JP2001122135A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power steering device capable of reducing hammering noise while maintaining accuracy of a worm. SOLUTION: Tooth flank abrasion of the worm 30a is suppressed by applying electroless plating process on at least a tooth flank of the worm 30a for improving tooth flank strength of the worm 30a meshed with a worm wheel 13 even when a resin including glass fiber or the like is used for the worm wheel 13. By employing the electroless plating process, it is possible to provide a uniform plating film thickness and to accurately reproduce a basis material shape for easily securing gear accuracy of the worm 30a. Because the worm 30a is substantially not subjected to heat treatment after tooth flank forming, problems of accuracy degradation due to thermal deformation do not occur and deterioration of surface roughness of the tooth flank is prevented, and thereby slip loss occurring between the worm 30a and the worm wheel 13 during power transmission can be reduced. Thus, transmission efficiency can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering device, and more particularly to an electric power steering device capable of reducing gear noise.

[0002]

2. Description of the Related Art As an electric power steering device for a vehicle, the rotational output of an electric motor, which is an auxiliary steering torque, is reduced by a gear device and transmitted to an output shaft of a steering mechanism, and the steering force applied to a steering wheel is assisted. Then, there is known one configured to steer wheels. In such an electric power steering device, a power transmission mechanism provided in a housing is used to transmit power to an output shaft while reducing the rotation of an electric motor.

[0003]

Incidentally, for example, in an electric power steering apparatus using a worm gear mechanism as a power transmission mechanism, it is necessary to set an appropriate backlash between the worm and the tooth surface of the worm wheel. That is, if the backlash is too small, the meshing teeth compete with each other, the operating torque becomes heavy, and the steering wheel returns poorly.

On the other hand, if the backlash is increased to some extent, competition between the teeth does not occur. However, when the power transmission direction is reversed between the worm and the worm wheel, the tooth surfaces abut each other. As a result, a tapping sound may be generated, which may cause discomfort to the occupants of the vehicle.

[0005] In order to alleviate such hitting noise, the outer peripheral portion of a worm wheel including teeth is formed of resin. By the way, in order to secure mechanical strength, it is necessary to mix glass fiber or the like into the resin forming the outer peripheral portion. Here, if the worm tooth surface is untreated, the end of the hard glass fiber exposed on the worm wheel tooth surface will cause the worm tooth surface to wear out early, so that the nitriding can be performed at a relatively low temperature. The worm is subjected to a treatment or nitrocarburizing treatment to increase the tooth surface hardness of the worm, thereby preventing premature wear. However, by performing such a heat treatment, a certain degree of thermal deformation occurs in the worm, which reduces the gear accuracy of the worm. In addition, in the worm after the heat treatment, scale and the like may be generated and the surface roughness may be deteriorated. However, when barrel processing is further performed to improve the surface roughness, there is a problem that the accuracy of the worm is further reduced. is there.

SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide an electric power steering device capable of reducing a tapping sound while maintaining worm accuracy.

[0007]

In order to achieve the above object,
An electric power steering apparatus according to the present invention includes a housing, a motor attached to the housing, the motor generating an auxiliary steering force on a rotating shaft, an output shaft transmitting a steering force for steering wheels, and the output shaft. A worm gear that has a bearing rotatably supported, a worm that rotates integrally with the rotation shaft, and a worm wheel that rotates integrally with the output shaft, and that transmits an auxiliary steering force of the motor to the output shaft. An electroless plating process is applied to at least the tooth surface of the worm.

