# Determine moment of Inertia from resistance

1. Feb 11, 2013

### barvas11

Hi Guys,
I am trying to calculate a starting torque required to move the drum at 6rpm that weights 2000kg and has 1200mm in diameter.

There would be no problem to do this calc in normal circumstances as I can find a moment of inertia formula for a pipe in my book.

But in this case there is a tension of 3000N acting on the rim of my drum.

What will be the formula for the moment of inertia in this case?

Thanks

2. Feb 11, 2013

### SteamKing

Staff Emeritus
The moment of inertia of the drum is not affected by the tension. However, overcoming this tension will figure significantly in the torque required to spin the drum.

What is this tension? Is it friction?

3. Feb 12, 2013

### barvas11

Hi,
No it's a wire injected in the steel coil tube and I am trying to take it out. I am currently building a special machine for it and need to select the right type of motor. I was thinking of powering the drum with light duty bonfiglioli gear box housed from one side of the drum or having a sprocket arrangement with the ratio 6:1. Sprockets would significantly drive down the torque but the gearbox would suit much better for this arrangement.

Yes that's right the tension will increase the torque value significatly but I don't know by how much.

4. Feb 12, 2013

### SteamKing

Staff Emeritus
Well, as a first guess, I would say 3000N times the radius of the drum.

5. Feb 15, 2013

### barvas11

Thanks,
Have you got any idea how to incorporate a sprocket ratio into my torque calc?

Diameter of the wheel that moves the drum has 38mm and drum has a diameter of 1200mm

I incorporated the ratio into my moment of inertia of the drum.

So the formula would be the following: I=Id*(38/1200)^2

Id is the moment of inertia of the drum

What do you think of that?

6. Feb 15, 2013

### SteamKing

Staff Emeritus
The inertia of the drum is a physical property of the mass of the drum and how that mass is distributed about the axis of rotation.

7. Feb 15, 2013

### barvas11

It is but how does my pulley ratio affect that property?

8. Feb 15, 2013

### SteamKing

Staff Emeritus
That's what I am trying to say: the inertia property is NOT affected by pulley ratios. The torque required to turn the drum will be affected by the pulley ratio, but not the inertia.