JP2002327829A - Gear and motor-driven power steering device provided with this - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gear capable of extending its service life by improving fatigue strength and wear resistance of teeth. SOLUTION: In the gear 10 made of synthetic resin, 0.1-0.3 wt.% of aluminum stearate is added to its synthetic resin material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reduction gear for rotating an electric actuator for generating a steering assist force, for example, by a drive gear such as a worm and a driven gear such as a worm wheel meshing with the drive gear. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gear suitable for use as a driven gear in an electric power steering device that transmits a steering angle to a wheel through a mechanism so as to change, and an electric power steering device including the gear.

[0002]

2. Description of the Related Art In a small car, a reduction gear mechanism is provided for reducing the noise and weight of an electric power steering apparatus for transmitting the rotation of an electric actuator for generating a steering assist force to wheels via a reduction gear mechanism. Such driven gears are made of a synthetic resin material.

In recent years, there has been a demand for low fuel consumption of vehicles in order to deal with environmental problems. Therefore, it is also desired to use a driven gear made of a synthetic resin material in an electric power steering device used for a large vehicle. ing. However, since the output of the electric actuator is higher than that of a mini vehicle, the transmission torque is increased, and the fatigue strength and wear resistance of the driven gear are reduced. A general synthetic resin material such as nylon which forms a conventional gear does not have sufficient fatigue strength and wear resistance to withstand an increase in transmission torque.

[0004]

In order to improve the fatigue strength and wear resistance of the synthetic resin gear as described above, it is conceivable to fill the synthetic resin material with a reinforcing fiber such as whisker. However, when molding gears from synthetic resin materials, the teeth are finished by machining after molding,
The reinforcement causes the reinforcing fibers to protrude from the tooth surface. There is a problem that the fracture occurs from the interface between the protruding reinforcing fiber and the synthetic resin material, so that the fatigue strength is reduced.

[0005] Further, as a synthetic resin material forming a gear,
It is conceivable to improve fatigue strength and wear resistance by mixing a synthetic resin material such as nylon with a highly lubricious synthetic resin material such as ethylene tetrafluoride resin (PTFE). However, such a synthetic resin material having a high lubricating property is inferior in strength.If the mixing ratio is increased as much as necessary to sufficiently improve the lubricating property, the tensile strength, bending, and the There is a problem that strength, elastic modulus and the like are reduced.

[0006]

According to the present invention, there is provided a gear made of a synthetic resin material, wherein 0.1% to 0.3% by weight of aluminum stearate is added to the synthetic resin material. . According to the configuration of the present invention, the lubricity of the gear can be improved by adding aluminum stearate to the synthetic resin material forming the gear. Due to the improved lubricity, the load acting on the gear during rotation transmission is reduced, fatigue strength and wear resistance are improved, and the life of the gear can be extended. Moreover, since the amount of aluminum stearate added to the synthetic resin material is as small as 0.3% by weight or less, the molecular weight of the synthetic resin material is hardly reduced, and it is necessary to transmit the rotational torque. Characteristics such as the tensile strength, bending strength, and elastic modulus of the gear can be maintained. When the amount of the aluminum stearate added is 0.1% by weight or more, the lubricity can be sufficiently improved.

The relative viscosity of the formed gear by the formic acid method is preferably 100 or more and 300 or less. By setting the relative viscosity to 100 or more, the fatigue strength can be further improved by improving the tooth root strength of the gear, and the formability can be ensured by setting the relative viscosity to 300 or less. That is, according to the general theoretical formula applied to the metal gear, the tooth root strength of the gear is correlated with the tensile strength and bending strength of the material. However, in the case of a synthetic resin gear, the root strength does not correlate with the tensile strength or bending strength of the material. In fact, when synthetic fibers were filled with reinforcing fibers in order to improve the tensile strength and the bending strength, when the amount of filling was too large, the tooth root strength was lower than when not filled. This is because the root strength of the synthetic resin gear is determined by the balance between the relaxation of the surface pressure acting on the teeth based on the elasticity of the synthetic resin material and the strength of the synthetic resin material. The present inventors have found that by setting the relative viscosity of a gear formed of a synthetic resin material to a value of 100 or more, which is larger than the conventional value, the tooth root strength can be increased and the fatigue strength can be improved.

[0008] The electric power steering apparatus of the present invention comprises:
In the electric power steering apparatus having a reduction gear mechanism for transmitting rotation of the electric actuator for generating steering assist force to wheels via a drive gear and a driven gear meshing with the drive gear, the gear of the present invention is used as the driven gear. Is used. As a result, even in an electric power steering device for a vehicle that requires a large steering assist force, the driven gear in the reduction gear mechanism that transmits the steering assist force is made of synthetic resin to improve quietness during operation and reduce weight. Can be planned.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An electric power steering apparatus 1 for a vehicle shown in FIG. 1 transmits a steering torque generated by steering of a steering wheel 2 to a pinion 4 by a steering shaft 3 so that the rack meshes with the pinion 4. Move 5 The movement of the rack 5 is controlled by wheels 6 via tie rods or knuckle arms (not shown).
The steering angle changes by being transmitted to the steering wheel.