[0008]

According to the electric power steering apparatus of the present invention, a housing, a motor mounted on the housing and generating an auxiliary steering force on a rotating shaft, and an output shaft for transmitting a steering force for steering wheels are provided. A bearing that rotatably supports the output shaft, a worm that rotates integrally with the rotation shaft, and a worm wheel that rotates integrally with the output shaft, and outputs the auxiliary steering force of the motor to the output shaft. A worm gear mechanism for transmitting to the shaft, and at least the tooth surface of the worm is subjected to electroless plating, so that the tooth surface hardness of the worm meshing with the worm can be increased. Even if a resin mixed with glass fiber or the like is used, wear on the worm tooth surface can be suppressed. In addition, according to the electroless plating process, since the plating film thickness is uniform and the base shape can be accurately reproduced, it is easy to ensure the gear accuracy of the worm. In addition, since the worm is not substantially heat-treated after the tooth flanks are formed, there is no problem of accuracy deterioration due to thermal deformation and the surface roughness of the tooth flanks is not deteriorated. Can reduce the sliding loss, and thus can improve the transmission efficiency. The electroless plating is preferably an electroless nickel-phosphorus or a composite plating containing nickel-phosphorus.

[0009] When the worm made of structural steel is nitrided,
The surface hardness increases to about Hv500, depending on the processing time. On the other hand, when electroless nickel-phosphorus plating is applied to the same worm, the hardness at the time of deposition of the plating film differs depending on the phosphorus content. In the case of plating, a tooth surface hardness of about Hv650, which is equal to or higher than that in the case of nitriding, can be obtained. Further, in the electroless nickel-phosphorus-boron plating, a high hardness of about Hv700 can be obtained. When PTFE (ethylene tetrafluoride) is dispersed and codeposited in the electroless nickel-phosphorus plating, although the hardness is slightly lowered, the friction coefficient is lowered, so that a better transmission efficiency can be obtained. .

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a partial cross-sectional view of an electric power steering apparatus 100 according to a first embodiment of the present invention.

In FIG. 1, an electric power steering apparatus 100 includes a tube 101 extending horizontally and a housing 1 connected to the left end of the tube 101. The tube 101 is fixed to a vehicle body (not shown) by a bracket 101a, and the housing 1 is fixed to the vehicle body by a bracket 1a.

In the housing 1, an input shaft 2 connected to a steering wheel (not shown) extends from the right side, and is connected to a right end (not shown) of the output shaft 3 via a torsion bar (not shown) in the tube 101. Are linked. The left end of the output shaft 3 is connected to a steering mechanism (not shown). The center of the output shaft 3 is rotatably supported by two bearings 4a and 4b. The outer rings of the bearings 4a and 4b are supported by a bearing holder 5, and the bearing holder 5 is fixed to the housing 1 by bolts 7. A lock nut 6 is screwed onto the output shaft 3 to apply a preload to the inner ring of the bearing 4a.

A worm wheel 13 composed of a metal core 13a and a resin outer peripheral portion 13b is fixed to the outer periphery near the right end (not shown) of the output shaft 3.

The worm wheel 13 meshes with a worm 30a, and the worm 30a is formed on a rotary shaft 30 connected to a rotor 21a (FIG. 2) of an electric motor 21 fixed to the housing 1. This electric motor 2
1 is connected to a CPU (not shown).
Is for inputting information such as the output of a torque sensor (not shown) and vehicle speed, and supplying predetermined electric power to the electric motor 21 to generate an appropriate auxiliary torque.

FIG. 2 is a view of the electric power steering apparatus 100 of FIG. 1 cut along the line II-II and viewed in the direction of the arrow. In FIG. 2, a serration hole 21b is formed in the center of a rotor 21a of an electric motor 21 to which driving power is supplied by a power supply line 21c. On the other hand, the rotating shaft 30 arranged coaxially with the rotor 21a
Has a serration 30b at its left end,
By engaging the serration portion 30b with the serration hole 21b, the rotating shaft 30 is relatively displaceable in the axial direction with respect to the rotor 21a, but rotates integrally therewith.

The rotating shaft 30 has a worm 30a at its center.
Are formed, and flange portions 30c and 30d are formed on both sides of the worm 30a. Further, a bearing 8a is provided on the left side of the left flange portion 30c, and rotatably supports the vicinity of the left end of the rotating shaft 30 with respect to the housing 1. On the other hand, a bearing 8b is provided on the right side of the right flange portion 30d, and similarly, the vicinity of the right end of the rotating shaft 30 is rotatably supported with respect to the housing 1.

