Electric power steering device

An electric power steering device includes a motor, a drive pulley that is connected to the output shaft of the motor and includes helical teeth, a driven pulley that is arranged coaxially with a rack shaft and includes helical teeth, and a belt that is wound around these pulleys and includes helical teeth meshing with the respective helical teeth of both pulleys. Internal teeth on a belt each are formed to have a tooth thickness that is reduced toward both ends thereof along the width direction of the belt.

INCORPORATION BY REFERENCE

The disclosures of Japanese Patent Applications No. 2014-025501 and No. 2014-230971 respectively filed on Feb. 13, 2014 and Nov. 13, 2014, each including the specification, drawings and abstract, are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electric power steering device.

2. Description of Related Art

As an electric power steering device applying assist force to a steering mechanism, an electric power steering device as disclosed in Japanese Patent Application Publication No. 2004-314770 (JP 2004-314770 A) is known. In the electric power steering device, a motor is arranged in parallel to a steered shaft and the output from the motor is applied as assist force via a transmission mechanism including one set of pulleys and a belt.

It has been generally known that, in this type of electric power steering device, an operating sound is generated when the set of pulleys and belt is operated and their teeth are meshed with each other. How to reduce such operating sound is regarded as a matter to be addressed.

For the aforementioned electric power steering device, one technical solution to reduce the operating sound is to spread the clearance between the pulley tooth and the belt tooth when they mesh with each other (referred to as “meshing clearance” hereinafter). However, if the meshing clearance between the pulleys and the belt is spread, tooth skipping may occur between the pulley teeth and the belt teeth more frequently, which is a new matter to be addressed.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide an electric power steering device capable of reducing an operation sound without raising a new matter to be addressed.

An electric power steering device according to one aspect of the present invention includes a steered shaft that changes a steering angle of steered wheels responding to steering operation,

a motor that applies assist force to the steered shaft,

a drive pulley that includes helical teeth and is connected to an output shaft of the motor,

a driven pulley that includes helical teeth and is arranged coaxially with the steered shaft, and

a belt that includes helical teeth meshing with the helical teeth of a set of the drive pulley and the driven pulley, and is wound around the set of the pulleys.

The helical teeth of at least one of the pulleys and the belt each have a tooth thickness that is reduced toward both ends of the helical tooth in the width direction of the pulleys or the belt.

When the helical teeth mesh with each other between the pulleys and the belt, one end of each helical tooth is the beginning of meshing during the rotation. With the aforementioned configuration, the meshing clearance is spread between the pulleys and the belt at the beginning of the meshing between the helical teeth thereof, which are formed in such a shape that the tooth thickness that is reduced toward both ends along the width direction of the pulleys and the belt. That is, under this condition, the tooth contact is adjusted at the beginning of the tooth meshing between the pulleys and the belt to consequently reduce the operating sound. However, if the meshing clearance is spread along the entire width of the meshing portion on the helical tooth, tooth skipping will occur. In this configuration, except for the beginning of the tooth meshing between the pulleys and the belt, the helical teeth of the pulleys and the belt is formed in such a shape that the tooth thickness is reduced toward both ends of the helical teeth in the width direction of the pulleys and the belt. This makes it easier to maintain the meshing between the pulleys and the belt because the meshing clearance at the part other than the beginning of the tooth meshing is narrowed between the pulleys and the belt. It is thus possible to reduce the cause of the operating sound and the occurrence of tooth skipping and therefore to reduce the operating sound effectively.

DETAILED DESCRIPTION OF EMBODIMENTS

The following will describe a first embodiment of the present invention.

As shown inFIG. 1, an electric power steering device1includes a pinion shaft2that rotates responding to steering operation by a driver and a rack shaft3serving as a steered shaft that changes the steering angle of steered wheels with linear reciprocating movement in the axial direction responding to the rotation of the pinion shaft2. The electric power steering device1includes a rack housing4formed in a generally cylindrical shape into which the rack shaft3is inserted.

