Q:

How much torque is required to operate a barley crusher?

Question:
How many in-lbs or ft-lbs of torque are required to crush barley using a mill such as the Barley Crusher?
Motivation:
I'm interested in motorizing my Barley Crusher.  There are endless blogs entries and such on how to do this but the parts used often vary based on industrial salvage or the cheap motor of the week.  Selecting equipment for this type of project requires a minimum torque specification I don't have, hence the question.
No guessing please; I'm looking for a definitive answer for the community.

A:

I've built 2 motorized grain mills. The first used a 600w (input power) 3/8" drill. It worked well enough with a loose crush (0.040") but I ended up frying the drill when trying to crush at 0.035", so this is a good candidate to gauge the minimum torque.
A 600W drill produces about 380W mechanical output. I was using this on just under half power - the drill used PWM to reduce the motor speed. The rotational speed, was 240rpm, or 4 rps. This gives a torque of 
 T = 190 / 2 x 3.141 x 4
   = 190 / 25.128
   = 7.5 Nm

That's about 66 lb.in.
However, unless you can get a motor with a rpm that matches the sweet spot for milling grain, 120-260 rpm, then you'll have to use gears, sheaves, PWM, variac or some other speed controller to regulate the rotational speed of the mill. All of these have an affect on the torque - most motors spin too quickly and so you'll need to slow them down, which increases the torque if using mechanical speed control. This makes the torque of the motor less of a pressing issue. This leads to the typical advice found in the forums that a 1/2 hp motor is more than sufficient when used with speed reducing transmission.
My second mill used a 340W washing machine motor, with a 1:15 sheave. This offered more than enough torque - the limiting factor was the belt slipping, which happens on the rare occaision I get a rock in there, or when I inadvertently drop the feeler gauge in the grain chute!