The outer ring of the bearing 8a is in contact with the retaining ring 9 attached to the housing 1 on the left side. On the other hand, between the inner ring of the bearing 8a and the flange portion 30c, a pair of disc springs 10a, which are elastic bodies, are arranged so that their peripheral edges are in contact with each other.

An annular pressing plate 11 is arranged on the right side of the bearing 8b. The pressing plate 11 is pressed from the right side by a bolt member 12, and its peripheral edge is brought into contact with the outer ring of the bearing 8b. A lock nut 14 is screwed in to prevent the bolt member 12 from coming off, and a cover member 15 is provided outside the lock nut 14. Between the inner ring of the bearing 8b and the flange portion 30d, a pair of disc springs 10b, which are elastic bodies, are arranged so as to abut against each other.

The disc springs 10a and 10b are bent to a certain extent and the bearings 8a and 8b and the flange portions 30c and 30c are bent.
d, a predetermined preload is applied to the bearings 8a and 8b, whereby the rotating shaft 30 is supported so that there is no backlash in the axial direction. Further, when a normal steering assist force is transmitted from the worm 30a to the worm wheel 13, even if one of the disc springs is bent and the rotary shaft 30 is maximally displaced in one direction, the other disc spring remains bent. So that the amount of bending is set.

Next, the operation of this embodiment will be described below. Assuming that the vehicle is in a straight running state and no steering force is input to the input shaft 2 via a steering wheel (not shown), a torque sensor (not shown) does not generate an output signal, and thus the electric motor 21 outputs an auxiliary steering force. Does not occur.

When the driver steers a steering wheel (not shown) when the vehicle is going to make a curve, a torsion bar (not shown) is twisted on the input shaft 2 according to the steering force.
Relative rotation occurs between the motor and the output shaft 3. The torque sensor outputs a signal according to the direction and amount of the relative rotation to the CPU.
(Not shown). Under the control of the CPU based on this signal, the electric motor 21 rotates to generate an auxiliary steering force. The rotation of the electric motor 21 is reduced by the worm gear mechanism and transmitted to the output shaft 3.

By the way, when the steering wheel is turned in one direction and then turned in the opposite direction immediately when the width of the vehicle is adjusted, in such a case, the power transmission direction suddenly reverses, and The tooth surfaces of the worm 30a and the worm wheel 13 that are separated from each other by the rush amount are in contact with each other. Further, the tooth surfaces may come into contact with each other due to vibration transmitted from the wheels during traveling. However, according to the present embodiment, the impact force generated between the tooth surfaces is mitigated by further bending the disc spring 10a or 10b and displacing the rotating shaft 30 in the axial direction, thereby reducing the tapping sound. Can be reduced. The rotor 21a of the electric motor 21
And the rotating shaft 30 are serrated, so that the rotating shaft 30 can be displaced in the axial direction relative to the rotor 21a.

In this embodiment, the rotating shaft 30 is
After the gear cutting of the worm 30a, the surface including the tooth surface is subjected to electroless nickel-phosphorus plating. Therefore, even if the outer peripheral portion 1 of the worm wheel 13 is
Even if 3b is formed from a resin mixed with glass fiber or the like, the tooth surface hardness of the worm 30a meshing with the resin can be increased, so that the tooth surface wear of the worm 30a can be suppressed. In addition, according to the electroless plating process, the plating film thickness is uniform and the substrate shape can be accurately reproduced, so that it is easy to secure the gear accuracy of the worm 30a. In addition, since the worm 30a is not substantially heat-treated after the tooth surface is formed, there is no problem of deterioration in accuracy due to thermal deformation, and the surface roughness of the tooth surface is not deteriorated.
3 can reduce the slip loss generated between the tooth flank surfaces, thereby improving the transmission efficiency. The electroless plating is preferably an electroless nickel-phosphorus or a composite plating containing nickel-phosphorus (for example, electroless nickel-phosphorus-boron plating). In addition, electroless nickel
Phosphorus plating may be used by dispersing and eutecting PTFE (tetrafluoroethylene). In such a case, although the hardness is slightly reduced, the coefficient of friction is reduced, so that better transmission efficiency can be obtained. .

