A universal joint for use in an automotive drive shaft or steering assembly, for example, is required of muddy-water resistance because the universal joint is exposed to splashes of muddy water from tires or road surface. FIG. 13 is a front view (partly in section) showing a spider joint 10 as a principal member constituting such a universal joint. Referring to the figure, a bearing cup 3 is assembled on each of four races 21 of a cross shaft 20. The bearing cup 3 is provided with needle rollers 32 on an inside surface of a cup 31, so that the cup 31 is free to rotate relative to the race 21. The cup 31 is formed with a circumferential groove 3a in an outer peripheral surface thereof (There is also known a cup free from the circumferential groove 3a). A ring-like seal 40 is attached to an end of the bearing cup 3, thereby sealing a gap between a neck 23 of the cross shaft 20 and the cup 31.
FIG. 14 is an enlarged view of a part XIV in FIG. 13 (similar structures disclosed in, for example, Japanese Unexamined Patent Publication No. 2002-106594, Japanese Unexamined Utility Model Publication No. S53 (1978)-88646 and Japanese Unexamined Patent Publication No. H11 (1999)-223223). Referring to the figure, the cross shaft 20 includes the neck 23 on a proximal side of the race 21, such that the seal 40 may be pressed against the neck 23. In the sectional shape shown in the figure, the neck 23 includes: a straight portion 231 defined by a straight line parallel to the central axis of the race 21 (three-dimensionally defined by a cylindrical surface); a neck slant portion 232 defined by a straight line inclined at a steep angle (e.g., approximately 75°) relative to the straight portion 231 (three-dimensionally defined by a conical surface); and a neck-R portion 233 defined by an arcuate line interposed between these portions. On the other hand, the seal 40 includes: a metal annular body 41 press-inserted and fixed in the cup 31; and a rubber seal body 43 formed integrally with the metal annular body 41. Although the seal body 43 is depicted in a shape of a free state, the seal body is actually elastically deformed as pressed against the cross shaft 20.
The seal body 43 Includes: an axial lip 43a pressed against the neck slant portion 232; and a radial lip 43r pressed against the straight portion 231. In this seal 40, an extension direction of the axial lip 43a is angled at about 25°, for example, relative to the central axis of the race 21, whereas an extension direction of the radial lip 43r is angled at about −40°, for example, relative to the central axis of the race.
The above cross shaft 20 may be assembled with unillustrated yokes as follows. The race 21 is inserted in a hole of the yoke and then, the bearing cup 3 (with the seal 40 press-inserted therein) is assembled on the race 21. Subsequently, a snap ring (not shown) is fitted in the circumferential groove 3a of each of the two bearing cups 3 on the opposite ends of one shaft of the cross shaft. Thus, the spider joint 10 is connected with one of the yokes. The other yoke is connected with the spider joint in the same way.
By making connection in the aforementioned manner, the bearing cup 3 is forcibly moved to a predetermined axial position relative to the cross shaft 20, so that the axial lip 43a is pressed against the neck slant portion 232 by a predetermined amount of interference. On the other hand, the radial lip 43r is pressed against the straight portion 231 irrespective of the interference. Thus is realized a sealing structure having muddy-water resistance.
In the conventional spider joint as described above, the axial position of the bearing cup 3 relative to the cross shaft 20 is determined by its positional relation with the yoke, while the amount of interference of the seal 40 is determined by the axial position of the bearing cup. However, the size tolerances and assembly tolerances of individual parts make it difficult to maintain the amount of interference of the seal 40 exactly at a fixed value. In actual fact, the seal is varied in the amount of interference. The axial lip 43a is pressed against the steeply inclined neck slant portion 232.
Therefore, even a minor axial displacement of the axial lip 43a leads to a significant variation of the way the axial lip 43a is pressed against the neck slant portion. This entails a problem that the seal suffers an inconsistent muddy-water resistant performance. On the other hand, there is no problem about the above reassure-contact variations of the radial lip 43r so long as the radial lip is pressed against the straight portion 231. In the case of the maximum allowable error, however, the radial lip may be pressed against the neck-R portion 233. With the radial lip pressed against the neck-R portion 233, the seal may fail to achieve an adequate sealing performance.