FIG. 7 shows a general type of steering mechanism of an automobile. In FIG. 7, the symbols a and b represent telescopic shafts. The telescopic shaft a is constructed by spline-fitting a male shaft and a female shaft to each other. This type of telescopic shaft is, however, required to have performance that absorbs an axis-directional displacement occurred when the automobile travels but transmits neither the displacement nor vibrations onto a steering wheel. This performance is required generally in such a structure that a vehicle body takes a sub-frame structure, a portion c for fixing an upper portion of the steering mechanism and a frame e to which a steering rack d is fixed are different units, and the frame e and the steering rack d are fixed by fastening through an elastic member f such as rubber interposed therebetween. Further, there is other case in which a worker, when fastening a steering shaft joint g to a pinion shaft h, temporarily contracts the telescopic shaft and thereafter fits and fastens the joint g to the pinion shaft h, and therefore a telescopic function is needed. Moreover, the telescopic shaft b provided in the upper portion of the steering mechanism is also constructed by spline-fitting the male shaft and the female shaft to each other. This type of telescopic shaft b is required to have a function of shifting a position of a steering wheel i in the axial direction in order to obtain an optimal position when a driver drives the car and adjusting this position, and hence an axis-directional telescoping function is requested of this shaft. In all cases described above, the telescopic shaft is requested to reduce backlash noises at the spline-fitting portion, a feeling of backlash on the steering wheel and a slide resistance when sliding in the axial direction.
Such being the case, according to European Patent Application Laid-Open Publication No. EP1078843A1, plural sets of torque transmission members (cylindrical members) are fitted in plural sets of axis-directional grooves formed in an outer peripheral surface of the male shaft and in an inner peripheral surface of the female shaft.
Each set of torque transmission members (the cylindrical members) are constructed of a plurality of needle rollers arranged side by side in the axial direction.
With this arrangement, when the torque is not transmitted, the backlash between the male shaft and the female shaft can be prevented, whereby the male shaft and the female shaft can slide in the axial direction with a stable slide load without any backlash. Further, when transmitting the torque, the male shaft and the female shaft can transmit the torque in a high-rigidity state by preventing the backlash in a rotating direction.
According to European Patent Application Laid-Open Publication No. EP1078843A1, however, each set of torque transmission members (the cylindrical members) are constructed of the plurality of needle rollers arranged side by side in the axial direction, and therefore, the assembly is complicated enough to take an excessive period of assembling time.
Accordingly, if the number of the needle rollers in each row is set to one or two, accuracy of straightness of each of the male shaft and the female shaft must be enhanced. This inevitably involves machining the grooved portions and induces a rise in manufacturing cost. Hence, enhanced performance (which reduces the backlash in the peripheral direction while restraining the slide resistance low) and a decrease in cost can not be compatible.
Moreover, if the accuracy of straightness of each of the male shaft and the female shaft is not enhanced, a high surface pressure might occur in the vicinity of an end face of the needle roller when transmitting the torque.
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