Next, a second embodiment of the present invention will be described with reference to the drawings. FIG. 3A shows an electric power steering apparatus 200 according to a second embodiment of the present invention.
3 is a sectional view similar to FIG. FIG.
It is a figure which expands and shows IIIB part of a structure of (a). In addition,
The second embodiment will be described with a focus on differences from the first embodiment shown in FIG. 2, and a description of common parts will be omitted.

The second embodiment shown in FIG. 3 differs from the first embodiment in the manner in which the rotating shaft 30 is supported.
More specifically, a cylindrical bush 201 with a one-sided flange is interposed between the outer periphery of the rotating shaft 30 and the left bearing 8a, and between the outer periphery of the rotating shaft 30 and the right bearing 8b. Has a cylindrical bush 202 with a flange on one side. The bushes 201 and 202 are made of PTFE and have a low coefficient of friction, and facilitate displacement of the rotary shaft 30 in the axial direction with respect to the bearings 8a and 8b.

The elastic member 203 is sandwiched between the left bearing 8a and the flange 30c of the rotary shaft 30 with the flange of the bush 201 interposed therebetween. As shown in FIG. 3B, the elastic member 203 includes a pair of ring portions 203a and 203b spaced apart from each other, and a rubber elastic portion 203c connecting between the ring portions. Part of the elastic portion 203c is covered along the surface of the ring portion 203a facing the step portion 30e formed adjacent to the flange portion 30c of the rotating shaft 30. In the neutral state shown in FIG.
A gap Δ is formed between the step 30e and the flange 30c. Although not described in detail, the elastic member 204 has the same configuration, that is, a part of the elastic portion of the elastic member 204 is formed by a stepped portion formed adjacent to the flange portion 30d of the rotary shaft 30. It is coated along the surface of the ring portion facing 30f.

According to the present embodiment, when the tooth surfaces of the worm 30a and the worm wheel 13 abut against each other, the impact force generated between the tooth surfaces is absorbed by the elastic members 203 and 204, and the generation of a tapping sound is generated. Suppress. Further, an excessive impact force is generated, and the rotating shaft 30 is moved away from the bearings 8a and 8b by a distance Δ.
When the relative displacement is caused by the elastic member 203 (or 20),
The ring portion 4) and the step portion 30e (or 30f) are in contact with each other to prevent further displacement in the axial direction. As is clear from FIG. 3B, the ring portion 203b
Is larger than the outer diameter of the step 30e,
Since the inner diameter of 3a is smaller than the outer diameter of the step portion 30e, there is no possibility that the elastic member 203 (204) is wrongly attached to the rotating shaft 30 in the wrong direction.

In this embodiment, if the entire rotating shaft 30 is subjected to an electroless plating process in which PTFE is dispersed and codeposited, the friction coefficient of the rotating shaft 30 is reduced. Even if not used, the relative displacement of the rotating shaft 30 in the axial direction with respect to the bearings 8a and 8b becomes easy. Therefore, since the bushes 201 and 202 can be omitted, the assemblability can be improved and the cost can be reduced by reducing the number of parts.

Next, a third embodiment of the present invention will be described with reference to the drawings. FIG. 4 is a sectional view similar to FIG. 2 of an electric power steering device 300 according to a third embodiment of the present invention. The third embodiment will be described with a focus on differences from the first embodiment shown in FIG. 2, and a description of common parts will be omitted.

