. The third transmission member <NUM> is formed into a cylindrical (or columnar) shape by a method such as injection molding, casting, forging, sintering, or cutting.

The elastic members <NUM> and <NUM> are formed from elastic material having rigidity lower than that of the third transmission member <NUM>. For example, the elastic members <NUM> and <NUM> are formed into a circular ring shape as a whole using elastic material such as rubber (such as NBR and HNBR) or an elastomer (such as polyurethane and silicon).

The elastic member <NUM> and the elastic member <NUM> are formed from elastic material, thereby further enhancing the tolerance to allow the first transmission member <NUM> and the second transmission member <NUM> to be coupled to each other with the elastic members <NUM> and <NUM> therebetween in a positional relation in which the first transmission member <NUM> and the second transmission member <NUM> are not limited to be coaxial to each other. The third transmission member <NUM> is formed from material having rigidity higher than that of the elastic members <NUM> and <NUM>, for example, synthetic resin, thereby enhancing, due to elasticity, the tolerance to allow the first transmission member <NUM> and the second transmission member <NUM> to be coupled to each other in a positional relation in which the first transmission member <NUM> and the second transmission member <NUM> are not limited to be coaxial to each other.

<FIG> is a diagram illustrating a configuration example of the electric motor <NUM> in the first embodiment. The electric motor <NUM> includes a rotor <NUM> and a stator <NUM>. The electric motor <NUM> illustrated in <FIG> is provided with the stator <NUM> on the outer peripheral side of the rotor <NUM>. The rotor <NUM> has a multipolar magnet <NUM> provided on the outer peripheral side. The stator <NUM> has a plurality of windings <NUM> provided on the inner side of a casing 10a of the electric motor <NUM>. For example, the windings <NUM> are wound around grooves (slots) provided in the stator <NUM>. The rotor <NUM> rotates about the output shaft <NUM> in response to energization of the windings <NUM>.

The number of poles of the magnet <NUM> in the first embodiment is <NUM>. In other words, three N poles and three S poles are alternatingly disposed circularly at intervals of <NUM> degrees about the rotation axis center 11a. The number of the windings <NUM> in the first embodiment is <NUM>. The electric motor <NUM> in the first embodiment is what is called a "<NUM>-pole <NUM>-slot" motor.

The smallest rotation order among rotation orders of the electric motor <NUM> and the rotation order of the worm 94a are prime to each other. The smallest rotation order among the rotation orders of the electric motor <NUM> and the rotation order of the coupling <NUM> are prime to each other. The rotation order of the worm 94a and the rotation order of the coupling <NUM> are prime to each other. The rotation order is the number (integer) of periodic components appearing per rotation of a rotating body. For example, a component that appears as one period per rotation of a rotating body is called a 1st order. In other words, the rotation order of a rotating body in which a component appears as one period is <NUM>. Similarly, a component that appears n periods per rotation of a rotating body is called an n-th order (n is an integer of <NUM> or greater). The rotation order of a rotating body in which a component appears as n periods is n.

When a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet <NUM> in the rotor <NUM>, the rotation order of the electric motor <NUM> is n times a larger natural number of the two natural numbers. n is a natural number of <NUM> or greater. The number of poles of the magnet <NUM> in the rotor <NUM> is <NUM> and can be expressed as <NUM>=<NUM>×<NUM>. Thus, in the first embodiment, when a multiplication (<NUM>×<NUM>) of two natural numbers other than <NUM> expresses the number of poles (<NUM>) of the magnet <NUM>, a larger natural number of the two natural numbers (<NUM>,<NUM>) is <NUM>. The rotation order of the worm 94a is the number of teeth of the worm wheel <NUM> that advance in response to one rotation of the worm gear shaft <NUM>. In other words, the rotation order of the worm 94a in the first embodiment is <NUM>. The rotation order of the coupling <NUM> is the respective numbers of projections <NUM> and <NUM> of the first transmission member <NUM> and the second transmission member <NUM>. In other words, the rotation order of the coupling <NUM> in the first embodiment is <NUM>.

When the smallest rotation order among rotation orders of an electric motor such as the electric motor <NUM> is A, the rotation order of a worm such as the worm 94a is B, and the rotation order of a coupling such as the coupling <NUM> is C, A:B:C=<NUM>:<NUM>:<NUM> is established in the first embodiment. The electric motor <NUM>, a worm gear including the worm 94a and the worm wheel <NUM>, and the coupling <NUM> may cause vibration with periods corresponding to the rotation orders indicated by A, B, and C, respectively.

Ideally, it is desired that a rotating body have a uniform weight distribution over the entire circumference about its rotation axis in order to prevent vibration caused by rotation of the rotating body. Actually, however, it is difficult to completely eliminate errors and deviation in the weight distribution in processes of the formation of shapes, the assembly, etc. Thus, vibration can occur due to errors and deviation in weight distribution. In the case of the electric motor <NUM>, vibration can occur due to errors and deviation depending on the number of poles of the magnet <NUM>. In the case of the worm gear, vibration can occur due to errors and deviation in teeth of the worms 94a and <NUM>. In the case of the coupling <NUM>, vibration can occur due to errors and deviation in the arrangement and the shapes (such as thickness) of the projections <NUM> and <NUM>. It is difficult to form the worm gear shaft <NUM> to be completely linear. Thus, vibration (1st order variation) can occur due to distortion of the worm gear shaft <NUM> with respect to the ideal rotation axis center. The m-th order variation (m is a natural number) refers to load variation that occurs with a period that is m times the period of one rotation of the output shaft of the electric motor. The load variation can be a cause of vibration. When the smallest r among the r-th order variations caused in a rotating body is m, m is equal to the rotation order of the rotating body.

