Being a student, I vaguely remember from physics that what matters is not necessarily the RPM but what friction the wood bar will encounter on startup. If the force of gravity is pushing the wood bar against something else such as a plywood backing, there is potential for some additional static friction that would need to be overcome by the motor. However, in an ideal (frictionless) world, if you know your target angular velocity and the amount of time you require it to accelerate up to full speed in, then you can calculate your required angular acceleration. I assume 100 milliseconds to reach 5 rpm. f - frequency t - time r - radius m - mass w - angular velocity a - angular acceleration N - Newton (unit) rad - radian (unit) f =3D 5rpm =3D 5/60 rev/sec w =3D 2*pi*f =3D 1/6*pi rad/sec a =3D w/t t =3D 0.1 sec a =3D (1/6*pi rad/sec) / (0.1 sec) =3D 10/6*pi rad/sec^2 =3D 5/3*pi rad/sec= ^2 >From there you can calculate torque by multiplying the mass for the bar by the radius of its center of mass squared. I lumped the two halves of the rectangular bar into one for this calculation. r =3D 2m / 2 =3D 1m m =3D 5kg t =3D m*r^2*a =3D 5kg * 1m^2 * 5/3*pi rad/sec^2 =3D 25/3 * kg*m^2 / sec^2 = =3D 25/3 N * m =3D 8.33 N*m Now if I haven't made any mistakes, the calculation says that to accelerate your 5kg wood bar to full speed in 100 milliseconds (no friction) it will take a torque of 8.3 N*m. I would recommend a motor capable of providing at least 25% more than the calculated figure. So for a bar suspended in the air (low friction) I might suggest a motor with at least 10 N*m of torque. Looking at the torque rating of an 1/8 HP 8 RPM electric motor, I saw that it was rated for 40 N*m. I think you'll do fine with something similar. As always, this information comes with no warranty. Let us know how it goes. On Fri, Jan 9, 2015 at 5:28 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 Jason White --=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 .