# Gear ratio for ft•lbs

i’m having a problem finding the right gearbox ratio for an arm to left 60 ft•lbs with a falcon motor. any help is accepted.

ReCalc is a good option to calculate gear rations for an arm.

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Stall torque for a falcon 500 is about 3.46 ft-lbs, so you’ll want at least a 18:1 gearbox to achieve a stall torque of 60 ft-lbs. AndyMark had a 20:1 sport gearbox that should work.

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Good math, but you want a safety factor. I usually increase the calcualted reduction by 50% when looking at torque-limited scenarios like arms. So in this case, at least 27:1.

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I would go significantly further than that. If the goal is to “[lift] 60ft*lbs” I’m assuming that means they need to be able to hold position at that torque as well.

I would aim for a specific current to hold position at. Something more along the lines of 20-30A so that the system can be controlled precisely.

60ft*lbs = ~81Nm

@ 30A a Falcon produces ~0.55Nm

So the gear ratio should be ~147:1

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As it has not been mentioned yet, I would look in to “making your arm lighter” through the use of some gas springs or similar to help counter balance (teams have gone as simple as surgical tubing or screen door hinges in the past). This will reduce wear and tear on a gearbox, allow you to use less extreme reductions in the gearbox, not operate the motor at near peak power, etc.

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Another big factor in this is the changing load that the motor driving a rotating arm sees—the required torque changes with the cosine of the angle between the arm and horizontal. Finding a control scheme that is powerful enough near horizontal and yet controllable enough near vertical can be a challenge with heavy arms. Offsetting part of the weight can greatly simplify this.

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Can we back up a bit and ask how you came to the conclusion that you needed 60 ft-lbs of torque?

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