The rack housing4accommodates the pinion shaft2, which intersects the rack shaft3at an angle and allowed to rotate. With the pinion gear of the pinion shaft2engaged with the rack gear of the rack shaft3, a rack and pinion mechanism is configured. The pinion shaft2is connected with a steering shaft, to the end of which a steering wheel that is operated by the driver is fixed.

In the electric power steering device1, when the driver executes steering operation, the rotation movement of the pinion shaft2is accordingly converted into linear reciprocating movement in the axial direction of the rack shaft3through the rack and pinion mechanism. Thus, the steering angle of the steered wheels, that is, the traveling direction of a vehicle is changed.

The electric power steering device1includes a steering assist device6that applies assist force to a steering mechanism including the pinion shaft2, the rack shaft3, and other components with a motor5arranged in parallel to the rack shaft3as a driving source. A transmission mechanism7includes a metal-made drive pulley10connected to an output shaft5aof the motor5, a metal-made driven pulley20arranged in parallel to the drive pulley10and connected to the rack shaft3, and a rubber-made belt30connecting to the pulley10and the pulley20by meshing with the pulleys. Rotation torque from the output shaft5aof the motor5is transmitted to the rack shaft3via the transmission mechanism7. A ball screw mechanism8is provided between the rack shaft3and the driven pulley20.

In the steering assist device6, the rotation torque from the motor5is transmitted to the ball screw mechanism8via the transmission mechanism7and is subsequently converted into axial force in the axial direction of the rack shaft3through the ball screw mechanism8to apply assist force to the steering mechanism. The electric power steering device1in the first embodiment functions as what is called a rack-assist type electric power steering device in which the rack and the motor are arranged in parallel.

The following will describe the connecting structure between the pulleys10and20and the belt30.

As shown inFIG. 2, the drive pulley10is provided with external teeth11protruding radially outward. The driven pulley20is provided with external teeth22protruding radially outward. The belt30is provided with internal teeth31meshing with each of the external teeth11of the drive pulley10and the external teeth22of the driven pulley20. The belt30is wound around both the pulleys10and20such that the internal teeth31mesh with the external teeth11,22. The belt30is wound around the pulleys10and20in a slightly elongated state to generate a specified tension.FIG. 2only shows the external teeth11and22, and the internal teeth31for convenience.

As shown inFIG. 2, the external teeth11,22on the pulleys10,20are configured as helical teeth inclined at an angle relative to the axial direction of the rotation axis (width direction of each pulley) for the pulleys10,20. Helix angles of the external teeth11,22are set to be equal relative to the width direction of each pulley. Furthermore, each of the external teeth11,22includes an identical tooth profile along the width direction of each pulley. That is, each tooth has the same thickness both in the external teeth11and the external teeth22. Also, each external tooth11,22has a flank12,23, which meshes with a flank of each internal tooth31of the belt30. The flank12,23is a curved surface having a specific helix angle relative to the width direction of each pulley.

As shown inFIG. 2andFIG. 3, the internal teeth31on the belt30are configured as helical teeth inclined at an angle relative to the axial direction of the rotation axis of the pulleys10,20(width direction of the belt) when the belt30is engaged with the pulleys10,20. The helix angle of the internal teeth31relative to the width direction of the belt is set equal to the helix angle of the external teeth11,22. Each internal tooth31has a thickness that is reduced toward its both ends (tip ends) in the width direction of the belt. On the internal tooth31, a center part32with a constant tooth thickness is provided. The internal tooth31has a thickness that is reduced from the center part32toward its both ends in the width direction of the belt. That is, the internal tooth31has tapered parts33in which the thickness is reduced. The internal tooth31is provided with flanks34that mesh with flanks of the external teeth11,22on the pulleys10,20at the center part32. The flanks34are curved surfaces having an equal helix angle relative to the width direction of the belt. The flank34is inclined, at the tapered part33, at an angle relative to a portion of the flank34at the center part32along the width direction of the belt. The tapered parts33of the internal tooth31are configured in which the reduction ratio of the tooth thickness becomes larger from the tooth bottom toward the tip ends. The internal tooth31is formed such that each flank34has a smoothly curved contour at the changing point from the center part32to the tapered parts33. Thus, the internal teeth31on the belt30each have a reduced thickness at its both ends in the width direction, compared to those of the prior arts disclosed in JP 2004-314770 A, for example, as indicated by imaginary lines inFIG. 3.

