{"data":{"fields":"term_definition","term_definition":"<div class=\"definition\">To calculate tooth loading accurately, several factors must be known, including: - the gear ratio - center distance limits - gear or pinion speed - gear or pinion horsepower or torque - type and duration of loading - operating temperature - service life requirement - type of lubrication (if any) The gear ratio is determined by the desired rate of motion between the driving (or input) gear and the driven (or output) gear. Center distance is usually set by the space allocated to the gear pair. The speed of either the driving gear or the driven gear must be known, as well as either the horsepower to be transmitted or the torque of one of the two mating gears. The expected service life should be stated along with the operating conditions (loading, temperature and lubrication) for meaningful predictions to apply. In addition, pressure angle and diametral pitch are critical factors, since they determine the size and shape of a gear tooth and, consequently, determine the load-sharing ability and strength of the tooth. The most common pressure angles are 14.5, 20 and 25\u00b0 with the 20\u00b0 pressure angle being used most often. The 20\u00b0 pressure angle has a higher loadbearing ability than the 14.5\u00b0 and it allows using fewer pinion teeth to be used before undercutting becomes a concern. A 25\u00b0 pressure angle has an even higher load carrying capacity than the 20\u00b0 angle and may have better wear characteristics than lower pressure angles. However, it is also more sensitive to center distance variation and as a result it is usually not as smooth or quiet in operation. Lubrication (or the lack thereof) has an effect on gears in that lubricated gears tend to fail in flexure while unlubricated gears tend to fail through surface wear. Unlubricated gears must be designed for contact stress.<\/div>"}}