Torque calculation for thick pipe

[vtmike]

Hello,

I am stuck on what should be a fairly simple problem. I need to calculate the torque required to rotate a hoolow pipe weighing 300 lbs with OD=6" and ID=4" about its center axis. This pipe is resting on two sets of roller bearings.

This is how I calculated the torque,

mass, m = 300 lbs = 9.324 slug
Hollow tube with OD = 6" & ID = 4"
Initial angular velocity W1= 0 rpm
Final angular velocity W2= 20rpm
Change in time, Delta t = 3sec

So using,
Moment of Inertia, I=(m(OD^2 + ID^2))/2 = 242.42 lb-s^2/in
Angular acceleration, alpha = (W2-W1)/delta t = 6.67 rad/s^2

Torque, T = I*alpha = 1616.97 lb-in

The torque value looks too high....Not sure if my procedure is ok?

Also since the pipe is resting on roller bearings, the weight of the pipe is transmitted to the bearings, so should the complete mass be included in torque calculation to rotate the pipe?

It will have to overcome the friction in the bearings to start rotation. But how do I include that into the calculation?

Any help would be appreciated!

Thanks,
Mike

 

[rb1957]

alpha = W2/t = 20*(2pi/60)/3 = 0.7 rad/s^2 ... i think you forget to convert rpm to rps, no?

 

[vtmike]

Yes I did! I thought I had messed my mass units, but I guess it was angular speed.

The torque goes down to 200 lb-in after conversion. Now that looks like a sensible number.

Thanks,
Mike

 

[vtmike]

But still how would I factor the friction in the roller bearings in the torque calculation?

 

[ARenko]

You add the drag torque of the bearings to your calculated torque. Unfortunately the chance of finding bearing drag information is slim - it's difficult to predict and will change over time as well.

The roller is just resting on the bearings right? I mean there is no nip force on the roller holding it against those bearings, correct? Nip rolling resistance, air resistance, and drive losses all affect total drag.

There are ways to measure total drag on a roller, but that doesn't help you in the initial design unless you have something similar you could experiment with. Unless the bearing mfr can give you some friction info for your bearings just add some torque to cover the bearing and drive losses.

 

[hydtools]

You also would need to account for the roller inertia.

However, I would expect that compared to the pipe, the rest of the resisting loads are very small.

Ted

 

[zekeman]

The static drag and rolling friction are negligible compared with the accelerating torque you need.