# Ball Screws and torque

1. Dec 5, 2016

### SevenToFive

I have a set of 4 ball screws, 2 inch diameter by 0.50", one in each corner to lift up a 12,000lb transformer. Each side will have one motor driving a right angle worm gearbox in the center that will power the two other right angle gearboxes that turn the ball screws.
I was given that the torque for each ball screw will be 1800in-lbs. But I am curious on how they came up with that value. I don't have much experience working with ball screws usually we do all Acme threads, or is the calculation similar?

Any help in double checking their calculated torque would be greatly appreciated, as I feel it is a bit low.

2. Dec 5, 2016

### JBA

The torque is based upon your load and the pitch and radius of the ball screw. If they supplied an attached right angle gear drive then the gear ratio of that unit will also be a part of that calculation.

3. Dec 8, 2016

### Baluncore

The computation is the same as for acme threads, but there is much less friction in ball screws.

I assume the ball screw has an outside diameter of 2” and a single-start thread with a pitch of 0.5”.
The screw circumference is 2 * Pi = 6.28”.
The ball is in effect, on a ramp with a slope of 0.5” in 6.28”. That ratio is 12.57
The load weighs 12,000 lbs. That is 3000 lbs carried by each screw.
Divide by the ratio of 12.57 and you get 239. lbs, acting at the screw circumference contact with the ball nut.
The radius to the circumference of the screw is 1 inch, so the minimum torque to hold the load in place is 239 inch lbs.
Does that seem to be a more reasonable value ?

4. Dec 18, 2016