In the first embodiment, these rotation orders A, B, and C are prime to one another, and hence the periods of vibrations are less easily overlap. In other words, the possibility that vibrations of a plurality of configurations occurs at the same timing to amplify the vibration is reduced. In this manner, by setting the rotation orders of configurations, which rotate in response to one rotation of the output shaft <NUM>, or one rotation of the worm gear shaft <NUM>, to be prime to each other, load variation, vibration, and noise caused by vibration in the assist mechanism <NUM> can be reduced.

<FIG> is a conceptual graph illustrating load variation that occurs in the assist mechanism <NUM> in a period during which the worm gear shaft <NUM> rotates one turn in the first embodiment. This load variation is, for example, load variation caused in the bearing 95a, which pivotally supports the worm gear shaft <NUM> and is located closer to the output shaft <NUM> of the electric motor <NUM>, among the bearings 95a and 95b that pivotally support the worm gear shaft <NUM>. <FIG> illustrates an example of load variation in the electric power steering device <NUM> including the assist mechanism <NUM> in the first embodiment and load variation in an electric power steering device including an assist mechanism in a comparative example. In the comparative example, unlike the present embodiment, the smallest rotation order among rotation orders of the electric motor <NUM>, the rotation order of the worm 94a, and the rotation order of the coupling <NUM> are not prime to one another. Specifically, the relation of A:B:C=<NUM>:<NUM>:<NUM> is established in the comparative example. More specifically, the electric motor in the comparative example is, for example, an <NUM>-pole <NUM>-slot electric motor.

The electric motor of the comparative example is an <NUM>-pole <NUM>-slot electric motor in which A=<NUM> is established. Thus, the electric motor causes 4th order variation, 8th order variation, and 24th order variation in response to one rotation of the output shaft of the electric motor. In the comparative example, B=<NUM> is established. Thus, the worm gear causes 2nd order variation and 4th order variation in response to one rotation of the output shaft of the electric motor. In the comparative example, C=<NUM> is established. Thus, the coupling causes 8th order variation in response to one rotation of the output shaft of the electric motor. The state in which 8th order variation occurs corresponds to the state in which load variation overlapping 2nd order variation and 4th order variation occurs. The state in which 4th order variation occurs corresponds to the state in which load variation overlapping 2nd order variation occurs. Thus, in the comparative example, as indicated by a load variation width W2, all load variations in the electric motor, the worm gear, and the coupling overlap at timing of half rotation (<NUM>°), and a particularly large load variation occurs.

The electric motor of the first embodiment is, on the other hand, a <NUM>-pole <NUM>-slot electric motor in which A=<NUM> is established. Thus, the electric motor <NUM> causes 3rd order variation, 6th order variation, and 18th order variation in response to one rotation of the output shaft <NUM>. In the first embodiment, B=<NUM> is established. Thus, the worm gear causes 2nd order variation and 4th order variation in response to one rotation of the output shaft <NUM>. In the first embodiment, C=<NUM> is established. Thus, the coupling <NUM> causes 7th order variation in response to one rotation of the output shaft of the output shaft <NUM>. Accordingly, in the first embodiment, load variations in the electric motor <NUM>, the worm gear, and the coupling <NUM> extremely hardly overlap. Thus, even if a large load variation among load variations caused in the first embodiment is picked up, the load variation can be reduced to about a load variation width W1 smaller than the load variation width W2.

In the case of the column assist system, vibration and noise caused by assist operation of steering tends to relatively affect the vehicle interior, but according to the first embodiment, load variation, vibration, and noise can be reduced, and hence the influence of vibration and noise on the vehicle interior can be reduced. Even in environments where vibration and noise are more conspicuous due to relatively high silence of the vehicle such as an electric vehicle and a hybrid car, the influence of vibration and noise on the vehicle interior can be reduced. The influence by vibration and noise in a configuration whose rotation order is predetermined can be reduced, and hence when vibration and noise occur for some reason, the vibration and the noise can be easily detected. Thus, countermeasures for reducing the vibration and the noise can be easily taken.

As described above, according to the first embodiment, the rotation orders of the electric motor <NUM>, the worm gear, and the coupling <NUM> can be made different to shift their timings of occurrence of vibration and noise. This can prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Thus, vibration and noise can be reduced.

The rotation orders of the electric motor <NUM>, the worm gear, and the coupling <NUM> can be specified based on the number of configurations that move at positions away from the respective rotation axes along with the rotations of the electric motor <NUM>, the worm gear, and the coupling <NUM>, and the rotation orders can be made different. Therefore, their timings of occurrence of vibration and noise can be shifted. This can more reliably prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Thus, vibration and noise can be more reliably reduced.

