1. Field of the Invention
The invention relates to a universal joint that is used in, for example, a steering system of an automobile and a yoke that constitutes the universal joint.
2. Description of Related Art
For example, a steering system of an automobile, which is described in Japanese Patent Application Publication No. 2009-299706 (JP 2009-299706 A), includes a steering wheel, a steering shaft coupled to the steering wheel, an intermediate shaft coupled to the steering shaft, and a steering gear unit coupled to the intermediate shaft. As the steering wheel is steered, the steering torque (the rotational force of the steering wheel) is transmitted to the steering gear unit via the steering shaft and the intermediate shaft sequentially, and the steering gear unit turns wheels on the basis of the steering torque. The steering shaft and the intermediate shaft are generally not aligned along the same straight line. In the steering system described in JP 2009-299706 A, the steering shaft and the intermediate shaft are coupled to each other by a universal joint.
The universal joint includes a yoke coupled to an end portion of the steering shaft, a yoke coupled to an end portion of the intermediate shaft, and a joint cross. Each of the yokes has a proximal portion and a pair of arm portions that extend from an axial distal end periphery of the proximal portion. The proximal portion has a cylindrical shape having an insertion hole through which the end portion of the steering shaft or the end portion of the intermediate shaft is inserted. A slit is formed at one portion in the circumference of the proximal portion, and extends in the axial direction of the proximal portion to split the one portion in the circumference. A pair of flanges are respectively formed integrally at the proximal portion on both sides of the slit. A bolt insertion hole is formed in each of the flanges, and extends in a direction perpendicular to the axial direction of the proximal portion.
In the yoke, when the end portion of the steering shaft or intermediate shaft is inserted through the insertion hole of the proximal portion and then a bolt is assembled into the bolt insertion holes of the flanges, the flanges deform in mutually approaching directions such that the width of the slit is reduced. Accordingly, the insertion hole is narrowed. Therefore, the end portion of the steering shaft or intermediate shaft, which is inserted in the insertion hole, closely contacts the proximal portion, so the yoke and the steering shaft or intermediate shaft are firmly coupled to each other.
A circular hole is formed in each arm portion of the yoke. Among four shaft portions of the joint cross, a pair of the shaft portions arranged along the same straight line are respectively inserted through the circular holes of the arm portions of the steering shaft-side yoke via bearings, and a remaining pair of the shaft portions are respectively inserted through the circular holes of the arm portions of the intermediate shaft-side yoke via bearings. By so doing, the joint cross is rotatably supported by the arm portions of the steering shaft-side yoke and the arm portions of the intermediate shaft-side yoke. Thus, as the steering wheel is steered, steering torque of the steering wheel is initially transmitted to the steering shaft to cause the steering shaft to rotate and is subsequently transmitted from the steering shaft to the intermediate shaft via the joint cross to cause the intermediate shaft to rotate.
In JP 2009-299706 A, there are types of yoke, that is, a type in which bolt insertion holes extend parallel to a direction in which a pair of arm portions face each other (referred to as type 1, and see FIG. 1 of JP 2009-299706 A) and a type in which bolt insertion holes extend perpendicular to a direction in which a pair of arm portions face each other (referred to as type 2, and see FIG. 13 of JP 2009-299706 A). In the case where a pair of yokes in a universal joint are formed of only one of type 1 and type 2, in a finished universal joint, a direction in which bolt insertion holes extend in one of yokes and a direction in which bolt insertion holes extend in the other one of the yokes deviate by 90° and do not coincide with each other. Therefore, in the case where a universal joint is coupled between two shafts, that is, a steering shaft and an intermediate shaft, when the corresponding shafts are inserted through the insertion holes of the yokes and bolts are assembled into the bolt insertion holes of the yokes, the bolt insertion holes of each yoke are not seen from the same side, and the bolt insertion holes of only one of the yokes are seen. Then, it is required to, after the bolt is assembled into the bolt insertion holes of the one of the yokes, shift the universal joint by 90° around its axis as a whole to allow the bolt insertion holes of the other one of the yokes to be seen, and then assemble the bolt into the bolt insertion holes of the other one of the yokes. That is, it is not possible to assemble the bolt into the bolt insertion holes of each yoke at a time, so it is inconvenient.
On the other hand, in the case where a universal joint is formed using a yoke of type 1 and a yoke of type 2 one by one, in a finished universal joint, a direction in which bolt insertion holes extend in one of the yokes and a direction in which bolt insertion holes extend in the other one of the yokes coincide with each other. Therefore, in the case where the universal joint is coupled between two shafts, when the bolt is assembled into the bolt insertion holes of each yoke, the bolt insertion holes of each yoke face the same side, so it is possible to assemble the bolt into the bolt insertion holes of each yoke at a time. However, in this case, it is necessary to prepare yokes of two types, that is, type 1 and type 2, so cost increases in manufacturing and control of yokes.