Drill motor gearBox question

Does anyone now the actual stepped gear reduction ratio for the drill motor gearbox? I know the gearbox in high is approx. 20:1 and in low is approx 64:1. I’m looking for the actual gear ratio for each stage,(gearbox has 3 stages), in high gear as well as low?
I figured I would ask the community before I go through the trouble of cracking open a gearbox.

Hello Dennis

You might also look through the FIRST handbook. They might list the ratios for the gearbox. I know it lists the speed and torque of the various motors we use from the kit.
Hope this helps.

Wayne Doenges


You might as well just crack one open. The drill motor gearboxes are 3 stage planetary gears. The gear ratio depends on which stages are locked. Though as I recall all of the stages use the same gears.

A good mechanical engineering reference book like the Lindberg PE exam book or “Mechanical Engineering Design” by Shigley will have the formulas for the effective gear ratio if you have the numbers of teeth.

Actually, taking one apart would be a good educational exercise. Putting one back together is even more of one. Last year we had to do this several times to rebuild gearboxes when we broke components. Once you learn how it’s not a big deal.

I’d tell you how, but I don’t remember off hand and the nearest gearbox I know of is several miles away…



For each planetary set, the gear ratio can be calculated as follows:

  1. the small gear in the middle is the sun gear, the internal gear (the one with the teeth on the inside also called the fixed gear) is called the ring gear.

  2. the gear ratio for each stage is calculated as folows:

of teeth on ring gear

------------------------------ + 1

of teeth on sun gear

  1. Multiply each stage’s gear ratio together to get the total ratio. If in high speed, the second stage gear ratio is 1; so only multply the 1st and the 3rd stages.


Thanks Paul !!

If anyone is interested, this is what I calculated:

Planetary gear train stage 1: (motor gear as the sun gear)
Ratio= 4.5:1
Planetary gear train stage 2:
Ratio= 3.2:1
Planetary gear train stage 3:
Ratio= 4.3:1

Looks like it correlates to the published overall gearbox ratio’s I’ve see and used in the past of 64:1 (Low) and 20:1 (High)

Thanks Again