By setting A:B:C=<NUM>:<NUM>:<NUM>, all the rotation orders of the electric motor, the worm, and the coupling can be made different. This can more reliably prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Thus, vibration and noise can be more reliably reduced.

The rotation order of the coupling <NUM> can be made difficult to be equal to the rotation orders of other configurations. This can more reliably prevent larger vibration and noise, which would otherwise be caused due to a synergistic effect created by concurrence of vibrations and noises of these configurations. Thus, vibration and noise can be more reliably reduced.

Next, a modification of the first embodiment is described with reference to <FIG> and <FIG>. <FIG> is a perspective view of a coupling <NUM> in the modification. <FIG> is an exploded perspective view of the coupling <NUM> in the modification. In the modification, the coupling <NUM> is employed instead of the coupling <NUM> in the first embodiment. The coupling <NUM> includes a first transmission member <NUM>, a second transmission member <NUM>, and a third transmission member <NUM>. The third transmission member <NUM> is engaged with five projections <NUM> of the first transmission member <NUM> and five projections <NUM> of the second transmission member <NUM>, thereby coupling the first transmission member <NUM> and the second transmission member <NUM> together. The rotation order of the coupling <NUM> in the modification is <NUM>. The configurations of an assist mechanism and an electric power steering device in the modification are the same as in the first embodiment except for the difference between the coupling <NUM> and the coupling <NUM>. In other words, in the modification, A:B:C=<NUM>:<NUM>:<NUM> is established. In this manner, in the modification, the smallest rotation order among rotation orders of the electric motor <NUM>, the rotation order of the worm 94a, and the rotation order of the coupling <NUM> are prime to one another.

The electric motor of the modification is a <NUM>-pole <NUM>-slot electric motor in which A=<NUM> is established as in the case of the first embodiment. Thus, the electric motor <NUM> causes 3rd order variation, 6th order variation, and 18th order variation in response to one rotation of the output shaft <NUM>. In the modification, B=<NUM> is established as in the case of the first embodiment. Thus, the worm gear causes 2nd order variation and 4th order variation in response to one rotation of the output shaft <NUM>. In the modification, C=<NUM> is established. Thus, the coupling <NUM> causes 5th order variation in response to one rotation of the output shaft of the output shaft <NUM>. In this manner, in the first embodiment, load variations in the electric motor <NUM>, the worm gear, and the coupling <NUM> extremely hardly overlap. Thus, according to the modification, load variation, vibration, and noise can be reduced.

Specifically, for example, the third transmission member <NUM> has an outer peripheral portion <NUM> having a diameter greater than the outer diameters of the circular rings of the first transmission member <NUM> and the second transmission member <NUM>. The outer peripheral portion <NUM> has an outer peripheral surface having a columnar or cylindrical shape. On the inner side of the outer peripheral portion <NUM>, an inner peripheral portion <NUM> having a diameter less than that of the outer peripheral portion <NUM> is formed. The center of a circular ring drawn by the outer peripheral portion <NUM> coincides with the center of a circular ring drawn by the inner peripheral portion <NUM>. The third transmission member <NUM> has a cylindrical shape having a thickness of a wall surface corresponding to the difference between the outer peripheral portion <NUM> and the inner peripheral portion <NUM> or a columnar shape having a hole corresponding to the inner diameter of the inner peripheral portion <NUM>. The center axis of the third transmission member <NUM> described below refers to the center axis of the cylinder or the column.

The first transmission member <NUM> has a plurality of projections <NUM> formed radially from the center of the circular ring of the first transmission member <NUM> to the outer periphery. More specifically, the first transmission member <NUM> has a cylindrical portion <NUM> that surrounds the output shaft <NUM> fixed on the inner side thereof. The projections <NUM> are formed so as to extend radially outward from the outer peripheral surface of the cylindrical portion <NUM>. For example, end portions of the extending projections <NUM> are arranged along the circular ring of the first transmission member <NUM>. The diameter of a circular ring drawn by the end portions of the projections <NUM> may be less than the diameter of the circular ring of the first transmission member <NUM>. The thicknesses of the cylindrical portion <NUM> and the projections <NUM> in a rotation axis direction of the output shaft <NUM> are equal to or less than a half of the thickness of the third transmission member <NUM> in a center axis direction.

The first transmission member <NUM> and the second transmission member <NUM> have shapes mirror-symmetric with respect to the third transmission member <NUM>. Specifically, the second transmission member <NUM> has a plurality of projections <NUM> formed radially from the center of the circular ring of the second transmission member <NUM> to the outer periphery. More specifically, the second transmission member <NUM> has a cylindrical portion <NUM> that surrounds the rotation shaft portion 94b fixed on the inner side thereof. The projections <NUM> are formed so as to extend radially outward from the outer peripheral surface of the cylindrical portion <NUM>. For example, end portions of the extending projections <NUM> are arranged along the circular ring of the second transmission member <NUM>. The diameter of a circular ring drawn by the end portions of the projections <NUM> may be less than the diameter of the circular ring of the second transmission member <NUM>. The thicknesses of the cylindrical portion <NUM> and the projections <NUM> in a rotation axis direction of the rotation shaft portion 94b are equal to or less than a half of the thickness of the third transmission member <NUM> in the center axis direction.

