{"data":{"fields":"term_definition","term_definition":"
Also known as \"circumferential play\" or \"tangential clearance\", backlash is generally defined as the distance by which toothspace exceeds tooth thickness, as measured on the pitch circle. The purpose of backlash is to prevent gears from binding or seizing-up by making simultaneous contact on both sides of meshing teeth. Backlash is necessary for several reasons and is influenced by various factors, among them: - operating temperatures - thermal expansion - 14 - - mounting tolerances - tooth shape and position - center distance tolerances - tooth size and accuracy - post-mold dimensional changes - other components of the assembly - runout of shaft bearings - gear load, speed and running conditions All of these are interdependent to some degree, but the most critical factor is temperature. Gears operating under moderate loads and at moderate speeds at room temperature will be less affected by small variations in backlash. At high load, high speed or high temp ratures, gears should have both greater tooth accuracy and additional backlash to compensate for the effect of thermal expansion. Gears operating at room temperature at high permissible loads and speeds can experience a rise in tooth temperature of 100\u00b0F (38\u00b0C). Therefore, providing backlash by means of increased center distance is a method which should be approached with the utmost caution. In all cases, backlash and center distance measurements and adjustments should be made under actual operating conditions.<\/div>
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