How the teeth meshes with each other between the pulleys10,20and the belt30will be described, with the meshing between the drive pulley10and the belt30used as an example.

When the external teeth11on the drive pulley10and the internal teeth31on the belt30are meshed with each other, a portion of the flank34at the center part32of the internal tooth31is specifically in contact with the flank12of the external tooth11.

As shown inFIG. 4A, the flank34at the end of the tapered part33in the internal tooth31along the width direction of the belt and the flank12of the external tooth11face each other at the end portion of the meshing between the internal tooth31and the external tooth11(denoted by X inFIG. 2). Since the tooth thickness at the tapered part33is smaller than that at other portions such as the center part32, there is a clearance K with a length L1formed between the meshing teeth at the end of the internal tooth31and the external tooth11. This clearance K is formed at both ends of the internal tooth31and the external tooth11in the width direction of the belt and the drive pulley.

As shown inFIG. 4B, the flank34of the internal tooth31at an intermediate part of the tapered part33in the width direction of the belt and the flank12of the external tooth11face each other slightly inside from the end portion of the meshing between the internal tooth31and external tooth11(denoted by Y inFIG. 2). At the intermediate part of the tapered part33, there is a clearance K with a length L2, which is shorter than the length L1(L2<L1), formed between the internal tooth31and the external tooth11because the tooth thickness is reduced gradually toward the ends. This clearance K is formed at intermediate sections from the center to both ends of the internal tooth31and the external tooth11in the width direction of the belt and the drive pulley.

As shown inFIG. 4C, at the center portion of meshing between the internal tooth31and the external tooth11(denoted by Z inFIG. 2), the amount of the clearance K is approximately 0 (zero) due to the contact between the flank34at the center part32of the internal tooth31and the flank12of the external tooth11. Thus, there is a portion with the clearance K of approximately 0 formed along the center portion of the meshing between the internal tooth31and the external tooth11.

Along the meshing between the internal tooth31and the external tooth11, the clearance K is configured to increase from the meshing center portion toward both ends in the width direction of the belt and the drive pulley (clearance K at both ends > clearance K inside of both ends > clearance K at the center part). This configuration is also applied to the meshing between the internal tooth31of the belt30and the external tooth22of the driven pulley20.

For example, while the belt30on the further side inFIG. 2is moving in the direction A, an upper right part31aof the internal tooth31, that is, the right-hand part on the forward side in the direction of movement, is the beginning of the meshing with the external tooth11on the pulley10and the external tooth22on the pulley20. Whereas, while the belt30on the further side inFIG. 2is moving in the direction A, a lower left part31bof the internal tooth31, that is, the left-hand part on the backward side in the direction of movement, is the end of the meshing with the external tooth11on the pulley10and the external tooth22on the pulley20.

While the belt30on the further side inFIG. 2is moving in the direction B, the lower left part31bof the internal tooth31, that is, the left-hand part on the forward side in the direction of movement, is the beginning of meshing with the external tooth11on the pulley10and the external tooth22on the pulley20. Whereas, while the belt30on the further side inFIG. 2is moving in the direction B, the upper right part31aof the internal tooth31, that is, the right-hand part on the forward side in the direction of movement, is the end of meshing with the external tooth11on the pulley10and the external tooth22on the pulley20.

