When applying a steering angle to steered wheels of an automobile (generally, front wheels except for a special vehicle such as a forklift), a power steering device has been widely used as a device for reducing a force necessary for a driver to operate a steering wheel. As such power steering device, an electric power steering device configured to use an electric motor as an auxiliary power source has also been recently used. Various structures have been known for the electric power steering device. In any structure, auxiliary power of the electric motor is applied via a reducer to a rotary shaft configured to rotate in accordance with an operation of the steering wheel and to apply a steering angle to the steered wheels as it rotates. As the reducer, a worm reducer has been generally used. In the case of the electric power steering device using the worm reducer, a worm configured to rotate by the electric motor and a worm wheel configured to rotate together with the rotary shall are meshed with each other such that the auxiliary power of the electric motor can be transmitted to the rotary shaft. However, in the case of the worm reducer, when changing a rotational direction of the rotary shaft, an uncomfortable abnormal noise called as gear striking noise may be generated based on a backlash existing at the meshing portion of the worm and the worm wheel if any measure is not made.
As a structure of suppressing the generation of such gear striking noise, it has been considered to elastically press the worm toward the worm wheel by an elastic member such as a spring. FIGS. 21 and 22 show an example of the electric power steering device disclosed in Patent Document 1. A front end portion of a steering shaft 2 configured to rotate in a predetermined direction by a steering wheel 1 is rotatably supported within a housing 3, and a worm wheel 4 is fixed to this part. Worm teeth 5 configured to mesh with the worm wheel 4 are provided on an axially intermediate portion of a worm shaft 6, and both axial end portions of a worm 8 configured to rotate by an electric motor 7 are rotatably supported in the housing 3 by a pair of rolling bearings 9a, 9b such as deep groove ball bearings or the like. Further, a pressing piece 10 is externally fitted to a part of a tip portion of the worm shaft 6, which protrudes more than the rolling bearing 9a, and an elastic member such as a coil spring 11 is provided between the pressing piece 10 and the housing 3. By the coil spring 11, the worm teeth 5 provided on the worm shaft 6 are pressed toward the worm wheel 4 via the pressing piece 10. By this configuration, the backlash between the worm teeth 5 and the worm wheel 4 is suppressed to reduce the generation of the gear striking noise.
According to the conventional structure as described above, it is possible to suppress the generation of the gear striking noise at the meshing portion of the worm teeth 5 and the worm wheel 4. However, it is not possible to suppress an abnormal noise, which is to be generated at a coupling portion between a tip portion of an output shaft 12 of the electric motor 7 and a base end portion of the worm shaft 6. This is described as follows. According to the structure shown in the drawing, in order to couple the tip portion of the output shaft 12 of the electric motor 7 and the base end portion of the worm shaft 6 such that torque can be transmitted, the base end portion of the worm shaft 6 is formed with a spline hole 13 opening to a base end surface of the worm shaft 6. The tip portion of the output shaft 12 is formed with a spline shaft portion 14. The spline shaft portion 14 and the spline hole 13 are spline-engaged. Thereby, the output shaft 12 and the worm shaft 6 are coupled such that the torque can be transmitted.
If the spline shaft portion 14 and the spline hole 13 are spline-engaged without a circumferential gap (without the backlash), an abnormal noise is not generated at the coupling portion (the spline engagement portion) between the tip portion of the output shaft 12 and the base end portion of the worm shaft 6. However, in the actual situation, the backlash exists at the spline engagement portion. Particularly, when the backlash between the worm teeth 5 and the worm wheel 4 is suppressed by the structure as shown in FIG. 22, it is not possible to completely remove the backlash of the spline engagement portion because it is necessary to allow swingable displacement of the worm shaft 6. That is, there is a room for improvement for suppressing the abnormal noise.
Patent Document 2 discloses a structure where the output shaft of the electric motor and the worm shaft are coupled via a metallic power transmission member having a cylinder shape such that the worm shaft can be smoothly swingably displaced. In the invention disclosed in Patent Document 2, since the worm shaft is swingably displaced, the backlash exists at spline engagement portions between spline shaft portions (male spline) provided at both end portions of the power transmission member and spline holes (female spline) provided at respective end portions of the worm shaft and the output shaft of the electric motor, respectively. Therefore, there is a room for improvement in the aspect of suppressing the abnormal noise to be generated when changing the rotational direction of the rotary shaft.
Patent Document 3 discloses a star-shaped elastic shaft joint configured to couple a worm shaft of a worm gear mechanism and a motor shaft of an electric motor, which are concentrically positioned, where a star-shaped elastic member is interposed between a first joint member provided at an end of the motor shaft and a second joint member provided at an end of the worm shaft. Patent Document 4 discloses a shaft for a constant-velocity universal joint in which a low torque transmission portion having an elastic member and a high torque transmission portion having a gap are provided, and the elastic member and the gap are interposed between a concave groove of a first shaft member and a projection of a second shaft member inserted into a cylindrical part of the first shaft member. Patent Documents 3 and 4 disclose the techniques of suppressing rattling between the two shafts to reduce the vibrations or noise by the above configurations. However, since the elastic member is disposed at an axially overlapping position of the two shafts without using a coupling, an allowance of deviation between the two shafts at assembling is small, so that there is a room for improvement in the assembling efficiency.