A bearing（轴承） is a connector that permits the connected members to either rotate or translate (move to and fro) relative to one another but prevents them from separating in the direction in which loads are applied. In many cases one of the members is fixed. And the bearing acts as a support for the moving member.
The relative motion in bearings is always opposed by friction, and the work done in overcoming friction is lost power in all machines. Consequently, much thought and effort have been devoted to the development of bearings with minimum friction. In all bearings there are two surfaces (one belongs to each of the connected parts) that move relative to one another. To minimize friction, the co-acting surfaces may be partially or completely separated by a film of liquid or gas; these are known as sliding-contact bearings (Figure 10-1). The surfaces may be separated also by an assemblage of rolling elements such as balls and rollers; these are known as rolling-contact bearing.
Sliding bearings are the simplest to construct and, considering the multitude of pin-jointed devices and structures in use, are probably the most commonly used.
The essential parts of a ball bearing – the inner and outer ring, the balls, and the separator—are known in Figure 10-2. The inner ring is mounted on a shaft and has a groove in which the balls ride. The outer ring is usually the stationary part of the bearing and also contains a groove to guide and support the balls. The separator prevents contact between the balls and thus reduces friction, wear, and noise from the regions where severe sliding conditions would occur. In a few applications where operating conditions are mild, the rings and separator can be omitted and loose balls interposed between the shaft and housing. This type of bearing is sometimes found in bicycles. The vast majority of ball bearings, however, are preassembled units consisting of rings and separators and in the number of balls. They can be divided into classes according to their function: those that support a radial load, those that support a thrust load, or those that support a combination of thrust and radial loads. The last type is termed “angular-contact bearings”.
In deep-groove ball bearings, the races are approximately one fourth as deep as the ball diameter. A cross section of the ball and outer ring of a deep groove ball bearing is known in Figure 9. . Although deep-groove ball bearings are designed to carry a radial load, they perform well under a combined radial and thrust load. For this reason, this is the most widely used type of ball bearings.
Vocabulary
1.connector n. 连接器；接头
2.permit v. 许可；容许
3.to and fro 往返[复 ]地
4.relative to 关于；相对于
5.consequently ad. 因此
6.devote vt. 致力于
7.co-acting 共同作用；相对运动
8.separator n. 分离[分隔]器
9.interpose v. 放[插、介]入
10.angular-contact bearing 角接触球轴承
11.sliding-contact bearings 滑动轴承
12.rolling-contact bearings 滚动轴承
13.inner ring 内圈