Thus, as shown inFIG. 4AtoFIG. 4C, the meshing clearance K becomes largest at the beginning of the meshing between the internal tooth31and the external teeth11,22during the movement of the belt30regardless of the moving direction. When the internal tooth31and the external teeth11,22continue to mesh with each other, the meshing clearance K becomes smaller gradually along the tapered part33of the internal tooth31. Later on, the internal tooth31and the external teeth11,22continue further to mesh with each other and then, at the center part32of the internal tooth31, the meshing clearance K becomes even smaller and then is kept constant along the center part32. The internal tooth31and the external teeth11,22continue further to mesh with each other, the meshing clearance K becomes larger again and then becomes largest at the end of the meshing between the internal tooth31and the external teeth11,22.

The following will describe the actions of the electric steering device1.

When the teeth mesh with each other between the pulleys10,20and the belt30, one end of each tooth is the beginning of the meshing. At the beginning of the meshing through the internal teeth31on the belt30with a tooth thickness reduced toward both ends along the width direction of the belt, the clearance K becomes largest. That is, the tooth contact is adjusted at the beginning of the meshing between the pulleys10,20and the belt30to consequently reduce the generation of the operating sound. If the meshing clearance K is widened along the entire width of each meshing tooth, there will occur tooth skipping. In the internal teeth31on the belt30, the tooth thickness is reduced toward both ends in the width direction of the belt such that the meshing clearance K becomes smaller at the part other than the beginning of the meshing between the pulleys10,20and the belt30especially at the center part of the meshing than that at the beginning of the meshing. This makes it easier to maintain the tooth meshing between the pulleys10,20and the belt30. In the first embodiment, the internal teeth31on the belt30are configured to have a tooth thickness that is reduced toward both ends in the width direction of the belt.

At the center part32of the internal tooth31on the belt30, where the meshing clearance K is kept constant, the occurrence of tooth skipping can be reduced because the small meshing clearance K is maintained.

As described above in the first embodiment, the advantageous effects will be obtained as follows.

(1) When the internal teeth31on the belt30and the external teeth11,22on the pulleys10,20are meshed, the tooth contact is adjusted at the beginning of the meshing of the teeth, whereby the generation of the operating sound is reduced and the meshing of the teeth is easily maintained especially at the center portion of the meshing. It is thus possible to reduce the operation sound and the occurrence of tooth skipping.

(2) At the center portion of the meshing between the belt30and the pulleys10,20, a state in which the clearance K is kept small is maintained. This can reduce the occurrence of tooth skipping.

(3) For the internal teeth31on the belt30, the tooth thickness is reduced toward both ends in the width direction of the belt to consequently reduce the operating sound. There is no need to process the pulleys10,20, which can reduce the processing costs.

(4) When the belt30and the pulleys10,20are meshed, both ends of each tooth in the width direction of the belt and the pulleys at the beginning of the meshing can also be the end of the meshing. At the end of meshing, the tooth contact is adjusted to reduce the generation of the operating sound.

The following will describe an electric power steering device according to a second embodiment. For example, the same configurations and controls described in the first embodiment are denoted with the same numerals and descriptions thereof will be omitted.

As shown inFIG. 5A, in the external teeth11,22on the pulleys10,20in the second embodiment, the tooth thickness is reduced from the center part toward both ends in the width direction of each pulley. On the external teeth11,22, the center parts13,24are formed, respectively, having a constant tooth thickness, and the tooth thickness is reduced from each center part13,24toward both ends in the width direction of each pulley, that is, tapered parts14,25with reduced tooth thickness are formed. These tapered parts14and25are formed by machining process in which one side of each end of each external tooth11,22in the width direction of the pulleys is cut such that the cut sides are arranged diagonally opposite from each other The tapered parts14and25are formed by cutting a side of the beginning of the meshing between the internal teeth31on the belt30and the external teeth11,22on the pulleys10,20, that is, a side inclined to the forward direction of the movement, considering the inclining direction and the rotation direction of the helical teeth (upper right and lower left portions on the nearer side inFIG. 2). The external teeth11,22are formed such that the flanks12,23have smoothly curved contours at the changing point from the center parts13,24to the tapered parts14and25.

