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Ball Screws and Torque calculations

Ball Screws and Torque calculations

(OP)
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.

Thanks in advance to those who reply.

RE: Ball Screws and Torque calculations

One thing that can have a drastic effect on torque is the alignment of the ball screws. If the ball screws are rigidly mounted to the load and the support above(below), slight misalignment can result in binding. Think through your guidance and mounting so the the ballscrews do not bind.

I have always used Duff-Norton calculation methods for screw actuators. You can find good literature from most any supplier.

RE: Ball Screws and Torque calculations

Do you mean .5 inch advance per 2*pi of turns of the screw? If the internal friction is negligible the work in should match the work out. Force times distance should match torque times angle; divide by the number of screws and the angle per turn and get:

the torque should be (12,000 lbf/4 screws)*.5 inch/2*pi turns = 240 lbf-inch/radians of turn/screw.

This is the lowest limit; losses might double it, but I expect the largest amount is safety factor.

At a low amount of friction, the diameter of the ball screw no longer matters.

These guys suggest 90% efficiency: http://www.nookindustries.com/LinearLibraryItem/Ba...
So do these: https://www.festo.com/net/SupportForum/yaf_postst6...

RE: Ball Screws and Torque calculations

I came up with a slightly higher ball screw driving torque than 3DDave using the approach described by Nook, but I included drag torque based on 10% preload. As noted, Nook suggests using a 90% efficiency for driving torque, and 80% efficiency for back driving torque.

It was not clear from the OP exactly what point in the system the driving torque for each ball screw was being determined. For example, between the drive motor and ball screw shaft there is a worm gear stage and a bevel gear stage, both of which involve efficiency losses. If you take a look at catalogue ball screw jacks from Nook, they recommend using a 1.5 factor for starting torque versus running torque.

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