JP2002054695A - Electric power steering system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power steering system that can suppress a degradation due to heat in the mechanical characteristics of a reduction gear mechanism for transmitting the rotation of an electric actuator for steering auxiliary force generation, and withstand a high-temperature operation for a long time. SOLUTION: The rotation of an electric actuator 8 for steering auxiliary force generation is transmitted to wheels 6 to change the steering angle, via a reduction gear mechanism having a driving gear 9 of metal and a follower gear 10 meshed with the driving gear 9. The follower gear 10 is formed from a material using a base material of synthetic resin to which a filler larger in thermal conductivity than the base material is added. The thermal conductivity of the follower gear 10 is 0.5 W/m.K or more. The follower gear 10 is integrated on the periphery of a metallic member 11.

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 apparatus for transmitting rotation of an electric actuator for generating a steering assist force to wheels via a reduction gear mechanism such that a steering angle changes.

[0002]

2. Description of the Related Art Conventionally, in an electric power steering apparatus for transmitting the rotation of an electric actuator for generating a steering assist force to a wheel via a reduction gear mechanism so as to change a steering angle, the reduction gear mechanism is provided with a metal worm. By using a worm wheel made of a synthetic resin that meshes with the worm, weight reduction and noise reduction are achieved.

[0003]

Friction heat is generated at the meshing portion between the worm and the worm wheel.
Then, since the worm wheel is made of a synthetic resin, the worm wheel is easily deformed by the frictional heat, and the mechanical characteristics are reduced. As a result, the upper limit of the usable ambient temperature of the electric power steering device and the upper limit of the load torque of the reduction gear mechanism are reduced, which hinders an increase in the number of revolutions or a reduction in output of the electric actuator. The service life of the lubricating grease supplied to the vehicle is shortened, and the lubricating grease cannot withstand long-time high-temperature operation.

SUMMARY OF THE INVENTION An object of the present invention is to provide an electric power steering apparatus which can solve the above-mentioned problem by focusing on the improvement of heat dissipation of a reduction gear mechanism, which has not been focused on conventionally.

[0005]

SUMMARY OF THE INVENTION The present invention relates to an electric power steering apparatus for transmitting the rotation of an electric actuator for generating a steering assist force to a wheel via a reduction gear mechanism such that the steering angle changes. The mechanism has a metal driving gear and a driven gear that meshes with the driving gear, and the driven gear is a filler made of a synthetic resin base material having a higher thermal conductivity than the base material. And the thermal conductivity of the driven gear is 0.5 W / (m ·
K) and the driven gear is integrated with the outer peripheral side of the metal member. The material forming the driven-side gear of the present invention has a lower thermal conductivity than metal, so a filler having a higher thermal conductivity than the base material is added to a synthetic resin base material having poor heat dissipation efficiency. The thermal conductivity of the driven gear is 0.5 W / (m ·
K) or more. Thereby, the frictional heat generated at the meshing portion between the driving side gear and the driven side gear can be efficiently radiated to the metal member fitted with the driven side gear. Therefore, deformation of the driven gear due to the frictional heat can be suppressed, and a decrease in the mechanical characteristics of the reduction gear mechanism can be reduced.

[0006] The filler is preferably in the form of short fibers or whiskers. Thus, the strength of the driven gear can be improved by the filler. In this case, the material of the filler is preferably any one of SiC, AlN, and BeO. As a result, the life of the reduction gear mechanism can be extended.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. The electric power steering device 1 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 to move a rack 5 meshing with the pinion 4 and to move the rack 5 The movement is transmitted to the wheel 6 via a tie rod, a knuckle arm or the like (not shown) to change the steering angle.

