I’m working on a prototype and I’m having trouble picking out a motor for that job. I need it to be able to move a rope that is about 2-4 mm thick outward (like a winch in reverse almost) and I also need it to be quick. Quick being about 0.025 mm per second. How would I determine this exactly? Unfortunately, math has never been my strong suit and while I have been looking over datasheets I’m not understanding them too well. What I’m asking is how do you determine if a motor has enough torque AND enough speed for an application.

  • $\begingroup$ There is not enough data here to do the calculations. You would need to know the frictions to determine the maximum speed. 0.025mm per second seems very very slow. Are you sure your unit is right $\endgroup$
    – joojaa
    Commented Jun 27, 2017 at 18:13
  • $\begingroup$ Apart from the data for the calculations, what are you trying to accomplish? I am having trouble visualizing what you mean by a winch in reverse since without something pulling on the rope, you would basically be spewing it all over the place out of the reel/pulley. $\endgroup$
    – Secundus
    Commented Jun 28, 2017 at 20:53

1 Answer 1


There are a few things that are important, including the characteristics of the load. Is it a constant load? Does it change slowly? Quickly? Make sure to take this into account.

You can get some mileage out of the standard formula for mechanical power, which is $P=\tau\omega$. In other words, for a constant power motor, you can have high torque & low velocity, or low torque & high velocity. So, you can start by calculating the torque required to drive your load. Then, the angular velocity to move the rope at your desired rate of 250 microns/sec. It's straightforward to compute power from that, but don't forget to add a safety factor.

Once you've got the power requirement, you can consult the datasheets to see which motors can deliver that amount of power. I would also recommend using motor sizing tools provided by vendors. For example, this one from Portescap, or this one from Kollmorgen.

  • $\begingroup$ Well I kind of get it but not that much. How exactly do I find out how much torque will be needed? Also won't the amount of torque needed also effect the speed I can go at? So how do I take that into account. $\endgroup$
    – Sie
    Commented Jun 27, 2017 at 3:19
  • $\begingroup$ @Sie you use gearing to get the power into the correct speed range $\endgroup$ Commented Jun 27, 2017 at 7:35

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