The tooth thickness of the internal teeth31on the belt30is kept constant. The flanks12,23are formed as curved surfaces having an equal helix angle relative to the width direction of the belt.

As shown inFIG. 5BandFIG. 5C, the tapered parts14,25are formed such that the length of the tip ends is L3, or such that the length of the tip ends is L4, which is shorter than L3(L4<L3), so as to make a gradient toward the tip ends larger. Therefore, in a case where the length of the tip ends of the tapered parts14,25is L3(referred to as “small gradient” hereinafter), the meshing clearance K between the internal tooth31and the external teeth11,22is widened compared with that in a case where the length is L4(referred to as “large gradient” hereinafter) while the belt30is moving.

In the second embodiment, the external teeth11,22on the pulleys10,20are machined to set the meshing clearance K between the helical teeth of the pulleys10,20and the belt30, depending on the model for the transmission mechanism7of the electric power steering device1.

As shown inFIG. 6AtoFIG. 6D, it is possible to assume multiple models for the transmission mechanism7of the electric power steering device1(four models presented in the second embodiment) depending on the supporting method for the ball screw mechanism8on which the driven pulley20is arranged (categorized by A and B) and the supporting method for the drive motor5on which the drive pulley10is arranged (categorized by P1and P2).

More specifically, it is possible to assume a supporting method A by which the ball screw mechanism8is supported on the rack housing4by a supporting member B1such as a bearing on the opposite side of the belt30from the pinion shaft2, and a supporting method B by which the ball screw mechanism8is supported on the rack housing4by the supporting member B1on the same side of the belt30as the pinion shaft2. In addition, it is possible to assume a supporting method P1by which the drive motor5is supported on the rack housing4by a supporting member B2such as bolts on the same side of the belt30as the pinion shaft2, and a supporting method P2by which the drive motor5is supported on the rack housing4by the supporting member B2on the opposite side of the belt30from the pinion shaft2.

As shown inFIG. 7, by the relationship between the tension of the belt30and the supporting methods for the ball screw mechanism8and the drive motor5, the ball screw mechanism8on which the driven pulley20is arranged or the drive motor5on which the drive pulley10is arranged can be tilted. According to the tilting angle thereof, the meshing between the internal teeth31of the belt30and the external teeth11,22of the pulleys10,20can also be tilted. For the description below, directions are defined based on the external teeth11,22on the nearer side inFIG. 2, that is, helical teeth inclined upward toward the right with the upper right and lower left portions cut.

As shown inFIG. 6A, in the A/P1model, or the combination of the supporting method A and the supporting method P1, the ball screw mechanism8, namely, the driven pulley20, is tilted toward the side opposite from the supporting member B1and toward the drive motor5(toward the lower right inFIG. 6A). The drive motor5, namely, the drive pulley10, is tilted toward the side opposite from the supporting member B2and toward the ball screw mechanism8(toward the upper left inFIG. 6A).

As shown in the row of the A/P1model inFIG. 7, for this combination model, the tooth profile of the external teeth11on the drive pulley10is preferably configured such that the portion where the drive pulley10is not supported, that is, the tapered part14on the left-hand side has a large gradient, and the portion where the drive pulley10is supported, that is, the tapered part14on the right-hand side has a small gradient. In this case, the tooth profile of the external teeth22on the driven pulley20is also preferably configured such that the portion where the driven pulley20is supported, that is, the tapered part25on the left-hand side, has a small gradient, and the portion where the driven pulley20is not supported, that is, the tapered part25on the right-hand side has a large gradient.

