hello

i've a worm gear connected to a tiny motor, and in turn the worm gear turns a small plastic spur gear. The motor's rpm is 5000, the output i'd like to reduce to 120 rpm on the small spur gear.

What's the best way of calculating how much a worm gear reduces RPM?

thanks

You count "starts" on the worm pinion. Normally there is one "start" the same as a threaded bolt etc.

If there is one start, the worm gear turns ONE tooth for each revolution of the worm pinion.

So a typical worm gear with (say) 50 teeth on the gear will be a 50:1 gear ratio.

If there are 2 starts on the worm pinion (rare but they do make them) then the worm gear rotates 2 teeth for each rotation of the worm pinion.

To count the starts you can look at the end of the pinion and count the notches, OR you can rotate the pinion once and see if it rotates the worm gear 1 or more teeth.