# Rotational force of motor + potential gear reduction

I'm trying to determine how powerful of a stepper motor I need for a project. I am attempting to take a display that weighs 15lbs and transition it from laying flat to inclined (approximately 45 degrees) using a stepper motor. I had considered a linear actuator since it would have been an easier build but clearance under the display is an issue and I have more room horizontally.

I haven't done the exact measurements for the lifting arm as I'm not sure if a gear reduction will be needed yet but for example purpose I was thinking of a 5in moment arm from the center of the motor. Even though the display will pivot at the bottom and be supported I figured that when laying flat the motor still has to have enough force to lift the full 15lbs at the 5in displacement for a moment of 75lb-in.

Motors I have looked at talk of a "holding torque" value as what describes them. Did a little research on holding torque but still not 100% understanding the term. These values, for most ideal sized motors, rarely exceeds 250 oz-in or about 15.5lb-in.

Am I right in needing to optimize "holding torque" > moment? In which case if I want to utilize the 15.5lb-in motor I would then need to perform at least a 1:5 gear reduction in order to boost my torque to 77.5lb-in to accommodate the max moment?

Thanks for any input.

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Mech_Engineer
Gold Member
You are correct in assuming you'll want the holding torque of the motor/gearbox to be greater than the required torque (plus a safety factor). Ideally, you'll want the motor's "self-braking" torque to be greater than the required torque in the upright position, so that you don't need to power the motor to maintain position of the screen. In that case, the powered torque of the motor will be plenty strong, and you'll be limited by the resting torque of the motor/gearbox combo.

Is a motor's "self-braking" torque a factor of holding torque or is it variable based on motor design/application. When looking at most stepper motors holding torque is typically the only torque value given. Otherwise the specifications are just electrical and physical sizes. Thanks for the self-braking point as I do not want to have the motor constantly powered to resist the load.

I came upon the idea of replacing a stepper and gear reduction with a worm gear motor. I know with worm gears you can't drive the input with the output technically so with my output being the force from the upright screen, would I then be safe in finding a worm gear motor that has a torque rating higher than what my torque requirement is?

Mech_Engineer
Gold Member
I think a worm gear could be a good idea for the reason you're stating- they are very difficult to back-drive so you could position the screen and then power down the motor. It will also be easy to find larger gear reductions when using a worm gear. Will you be using limit switches to detect the position of the screen? If so, it might not be necessary to use a stepper motor at all, and you could instead use a simple DC motor.

That is to be my new problem because with the stepper motor I hadn't planned to use limit switches and base my code off of step counts to provide limits that way. I know I will have to add in limit switches now yes (not quite sure how or best options yet). I had less experience with limit switches and more with coding so that was the reason for stepper over DC motor but I will look into both worm gear and DC motor since I will need limit switches with either.

I appreciate all the info.... Civil Engineer attempting to be a Mechanical Engineer here lol. Now it is onward to limit switch options.

Mech_Engineer