As shown inFIG. 6B, for the A/P2model, or the combination of the supporting method A and supporting method P2, the ball screw mechanism8, namely, the driven pulley20, is tilted toward the side opposite from the supporting member B1and toward the drive motor5(toward the lower right inFIG. 6B). The drive motor5, namely, the drive pulley10, is tilted toward the side opposite from the supporting member B2and toward the ball screw mechanism8(toward the upper right inFIG. 6B).

As shown in the row of the A/P2model inFIG. 7, for this combination model, the tooth profile of the external teeth11on the drive pulley10is preferably configured such that the portion where the drive pulley10is supported, that is, the tapered part14on the left-hand side, has a small gradient, and the portion where the drive pulley10is not supported, that is, the tapered part14on the right-hand side has a large gradient. In this case, the tooth profile of the external teeth22on the driven pulley20is preferably configured as for the A/P1model.

As shown inFIG. 6C, for the B/P1model, or the combination of the supporting method B and the supporting method P1, the ball screw mechanism8, namely, the driven pulley20, is tilted toward the side opposite from the supporting member B1and toward the drive motor5(toward the lower left inFIG. 6C). The drive motor5, namely, the drive pulley10, is tilted toward the side opposite from the supporting member B2and toward the ball screw mechanism8(toward the upper left inFIG. 6C).

As shown in the row of the B/P1inFIG. 7, for this combination model, the tooth profile of the external teeth11on the drive pulley10is preferably configured such that the portion where the drive pulley10is not supported, that is, the tapered part14on the left-hand side, has a large gradient, and the portion where the drive pulley10is supported, that is, the tapered part14on the right-hand side, has a small gradient. In this case, the tooth profile of the external teeth22on the driven pulley20is preferably configured such that the portion where the driven pulley20is not supported, that is, the tapered part25on the left-hand side has a large gradient, and at the same time, the portion where the driven pulley20is supported, that is, the tapered part25on the right-hand side, has a small gradient.

As shown inFIG. 6D, for the B/P2model, or the combination of the supporting method B and the supporting method P2, the ball screw mechanism8, namely, the driven pulley20, is tilted toward the side opposite from the supporting member B1and toward the drive motor5(toward the lower left inFIG. 6D). The drive motor5, namely, the drive pulley10, is tilted toward the side opposite from the supporting member B2and toward the ball screw mechanism8(toward the upper right inFIG. 6D).

As shown in the row of the B/P2inFIG. 7, for this combination model, the tooth profile of the external teeth11on the drive pulley10is preferably configured such that the portion where the drive pulley10is supported, that is, the tapered part14on the left-hand side, has a small gradient, and the portion where the drive pulley10is not supported, that is, the tapered part14on the right-hand side, has a large gradient. In this case, the tooth profile of the external teeth22on the driven pulley20is preferably configured as for the B/P1model.

As mentioned above, by applying preferable configurations to the assumed combination models, the meshing clearance K between the internal teeth31and the external teeth11is widened on the opposite side from the supporting member B2, that is, the side on which the drive pulley10is not supported, compared with that on the same side as the supporting member B2, that is, the side on which the drive pulley10is supported. The meshing clearance K between the internal teeth31and the external teeth22is widened on the opposite side from the supporting member B1, that is, the portion where the driven pulley20is not supported, compared with that on the same side as the supporting member B1, that is, the side on which the driven pulley20is supported.

The following will describe the actions of the electric power steering device1in the second embodiment.

When the driven pulley20and the drive pulley10are independently tilted as seen in the assumed combination models in the second embodiment, if the tilting angles of the driven pulley20and the drive pulley10are not taken into consideration in relation to the external teeth11,22, the meshing clearance K in the meshing between the internal tooth31and the external teeth11,22will be smaller than it is expected even if the tooth thickness is reduced toward both ends along the width direction of each pulley.

In the second embodiment, the tooth thickness of the external teeth11,22on the pulley10and the pulley20is reduced toward both ends in the width direction of each pulley considering the tilting angles of the driven pulley20and the drive pulley10, thereby reducing the operating sound.

