A rotary（旋转的） cam is a part on a machine which mechanically changes cylindrical（圆柱的） motion to straight-line motion. The cam, which is mounted on a shaft, is usually driven by a motor either applied directly to the cam shaft or connected by pulley（皮带轮） or gears. The purpose of a cam is to transmit various kind of motion to other parts of a machine.
Practically every cam must be designed and manufactures（制造） to fit special requirements. Though each cam appears to be quite different from the other, all the cams work in a similar way. In each case, as the cam is rotated or turned, another part in contact with the cam, called a follower（从动件）, is moved either left or right, up and down, or in and out. The follower is usually connected to other parts on the machine to accomplish（实现） the desired action. The direction the follower moves depends upon the position of the framework（框架） which attaches the cam follower to the machine. If the follower loses contact with the cam, it will fail to work.
The faster the cam turns, the more rapid（快速） is the motion of the follower. The motion of the follower caused by rotating the cam is exactly the same for each complete turn of the cam.
Vocabulary
1.cylindrical a.圆柱形的；回转的
2.cam n. 凸轮
3.apply to 适用于；s用到
4.manufacture vt. & n. （机械）制造；制造业
5.appear vi. 出现；显现
6.follower n. 随动件；从动件
7.contact n. 接触
8.accomplish vt. 完成；实现
9.depend upon 依赖于；取决于
10.exactly ad. 正s；精确地
11.complete a. 完全的；整个的
