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

[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.

 

[SteamKing]

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.

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.

 

[CWatters]

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?

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.