As a machine component（成份） a shaft is commonly a cylindrical bar that supports and rotates with devices for receiving and delivering rotary motion and torque. The crankshaft（曲柄轴） of a reciprocating engine（往复移动式发动机）receives its rotary motion from each of the cranks, via the pistons and connecting rod (the slider-crank mechanism), and delivers it by means of couplings（连轴器）, gears, chains, or belts to the transmission, camshaft, pumps, and other devises. The camshaft, driven by a gear or chain from the crankshaft, has only one receiver or input, but each cam on the shaft delivers rotary motion to the valve-actuating mechanisms.
An axle is usually defined as a stationary（固定的） cylindrical member on which wheels and pulleys can rotate, but the rotation shaft that drive the rear wheels of a automobile（汽车） are also called axles, no doubt a carryover（传承） from horse-and-buggy days（马拉的轻型车）. It is common practice to speak of short shafts on machines as spindles（心轴）, especially tool-carrying or work-carrying shafts on machine-tools.
In operation, shafts are subjected to（容易遭受） a shearing stress（剪应力）, whose magnitude（大小） depends on the torque and the dimensions of the cross section（横截面）. This stress is a measure of the resistance that the shaft material offers to the applied torque. All shafts that transmit a torque are subjected to torsional shearing stress.
In addition to the shearing stresses（剪应力）, twisted shafts are also subjected to shearing distortions. The distorted state is usually defined by the angle of twist per unit length, i.e., the rotation of one cross section of a shaft relative to another cross section at a unit distance from it.
Vocabulary
1.component n. 元件；分量
2.receive v. 收到；接到
3.deliver vt. 输送、运送
4.reciprocate v. 往复运动
5.connecting rod 连杆
6.the slider-crank mechanisms 曲柄滑块机构
7.the transmission 变速箱
8.cam-shaft 凸轮轴
9.chain n.链
10.shearing stress 剪切应力
11.magnitude n. 大小、尺寸
12.offer v. 提供、提出
13.the angle of twist 扭转角