In order to apply a steering assist force in accordance with the steering torque transmitted by the steering shaft 3, a torque sensor 7 for detecting the steering torque and a motor (electric actuator) driven in accordance with the detected steering torque ) 8, a metal worm (drive gear) 9 provided on the outer periphery of a drive shaft 50 driven by the motor 8, and a worm wheel (driven gear) 10 that meshes with the worm 9 and is attached to the steering shaft 3. Is provided. By transmitting the rotation of the motor 8 from the steering shaft 3 via the worm 9 and the worm wheel 10 to the wheels 6 via the pinion 4 and the rack 5 so as to change the steering angle, a steering assist force can be applied. As shown in FIG. 2, the drive shaft 5 driven by the motor 8 attached to the housing 21
0 is supported by the housing 21 via bearings 62 and 63.

The steering shaft 3 includes a first shaft 3a connected to the steering wheel 2, a cylindrical second shaft 3b connected to the first shaft 3a by a pin 22, and an outer periphery of the second shaft 3b. And a third cylindrical shaft 3c that is rotatably fitted through a bush 25. Each shaft 3
The torsion bar 23 is inserted in the center holes of a, 3b, and 3c. One end of the torsion bar 23 is connected to the first shaft 3a and the second shaft 3b by the pin 22, and the other end is connected to the third shaft 3c by the pin 24. Thereby, the second shaft 3b and the third shaft 3c can be elastically rotated relative to each other in accordance with the steering torque. The second shaft 3b is supported via a bush 31 by a steering column 30 pressed into the housing 21. The third shaft 3c
Is supported by the housing 21 via bearings 26 and 27. The worm wheel 1 is mounted on the outer periphery of the metal sleeve 11 fitted on the outer periphery of the third shaft 3c.
0 is integrated. The sleeve 11 is press-fitted into the third shaft 3c or firmly fixed via a key or the like.

The torque sensor 7 includes a second shaft 3b
And a second detection ring 37 made of a magnetic material fixed to the third shaft 3c.
And a detection coil 33 that covers the space between the opposing detection rings 36 and 37. A plurality of teeth 36a provided on the end face of the first detection ring 36 along the circumferential direction;
7, an area facing the plurality of teeth 37a provided along the circumferential direction on the end surface of the second shaft 7 changes according to an elastic relative rotation corresponding to the steering torque of the second shaft 3b and the third shaft 3c.
Since the magnetic resistance to the magnetic flux generated by the detection coil 33 changes in accordance with the change, the steering torque can be detected based on the output of the detection coil 33. The torque sensor 7 may have a known configuration. According to the signal corresponding to the detected steering torque, the motor 8
The rotation of the motor 8 is transmitted to the steering shaft 3 via the worm 9 and the worm wheel 10, and a steering assist force is applied.

The worm wheel 10, which is a reduction gear for rotating the motor 8, is made of a synthetic resin material and is formed through an injection molding process. Aluminum stearate is added to the synthetic resin material of the worm wheel 10 by 0.1%.
% To 0.3% by weight. As an addition method, for example, when the worm wheel 10 is injection-molded, a synthetic resin material pellet as a raw material is immersed in liquid aluminum stearate, and thereby the pellet is coated with aluminum stearate.
The worm wheel 10 is formed by melting and injecting the pellet into a mold. The relative viscosity (VR) of the formed worm wheel 10 is preferably 100 or more and 300 or less as measured by a formic acid method. In this embodiment, the synthetic resin material is, for example, PA (polyamide) 6, PA66, PA46, PA12, PA1.
1, PA612, PA610, PA6T, PA6.6
T, PPA (polyphthalamide) high-viscosity nylon-based thermoplastic synthetic resin materials, and non-nylon-based PPS
Thermoplastic synthetic resin materials such as (polyphenylene sulfide), PEEK (polyether ether ketone), and PBT (polybutylene terephthalate) can be used.

At the time of the injection molding, the sleeve 11
The worm wheel 10 is molded and integrated with the sleeve 11 by injecting the synthetic resin material into the molding die in which the is inserted. After the molding, finishing of the teeth of the worm wheel 10 and the like are performed by machining. A tooth row is formed on the outer periphery of the sleeve 11, which is a joint portion with the worm wheel 10, and the synthetic resin material constituting the worm wheel 10 is filled between the teeth 11a, so that the worm wheel 10 and the sleeve 11 , The relative rotational displacement of the shaft center and the relative displacement in the radial direction are prevented.
In addition, since the worm wheel 10 is formed so as to embrace both end surfaces of the sleeve 11, the axial relative displacement between the worm wheel 10 and the sleeve 11 is prevented.

