英语翻译
Spur Gears
Gears,defined as toothed members transmitting rotary motion from one shaft to another,are among the oldest devices and inventions of man,In about 2600 B,C.the Chinese are known to have used a chariot incorporating a complex series of gears.Aristotle,in the fourth century B ,C,wrote of gears as if they were commonplace.In the fifteenth century A ,D,Leonardo da Vinci designed a multitude of devices incorporating many kinds of gears.
Among the various means of mechanical power transmission (including primarily gears,belts,and chains),gears are generally the most rugged and durable.Their power transmission efficiency is as high as 98 percent.On the other hand,gears are usually more costly than chains and belts.As would be expected,gear manufacturing costs increase sharply with increased precision---as required for the combination of high speeds and heavy loads,and for low noise levels.(Standard tolerances for various degrees of manufacturing precision have been established by the AGMA ,American Gear Manufacturers Association)
Of the non-ferrous metals,bronzes are most often used for making gears.Nonmetallic gears(nylon and other plastics ) are generally quiet,durable,reasonably priced,and often can operate under light loads without lubrication.Their teeth deflect more easily than those of corresponding metal gears.This promotes effective load sharing among teeth in simultaneous contact.Since nonmetallic materials have low thermal conductivity,special cooling provisions may be required.Furthermore,these materials have relatively high coefficients of thermal expansion,and this may requie installation with greater backlash than metal gears.