Re: [CAD_CAM_EDM_DRO] Re: anti-back lash nuts

Yahoo wrote:

> I have watched a lot of debates over ballscrews vs leadscrews, and best I
> can figure, there are thee main reasons ballscrews are better: 1) Ballscrews
> can be made with much greater precision than leadscrews, and 2) ballscrews
> wear much more slowly than leadscrews, because 3) the use of ball bearings
> reduces friction enough to allow much tighter tolerances. But there is no
> such thing as zero backlash between moving parts, and I suspect what that
> really means is negligible backlash.

Anti-backlash ball nuts have NEGATIVE clearance, also known as
preload. The metal is actually in compression, so even when
loads are applied to the nut, the negative clearance remains in
force, up until the load overcomes the preload. This is often
1000 Lbs on small ballscrews, and many tons on large ones.
But again, far as I can tell, all

> ballscrews are not created equal, and you can buy a cheesy ballscrew with
> lots of backlash.

Power transmission ballscrews have plenty of backlash.
Precision motion control ballscrews with selected-fit
anti-backlash ballnuts do not.
Otherwise we wouldn't see people talking about replacing

> the ball bearings. Any backlash due to wear would have the same problem as
> leadscrews: more wear at the center. So replacing the ball bearings would
> not solve that problem.

Worn ballscrews are reground by professional rebuilders for far
less than the cost of a complete new one, as they only have to
take off .001" or even less to restore it to new accuracy.

The wear on a ballscrew is not just a little less than an Acme
screw, it is several orders of magnitude less!
And with ballscrews there is no backlash adjustment.
Not neccessarily true. Many newer ballnuts are not adjustable,
other than by replacing the balls with oversize ones, which are
available in .0001" increments. But, there ARE adjustable
anti-backlash sets which have two simple nuts with a firm screw
adjustment between them, or a stack of Belleville springs to
apply the preload. these are a LOT less common now, but you
still see them from time to time.

Jon