The gear reduction in a right-angle worm drive is dependent on the amount 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 has 50 tooth, the resulting gear ratio is 25:1 (50 / 2 = 25).
Calculating the gear ratio in a planetary gear reducer is much less intuitive as it is dependent upon the amount of teeth of the sun and ring gears. The earth gears act as idlers and do not affect the apparatus ratio. The planetary equipment ratio equals the sum of the number of teeth on the sun and ring gear divided by the amount of teeth on the sun gear. For instance, a planetary established with a 12-tooth sun gear and 72-tooth ring gear has a gear ratio of 7:1 ([12 + 72]/12 = 7). Planetary gear units can achieve ratios from about 3:1 to about 11:1. If more gear reduction is necessary, additional planetary stages can be used.
If a pinion gear and its mating gear have the same quantity of teeth, no decrease occurs and the apparatus ratio is 1:1. The gear is named an idler and its own principal function is to change the direction of rotation instead of decrease the speed or raise the torque.
Parallel shaft gearboxes many times contain multiple gear units thereby increasing the gear reduction. The full total gear reduction (ratio) depends upon multiplying each individual equipment ratio from each equipment set stage. If a Planetary Wheel Drive gearbox contains 3:1, 4:1 and 5:1 gear models, the full total ratio is 60:1 (3 x 4 x 5 = 60). Inside our example above, the 3,450 rpm electric motor would have its speed reduced to 57.5 rpm by using a 60:1 gearbox. The 10 lb-in electric motor torque would be risen to 600 lb-in (before efficiency losses).