In the modification, the number of the projections <NUM>, the number of the projections <NUM>, and the number of the recesses <NUM> are <NUM>. The diameter of the circular ring drawn by the end portions of the projections <NUM> and the diameter of the circular ring drawn by the end portions of the projections <NUM> are equal to or less than the diameter drawn by the end portions of the recesses <NUM>. The shapes of the end portions of the projections <NUM> and the projections <NUM> are shapes housed inside the recesses <NUM>. Specifically, the shapes of the end portions of the projections <NUM> and the projections <NUM> are arc, for example. As illustrated in <FIG>, the projections <NUM> and the projections <NUM> are fitted so as to be housed inside the recesses <NUM>, so that the third transmission member <NUM> couples the first transmission member <NUM> and the second transmission member <NUM> to each other.

More specifically, in the modification, margins are provided between the diameter of the circular ring drawn by the end portions of the projections <NUM> and the diameter drawn by the end portions of the recesses <NUM> and between the diameter of the circular ring drawn by the end portions of the projections <NUM> and the diameter drawn by the end portions of the recesses <NUM>. This allows the first transmission member <NUM> and the second transmission member <NUM> to be coupled to each other with the third transmission member <NUM> therebetween in a positional relation in which the first transmission member <NUM> and the second transmission member <NUM> are not limited to be coaxial to each other. Thus, warpage of the output shaft <NUM> and the rotation shaft portion 94b due to misalignment of axes caused between the output shaft <NUM> and the rotation shaft portion 94b can be suppressed.

In the examples illustrated in <FIG> and <FIG>, elastic members <NUM> and <NUM> are respectively attached to end surfaces of the cylinder (or column) of the third transmission member <NUM>. For example, the elastic member <NUM> has a circular ring-shaped protruding portion <NUM> extending toward the third transmission member <NUM>. The protruding portion <NUM> is fitted to a circular ring-shaped groove <NUM> formed so as to be located between the outer peripheral portion <NUM> and the recesses <NUM> of the third transmission member <NUM>. The elastic member <NUM> fitted to the third transmission member <NUM> is located between the second transmission member <NUM> and the third transmission member <NUM>. In the elastic member <NUM>, an inner peripheral portion <NUM> having a diameter equal to that of the inner peripheral portion <NUM> is formed. The elastic member <NUM> has a plurality of recesses <NUM> formed radially outward from the inner peripheral portion <NUM> in a manner similar to the recesses <NUM> of the third transmission member <NUM>. The third transmission member <NUM> is provided with projections <NUM> provided so as to be located between the recesses <NUM> arranged circularly. The projections <NUM> extend from the inner peripheral portion <NUM> toward the rotation shaft portion 94b along the center axis direction of the third transmission member <NUM>. In a plate surface <NUM> of the elastic member <NUM>, holes <NUM> for fitting the projections <NUM> therein are formed. The positional relation between the recesses <NUM> and the projections <NUM> corresponds to the positional relation between the recesses <NUM> and the holes <NUM>. In this manner, as illustrated in <FIG>, the projections <NUM> are fitted so as to be housed inside the recesses <NUM>.

The materials and the forming methods of the first transmission member <NUM>, the second transmission member <NUM>, the third transmission member <NUM>, and the elastic members <NUM> and <NUM> may be the same as those of the first transmission member <NUM>, the second transmission member <NUM>, the third transmission member <NUM>, and the elastic members <NUM> and <NUM>, respectively.

Next, a second embodiment is described with reference to <FIG> and <FIG>. <FIG> is a diagram illustrating a configuration example of a worm 94c and a worm wheel <NUM> in the second embodiment. A worm gear shaft <NUM> in the second embodiment has the worm 94c instead of the worm 94a. The worm 94c is a triple thread worm. In other words, as indicated by a half rotation pitch P2 in <FIG>, gears of the worm wheel <NUM> are formed so as to advance by <NUM> in response to half rotation of the worm 94c. Thus, the gears of the worm wheel <NUM> advance by three in response to one rotation of the worm 94a. Thus, the rotation order of the worm 94c is <NUM>.

<FIG> is a diagram illustrating a configuration example of an electric motor 10A in the second embodiment. The electric motor 10A in the second embodiment includes a magnet 13A and a stator 14A instead of the magnet <NUM> and the stator <NUM> included in the electric motor <NUM> in the first embodiment. The electric motor 10A has the same configuration as that of the electric motor <NUM> except for the difference between the magnet <NUM> and the magnet 13A and the difference between the stator <NUM> and the stator 14A. The magnet <NUM> and the magnet 13A have the same configuration except for the number of poles of the magnet. The stator <NUM> and the stator 14A have the same configuration except for the number of windings <NUM> and the number of grooves provided with the windings <NUM>.