In order to apply a steering assist force corresponding to 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 and a worm (drive side 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 via a sleeve (metal member) 11. The steering shaft 3, the worm 9 and the sleeve 11 are made of metal such as steel. The worm 9 and the worm wheel 10 constitute a reduction gear mechanism. By transmitting the rotation of the motor 8 from the steering shaft 3 to the wheels 6 so that the steering angle changes, a steering assist force can be applied. As shown in FIG.
The drive shaft 50 driven by the motor 8 attached to the housing 21 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
A torsion bar 23 is inserted as an elastic member along the centers of a, 3b, and 3c. 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
The housing 21 is supported via bearings 26 and 27. The worm wheel 10 is integrated with the outer peripheral side of the sleeve 11 fitted on the outer periphery of the third shaft 3c. The sleeve 11 may be press-fitted into the third shaft 3c, or may be fixed via a key or the like. Further, a torque limiter mechanism may be provided between the sleeve 11 and the third shaft 3c so that the worm wheel 10 and the steering shaft 3 rotate relatively when an excessive torque acts.

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 face changes in accordance with 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
Is driven to rotate.

The worm wheel 10 is formed from a material obtained by adding a filler having higher thermal conductivity than a base material made of a synthetic resin to a base material made of synthetic resin, and is injection-molded in the present embodiment. In this embodiment, the base material is PA (polyamide).
6, PA66, PA46, PA12, PA11, PA1
2, PPA (polyparabanic acid), PA6T, PA6.6
It is a nylon synthetic resin material such as T. By performing injection molding with the sleeve 11 inserted into the molding die of the worm wheel 10, the worm wheel 10 is integrated with the outer peripheral side of the sleeve 11, and then the finishing of the teeth of the worm wheel 10 is performed by machining. Is going. The kind and amount of the filler are determined so that the thermal conductivity of the worm wheel 10 is 0.5 W / (m · K) or more.

As the filler, for example, silver (Ag),
Copper (Cu), gold (Au), nickel (Ni), iron (F
e), aluminum (Al), carbon (C), silicon carbide (SiC), aluminum nitride (AlN), beryllium oxide (BeO), and the like. Further, the form of the filler is, for example, powder, flexible long fiber,
Short fibers or whiskers can be used, and short fibers or whiskers are preferable from the viewpoint of improving the strength of the worm wheel 10. When the short fibers or whiskers are used, the material of the filler is SiC, AlN, By using any one of BeO, the life of the reduction gear mechanism can be extended.

Table 1 below shows a plurality of materials that can be used as fillers, and shows an example of the thermal conductivity of each material and the composition ratio (weight percent) when the filler is added to the base material. . The lower limit of the composition ratio may be set so as to increase the thermal conductivity of the worm wheel 10 to achieve a good heat radiation effect, and the upper limit of the composition ratio may be set so as to maintain good formability. .

[0015]

[Table 1]

The material forming the worm wheel 10 is made of a synthetic resin base material having a low thermal conductivity as compared with a metal and thus having a low heat radiation efficiency, and a filler having a higher heat conductivity than the base material. By being added, the thermal conductivity of the worm wheel 10 is set to 0.5 W / (m · K) or more. Thereby, the frictional heat generated at the meshing portion between the worm 9 and the worm wheel 10 is transferred to the metal sleeve 11 to which the worm wheel 10 is fitted.
Furthermore, heat can be efficiently radiated to the metal steering shaft 3 into which the sleeve 11 is fitted. Therefore, deformation of the worm wheel 10 due to the frictional heat can be suppressed, and a decrease in the mechanical characteristics of the reduction gear mechanism can be reduced.

