History |
|
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 though they were commonplace. In the fifteenth century A.D., Leonardo da Vinci designed a multitude of devices incorporating many kinds of gears. The basic form of the Hansen gearbox as we know it today, has evolved a great deal.
Figure 1 A large Hansen gearbox internals.
In industry most pieces of equipment are driven by electric motors which, because of our 50 cycle power supply, run at approximately 3000 rpm, 1500 rpm, 750 rpm, etc. Any equipment that needs to be driven at a different speed, needs a speed changing device such as pulleys, sprockets, gears, etc., to reach the required speed. Gears are therefore used to transmit power from one shaft to another, change speed and torque and, where required, the direction of the drive.
Figure 2 shows a very crude comparison of the various means of transmitting power from one shaft to another.
Figure 2 Comparison of driving methods.
In contrast with timing belts, chains and gears do not depend on friction to transmit power. Gears can transmit power at much higher speeds than chains. However, adequate lubrication is then of paramount importance. 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).
The engineer seriously involved with gears of any kind should consult the pertinent standards of AGMA, as well as other contemporary gear literature.