ABSTRACT
Gears are machine elements that transmit power and rotary motion from one shaft to another. An advantage they have over friction and belt drives is that they are positive in their action, a feature that most of the machine tools require, as exact speed ratios are sometimes essential. Thread cutting and indexing movements in gear cutting are typical examples, which require synchronized rotary and linear movements without any slip. As drive elements, gears are speci cally used to
Change the speed of rotation Change the direction of rotation Increase or reduce the magnitude of speed and torque Convert rotational movement into linear or vice versa (rack and pinion drive) Change angular orientation (bevel gears) Offset the location of rotating movement (helical gears and worm gear sets)
Depending on the speci c application, gears can be selected from the following types:
Spur gears. These are the most common type, which transmit power or motion between parallel shafts or between a shaft and a rack. They are simple in design and measurement. If noise is not a serious problem, spur gears can be used. For aircraft gas turbines, spur gears of extra high quality can operate at pitch-line speed above 2000 m/min. In general applications, spur gears are not allowed to work at speeds over 1200 m/min.