The differences in the maximum service temperature, load torque, motor characteristics, and life depending on the presence or absence of the filler to the worm wheel 10 were as follows. The worm wheel 1 was added with 25% by weight of powdered SiC as a filler.
The base material of No. 0 was PA66, and the thermal conductivity of the worm wheel 10 was 0.66 W / (m · K). For example, when the module of the worm 9 and the worm wheel 10 is 1.65, the load torque is 35 N · m, and the other conditions are the same, the upper limit use temperature is 65 ° C. without adding the filler. 140 ° C. was achieved by the addition of the material. Also,
The upper limit of the load torque of the reduction gear mechanism can be increased. For example, the module of the worm 9 and the worm wheel 10 is 1.6
5 and the other conditions were the same, the upper limit of the load torque in use at room temperature was 35 N ·
m was increased to 60 N · m by adding a filler. Further, the output of the motor 8 can be reduced. For example, when the module of the worm 9 and the worm wheel 10 is set to 1.65 and other conditions are the same, the rated current of the motor 8 is such that the filler is not added. What required 50A was sufficient, but 30A was sufficient by adding a filler. When motors having the same output are used, the motor 8
Can be increased to only 100 rpm by adding the filler, whereas the rotation speed of can be reduced to only 60 rpm when the filler is not added. Furthermore, the material of the filler is SiC, AlN,
When it is any of BeO, the upper limit of the usable ambient temperature of the electric power steering device 1 can be increased, and the fatigue life of the worm wheel 10 and the life of the lubricating grease supplied to the meshing portion are prolonged. For example, in a service test under the same conditions, it was possible to withstand only 520,000 cycles of operation without adding a filler, but up to 700,000 cycles by adding a filler. Drivable.

When the base material is PA66, the worm wheel 10 has a thermal conductivity of 0.66 W / (m · K) when 40% by weight of SiC is added as a powdery filler, and a service life of 700,000. Cycle and AlN 40
When added by weight, the thermal conductivity is 0.63 W / (m · K),
The service life cycle becomes 620,000 cycles, and BeO becomes 4
When 0% by weight is added, the thermal conductivity becomes 0.64 W / (m ·
K), the durable operation cycle was 660,000 cycles, and the durable atmosphere temperature could be 120 ° C. This is because the increase in thermal conductivity suppresses a rise in temperature, increases the load at which deformation starts, and extends the time until fatigue failure. On the other hand, in a worm wheel in which the base material is PA66 and only glass fiber, which is a general reinforcing material, is added, the thermal conductivity is 0.35 W when the addition amount is 25% by weight.
/ (M · K), the durable atmosphere temperature was lowered. When the amount of glass fiber added was 33% by weight, the thermal conductivity was increased and the durable atmosphere temperature could be set to 120 ° C. The glass fiber is easier to grind the worm 9 than SiC, AlN, and BeO. The base material is PA6
When only a solid lubricant such as polytetrafluoroethylene (PTFE) and molybdenum disulfide (MoS ₂ ) was added to the worm wheel set to No. 6, the load of the reduction gear mechanism was reduced by setting the addition amount to 25% by weight. Although the torque could be set to 80 N · m or the number of rotations of the worm wheel 10 could be increased, the thermal conductivity was low at 0.33 W / (m · K), which was longer than SiC, AlN, and BeO. Cannot withstand the high temperature operation, and the number of operation cycles was 50,000, and the backlash had already been excessively increased, so that it could not be used before it was destroyed. Therefore, the material of the filler is SiC, Al
N and BeO are more preferable.

The present invention is not limited to the above embodiment. For example, a synthetic resin material other than a nylon-based material may be used as the base material of the worm wheel. Further, the driving side gear and the driven side gear may be those that constitute a reduction gear mechanism, and may be, for example, bevel gears.

[0020]

According to the present invention, the mechanical power of the reduction gear mechanism for transmitting the rotation of the steering assist force generating electric actuator is prevented from being reduced by heat, and the electric power capable of withstanding a long-time high-temperature operation is suppressed. A steering device can be provided.

[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 9 worm 10 worm wheel

Claims (3)

[Claims] 1. An electric power steering device for transmitting rotation of an electric actuator for generating a steering assist force to a wheel via a reduction gear mechanism such that a steering angle is changed, wherein the reduction gear mechanism includes a metal driving gear. And a driven gear that meshes with the driving gear, and the driven gear is formed from a material obtained by adding a filler having a higher thermal conductivity than the base material made of a synthetic resin base material, An electric power steering apparatus, wherein the driven gear has a thermal conductivity of 0.5 W / (m · K) or more, and the driven gear is integrated with an outer peripheral side of a metal member. 2. The electric power steering apparatus according to claim 1, wherein the filler is in the form of short fibers or whiskers. 3. The material of the filler is SiC, AlN, Be.
The electric power steering apparatus according to claim 2, wherein the electric power steering apparatus is any one of O.