Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2003-01-31
2004-07-13
Lerner, Avraham (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
Reexamination Certificate
active
06761244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric power steering apparatus using an electric motor as a source for generating a steering assist force.
2. Description of Related Art
An electric power steering apparatus for vehicles is, for example, constructed so as to detect steering torque applied to an input shaft, based on a relative angular displacement between the input shaft connected to a steering wheel and an output shaft connected coaxially to the input shaft through a torsion bar, to drive a steering force assisting electric motor based on the detected torque, and to transmit the torque of this motor to a steering mechanism through a reduction gear mechanism. In such a structure, the operation of the steering mechanism corresponding to the rotation of the steering wheel is assisted by the rotation of the electric motor, and the labor of a driver for steering is reduced.
The reduction gear mechanism comprises a worm coupled to the driving shaft of the steering assisting electric motor, and a worm wheel meshed with this worm. This worm wheel is fitted and fixed at a midway point of a transmission path running from the output shaft to the steering mechanism.
Further, both ends of the worm are supported by a pair of rolling bearings to improve the rotating performance of the worm. However, the rolling bearing has a thrust clearance between an inner ring and outer ring fitted with a plurality of rolling elements therebetween, and it is necessary to eliminate the thrust clearance for the following reasons. Specifically, it is necessary to eliminate the thrust clearance because the thrust clearance of the rolling bearing causes a backlash of the worm in the axial direction, and abnormal noise caused by the backlash leaks into the room of a vehicle and makes the driver uncomfortable.
For such reasons, there is provided a screw ring which is in contact with one end of the outer ring of one rolling bearing supporting one end of the worm and capable of being turned. By turning this screw ring, the outer ring and inner ring of the one rolling bearing are moved relatively in the axial direction, and also the inner ring and outer ring of the other rolling bearing are moved relatively in the axial direction through the worm, thereby eliminating the thrust clearance and backlash of the worm in the axial direction.
By the way, the worm of the electric power steering apparatus constructed as described above is supported so that the worm can not move in the axial direction relative to the rolling bearings that support both ends thereof. Therefore, in the case where the electric power steering apparatus is constructed so that the electric motor is rotated from an early stage of steering and steering is assisted by turning the steering wheel in the left or right direction from the steering neutral position, the steering assist is provided even when the steering wheel is turned at a small angle of around 1° during driving of the vehicle at high speed, causing a reduction in the steering feeling. For this reason, in general, the electric power steering apparatus is constructed so that the electric motor is not driven when the steering angle is as small as around but is driven when the steering angle exceeds a suitable steering angle.
In such a structure where the electric motor is not driven until the steering angle exceeds a suitable angle, however, in a steering range in which the electric motor is not driven, i.e., steering in a range near the steering neutral position, the steering force of the steering wheel is transmitted to the driving shaft of the electric motor through the input shaft, torsion bar, output shaft, worm wheel and worm sequentially, and consequently the driving shaft is rotated. As a result, load to rotate the driving shaft of the electric motor is applied to the steering wheel through the worm, worm wheel, output shaft, torsion bar and input shaft sequentially, and the steering load increases.
By the way, in order to decrease the steering load in the steering range in which the electric motor is not driven, for example, there is a known technique disclosed in Japanese Patent Application Laid-Open No. 11-43062 (1999). More specifically, this technique adopts a structure where two rolling bearings separated in the axial direction support the worm coupled to the driving shaft of the electric motor so that the worm is movable in the axial direction, two plate springs are provided between the worm and the inner rings of the two rolling bearings, and the inner rings are forced in the axial direction relative to the outer rings by the elastic restoring force of the plate springs. Accordingly, it is possible to eliminate the thrust clearances of the rolling bearings and restrain the movement of the worm in both directions along the axial direction, thereby solving the above problems.
In this structure, when the steering force of the steering wheel is transmitted from the worm wheel to the worm by steering in a steering range in which the electric motor is not driven, the worm overcomes the elastic restoring force of the plate springs by a component force applied to the worm in the axial direction and moves in the axial direction, the rotation angle of the worm becomes smaller, and the transmission power from the worm to the driving shaft of the electric motor is reduced.
However, in the structure as disclosed in the Japanese Patent Application Laid-Open No. 11-43062 (1999), two rolling bearings and two plate springs are necessary to eliminate the thrust clearances of the rolling bearings, resulting in a complicated structure.
Moreover, since the plate springs for restraining the movement of the worm in both directions along the axial direction also perform the function of eliminating the thrust clearances of the rolling bearings, the elastic restoring force of the plate springs needs to be set relatively high, and accordingly the effect of reducing the steering load in the steering range in which the electric motor is not driven is reduced.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made with the aim of solving the above problems, and it is an object of the present invention to provide an electric power steering apparatus capable of decreasing a steering load in a steering range in which a steering assisting electric motor is not driven, even when the electric power steering apparatus comprises a single rolling bearing for supporting a worm which is rotated by the steering assisting electric motor.
An electric power steering apparatus according to the first aspect of the present invention comprising a worm that is rotated by a steering assisting electric motor, bearing means for supporting the worm, and a worm wheel which meshes with the worm and is connected to a steering mechanism, for assisting steering by the rotation of the electric motor, characterized in that the bearing means comprises: a rolling bearing with an inner ring into which a shaft portion on one end side of the worm is inserted, for supporting the worm rotatably; a first resilient member disposed in contact with an inner circumference of the inner ring of the rolling bearing, and capable being extended in a axial direction of the worm by a force applied to the worm in the axial direction; and two second resilient members for restraining a movement of the worm in the axial direction, the second resilient members being held on the worm or the inner ring so as to be in contact with both ends of the inner ring of the rolling bearing.
In the electric power steering apparatus of the first aspect of the present invention, the first resilient member is in contact with the inner circumference of the inner ring of one rolling bearing, and the second resilient members are in contact with both ends of the inner ring of the rolling bearing. Therefore, with a force in the axial direction applied to the worm when steered in either the left or right direction from a steering neutral position, the first resilient member is extended, and either of the seco
Sano Osamu
Sasaki Hiroto
Darby & Darby
Koyo Seiko Co. Ltd.
Lerner Avraham
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