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

Phil is of course correct in his assumption that even ball screws will have
some backlash but in a properly designed system that backlash will be in the
low tenths range or even less in high precision systems. I am not aware of
a single high precision CNC machine that does not use ball screws in some
way shape or form.



Another reason besides backlash to use ball screws has to do with the
efficiency of a ball screw versus the efficiency of a lead screw. A lead
screw is very inefficient in converting rotary motion to linear motion,
usually in the 15% efficiency range. A ball screw will approach 85%
efficiency (simply reason is a lead screw has sliding friction between the
screw and the nut, a ball screw has a much larger portion of its load
carried as rolling friction by the balls themselves). This allows for much
smaller drive motors as well as much faster movement of an axis...



When searching for ball screws you need to look for "preloaded" ball screws.
This means the manufacturer has taken steps to eliminate back lash, usually
by having two ball screw runs in a single housing that is either spring
loaded or made in such a way as there are two races in a very slight
interference fit that preloads the bearing for near "no backlash".,



Jim DuBois

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Yahoo
Sent: Thursday, November 08, 2007 9:00 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: anti-back lash nuts




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. But again, far as I can tell, all
ballscrews are not created equal, and you can buy a cheesy ballscrew with
lots of backlash. 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. And with ballscrews there is no backlash adjustment.
You get what you get. So you have to rely on the manufacturer's reputation.
Either that or empirical measurement, which is rarely practical before you
buy. So unless you are prepared to spend the money for a new brand-name
ballscrew, you're still at risk. It's really a question of whether it's
worth the investment, how much you will use the machine, how much precision
you really need, etc. But for CNC, if you choose leadscrews, I would suggest
that backlash adjustment capability is pretty much essential.
--
Phil Mattison
http://www.ohmikron.com/
Motors::Drivers::Controllers::Software






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