[Joe Johnson]

speed is something you can apply a ratio to and get the right answer (a motor spinning at 15000 RPM will have an output speed of 3000RPM after a 5:1 reduction), but torque does not work that way. You need to apply an efficiency factor to the gearset.

I typically use 90-95% PER STAGE for straight spur gear pairs if things are designed right.

I use 85% (again per stage) for planetary gearsets if the ratio for that stage is below 7:1. The efficiency of a planetary gearset falls off sharply with higher ratios so use multiple stages rather than 1 high ratio stage (2 4:1 stages are MUCH more efficient than 1 16:1 stage).

Helical and bevel gears can be much lower than spur gears but they can be pretty close as well, it all depends on how the thrust loads are managed.

Worm gears? Count on 50% is the best you'll get. Many are are 10-20% efficient (i.e. you have to put in 5X to 10X the amount of power into the gearbox as you get out of it).

You really need to keep in mind the "effective" ratio when thinking about torque and gearboxes. A 110:1 gearbox that gets the ratio in 3 spur gear stages will have an effective ratio of between 80 and 95 to 1 depending on the efficiency per stage.

Joe J.