Keys（键） are used to prevent relative motion between a shaft and machine elements such as gears, pulleys, sprockets（链轮）, cams（凸轮）, levers（控制杆）, flywheels（飞轮）, impellers（叶轮）, and so on. There are numerous（无数的） kinds of keys (some of which have been standardized（标准化）) for various design requirements. The keys most frequently used are the square keys（方头键）, the tapered keys（锥形键）, and the woodruff key（半圆形键）.
Figure7-1 (a) square key (c) tapered key (b) woodruff key
Perhaps the most common of these torque-transmitting shaft-to-hub（毂） connections are keys, of which the most common type is the square key. Standard proportions call for the key width to be approximately one-fourth the shaft diameter. Keys are usually made of cold-finished（冷加工的） low-carbon steel, but heat-treated alloy steel is used when greater strength is required.
Mechanical springs are used in machines to exert force, to provide flexibility（弹性） and to store or to absorb（吸收） energy. In general, springs can be classified as either wire springs or flat springs, although there are variations within these pisions（区分）. Wire springs include helical springs of round, square, or special-section wire and are made to resist tensile（拉力）, compression（压力） or torsional（扭矩） loads. Under flat springs are included the cantilever（悬臂） and elliptical（椭圆） types, the clock-type power-springs and the flat spring washers（垫片）, usually called Belleville springs（盘簧）.
In one word，springs are elastic（弹性的） members that exert forces (or torques) and absorb energy (which is usually stored and later released). Springs are usually, but not necessarily（必要地）, made of metal. Plastics can be used when loads are light.
Vocabulary
1.key n. 键
2.prevent v. 阻止；防止
3.prevent M (from) +ing
4.sprocket n. 链轮；星轮
5.flywheel n. 飞轮
6.impeller n. 叶轮；涡轮
7.square key 方键
8.tapered key 锥形键
9.woodruff key 半圆键
10.variation n. 变化；差异
11.compressure n. 压缩力
12.cantilever n.悬臂；伸臂
13.elliptical a.椭圆
14.washer n. 垫片；垫圈
15.Belleville springs 贝氏弹簧
16.elastic a. 弹性的
17.torque n. 扭矩；力矩
18.proportion n. 部分
19.diameter n. 直径
20.alloy n. 合金
21.call for 要求；需要
22.cold finished 冷加工；冷轧制
23.heat-treaded alloy 热处理过的合金钢或热轧合金
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