The number of poles of the magnet 13A in the second embodiment is <NUM>. In other words, four N poles and four S poles are alternatingly disposed circularly at intervals of <NUM> degrees about the rotation axis center 11a. The number of windings <NUM> in the second embodiment is <NUM>. The electric motor 10A in the second embodiment is what is called a "<NUM>-pole <NUM>-slot" motor. That is, the number of poles of the magnet 13A can be expressed as <NUM>=<NUM>×<NUM>. Thus, in the second embodiment, when a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet 13A, the larger natural number of the two natural numbers is <NUM>.

The configurations of an assist mechanism and an electric power steering device in the second embodiment are the same as in the first embodiment except for the points described above. In other words, in the second embodiment, A:B:C=<NUM>:<NUM>:<NUM> is established. In this manner, in the second embodiment, the smallest rotation order among rotation orders of the electric motor 10A, the rotation order of the worm 94c, and the rotation order of the coupling <NUM> are prime to one another.

The second embodiment is an <NUM>-pole <NUM>-slot electric motor in which A=<NUM> is established. Thus, the electric motor 10A causes 4th order variation, 8th order variation, and 24th order variation in response to one rotation of the output shaft <NUM>. In the second embodiment, B=<NUM> is established. Thus, the worm gear causes 3rd order variation and 6th order variation in response to one rotation of the output shaft <NUM>. In the second embodiment, C=<NUM> is established. Thus, the coupling <NUM> causes 7th order variation in response to one rotation of the output shaft of the output shaft <NUM>. In this manner, in the second embodiment, load variations in the electric motor 10A, the worm gear, and the coupling <NUM> extremely hardly overlap. Thus, according to the second embodiment, load variation, vibration, and noise can be reduced.

In the second embodiment, the modification may be applied as in the case of the first embodiment. In other words, in the second embodiment, the coupling <NUM> may be employed instead of the coupling <NUM>. In this case, A:B:C=<NUM>:<NUM>:<NUM> is established. In this manner, also in the modification of the second embodiment, the smallest rotation order among rotation orders of the electric motor 10A, the rotation order of the worm 94c, and the rotation order of the coupling <NUM> are prime to one another. Thus, according to the modification of the second embodiment, load variation, vibration, and noise can be reduced.

Next, a third embodiment is described with reference to <FIG> is a diagram illustrating a configuration example of an electric motor 10B in the third embodiment. The electric motor 10B in the third embodiment includes a magnet 13B instead of the magnet 13A included in the electric motor 10A in the second embodiment. The electric motor 10B has the same configuration as that of the electric motor 10A except for the difference between the magnet 13A and the magnet 13B. The magnet 13A and the magnet 13B have the same configuration except for the number of poles of the magnet.

The number of poles of the magnet 13B in the third embodiment is <NUM>. In other words, five N poles and five S poles are alternatively disposed circularly at intervals of <NUM> degrees about the rotation axis center 11a. The electric motor 10B in the third embodiment is what is called a "<NUM>-pole <NUM>-slot" motor. That is, the number of poles of the magnet 13B can be expressed as <NUM>=<NUM>×<NUM>. Thus, in the third embodiment, when a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet 13B, the larger natural number of the two natural numbers is <NUM>.

The configurations of an assist mechanism and an electric power steering device in the third embodiment are the same as in the first embodiment except for the points described above. In other words, in the third embodiment, A:B:C=<NUM>:<NUM>:<NUM> is established. In this manner, in the third embodiment, the smallest rotation order among rotation orders of the electric motor 10B, the rotation order of the worm 94a, and the rotation order of the coupling <NUM> are prime to one another.

The third embodiment is a <NUM>-pole <NUM>-slot electric motor in which A=<NUM> is established. Thus, the electric motor 10B causes 5th order variation, 10th order variation, and 60th order variation in response to one rotation of the output shaft <NUM>. In the third embodiment, B=<NUM> is established. Thus, the worm gear causes 2nd order variation and 4th order variation in response to one rotation of the output shaft <NUM>. In the third embodiment, C=<NUM> is established. Thus, the coupling <NUM> causes 7th order variation in response to one rotation of the output shaft of the output shaft <NUM>. In this manner, in the third embodiment, load variations in the electric motor 10B, the worm gear, and the coupling <NUM> extremely hardly overlap. Thus, according to the third embodiment, load variation, vibration, and noise can be reduced.

Next, a fourth embodiment is described with reference to <FIG> and <FIG>. <FIG> is a perspective view of a coupling <NUM> in the fourth embodiment. <FIG> is an exploded perspective view of the coupling <NUM> in the fourth embodiment. In the fourth embodiment, the coupling <NUM> is employed instead of the coupling <NUM> in the first embodiment. The coupling <NUM> includes a first transmission member <NUM>, a second transmission member <NUM>, and a third transmission member <NUM> that couples the first transmission member <NUM> and the second transmission member <NUM>.

