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dodgy lead screws?

Posted by friarfish 
dodgy lead screws?
September 14, 2016 10:19AM
Hi folks,
a while ago I purchased some 8mm lead screws off of ebay. Just gotten
around to playing with them today. However there is noticeable wobble
by the brass nut along the length of the screw as well as angular wobble
around the shaft.
Is this normal for lead screws or, as I suspect, just another example of
c**p chinese products on ebay?

Re: dodgy lead screws?
September 14, 2016 11:40AM
Normal. Lead screws are used to provide motion, not guidance. That's why they are normally paired with guide rails or linear guides. You can reduce the axial slop by using antibacklash nuts- really just two nuts that are pushed apart by a spring which forces them to be maintain contact with the screw threads at all times. It also increases friction.

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Re: dodgy lead screws?
September 14, 2016 11:47AM
Normal for TR8 style leadscrews and nuts. They are not intended to be a tight fit. In the radial direction the platform should be constrained by the linear rails, not the leadscrew. Axially these are not intended to be lash-free, or even low-lash, so they are only really suitable in applications where the screw is either operating uni-directionally or against a constant force. It's ok for Z as long as the platform moves smoothly on the rails because gravity will hold the platform down against the same bearing surface of the nut at all times.

[DD beat me to it smiling smiley]

Edited 1 time(s). Last edit at 09/14/2016 11:50AM by JamesK.
Re: dodgy lead screws? solved
September 15, 2016 03:32AM
Thanks folks,
I wasn't quite sure. The expensive "professional " lead and ball screws with
very substantial price had me wondering if it was just an issue of price vs quality.
The use of a backlash mechanism or just letting gravity keep the nut flush
are probably what i'll do.

Thanks again.
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