Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-10-05
2002-05-21
Hurley, Kevin (Department: 3619)
Motor vehicles
Steering gear
With electric power assist
C074S422000
Reexamination Certificate
active
06390230
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates 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 this assist torque to the rack and pinion mechanism of the steering system, as taught in, for example, Japanese Patent Laid-Open Publication (kokai) No. HEI-9-193815.
More specifically, the electric power steering system produces assist torque according to the steering torque by means of an electric motor, transfers this assist torque through small and large bevel gears to a rack and pinion mechanism comprising a pinion and rack shaft, and steers the steering wheels by means of this rack and pinion mechanism. The rack shaft is a round rod having a rack formed thereon at the place opposite the pinion. The pinion and rack both have involute teeth.
An automotive steering system also usually has a stopper mechanism for limiting the maximum turning angle of the steering wheels. More specifically, this 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.
The rack and pinion of the rack and pinion mechanism used in the electric power steering apparatus taught in Kokai HEI-9-193815 uses spur or helical gears. The tooth profile of these spur or helical gears is also involute. Helical gears are widely used in high load, high speed gear applications because they mesh more smoothly than spur gears and produce less vibration and noise.
Small, high load helical gears are also used in the rack and pinion mechanism of the above-noted electric power steering apparatus. Helical gears produce a constant thrust corresponding to torque because the tooth profile has a specific helix angle. The thrust produced during normal steering conditions is determined by the total torque acting on the pinion, that is, the steering torque applied by the driver plus the assist torque produced by the motor.
Once the rack shaft slides the above-noted specific distance, further movement is restricted by the stopper mechanism. When the rack shaft is thus stopped, the total torque acting on the pinion is greater than during normal steering, and high thrust corresponding to this total combined torque is at work. Note that maximum combined torque and maximum thrust are produced at this time.
The power transfer section, bearings, housing, and other components of the electric power steering apparatus must also be strong enough to withstand this maximum thrust. Increasing the strength of these components requires relatively high quality materials and relatively large components. The electric power steering apparatus itself thus becomes larger and more expensive, leaving room for improvement.
In the above-noted electric power steering apparatus the assist torque (assist force) produced by the motor is increased by a reduction mechanism comprising small and large bevel gears, and the increased assist force is converted to thrust assistance by the rack and pinion mechanism. The assist force of the motor is converted to rack output at the combined efficiency &eegr;
T
=&eegr;
G
×&eegr;
R
where &eegr;
G
is the transfer efficiency of the reduction mechanism and &eegr;
R
is the transfer efficiency of the rack and pinion mechanism. The product of motor assist force and (1−&eegr;
T
) is output loss, which is converted to parts wear and heat, and contributes to a drop in system durability and output due to heat.
The effect of output loss is particularly great, and it is therefore desirable to improve transfer efficiency &eegr;
G
and transfer efficiency &eegr;
R
, in electric power steering systems that convert motor assist force from a high output motor to rack thrust by way of a reduction mechanism and rack and pinion mechanism.
SUMMARY OF THE INVENTION
It is therefore a first object of the present invention is to provide a compact, low cost electric power steering apparatus having a rack and pinion mechanism with durability sufficient to withstand the torque load of motor inertia.
A second object of the present invention is to provide particularly technology for improving the transfer efficiency of the rack and pinion mechanism.
A third object of the present invention is to provide an electric power steering apparatus having a rack and pinion mechanism with sufficient strength relative to motor inertia by maintaining good mesh between the pinion and rack.
To achieve the above objects, an electric power steering apparatus according to the present invention has a motor for producing an assist torque in correspondence with a steering torque, a rack and pinion mechanism for a steering system, and a geared reduction mechanism for transferring the assist torque to the rack and pinion mechanism. The pinion and rack of the rack and pinion mechanism are both helical gears. The helix angle of the pinion is less than the helical gear friction angle. One of the helical gears has a tooth profile wherein at least the addendum is a circular arc substantially centered on the reference pitch line. The other of the helical gears has a tooth profile wherein at least the dedendum is a circular arc practically centered on the reference pitch line.
By using helical gears, the rack and pinion mechanism can transfer higher torque than a conventional spur gear.
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, torque 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. Thrust acting on the input shaft is therefore low, and thrust acting on the bearings supporting the input shaft and the geared reduction mechanism linked to the input shaft is low. It is therefore not necessary to increase the strength of the input shaft, bearings, and geared reduction mechanism even though helical gears are used. These components can therefore be downsized and less expensive.
The tooth profile of the pinion and rack of the 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, and 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 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 combined torque achieved by adding the assist torque from the motor to the steering torque, even when this combined torque is greater than during normal conditions.
The present invention can thus provide a c
Ohyama Yasuharu
Shimizu Yasuo
Terada Yasuhiro
Watanabe Katsuji
Yamawaki Shigeru
Honda Giken Kogyo Kabushiki Kaisha
Hurley Kevin
Merchant & Gould P.C.
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