The first transmission member <NUM> has four projections <NUM> extending toward the third transmission member <NUM>. For example, the projection <NUM> is a fan-shaped projection whose outer side is along the outer circumference of the first transmission member <NUM> and whose inner side is obtuse. The first transmission member <NUM> is fixed to the output shaft <NUM> in a manner similar to the first transmission member <NUM>. The second transmission member <NUM> has four projections <NUM> extending toward the third transmission member <NUM>. Specifically, two pairs of two projections <NUM> are disposed at positions opposed to each other across the center axis of the rotation shaft portion 94b fixed to the second transmission member <NUM>. The two pairs of the projections <NUM> have a positional relation in which straight lines connecting the second transmission members <NUM> in the pairs are orthogonal to each other.

The third transmission member <NUM> is a plate-shaped member in which L-shaped side portions 236a and 236b continuous so as to draw an obtuse angle and a peripheral portion <NUM> that connects the side portion 236a of one of the two L shapes and the side portion 236b of the other L shape are continuous so as to form holes corresponding to the shapes of outer peripheral-side cutout portions corresponding to obtuse angles of the first transmission member <NUM> and the shapes of the projections <NUM>. In the third transmission member <NUM>, the side portion 236a, the side portion 236b, and the peripheral portion <NUM> sequentially arranged continuous to form a ring. The cutout portion in the form of the L shape drawn by the continuous side portions 236a and 236b is directed to the outer side of the third transmission member <NUM>. The cutout portion in the form of the L shape is engaged with the projection <NUM>. A hole <NUM> on the inner peripheral side that is sandwiched between the side portion 236a of one of two L shapes continuous across the peripheral portion <NUM> and the side portion 236b of the other L shape is engaged with the projection <NUM>. The rotation order of the coupling <NUM> in the fourth embodiment is <NUM>.

In the fourth embodiment, an elastic member <NUM> is interposed between the first transmission member <NUM> and the third transmission member <NUM>. An elastic member <NUM> is interposed between the second transmission member <NUM> and the third transmission member <NUM>. The elastic members <NUM> and <NUM> are plate-shaped members in which L-shaped side portions 238a and 238b continuous so as to draw an obtuse angle and a peripheral portion <NUM> that connects the side portion 238a of one of the two L shapes and the side portion 238b of the other L shape are continuous. The elastic members <NUM> and <NUM> further include claws <NUM> and cutout portions <NUM>. The claws <NUM> extend from two opposed peripheral portions <NUM> of the four peripheral portions <NUM> toward the third transmission member <NUM>. The cutout portions <NUM> are provided in two peripheral portions <NUM> from which the claws <NUM> do not extend. A hooking structure of the claw <NUM> is fitted to a cutout structure of the cutout portion <NUM>. The positions of the claw <NUM> and the cutout portion <NUM> in the elastic member <NUM> are out of phase with the positions of the claw <NUM> and the cutout portion <NUM> in the elastic member <NUM> by <NUM>°. In this manner, the claws <NUM> of one of the elastic member <NUM> and the elastic member <NUM> are fitted to the cutout portions <NUM> of the other elastic member, and the claws <NUM> of the other elastic member are fitted to the cutout portions <NUM> of the one elastic member. Consequently, the elastic member <NUM> and the elastic member <NUM> are fixed to each other with the third transmission member <NUM> therebetween.

The third transmission member <NUM> has a disc-shaped support portion <NUM> at a center part of the plate surface. In this manner, the side portions 236a and 236b can be supported from the inner peripheral side, and this further enhances the rigidity. The materials and the forming methods of the first transmission member <NUM>, the second transmission member <NUM>, the third transmission member <NUM>, and the elastic members <NUM> and <NUM> may be the same as those of the first transmission member <NUM>, the second transmission member <NUM>, the third transmission member <NUM>, and the elastic members <NUM> and <NUM>, respectively.

In the fourth embodiment, the electric motor 10B in the third embodiment is employed (see <FIG>). Thus, in the fourth embodiment, when a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet 13B, the larger natural number of the two natural numbers is <NUM>. In the fourth embodiment, as in the case of the second embodiment, the worm gear shaft <NUM> has the worm 94c (see <FIG>). Thus, in the fourth embodiment, the rotation order of the worm 94c is <NUM>. In other words, in the fourth embodiment, A:B:C=<NUM>:<NUM>:<NUM> is established. In this manner, in the fourth embodiment, the smallest rotation order among rotation orders of the electric motor 10B, the rotation order of the worm 94c, and the rotation order of the coupling <NUM> are prime to one another. The configurations of an assist mechanism and an electric power steering device in the fourth embodiment are the same as in the first embodiment except for the points described above.

The fourth embodiment is a <NUM>-pole <NUM>-slot electric motor in which A=<NUM> is established. Thus, the electric motor 10B causes 5th order variation, 10th order variation, and 60th order variation in response to one rotation of the output shaft <NUM>. In the fourth embodiment, B=<NUM> is established. Thus, the worm gear causes 3rd order variation and 6th order variation in response to one rotation of the output shaft <NUM>. In the fourth embodiment, C=<NUM> is established. Thus, the coupling <NUM> causes 4th order variation in response to one rotation of the output shaft of the output shaft <NUM>. In this manner, in the fourth embodiment, load variations in the electric motor 10B, the worm gear, and the coupling <NUM> extremely hardly overlap. Thus, according to the second embodiment, load variation, vibration, and noise can be reduced.

