The gear decrease in a right-angle worm drive would depend 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 begins and the mating worm wheel provides 50 tooth, the resulting equipment ratio is 25:1 (50 / 2 = 25).
Calculating the apparatus ratio in a Planetary Wheel Drive planetary gear reducer is less intuitive since it is dependent on the amount of teeth of the sun and band gears. The planet gears become idlers , nor affect the apparatus ratio. The planetary gear ratio equals the sum of the amount of teeth on the sun and ring equipment divided by the number of teeth on the sun gear. For example, a planetary set with a 12-tooth sun gear and 72-tooth ring gear includes a equipment ratio of 7:1 ([12 + 72]/12 = 7). Planetary gear sets can achieve ratios from about 3:1 to about 11:1. If more gear reduction is necessary, additional planetary stages may be used.
If a pinion gear and its mating equipment have the same number of teeth, no reduction occurs and the gear ratio is 1:1. The apparatus is called an idler and its primary function is to change the path of rotation instead of reduce the speed or boost the torque.
Parallel shaft gearboxes many times contain multiple gear sets thereby increasing the apparatus reduction. The total gear decrease (ratio) is determined by multiplying each individual equipment ratio from each gear set stage. If a gearbox contains 3:1, 4:1 and 5:1 gear sets, the full total ratio is 60:1 (3 x 4 x 5 = 60). In our example above, the 3,450 rpm electric electric motor would have its rate reduced to 57.5 rpm by using a 60:1 gearbox. The 10 lb-in electric engine torque would be increased to 600 lb-in (before efficiency losses).