I am building a robotic arm with these specifications:

  • 1 meter long
  • approx. 1kg weight
  • it is made of 4 motors. (2 at the base. One for rotating the whole arm left and right and another one for rotating up down. 1 motor for rotating the second half of the arm, only up and down. 1 for the claw used for grabbing)
  • it must be able to lift at least 4kg + 1kg (it's own weight), and have a speed of 180 degrees in 2 seconds = 360 degrees in 1 second resulting 60rpm.

Which kind of motor would be best for the project (servo or stepper) and how much torque will it need? (Please also give me an explanation of how I can calculate the torque needed). Could you also give me an example or two of the motors I would need and/or a gearbox for them (models or links).

  • 1
    $\begingroup$ "180 degrees in 2 seconds = 360 degrees in 1 second resulting 60rpm." That doesn't sound right... That's rather 30 rpm (if you ignore acceleration). This is some serious speed, are you sure you need or want that? This will be a rather massive arm and expensive if you want some accuracy. $\endgroup$
    – FooTheBar
    Commented Aug 23, 2016 at 14:49
  • $\begingroup$ Yes, you are right.. I divided instead of multiplying the seconds.. Well yeah I would like to build an arm that can be moved by buttons(for example button 1 moves the arm up, button 2 moves it down.. The more you press it, the more it rotates).. I would like to mount the arm on a moving platform and be able to lift things around it(about 4 kg.. maybe 3).. I bought everything I need + some small motors to do the testing(they are dc motors with H-bridges), but now I need the motors that will be used for the actual arm(The body of the arm will be 3d printed).. $\endgroup$ Commented Aug 23, 2016 at 20:57
  • $\begingroup$ I searched the web for motors for about 10 hours in total the last week or so but I couldn't really find something that would do and be in the price range as well(And if it was it would deliver only if you buy something like 200). Can you help me find a model or give me some idea for it(The thing is that I am still in collage so I don't really have a source of money and that's why I can only put about 200 euro, max. 300 for the motors) $\endgroup$ Commented Aug 23, 2016 at 20:59
  • $\begingroup$ I'm sorry to tell you, but with that funding, you can totally forget your project. You describe an arm that costs (ballpark) 20.000$ if you build hundreds or thousands of them. You also want it to lift 4kg with a 1kg arm which means really expensive materials. 3D printed stuff (if you don't print in titanium) will just evaporate at this speed. $\endgroup$
    – FooTheBar
    Commented Aug 23, 2016 at 21:14
  • $\begingroup$ So, to clarify, following on from FooBar's initial comment your statement should read: "180 degrees in 1 second => 360 degrees in 2 seconds, resulting in 30 rpm" or "180 degrees in 2 seconds => 360 degrees in 4 seconds, resulting in 15 rpm"? $\endgroup$ Commented Aug 26, 2016 at 9:31

1 Answer 1


This type of question "how much torque does my robot arm need" has been answered many times on this site:

The equation for torque is pretty simple. It is just force times moment arm length. So the lift motor at the base of the arm will require: 4 kg * 1 m + 1 kg * 0.5 m = 4.5 kg m, or about 44 Nm. The elbow motor will require 4 kg * 0.5 m + 0.5 kg * 0.25 m = 2.125 kg m, or about 21 Nm.

  • 2
    $\begingroup$ And that is only the gravity compensating part of the torque, if you want to move that thing, you need even more. $\endgroup$ Commented Aug 24, 2016 at 7:32
  • $\begingroup$ Yes, good point. I believe some of the above links discuss this. Remember that it is not velocity that drives the torque requirements, but acceleration. $\endgroup$
    – Ben
    Commented Aug 26, 2016 at 13:17

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