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Pulley ball bearing and pulley

The present invention has been devised in view of the aforementioned circumstances, and it is a technical object of the present invention to prolong the life of pulley ball bearings, and in turn, the life of the pulleys by increasing the sealed amount of grease at the optimum ratio by sufficiently securing the space volume between inner and outer rings while effectively solving the problems of shoulder stranded balls, and at the same time by reducing the expansion and deformation caused by large centrifugal forces at the time the outer ring revolves at high speed as much as possible by increasing the rigidity of the retainer is heightened.

In order to achieve the above-mentioned technical object, in a ball bearing for a pulley that is inter-fitted to a pulley main body having pulley circumferential surface coming into contact with a belt, the present invention is characterized by setting ball groove depths formed in an inner ring and an outer ring of the ball bearings so that a risk rate of shoulder stranded balls is approximately equal between the inner ring and the outer ring under pure thrust load.

In other words, the present invention has been devised by paying our attention to the point that the risk rate of the shoulder stranded contact ellipse based on contact of the ball at the time pure thrust load (pure axial load) acts, varies between the inner ring and the outer ring, in case where the depths of ball grooves of the inner ring and the outer ring are both made the same as in the conventional ones. In other words, this is based on paying attention to the point that in case where the depths of the ball grooves for the inner ring and the outer ring are made equal to each other, the permissible thrust load of the inner ring side that becomes the judgment standard of whether the ball becomes stranded on the shoulder of the inner ring or not, and the permissible thrust load of the outer ring side that becomes the judgment standard of whether the ball becomes stranded on the shoulder of the outer ring or not, are different.

The reason why the risk rate of the ball becoming stranded on the shoulder varies between the inner ring and the outer ring, is that the radius of curvature for the ball groove of the inner ring is smaller than the radius of curvature for the ball groove of the outer ring, and that the ball groove of the inner ring is convex with respect to the circumferential direction whereas the ball groove of the outer ring is concave with respect to the circumferential direction. In case where the depths of the ball grooves are the same for the inner ring and the outer ring, the risk rate of the ball being stranded on the shoulder is greater for the inner ring than for the outer ring. In other words, it is evident that the permissible thrust load of the inner ring is smaller than that of the outer ring. (Details will be mentioned later on.)

Therefore, in the above-mentioned constitution, in order to set the ball groove depths so that the risk rate of the ball being stranded on the shoulder becomes approximately the same between the inner ring and the outer ring under the condition of pure thrust load, the depth of the ball groove for the outer ring shall be made shallower than the depth of the ball groove for the inner ring by a dimension corresponding to the difference in risk rate of ball being stranded on the shoulder of the two rings, that is, a dimension corresponding to the difference in the permissible thrust load between the two.

If composed in the above manner, by the dimension made shallower for the ball groove of the outer ring, the space volume between the outer ring and the inner ring becomes large, it is possible to make the amount of grease to be filled increased and as a result, the life of the grease, and in turn, the life of the ball bearings can be extended. Furthermore, since the ball being stranded on the shoulder occurs under approximately the same thrust load conditions between the outer ring and the inner ring, it becomes possible to cope with the problem of the ball being stranded on the shoulder effectively without any waste.

It is desirable to set the dimension between the inside diameter and the outside diameter of the outer ring (thickness of the outer ring) to be shorter by the dimension corresponding to approximately the difference in the depth of the ball groove between the outer ring and the inner ring than the dimension between the inside diameter and outside diameter of the inner ring (thickness of the inner ring). If it is composed in this manner, , the thickness of the outer ring can be made thinner and light-weight, while making the depth of the ball groove for the outer ring shallower. In addition, in spite of the thickness for the outer ring becoming thinner, since the ball groove also becomes shallower accordingly, the thickness in the vicinity of the deepest portion of the ball groove for the outer ring does not become thin, and the rigidity of the outer ring is maintained sufficiently.

It is desirable to set the depth of the ball groove for the outer ring within a range of about 12-19% of the ball diameter. This setting condition is determined by taking into consideration the fact that if the depth of the ball groove for the outer ring is less than 12% of the ball diameter, the ball being stranded on the shoulder appears significantly, and if the depth of the ball groove for the outer ring is greater than 19% of the ball diameter, sufficient space volume cannot be secured for grease filling. Thus, in the case of such a setting, sufficient space volume can be secured between the inner and outer rings while coping with the problem of the ball being stranded on the shoulder of the outer ring without any waste, and it becomes possible to suitably increase the amount of grease to be filled.

A retainer which is installed between the outer ring and the inner ring and which holds the balls so that they can roll freely may be preferably formed as a snap-on type made of synthetic resin. In addition, it is preferable that the outside diameter thereof is formed with a larger dimension than the inside diameter thereof with respect to the pitch circle diameter of the ball. In other words, as mentioned above, under a setting condition in which the depth of the ball groove for the outer ring is shallower than the depth of the ball groove for the inner ring, the space volume between the outer ring and the inner ring becomes larger for the outside diameter side (outer ring side) than the inside diameter side (inner ring side) with respect to the pitch circle diameter of the ball. Therefore, in accordance with this, if the retainer is formed so that the outside diameter side becomes larger than the inside diameter side with respect to the pitch circle diameter, the thickness in the radial direction of the retainer can be made thicker until both clearances that exist between the inner and outer rings become almost equal, and the rigidity of the retainer can be heightened as much as possible. As a result, at the time of high speed revolution of the outer ring, the situation in which the retainer is expanded and deformed by the effect of centrifugal force can be suppressed as much as possible. Consequently, abnormal friction and heat build-up at the contact surface between the balls and the retainer as well as interference with the seals caused by the balls being pushed out from the retainer can be avoided.

It is desirable to fill the grease within the range of 27.5%-32.5% with respect to the space volume that exists inside of the ball bearings. If it is composed in this manner, it is possible to fill the grease into the space volume that is increased by making the ball groove of the outer ring shallow at the optimum rate of 27.5%-32.5%, preferably at 30%, determined by experiments conducted beforehand. As a result, it is possible to increase the amount of grease to be filled without occurring agitation heat build-up or grease leakage due to excessive rate of grease filled amount with respect to the space volume. This leads to long life of the grease, and in turn, long life of the ball bearings for the pulleys can be aimed.

As base oil of the grease, it is preferable to use ester type synthetic oil. By doing so, grease that has long life against high temperature and excellent low temperature characteristics can be obtained, and long life of the grease, and in turn, the long life of the ball bearings for the pulley can be aimed at all the more.

It is desirable for the pulley ball bearings having the above-mentioned constitution to be used as pulley bearings that is a component of a wrapping connector transmission device that is driven by an engine for an automobile. It is suitable for pulleys in such a case to be floating pulleys such as idler pulleys used for increasing the contact angle of the wound belt, and tension pulley used for adding the required tension to the belt. In this case, the term ˇ°pulleyˇ± means not only timing pulley and V pulley but also a concept that includes sprockets. Furthermore, the term ˇ°beltˇ± means a concept that includes not only timing belts and V-belts but also chain. According to the resulted constitution, it is possible to correspond suitably to high revolution of the engine for an automobile, and in turn, the high revolution request of the outer ring for bearings.

The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.