The simple answer is the old standby of f=3Dm*a. The angular acceleration version of that is a little different ( http://en.wikipedia.org/wiki/Angular_acceleration), but the same concept. In the end, the answer is going to mostly depend on information you did _not_ give us. How quickly does it need to come up to speed? What are the other loads on it? Will people or other objects come in contact with it? If so, How will their safety be assured? Will it be stop - start, or continuously run? What environment will it run in? How fast do you need it to turn? (a "few RPM" is vague, 2-3 RPM is vastly different than 20-30 RPM) And the one that is probably hardest to understand and most important - why are you doing this? Production product vs. one-off -> different answer. Hobby or "art" vs. paid product -> different answer. On Fri, Jan 9, 2015 at 3:28 AM, David C Brown wrote: > I am thinking on an application where a DC motor has to rotate a wooden b= ar > in the horizontal plane - motor spindle vertical. The bar will be about= 4 > metres long, weigh about 5kG and is balance about the rotation axis. > Rotation speed a few RPM. > > How do I calculate the torque rating of the motor? > -- > __________________________________________ > David C Brown > 43 Bings Road > Whaley Bridge > High Peak Phone: 01663 733236 > Derbyshire eMail: dcb.home@gmail.com > SK23 7ND web: www.bings-knowle.co.uk/dcb > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .