An oil-impregnated sintered bearing includes a sintered body in which lubricating oil is impregnated in advance, and thermally expands due to pumping action and frictional heat caused by rotation of a shaft so that lubricating oil comes out of the sintered body to lubricate a friction surface. Since the oil-impregnated sintered bearing can be used for a long time without supply of oil, the oil-impregnated sintered bearing has been widely used as a bearing for supporting a rotating shaft of a vehicle or home electric appliances, an acoustic equipment, etc.
In the conventional oil-impregnated sintered bearing, in order to center the rotating shaft inserted into a bearing hole, a portion of the bearing hole is formed to have a diameter smaller than diameters of other portions in the bearing hole, and the only portion having the smaller diameter comes into contact with the rotating shaft.
In the meantime, as described above, in a case where a portion of the bearing hole is formed to have a diameter smaller than diameters of other portions in the bearing hole, since the length of a portion actually contacting the rotating shaft is shorter than the entire length of the bearing, there has been problems in that the supporting state of the rotating shaft is likely to be unstable and the rotating shaft is likely to be deviated.
Consequently, in the conventional oil-impregnated sintered bearing, the bearing hole is formed to have a journal part that supports the rotating shaft and enlarged diameter parts that are connected with the journal part and have constant diameters to be enlarged toward the tips thereof. Furthermore, the enlarged diameter parts are formed to have sintering density higher than that of the journal part in order to suppress deviation from the center of the rotating shaft (for example, see Japanese Laid-Open Patent Application Publication No. 64-030922).
In the bearing having the above-mentioned structure, when a shear load is applied to the rotating shaft, the lubricating oil lubricating between the rotating shaft and the journal part is pushed toward the enlarged diameter parts due to the occurrence of runout of the rotating shaft then filled between the rotating shaft and the enlarged diameter parts. The lubricating oil filled between the rotating shaft and the enlarged diameter parts is pressed by the runout of the rotating shaft so as to be impregnated into the enlarged diameter parts. However, since the enlarged diameter parts are thickly formed, the lubricating oil is not impregnated and remains between the rotating shaft and the enlarged diameter parts to apply reaction forces to the rotating shaft. The runout of the rotating shaft is suppressed by the reaction forces so as to prevent the deviation from the center of the rotating shaft with respect to the bearing.
The bearing having the above-mentioned structure is very effective in suppressing deviation from the center of the rotating shaft. Accordingly, in a case where the conventional oil-impregnated sintered bearing is used to support the rotating shaft, for example, if torque is transmitted to rotate the rotating shaft in a certain direction, a shear load is applied to the rotating shaft. However, when the shear load is very large and the rigidity of the rotating shaft is not sufficiently high, the rotating shaft is deflected due to the shear load and the rotating shaft is rotated while an axis thereof is inclined in a bearing body. Therefore, there is a possibility of becoming the state (motion that the rotating shaft scrapes out the inner surface of the bearing) where the surface of the rotating shaft does not correctly come into contact with the friction surface of the bearing. In such a state, since the rotating shaft receives strong resistance, it is difficult to rotate the rotating shaft, thereby not sufficiently functioning as a bearing. Moreover, if the state is repeated, it is also considered that durability of the rotating shaft and the bearing deteriorates.
In addition, when a shear load causing runout of the rotating shaft is remarkably large and a push-back action caused by the lubricating oil remaining between the rotating shaft and the enlarged diameter parts does not sufficiently function, the rotating shaft is supported while the axis of the rotating shaft is inclined in the bearing body. In this case, since the surface of the rotating shaft is pushed against boundaries between the journal part and the enlarged diameter parts thereby coming in contact with the bearing body at a point. In this case, if the rotating shaft acts as described above, both end of the journal part are scraped out and stress concentration occurs at the contact portion between the rotating shaft and the bearing body. In this way, if the stress concentration occurs, excessive abrasion and overheating occurs from the contact portion. This phenomenon does not occur as long as a push-back action caused by the lubricating oil remaining between the rotating shaft and the enlarged diameter parts function. However, when a large unexpected shear load is suddenly applied, there is a possibility that durability of the rotating shaft and the bearing deteriorates.
In the bearing, since taper angles (Angles formed by an inclined plane of the enlarged diameter parts with respect to a longitudinal direction of the bearing along an axis of the journal part, that is, a longitudinal direction of the rotating shaft supported by the bearing, and angles formed between an inner surface of the journal part and inclined planes of the enlarged diameter parts are the same) are set to very small angles of 2° to 3°, a very high machining accuracy is required. If the taper angles are not set accurately, there is a possibility of not sufficiently exhibiting center deviation-suppressing action for the rotating shaft.