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Torque required to drive shaft 
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#1
Apr1114, 07:03 AM

P: 32

Hi,
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. 


#2
Apr1114, 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. 


#3
Apr1414, 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. Thanks. 


#4
Apr1414, 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? 


#5
Apr1414, 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...



#6
Apr1414, 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.



#7
Apr1514, 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.



#8
Apr1514, 04:27 AM

P: 558

Yes, there most certainly is:
∑T = I_{CM}[itex]\alpha[/itex] where ∑T = summation of all the torques acting on the object about the axis I_{CM} = 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 


#9
Apr1514, 05:05 AM

P: 32

Thanks



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