Next, a fifth embodiment is described with reference to <FIG> is a diagram illustrating a configuration example of an electric motor 10C in the fifth embodiment. The electric motor 10C in the fifth embodiment includes a magnet 13C instead of the magnet 13A included in the electric motor 10A in the second embodiment. The electric motor 10C has the same configuration as that of the electric motor 10A except for the difference between the magnet 13A and the magnet 13C. The magnet 13A and the magnet 13C have the same configuration except for the number of poles of the magnet.

The number of poles of the magnet 13B in the fifth embodiment is <NUM>. In other words, two N poles and two S poles are alternatingly disposed circularly at intervals of <NUM> degrees about the rotation axis center 11a. The electric motor 10C in the fifth embodiment is what is called a "<NUM>-pole <NUM>-slot" motor. That is, the number of poles of the magnet 13C can be expressed as <NUM>=<NUM>×<NUM>. Thus, in the fifth embodiment, when a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet 13C, the larger natural number of the two natural numbers is <NUM>.

In the fifth embodiment, the rotation order of the worm and the rotation order of the coupling that are prime to the smallest rotation order among rotation orders of the electric motor 10C are employed. Specifically, in the fifth embodiment, for example, as in the case of the second embodiment, the worm gear shaft <NUM> has the worm 94c (see <FIG>). In the fifth embodiment, for example, the same coupling <NUM> as in the first embodiment or the same coupling <NUM> as in the modification is employed. Thus, in the fifth embodiment, A:B:C=<NUM>:<NUM>:<NUM> or A:B:C=<NUM>:<NUM>:<NUM> is established. The configurations of an assist mechanism and an electric power steering device in the fifth embodiment are the same as in the first embodiment or the modification except for the points described above. In this manner, also in the fifth embodiment, load variations in the electric motor 10C, the worm gear, and the coupling <NUM> or the coupling <NUM> extremely hardly overlap. Thus, according to the fifth embodiment, load variation, vibration, and noise can be reduced.

Next, a sixth embodiment is described with reference to <FIG> is a diagram illustrating a configuration example of an electric motor 10D in the sixth embodiment. The electric motor 10D in the sixth embodiment includes a stator 14B instead of the stator 14A included in the electric motor 10B (see <FIG>) in the third embodiment. The electric motor 10D has the same configuration as that of the electric motor 10B except for the difference between the stator 14A and the stator 14B. The stator 14A and the stator 14B have the same configuration except for the number of windings <NUM> and the number of grooves provided with the windings <NUM>.

The number of poles of magnets 13B in the sixth embodiment is <NUM>. The number of windings <NUM> in the sixth embodiment is <NUM>. The electric motor 10D in the sixth embodiment is what is called a "<NUM>-pole <NUM>-slot" motor. Thus, in the sixth embodiment, when a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet 13B, the larger natural number of the two natural numbers is <NUM>.

In the sixth embodiment, the rotation order of the worm and the rotation order of the coupling that are prime to the smallest rotation order among rotation orders of the electric motor 10D are employed. For example, in the sixth embodiment, for example, the same worm 94a and the same coupling <NUM> as in the first embodiment are employed. Thus, in the sixth embodiment, A:B:C=<NUM>:<NUM>:<NUM> is established. In the sixth embodiment, the worm 94c (see <FIG>) may be employed instead of the worm 94a. In this case, A:B:C=<NUM>:<NUM>:<NUM> is established. The configurations of an assist mechanism and an electric power steering device in the sixth embodiment are the same as in the first embodiment except for the points described above. In this manner, also in the sixth embodiment, load variations in the electric motor 10D, the worm gear, and the coupling <NUM> extremely hardly overlap. Thus, according to the sixth embodiment, load variation, vibration, and noise can be reduced.

Next, a seventh embodiment is described with reference to <FIG> is a diagram illustrating a configuration example of an electric motor 10E in the seventh embodiment. The electric motor 10E in the seventh embodiment includes a stator 14C instead of the stator 14A included in the electric motor 10A (see <FIG>) in the second embodiment. The electric motor 10E has the same configuration as that of the electric motor 10A except for the difference between the stator 14A and the stator 14C. The stator 14A and the stator 14C have the same configuration except for the number of poles of a magnet.

The number of poles of a magnet 13A in the seventh embodiment is <NUM>. The number of windings <NUM> in the seventh embodiment is <NUM>. The electric motor 10E in the seventh embodiment is what is called a "<NUM>-pole <NUM>-slot" motor. Thus, in the seventh embodiment, when a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet 13A, the larger natural number of the two natural numbers is <NUM>.

In the seventh embodiment, the rotation order of the worm and the rotation order of the coupling that are prime to the smallest rotation order among rotation orders of the electric motor 10E are employed. Specifically, in the fifth embodiment, for example, as in the case of the second embodiment, the worm gear shaft <NUM> has the worm 94c (see <FIG>). In the seventh embodiment, for example, the same coupling <NUM> as in the first embodiment or the same coupling <NUM> as in the modification is employed. Thus, in the seventh embodiment, A:B:C=<NUM>:<NUM>:<NUM> or A:B:C=<NUM>:<NUM>:<NUM> is established. The configurations of an assist mechanism and an electric power steering device in the seventh embodiment are the same as in the first embodiment or the modification except for the points described above. In this manner, also in the seventh embodiment, load variations in the electric motor 10E, the worm gear, and the coupling <NUM> or the coupling <NUM> extremely hardly overlap. Thus, according to the seventh embodiment, load variation, vibration, and noise can be reduced.

