How To Calculate Torque for a Clutch??

Is this the correct way to determine the clutch rating?

Shaft Speed1352 RPM
Power Input 4000 watts = 4000/746 = 5.36 HP

HP = Torque * RPM / 5252

Torque = 5.36 / 1352 * 5252 = 20.82 ft-lb
20.82 ft-lb = 249.87 in-lb

So this clutch should be close enough to work?

looks right.

how do these type clutches not slip on the shaft? Do they bite in to the shaft somehow?

They do bite the shaft. This pdf shows how they work. http://stockdriveproducts.com/D785/PDF/D785BC014.pdf

Bubba