Gears are vital factors in machinery. One of the first mechanism invented using gears was clock. In fact, a clock is little more than a train of gears. Considerable study and research have been made on gears in recent years because of their wide use under exacting conditions. They have to transmit heavier loads and run at higher speeds than ever before. The engineers and the machinists all consider gearing the prime element in nearly all classes of machinery.
1.Spur Gears（直齿轮）
Spur gears are used to transmit power and rotary motion between parallel shafts. The teeth are cut parallel to axis of the shaft on which the gears are mounted. The smaller of two gears in mesh(啮合) is called the pinion（小齿轮）, and the larger is customarily（习惯上） designated as the gear. In most applications, the pinion is the driving element whereas the gear is the driven element.
Figure 3-1 Spur Gear Figure 3-2 Speed Reducer
2.Helical Gears（斜齿轮）
Helical gears have certain advantages, for example, when connecting parallel shafts they have a higher load-carrying capacity（能力） than spur gear with the same tooth numbers and cut with the same cutter.
Helical gears can also be used to connect nonparallel, non-intersecting（非相交的） shafts at any angle to one another.
The maximum gear ratio obtainable with a single pair of gears varies with type of gear and the application. The following are approximate maximum for the various type for average load conditions: spur, 8; parallel-shaft helical, 10: straight bevel（）,6: spiral bevel, 8: hypoid（准双曲面齿轮）, 12: and worm, 80. For highly loaded, instrument, and positioning gears, these ratios can be exceeded. Ratios as high as 400 or higher can be obtained with gears that resemble tapered worms meshing with hypoid gear. For heavily loaded gears, the given ratios may be so high that a reasonable gear size precludes（排出） a satisfactory pinion.
Since the ratio in a single pair of gears is the quotient(商) of the tooth number, and since there usually are limitations on both the minimum and maximum numbers of teeth on the available gears, it follows that the number of ratios obtainable in a single pair is limited. To enlarge the coverage it is necessary to use multiple pairs, or trains. The overall speed ratio in a train is the product of the ratios in each pair. In certain cases an exact ratio cannot be obtained with gears, but by using two or more pairs, the desired ratio can be approximated to any degree of precision.
As a convenience（方便） for machine builders and users, packaged speed reducers, following an industry accepted pattern, are manufactured in a wide variety of type, configuration（配置）, speed ratios, and capacities; this consists of a box or housing containing bearings, shafts, lubricant, and shaft oil seals. Speed increasers（加速器） are usually custom built.
All speed reducers when operating continuously become hot because of fiction in t
he teeth, in the lubricant, in the bearings, and in the oil seals. If the heat is generated at a faster rate than it can be dissipated（消散） to the atmosphere, the lubricant may deteriorate（使恶化） and the gears or bearings fail（失效）.
Vocabulary
1.vital a. 活的；有生命力的
2.factor n. 系数；因数；要素
3.machinery n. 机器；机械；机构
4.invent vt. 发明；创造
5.a train of … 一系列
6.transmit v. 传（递、动、送、输）
7.machinist n. 工程师；工程技术人员
8.obtainable a.能获得的；可得到的
9.for average load condition 于（相应的）平均载荷下
10.parallel-shaft helical 平行轴斜齿轮
11.straight bevel 直齿锥齿轮
12.spiral bevel 曲线齿锥齿轮
13.hypoid gear 准双曲面齿轮
14.positioning gears 定位齿轮
15.resemble vt. 类似；像
16.preclude vt. 妨碍；使不可能
17.fail v. & n. 失败；失效；失灵
18.rotary motion 回转运动
19.mount v. 安装；装配
20.mesh v. 啮合；配合
21.customarily ad.照例；习惯上
22.the driving element 主动件
23.the driven element 被动件
24.helical n. & a. 螺旋线；螺旋面
25.a higher load-carrying capacity 较高的承载能力
26.cutter n. 刀具；
27.nonparallel 非平行的
28.non-intersecting 非相交的
29.spur n. 直齿圆柱齿轮
30.satisfactory a. （令人）满意的
31.a single pair of 一对（齿轮）；单一（齿轮）副
32.quotient n. 商
33.it follows that 因此
34.coverage n. 范围
35.multiple a. 多（重，倍，数级）的
36.configuration n. 排列；组合；配置
37.deteriorate v. 损坏；磨损
38.dissipate v. （使）消散；散逸
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