# How to calculate required hp for machine

Tags: machine, required
P: 4,663
 Quote by OldEngr63 Realism means that you really believe that 41 watts will do this job. I don't. Look at your overall calculation and tell us that the whole scenario is realistic, if you believe it is.
First, you have to do the math.
If the stored rotational energy after 60 seconds is
$$E=\frac{1}{2}I\omega^2=\frac{1}{2}mr^{2}\omega^2 = 1234 \text{ Joules}$$ then you have to add energy at the rate of 1234/60 joules/sec = 21 watts for 60 seconds.
This does not include motor inefficiency, friction losses, not using the right gear ratios, etc. Please check my math.
 P: 343 @ Bob_S: I did do the math, in considerably more detail than it appears you have done. It appears that you are simply saying that I = MR2 = 17.956 kg-m2 Note that the radius value you used is to the outer diameter of the rubber layer, well past the last of the steel structure. This gives an excessively large estimate for R. By a more detailed reckoning, based on the drawing, I came up with the figure that I gave previously I = 16.5 kg-m2 I think that you have somewhat over estimated the MMOI by putting it all at the rubber layer, well outside the steel, considering that part of it is internal structure. N = 100 rpm → ω = 10.47 rad/s ω2 = 109.66 1/s2 Because of the difference in the estimates for I, we differ substantially in the total energy content of the rotating roll. This is where our differences lie. My comments about these values being minor compared to the losses in the system and therefore not useful for determining drive power requirements still stand.
P: 4,663
 Quote by OldEngr63 @ Bob_S: I did do the math, in considerably more detail than it appears you have done. It appears that you are simply saying that I = MR2 = 17.956 kg-m2
You are correct. I decided to put the maximum value in $I=mr^2$, which is exactly twice the moment of inertia for a solid cylinder. It adds a few % to the result, as you found out.

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