In the second embodiment focusing on the beginning of the meshing between the pulleys10,20and the belt30, only a single flank of each helical tooth needs to be considered in relation to the inclining direction of the helical tooth and the rotation direction. That is, in the external teeth11,22on the pulleys10,20, portions where the tooth thickness is reduced toward both ends in the width direction of each pulley can be minimized. It is thus possible, in the pulleys10,20and the belt30, to secure as large a portion of each tooth as possible where the meshing clearance K is small.

As described above, the second embodiment will ensure the following advantageous effects as well as the effects (1) to (3) in the aforementioned first embodiment.

(4) For the external teeth11,22on the pulleys10,20, the tooth thickness is reduced toward both ends in the width direction of each pulley so as to provide a device for reducing the operating sound to each of the pulley10and the pulley20while providing a preferable meshing clearance.

(5) For the external teeth11and22on the pulleys10,20, portions where the tooth thickness is reduced toward both ends in the width direction of each pulley can be minimized. This makes it easier to secure a portion where the meshing clearance K between the teeth on the pulleys10,20and the belt30is narrowed, whereby the occurrence of tooth skipping can be reduced.

Each embodiment described above may also be applicable in other modes by making appropriate modifications as follows.

In the first embodiment, the tapered parts33of the internal tooth31may be formed by cutting one side of each end of each tooth in the width direction of the belt such that the cut sides are arranged diagonally opposite from each other considering only the beginning of the meshing between both pulleys10and20and the belt30.

In the first embodiment, the tip ends of the internal tooth31may have a gradient considering the models for the transmission mechanism7of the electric power steering device1, as in the second embodiment. In this case, the gradient to be applied to the tip ends of the internal tooth31may be set considering the meshing with the pulley whose tilting angle is larger in the aforementioned models.

The first embodiment may be realized with the internal tooth31having a barrel-like shape as a whole with a configuration where the tooth thickness of the center part32of the internal tooth31in the width direction of the belt is largest at the center portion of the internal tooth31and the tooth thickness is reduced gradually toward both ends in the width direction of the belt.

In the second embodiment, the tapered parts14and25of each of the external teeth11and22may be formed by cutting both sides of each end in the width direction of each pulley, considering the end of the meshing between the pulleys10,20and the belt30. In this case, as in the beginning of the meshing between the pulley10,20and the belt30, the ends of the external tooth can be tapered considering the model for the transmission mechanism7of the electric power steering device1.

In the second embodiment, the ends of the external tooth on either one of the pulleys10,20may be tapered. That is, the ends of the external tooth on the pulley whose tilting angle is larger during meshing can be tapered, considering the meshing with the pulley whose tilting angle is larger in the aforementioned models. With this configuration, a device is provided to at least one of the external-toothed pulleys10and20so that the operating sound is reduced compared to the case where no device is provided.

The second embodiment may be realized with the external teeth11,22having an approximately barrel-like shape as a whole with a configuration in which at the portion where the tip ends of each of the external teeth11,22are tapered, the tooth thickness at each center part13,24along the width direction of each pulley becomes largest at the center of each of the external teeth11,22, whereas the tooth thickness is reduced gradually toward both ends along the width direction of each pulley.

In each embodiment, to taper the tip ends of the helical tooth on the pulleys10,20and the belt30, the tapering process may not be applied to every helical tooth. For example, it is acceptable to apply the tapering process to every two or three teeth.

In each embodiment, the supporting methods for the ball screw mechanism8and the supporting methods for the drive motor5are exemplified as a cantilever-supporting model, but may also be applicable to a double-supporting model.

In each embodiment, the tip ends of the helical tooth on the pulleys10,20and the belt30may be tapered with a curvature. In the second embodiment, especially, it is acceptable to adjust the amount of the meshing clearance K with a curvature.