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Torque required to drive shaft

by deadstar33
Tags: drive, required, shaft, torque
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Apr11-14, 07:03 AM
P: 32

I'm trying to choose the right motor for a test rig I've built. I have a shaft attached to a cylinder that weighs 6kg, and I need to know how much torque would the motor I'm getting need to be able to produce in order to rotate the cylinder at 100RPM.

My attempt:
Torque = Force x r
where r = radius of the cylinder = 0.08m

Force = mass x acceleration
where mass = 6kg

accel = velocity/time
velocity = r x ω (rotational speed)
where ω at 100RPM = 6.28 rad/sec
So v = 0.08 x 6.28 = 0.5 m/s
Therefore accel = 0.5/t

Therefore T = 6 x 0.5/t x 0.08
= 0.24/t Nm

Is it as simple as this or is there more to it? And how do I know what t should be to work out the acceleration if all I know is what velocity I want it to rotate at? Would every motor have the acceleration spec listed with it or something?

Thanks in advance.
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Apr11-14, 03:30 PM
P: 558
There is more to it than that:

1) Is the shaft the same diameter as the cylinder?

2) ω = 100RPM = (100 *2∏¸rad/revolution)/(60s/min.) = 10 rad/s approx.

3) There must be some friction or external force acting on the cylinder (other than your shaft) else once you start rotating it will continue to spin in perpetuity.
Apr14-14, 05:05 AM
P: 32
1. The diameter of the shaft is 0.017m (17mm) and the diameter of the cylinder is 0.16m.

2. Sorry you're right, it's 6.28 rad/s at 60RPM and ~10rad/s at 100RPM.

3. Do you mean like air resistance? Or friction due to having imperfect bearings on the shaft? I don't have much experience with motors so I'm just trying to understand as much as I can about the problem and how to select the right motor.


Apr14-14, 09:38 AM
P: 558
Torque required to drive shaft

I mean anything that provides resistance that requires a torque to keep the 100rpm going.

Or maybe you meant your question was how much torque is required to accelerate the cylinder from 0 to 100rpm?
Apr14-14, 11:37 AM
P: 32
Yeah that's what I meant. I'm guessing the torque will then depend upon how fast I'm accelerating it from 0 to 100RPM? i.e. if I want to be able to accelerate it from 0 to 100 in 1 second it would require more torque than the same increase over 10 seconds. If that's the case, it presumably depends upon the motor in question...
Apr14-14, 11:48 AM
P: 558
Yes, it will depend on how fast you want to accelerate it but also depend on the mass moments of inertia of the cylinder and shaft.
Apr15-14, 04:05 AM
P: 32
I see. Is there an equation that relates the mass moment of inertia to the torque in some way then? I calculated the mass moment of inertia of the cylinder and shaft together to be 0.033kgm^2.
Apr15-14, 04:27 AM
P: 558
Yes, there most certainly is:

∑T = ICM[itex]\alpha[/itex]


∑T = summation of all the torques acting on the object about the axis
ICM = the mass moment of inertia of the object about the axis passing through the center of mass
[itex]\alpha[/itex] = the angular acceleration of the object about the axis
Apr15-14, 05:05 AM
P: 32

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