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I need to specify a stepper motor to drive a system of gears that isn't yet produced. I have almost no information about the system except that it will involve a stack of planetary gear sets with a common 3/8" shaft, with a total weight of about 100lbs (mixed metal and plastic). It will need to rotate very slowly, about 1 rpm, and occasionally reverse.

I'll be able to get hold of one planetary gear set soon. What would anyone recommend to get a handle on the needed motor? I figured I'd wrap a wire around the shaft and pull it with a fish scale to see how many ounces it takes to move it, then multiply that by the number of sets. Given that value, how could I convert that to a motor spec? I'm assuming that most of the requirement will be in overcoming the initial inertia & friction.

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It is the same as everywhere else. You do not need to calculate torque based on the design of the reducer/motor.

you need to figure out the ratio that specific gear vs motor will spin at. Once you do that, calculate the power of the pull, are you pulling with 5lbs or 10lbs of force? once you figure out the pull/push factors, you can translate these values into usable equations. since pull/push is power distribution it will always be able to be converted.

once you calculate the ratio and pull factors, you can then translate these values into torque. These calculators are available all over the net.

(you can use a food scale weight hook to determine the amount of pull in lbs tied to the end of the string)

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    $\begingroup$ Thanks for the comment. I've been away and couldn't reply. One issue is that each of the 20+ gear sets will use a different ratio. So it seemed as if a purely empirical method would be best: how many pounds of pull does it take to start it moving and then to keep it moving? With those values in hand, what else do I need to know to calculate torque? Can you recommend an online calculator as you mentioned? $\endgroup$
    – Jim Mack
    Commented Feb 26, 2018 at 13:38
  • $\begingroup$ Is it possible to weigh each of the Gears and get a measurement of the diameter of each one? You can also go above and beyond to try to get the measurements of the gears themselves. For instance if the pitch is lower = higher torque required, if pitch is higher = lower torque required. Some colleagues might disagree with me though. I will look for a calc for you. $\endgroup$
    – DeerSpotter
    Commented Mar 1, 2018 at 19:15
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    $\begingroup$ As is often the case with projects I work on, a great deal will remain unknown until well after buying decisions must be made. I'm forced into a 'best guess' or at least an 'OK guess' here. As a result I've spec'd a motor that I'm sure is overkill. Thanks for trying to help. $\endgroup$
    – Jim Mack
    Commented Mar 1, 2018 at 22:37

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