The gear decrease in a right-angle worm drive is dependent on the number of threads or “starts” on the worm and the number of teeth on the mating worm wheel. If the worm has two starts and the mating worm wheel offers 50 teeth, the resulting gear ratio is 25:1 (50 / 2 = 25).
Calculating the gear ratio in a planetary gear reducer is less intuitive since it is dependent upon the amount of teeth of sunlight and band gears. The earth gears act as idlers and don’t affect the gear ratio. The planetary gear ratio equals the sum of the number of teeth on sunlight and ring equipment divided by the amount of teeth on sunlight gear. For example, a planetary arranged with a 12-tooth sun gear and 72-tooth ring gear has a equipment ratio of 7:1 ([12 + 72]/12 = 7). Planetary gear models can perform ratios from about 3:1 to about 11:1. If more gear reduction is necessary, additional planetary stages may be used.
If a pinion equipment and its mating gear have the same amount of teeth, no reduction occurs and the gear ratio is 1:1. The gear is named an idler and its major function is to change the direction of rotation rather than decrease the speed or raise the torque.
Parallel shaft gearboxes often contain multiple gear units thereby increasing the apparatus reduction. The total gear decrease (ratio) depends upon multiplying each individual equipment ratio from each gear established stage. If a gearbox contains 3:1, 4:1 and 5:1 gear pieces, the total ratio is 60:1 (3 x 4 x 5 = 60). In our Planetary Wheel Drive example above, the 3,450 rpm electric motor would have its rate decreased to 57.5 rpm by using a 60:1 gearbox. The 10 lb-in electric motor torque would be increased to 600 lb-in (before effectiveness losses).