# Calculate hold down force for rotating shaft

1. Jul 14, 2015

### deserttech

I have a .850" diameter steel shaft that will have a torque of 125 ft-lbs put on it. I need to calculate how much down force I need to stop the shaft from rotating. I suppose we can assume that it will be steel on steel.

Going with a coefficient of friction of about .35 I tried to calculate with the typical CoF problem of a box and an incline, but I don't know how steep to make the incline.

How can I figure out how much force it will take to stop a .850" diameter shaft from rotating under a torque of 125 ft-lbs?
Thanks

2. Jul 14, 2015

### A.T.

Depends on how quickly you want to stop it.

3. Jul 14, 2015

### deserttech

Maybe, I didn't describe it fully. I need to know how much force it will take to clamp the shaft before I apply the torque so that it doesn't move. I'm not sure how to calculate that.

4. Jul 14, 2015

### Staff: Mentor

You can get a pretty good start by looking up the coefficient of static friction between the material of the shaft and the material of the clamp block. The force that you apply to the clamp block will be the normal force generating a static frictional force; this frictional force acting on a radius of .425" has to generate enough torque to resist the 125 lb-ft that you'll be applying.

You'll want to consider the strength of everything involved to make sure nothing will break.

Last edited: Jul 14, 2015