According to the above configuration, the worm wheel 10 for transmitting the rotation of the motor 8 is made of a synthetic resin material, so that the weight and noise can be reduced. By adding aluminum stearate to the synthetic resin material forming the worm wheel 10, the worm wheel 1
0, the load acting upon rotation transmission is reduced, fatigue strength and wear resistance are improved, and the life can be extended. Moreover, since the amount of aluminum stearate added to the synthetic resin material is as small as 0.3% by weight or less, there is almost no reduction in the molecular weight of the synthetic resin material.
The gear's tensile strength required to transmit rotational torque,
Characteristics such as bending strength and elastic modulus can be maintained. When the amount of aluminum stearate added is 0.1% by weight or more, lubricity can be sufficiently improved. Further, by setting the relative viscosity of the formed worm wheel to 100 or more, the root strength and fatigue strength of the worm wheel can be improved, and by setting the relative viscosity to 300 or less, formability can be ensured. In addition, a worm wheel made of high-viscosity nylon has a high root strength, and performance with respect to water absorption and thermal deterioration does not decrease even if the viscosity increases. The molding cost can be reduced by injection molding the worm wheel 10 from a synthetic resin material. Further, since the synthetic resin material does not contain the reinforcing fiber, there is no problem that the fatigue strength is reduced even if the teeth are machined.

The present invention is not limited to the above embodiment. For example, instead of the worm and the worm wheel in the above embodiment, another gear such as a bevel gear or a hypoid gear may be used as the drive gear and the driven gear. Also,
The gear of the present invention is not limited to the gear used as the driven gear in the electric power steering device as in the above embodiment, and any gear that meshes with another gear to form a rotation transmission gear mechanism may be used. Can be used.

[0017]

Example 1 A conventional sample material formed from pure PA66 to which aluminum stearate was not added, and PA6 to which 0.1% by weight of aluminum stearate was added.
Suzuki-type friction and wear test was performed on the sample material of the present invention molded from No. 6 under the same conditions. As a result, while the wear amount of the conventional sample material was 130 mg and the friction coefficient was 0.43, the wear amount of the sample material of the present invention was 7 m.
g and coefficient of friction were 0.32. That is, it can be confirmed that the lubrication and wear resistance of the gear are significantly improved by applying the present invention.

[0018]

Example 2 A durability test was performed on the worm wheel of the above embodiment. A worm wheel injection-molded from pure PA66 to which aluminum stearate was not added and having a relative viscosity of 60 by the formic acid method was used as a comparative example, and injection-molded from PA66 to which 0.1% by weight of aluminum stearate was added. A worm wheel having a relative viscosity of 60 according to the method was injection-molded from Example 66, PA66 containing 0.1% by weight of aluminum stearate,
A worm wheel having a relative viscosity of 180 by the formic acid method was used as Example b. The durability test was performed by rotating the steering wheel back and forth at a constant rotation angle while applying a constant load from the wheel side. As a result, the worm wheel teeth of the comparative example failed in 15,000 reciprocating cycles, whereas the worm wheel teeth of Example a did not break until 36000 reciprocating cycles, and the worm wheel teeth of Example b rejected 100,000 reciprocating cycles. It did not break over the cycle. That is, it can be confirmed that the fatigue strength of the gear is significantly improved by applying the present invention.

[0019]

According to the present invention, a gear whose life can be extended by improving the fatigue strength and wear resistance of the teeth, and the provision of such gears provide quietness and light weight even when a large steering assist force is applied. It is possible to provide a long-life electric power steering device that can be realized.

[Brief description of the drawings]

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

FIG. 2 is a sectional view taken along line II-II of FIG.

[Explanation of symbols]

 6 wheel 8 motor (electric actuator) 9 worm (drive gear) 10 worm wheel (driven gear)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3D033 CA02 CA04 CA16 CA21 CA28 3J030 AA02 AC02 AC10 BA01 BA08 BC01 BC08 4J002 AA001 CL031 EG046 FD176 GM00 GM02

Claims (3)

[Claims] 1. A gear made of a synthetic resin material, wherein the synthetic resin material contains 0.1% by weight of aluminum stearate to 0.1% by weight.
A gear characterized in that 3% by weight is added. 2. The gear according to claim 1, wherein the formed gear has a relative viscosity of not less than 100 and not more than 300 by a formic acid method. 3. An electric power steering apparatus having a reduction gear mechanism for transmitting rotation of an electric actuator for generating a steering assist force to wheels via a driving gear and a driven gear meshing with the driving gear. An electric power steering apparatus comprising the gear according to claim 1.