Torque needed to turn a steel shaft on sandstone

[Lazorbeam]

I'm auditing someone's work and it seems I've found an error.

We have a very simple scenario where a circular steel shaft lies on sandstone. What is the torque required to turn the shaft (the shaft is in a static position)?

Torque = weight of shaft × radius of shaft × friction of steel on sandstone

It seems that he has divided the total by 2 which doesn't make sense to me. Then again my physics days are way behind me.

If someone could confirm that I'm crazy/lucid that would be great.

 

[Delta2]

Yes i think you are right , all you need to do is to counter the torque of friction which seems to be equal to what you state. er to be more precise torque=weight x radius x friction coefficient

But if by shaft you mean a solid shaft where the mass is distributed in the whole surface of shaft and not in an outer ring of small thickness then the torque of friction is different.

 

[tygerdawg]

Two torque components.
(1) torque to overcome friction of body at contact point T1 = (force)(radius). Force = (weight)(coeff of friction). (??...perhaps)
(2) torque to accelerate the mass of the bar from rest T2 = (mass moment of inertia)(angular acceleration)

 

[Randy Beikmann]

Is the shaft supported at the driven end? If only one end of the shaft rests on the sandstone, then the normal force causing friction is only half the shaft's weight. That may be why he divided by 2.

 

[jack action]

Are you sure he did not use the shaft diameter in his calculation instead of the radius? In that case, you have to divide the diameter of the shaft by 2 to get its radius.