Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Motor torque and size

  1. Feb 21, 2016 #1

    I'm looking for some info and advice. I've looked across the Internet and forums looking at different formulas for working out the torque requirements for a motor but can't find anything that helps me.

    I have designed a small amusement ride. Please see pic below.


    The outer ring (yellow) needs to rotate at approx 35rpm.

    The mass of the ring and everything inside it is approx 750kg.

    The diameter of the ring is 3m

    How do I work out the torque required to spin the disc? And does adding in a gear box effect the torque needed at the motor? It's hard for me to select a suitable motor without knowing the power required. But let's say for example the motor is 1800rpm. For me to gear that down to 35rpm how does that effect the torque needed by the motor?

    Any help and advice appreciated.
  2. jcsd
  3. Feb 21, 2016 #2


    User Avatar
    Science Advisor

    Excuse my bluntness but if you don't have the engineering skills to spec a motor how have you managed to do the structural design?

    As for motor selection, are you familiar with these?

    At constant speed motor torque has to only overcome friction, and will probably be a small fraction of the torque required for a reasonable start up acceleration. In other words, design for start up, not constant speed.
  4. Feb 21, 2016 #3
    I agree with everything billy_joule said...

    anyhow, first figure out your reduction ratio.. 1800/35 = ~50:1, so whatever torque you require to spin the ride, the motor needs to provide at least 1/50th of that before reduction.

    It seems to be designed so that it's relatively balanced, or should I say the center of mass is close to the rotating axis? You need to find out what the maximum torque is due to the that, and then add in any acceleration over and above that... from there you can choose a motor that will provide that kind of torque..
  5. Feb 23, 2016 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member


    Torque = Moment of Inertia * angular acceleration.

    If you know how fast it must spin up you can calculate the angular acceleration. Depending on how accurate you need the answer you might approximate the moment of inertia to that of a disc...

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook