External helical gears are generally used when both high speed and high power are involved. However, some axial thrust is inevitable in helical gears, causing loss of power and requiring thrust bearings. This condition permits smoother action than with spur gears. Because of this angle, teeth mating occurs in such a way that more than one tooth of each gear is always in mesh. These are used to transmit motion between parallel or crossed shafts, or between a shaft and a rack by meshing teeth that lie along a helix at an angle to the shaft. In general applications, spur gears are not allowed to work at speeds over 1200 m/min. For aircraft gas turbines, spur gears of extra high quality can operate at pitch-line speed above 2000 m/min. If noise is not a serious problem, spur gears can be used. They are simple in design and measurement. These are the most common type, which transmit power or motion between parallel shafts or between a shaft and a rack. Depending on the specific application, gears can be selected from the following types:
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