机械工程英语——Lesson 3 Gear and Reducers

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