The gear decrease 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 provides 50 the 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 on the number of teeth of sunlight and ring gears. The planet gears act as idlers and do not affect the gear ratio. The planetary gear ratio equals the sum of the amount of teeth on the sun and ring equipment divided by the amount of teeth on the sun gear. For example, a planetary arranged 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 perform ratios from about 3:1 to about 11:1. If more gear reduction is needed, additional planetary stages can be used.
If a pinion equipment and its mating equipment have the same amount of teeth, no reduction occurs and the apparatus ratio is 1:1. The apparatus is called an idler and its own major function is to change the direction of rotation rather than reduce the speed or increase the torque.
Parallel shaft gearboxes many times contain multiple gear sets thereby increasing the gear reduction. The total gear decrease (ratio) is determined by multiplying each individual equipment ratio from each equipment established stage. If a gearbox contains 3:1, 4:1 and 5:1 gear sets, the total ratio is 60:1 (3 x 4 x 5 = 60). Inside our Planetary Wheel Drive example above, the 3,450 rpm electric electric motor would have its quickness reduced to 57.5 rpm by using a 60:1 gearbox. The 10 lb-in electric engine torque would be risen to 600 lb-in (before efficiency losses).