# Torque / HP to move a disc with 20,000 lbs on it

1. Jun 1, 2015

### josh0916

I am trying to spec out a motor to move a disc on a bearing, the disc has a mass on it weighing 20,000lbs. The diameter of the disc is 3.33ft. Angular acceleration of 0.1047 rad/s^2 to 3RPM. I am assuming coefficient of friction of 0.5.

1. Can I use the assumption that the mass on the disc doesn't matter much for my calculation and use that weight (20,000lbs) for my I=(1/2)*m*r^2?
2. Is the 0.5 coefficient pretty conservative in general for a disc on a bearing?

If so, I am getting I=55778
T=I*a*(coefficient of friction) = 2920
HP = 1.668

This sound about right? Thanks for any help.

2. Jun 6, 2015

### Greg Bernhardt

Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Jun 6, 2015

### SteamKing

Staff Emeritus
You don't use units, so it's hard to tell what you have calculated.

The equation for the mass moment of inertia, I = mr2 is probably OK for calculating the MMOI of the disk, but you haven't described the shape or distribution of the 20,000-lb weight on the disk, so who knows what to use for the MMOI.

Another problem is, in the MMOI formula, m stands for the mass of the object, not its weight, which is W = mg, where g is the acceleration due to gravity, or g = 32.2 ft/s2. Physics is pretty picky about these things.

Where did you obtain a coefficient of friction of 0.5? What kind of friction? To me, this seems a bit high, especially if a bearing is involved.

4. Jun 7, 2015

### CWatters

The mass of the disc might not matter if it's small in relation to the load.

What Steamking said.... The shape of the load might matter a lot.