enter image description here I often need to figure out the linear velocity to the rotational velocity. If a roller is rotating at #frame/n and the roller has a diameter D, what is the resulting linear velocity?

  • $\begingroup$ this might help as well en.wikipedia.org/wiki/Angular_velocity $\endgroup$
    – Chris
    Jan 14 at 9:26
  • $\begingroup$ In some cases, this rig might be easier to animate backwards.. with the box driving the rollers. But you've probably already decided on that. $\endgroup$
    – Robin Betts
    Jan 14 at 11:09

1 Answer 1


Your question is not very clear about what you mean by "is rotating at #frame/n" because this means technically an acceleration/deceleration. Also the meaning of n is not clearly stated so I will assume it's a constant number. I can give a general explanation:

If you don't want to include simulated friction or slip losses, you simply need the Peripheral Speed of one cylinder. It can be calculated by P_s = pi * D * RPM where:

  • pi ~ 3.1416 [no unit]
  • D is your diameter [meter]
  • RPM "rounds per minute" [1/minute] (in case of Blender it might be more useful to give it as RPS "rounds per seconds") resp. [1/second]

pi and D are known, so you only have to derive RPS from Blender's driver expression #frame/n which has to be in [1/minute] resp. [1/second]. For that you probably have to calculate the duration of one frame. If you have 24 FPS (see your Output Properties) then one frame lasts 1.0 seconds / 24.0 ~= 0.042 seconds. If you clarify your question, I can give a more detailed answer.

  • 2
    $\begingroup$ your answer is totally right (+1 from me), but maybe you should write additionally "if n is a constant number" which you assumed, but maybe JB Riley meant something different - so he you might misunderstood each other - and i think he should have mentioned what "n" is. $\endgroup$
    – Chris
    Jan 14 at 9:25
  • $\begingroup$ yes. you are right. i updated my answer. $\endgroup$ Jan 14 at 13:21

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