The third embodiment shown in FIG. 4 differs from the first embodiment in the manner in which the rotating shaft 130 is supported. That is, in FIG. 4, the vicinity of the left end of the rotating shaft 130 is supported by the two bearings 8a and 8b in series. The bearings 8a and 8b are connected to the housing 1 by a retaining ring 9.
Are mounted so that they cannot be displaced in the axial direction. A flange portion 130c is formed on the right side of the bearings 8a and 8b, while an outer peripheral groove 130e is formed on the left side thereof.

A C-shaped clip 130f forming the other flange portion is fitted in the outer peripheral groove 130e. On the other hand, a sliding bearing 1 a is formed around the right end 130 g of the rotating shaft 130, and the output shaft 130
Are rotatably supported relative to each other in the axial direction.

In FIG. 4, one disc spring 210a
Are arranged between the clip 130f and the inner ring of the bearing 8a. On the other hand, one disc spring 210b is arranged between the flange portion 130c and the inner ring of the bearing 8b. The other configurations and operations are the same as those of the first embodiment, and the description thereof is omitted. According to the present embodiment, the configuration around the slide bearing 1a can be made more compact than in the above-described embodiment, thereby increasing the degree of freedom in design around the electric power steering device 300.

Although the present invention has been described with reference to the embodiments, it should be understood that the present invention should not be construed as being limited to the above embodiments, and that modifications and improvements can be made as appropriate. is there. For example, regarding electroless nickel-phosphorus plating or electroless nickel-phosphorus-boron plating, the contents of phosphorus and boron are not particularly limited.

[0034]

As described above, according to the electric power steering apparatus of the present invention, the housing, the motor mounted on the housing and generating the auxiliary steering force on the rotating shaft, and the wheel for steering the wheels are provided. An output shaft transmitting a steering force, a bearing rotatably supporting the output shaft, a worm rotating integrally with the rotation shaft, and a worm wheel rotating integrally with the output shaft, A worm gear mechanism for transmitting an auxiliary steering force of a motor to the output shaft; and since at least a tooth surface of the worm is subjected to electroless plating, the tooth surface hardness of the worm meshing with the worm is increased. Therefore, even if a resin mixed with glass fiber or the like is used for the worm wheel, wear on the tooth surface of the worm can be suppressed.
In addition, according to the electroless plating process, since the plating film thickness is uniform and the base shape can be accurately reproduced, it is easy to ensure the gear accuracy of the worm. In addition, since the worm is not substantially heat-treated after the tooth flanks are formed, there is no problem of accuracy deterioration due to thermal deformation and the surface roughness of the tooth flanks is not deteriorated. Can reduce the slip loss that occurs in
Therefore, transmission efficiency can be improved. The electroless plating is preferably an electroless nickel-phosphorus or a composite plating containing nickel-phosphorus.

[Brief description of the drawings]

FIG. 1 is an axial partial cross-sectional view of an electric power steering device 100 according to a first embodiment of the present invention.

FIG. 2 shows the electric power steering device of FIG.
FIG. 3 is a cross-sectional view cut along an I line.

FIG. 3A is a sectional view similar to FIG. 2 of an electric power steering apparatus 200 according to a second embodiment of the present invention, and FIG. 3B is a sectional view of FIG. II) of the configuration of a)
It is a figure which expands and shows an IB part.

FIG. 4 is a sectional view similar to FIG. 2 of an electric power steering device 300 according to a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Housing 2 Input shaft 3 Output shaft 5 Bearing holder 6 Lock nut 10a, 10b, 210a, 210b Disc spring 13 Worm wheel 21 Electric motor 30, 130 Rotating shaft 30a Worm 203, 204 Elastic member

Claims (1)

[Claims] 1. A housing, a motor mounted on the housing and generating an auxiliary steering force on a rotating shaft, an output shaft transmitting steering force for steering wheels, and rotatably supporting the output shaft. Bearing, a worm rotating integrally with the rotating shaft, and a worm wheel rotating integrally with the output shaft, comprising a worm gear mechanism transmitting auxiliary steering force of the motor to the output shaft; An electric power steering device, wherein at least a tooth surface of the worm is subjected to an electroless plating process.