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understanding screw torque

Posted: February 22nd, 2018, 12:03 am
by farmer
Looking at different worm gearheads, I notice that "full load" spinning torque does not double when the gear ratio doubles. I assume this is because when the screw turns twice as far to spin the gear the same distance, it generates twice as much friction and heat. I have two questions:

1) Assuming the motor is stalled, does the torque ratio begin to match the gear ratio? Meaning one doubles when the other doubles? Because the instantaneous force on the gear has to overcome the same amount of static friction.

2) In general, I think the torque and the power consumption is highest when a permanent magnet DC motor is stalled. Is this true, and does the exact level of torque when stalled depend on the position in rotation of the inner magnets relative to the outer ones?

3) What is a typical stall torque compared to spinning torque in a 60:1 worm gearhead? Steel worm, bronze gear 1-inch radius immersed in gear oil.

Here are some references:

torque calculator: ... lation.htm
torque examples: ... and-torque
stall torque: ... tor-types/

Re: understanding screw torque

Posted: February 22nd, 2018, 12:37 am
by farmer
I think I figured it out. A gearhead with a low ratio will not burn a lot of energy just spinning. But the worm has a bad angle on the gear, so it is not very efficient at low speed. So a 10:1 gearhead might have a stall torque that is no greater than the full speed torque. But as the ratio goes up. to say 60:1. the full speed torque does not rise 6 times, because a lot is spent spinning. But the work has a better angle on the gear, so it when stalled it might push six times as much torque as when operating full speed.

So I could rough guess the stall to spinning torque, as the gear ratio divided by 10. As the gear ratio goes up, the stall torque does go up more proportionally than the spinning torque, which does not rise as much as the ratio.

Re: understanding screw torque

Posted: February 22nd, 2018, 12:56 am
by Traden4Alpha
Yep, wormgears are very inefficient but a lot simpler and cheaper than a multistage planetary. They also many types are self-locking which is nice because it means you can shut off the motor and the gear won't turn.