Next, an eighth embodiment is described with reference to <FIG> is a diagram illustrating a configuration example of an electric motor 10F in the eighth embodiment. The electric motor 10F in the eighth embodiment includes the magnet <NUM> (see <FIG>) included in the electric motor 10A in the first embodiment and the stator 14A (see <FIG>) included in the electric motor 10A in the second embodiment. The electric motor 10F has the same configuration as that of the electric motor <NUM> except for the difference between the stator <NUM> and the stator 14A.

The electric motor 10F in the eighth embodiment is what is called a "<NUM>-pole <NUM>-slot" motor. Thus, in the eighth embodiment, when a multiplication of two natural numbers other than <NUM> expresses the number of poles of the magnet <NUM>, the larger natural number of the two natural numbers is <NUM>.

In the eighth embodiment, the rotation order of the worm and the rotation order of the coupling that are prime to the smallest rotation order among rotation orders of the electric motor 10F are employed. Specifically, in the eighth embodiment, for example, the same worm 94a and the same coupling <NUM> as in the first embodiment are employed. Thus, in the eighth embodiment, A:B:C=<NUM>:<NUM>:<NUM> is established. The configurations of an assist mechanism and an electric power steering device in the eighth embodiment are the same as in the first embodiment except for the points described above. In this manner, also in the eighth embodiment, load variations in the electric motor 10F, the worm gear, and the coupling <NUM> extremely hardly overlap. Consequently, according to the eighth embodiment, load variation, vibration, and noise can be reduced. In the eighth embodiment, the modification may be applied as in the case of the first embodiment. In other words, in the eighth embodiment, the coupling <NUM> may be employed instead of the coupling <NUM>. In this case, A:B:C=<NUM>:<NUM>:<NUM> is established. Also in the modification of the eighth embodiment, load variation, vibration, and noise can be reduced.

The combination of the smallest rotation order among rotation orders of the electric motor, the rotation order of the worm, and the rotation order of the coupling is not limited to the ones exemplified in the above-mentioned embodiments and modifications. At least two rotation orders of the smallest rotation order among rotation orders of the electric motor, the rotation order of the worm, and the rotation order of the coupling only need to be prime to each other. The specific forms of the coupling are not limited to the couplings <NUM>, <NUM>, and <NUM>, and can be changed as appropriate within the range not departing from the matters specifying the present invention. The electric motor may be what is called an "outer rotor", in which a rotor is located on the outer peripheral side of a stator. The specific forms of the other configurations can also be changed as appropriate within the range not departing from the matters specifying the present invention.

The above-mentioned electric motor is a brushless motor, but may be a brushed motor. When the electric motor is a brushed motor, the rotor has a winding and the stator has a magnet. A commutator corresponding to the number of poles of the winding is provided to the rotor, and a brush to be brought into sliding contact with the commutator to supply current is provided. When the electric motor is a brushed motor, the rotation order of the electric motor is n times a larger natural number of two natural numbers other than <NUM> when a multiplication of the two natural numbers expresses the number of poles of the commutator in the rotor. For example, the smallest rotation order among rotation orders of a brushed motor in which the number of commutators is <NUM> (<NUM>×<NUM>) is <NUM>. The smallest rotation order among rotation orders of a brushed motor in which the number of commutators is <NUM> (<NUM>×<NUM>) is <NUM>. The smallest rotation order among rotation orders of a brushed motor in which the number of commutators is <NUM> (<NUM>×<NUM>) is <NUM>.

Claim 1:
An assist mechanism (<NUM>) comprising:
an electric motor (<NUM>,10A,10B,10C,10D,10E,10F) in which a rotor (<NUM>) has a magnet (<NUM>,13A,13B,13C) and a stator (<NUM>,14A,14B,14C) has a winding (<NUM>);
a shaft-shaped member (<NUM>) in which a worm (94a,94c) engaged with a worm wheel (<NUM>) is formed; and
a coupling (<NUM>,<NUM>,<NUM>) that couples an output shaft of the electric motor and the shaft-shaped member to each other, wherein
a rotation order of the worm and a rotation order of the coupling are prime to each other, characterised in that
the rotation order of the worm is the number of teeth of the worm wheel that advance in response to one rotation of the shaft-shaped member,
the coupling includes
a first transmission member (<NUM>,<NUM>,<NUM>) fixed to the output shaft,
a second transmission member (<NUM>,<NUM>,<NUM>) fixed to the shaft-shaped member, and
a third transmission member (<NUM>,<NUM>,<NUM>) engaged with a predetermined number of projections (<NUM>,<NUM>) of each of the first transmission member and the second transmission member to couple the first transmission member and the second transmission member to each other, and
the rotation order of the coupling is the predetermined number.