Belt drivers（带传动机构） offer a maximum of versatility（通用性） as power transmission（传输） elements. They allow the designer considerable flexibility in location（位置） of driver and driven machinery, and tolerances（公差） are not critical（重要的） as in the case with gear drivers. Another advantage of belt drivers is that they reduce vibration and shock transmission. Furthermore, belt drivers are relatively quiet.
Large flat leather belts were in general use many years ago, when the usual practice was to use on large engine to drive several different pieces of machinery. Some leather belts are in use at this time as well as flat steel, rubber, plastic, and fabric belts. Light, thin flat belts are practical（实用的） on high speed machinery where vibration may be a problem.
V belts are probably the most common means of transmitting power between electric motor and driven machinery. Conventional（传统的） V belts are made of rubber. Most often, both driver and driven pulleys lie in the same vertical plane, and the speed ratio（速比） is constant.
Smooth flat belts and V belts depends on friction（摩擦） on the pulleys and some slippage（滑动） is inherent（固有的） in their operation. Therefore, speed ratios are not precise. Most heavy duty（重载的） pulleys are made of cast iron（铸铁） or formed steel.
Note that the capacity of the belt drive is determined by the angle of wrap（包缠）, α, on the smaller pulley, and that this is particularly critical for drives involving pulleys of greatly different size, spaced closely together.
Vocabulary
1.belt n.带
2.versatility n.通用性；适应性
3.flexibility n.机动性；灵活性
4.several different pieces of machinery 若干台不同的机械设备
5.fabric belts 纤维带
6.electric a. 电（力、动、气）
7.slippage n. 滑动；打滑
8.inherent a. 固有的
9.duty n. 义务；负载[荷]
10.pulley n.皮带轮
11.capacity n. 能力；本领
12.wrap v. 包（裹、围、装、封）
13.wrap angle 包角
14.critical n. l临界点（值）；转折点（处）
15.differ vi. 不相同；不一致
