Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-10-06
2003-04-08
Morris, Lesley D. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C074S66500G, C074S3880PS, C074S458000
Reexamination Certificate
active
06543569
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an electric power steering apparatus, and more particularly to an improvement in a rack-and-pinion mechanism used in such an electric power steering apparatus.
2. Description of the Related Art
Electric power steering systems are commonly used to make steering easier by reducing a force needed to turn a steering wheel (referred to as the steering force below). Electric power steering systems use an electric motor to produce assist torque according to the steering torque, and transfer the assist torque to the rack-and-pinion mechanism of the steering system, as taught in, for example, Japanese Patent Laid-Open Publication (kokai) No. SHO-61-160359.
The disclosed electric power steering apparatus includes a single rack shaft designed for meshing engagement with first and second pinions to thereby steer right and left steered wheels. Steering torque produced by turning a steering wheel is transmitted via the first pinion to the rack shaft while an assist torque produced by an electric motor is transmitted via the second pinion to the rack shaft. The rack shaft steers the steered wheels by the combined steering torque and assist torque. In the conventional electric power steering apparatus, since the rack-and-pinion mechanism for transmitting the steering torque is separated from the rack-and-pinion mechanism for transmitting the assist torque, each rack-and-pinion mechanism can advantageously be made to have smaller strength than a unified rack-and-pinion mechanism.
An automotive steering system also usually has a stopper mechanism for limiting the maximum turning angle of the steering wheels. More specifically, the stopper mechanism has a rack end stopper attached at each longitudinal end of the housing in which the rack shaft is slidably disposed, and a ball joint, for example, is attached to each end of the rack shaft. When the rack shaft slides a specific distance, the ball joint contacts the rack end stopper. The maximum turning angle of the steering wheels is thus limited by limiting the movement of the rack shaft.
As the rack shaft is slid a specific distance, its further movement is restricted by the stopper mechanism. Upon stoppage of the rack shaft, the second pinion is fed with a torque proportionate to the square of a reduction gear ratio due to motor inertia and is thus supplied with a larger assist torque than it is in a normal operation. The assist torque becomes maximum at this time and larger than the steering torque. Consequently, the second rack-and-pinion mechanism needs to have strength sufficient to withstand the maximum torque. For this purpose, one may propose to make each component have increased strength but this requires a larger rack-and-pinion module and high quality materials, thereby rendering the rack-an-pinion mechanism large in size and expensive.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an electric power steering apparatus including a first rack-and-pinion mechanism for transmitting a steering torque and a separate second rack-and-pinion mechanism for transmitting an assist torque with strength and durability sufficient to with stand a torque load of motor inertia.
According to an aspect of the present invention there is provided an electric power steering apparatus which comprises: a rack shaft for steering wheels, the rack shaft having a first rack and a second rack provided separately axially thereof; a first rack-and-pinion mechanism for transferring a steering torque, produced by turning a steering wheel, to the rack shaft; an electric motor for producing an assist torque in accordance with the steering torque; and a second rack-and-pinion mechanism, comprised of a pinion and the second rack, for transferring via a geared reduction mechanism the assist torque to the rack shaft, the pinion and rack of the second rack-and-pinion mechanism both being helical gears, one of the helical gears having a tooth profile wherein at least an a dedendum is a circular arc generally centered on a reference pitch line, the other of the helical gears having a tooth profile wherein at least a addendum is a circular arc generally centered on the reference pitch line.
With the rack and pinion being formed of helical gears, the second rack-and-pinion mechanism can transfer a larger torque than a conventional spur gear.
The tooth profile of the pinion and rack of the second rack-and-pinion mechanism of the present invention is a curved arc. Because a conventional involute tooth profile is convex, meshing in a gear pair is contact between two convex surfaces. With the curved arc tooth profile of the present invention, however meshing in a gear pair occurs as contact between a convex surface and a concave surface. The contact area is thus increased, whereby contact pressure is reduced to approximately ⅙ that of an involute tooth profile. By thus using a curved arc tooth profile in the rack and pinion of the second rack-and-pinion mechanism, surface fatigue strength, bending strength, and bending fatigue strength are greater than with an involute tooth profile. This means that the rack-and-pinion mechanism of our invention can transfer the assist torque sufficiently, even when the assist torque from the motor is larger than that in a normal operation of the motor.
Since it transfers only a driver's steering torque, the first rack-and-pinion mechanism is not fed with a steering torque extremely large compared with one in normal driving conditions, even when the rack shaft is stopped. It is thus not necessary to increase rigidity of the mechanism.
When the steered wheels turn right or left to the maximum steering angle and the rack shaft meets the rack end stopper, that is, when the rack shaft moves to the end of its range of movement, the rack drops immediately. Because the torque at this time is impact torque and not static torque, it is significantly higher than during normal driving conditions. However, because the helix angle of the helical gear pinion is less than the helical gear friction angle, thrust does not act on the pinion. Thrust acting on the pinion is only an extremely weak force occurring during normal conditions when the rack is not stopped at the right or left end of its range.
The geared reduction mechanism of the present invention is preferably a combination of driver and driven gears in which the tooth surfaces of the driver gear and/or the tooth surfaces of the driven gear are coated with a low friction material coating, and the driver gear and driven gear mesh with no backlash. Coating with a low friction coefficient material can be achieved by imparting a coating made from a low friction coefficient material, or by impregnating the tooth surfaces with a low friction coefficient material.
By thus meshing driver gear and driven gear with no backlash, there is no play between the driver and driven gears, and impact torque due to motor inertia does not pass from the driver gear tooth surface to the driven gear tooth surface. Moreover, the tooth surfaces of one or both of the driver gear and driven gear are coated with a low friction coefficient material coating. By lowering the coefficient of friction between the tooth surfaces of the driver and driven gears by means of this coating, power transfer efficiency can be increased even though there is no play between the driver and driven gears.
It is further preferable to insert a torque limiter between the motor and the geared reduction mechanism to limit the transfer of assist torque exceeding a specific limit from the motor to the reduction mechanism. When the rack shaft hits the rack end stopper, excessive torque will not be produced as a reaction to the motor, and excessive torque will not be transferred to the load side.
It is yet further preferable to provide a steering torque sensor for detecting steering torque. Yet further preferably the steering torque sensor is a magnetostrictive sensor for detecting magnetostriction of the pinion shaft
Ohyama Yasuharu
Shimizu Yasuo
Terada Yasuhiro
Watanabe Katsuji
Yamawaki Shigeru
Honda Giken Kogyo Kabushiki Kaisha
Merchant & Gould P.C.
Morris Lesley D.
Yeagley Daniel
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