Re: [CAD_CAM_EDM_DRO] Re: Acme screws and pre-load nuts was:Re:Re:greetings

Ray wrote:

> > Dan Dowell asks,
> > >Here I go again. Whats a preload nut? Would like to see one.
> > >THANKS Dan
> >
>
> > So, what are we lookin' at:
> >
> > 1.There are two nuts on the screw.
> > 2.One is fastened to the machine(labeled NUT, above)
> > 3.The second one is not fastened to the machine,but is PUSHED AWAY
> > from the first nut(by a spring, shown with the 0's,above)
> > 4.The second nut is not allowed to rotate independant of the 1st
> NUT.
> >
> > The two nuts are pushing outward against the flanks of the acme
> > threads with the force of the spring. As long as we don't push on
> the
> > assembly more than the force of the spring can resist(the pre-load),
>
> > we have created a rigid nut with no backlash when changing
> > directions.

<snip>

> I believe that many bridgeports use two nuts in a cage. By rotating
> one
> with respect to the other, the teeth of the nut thread are made wider
> and expand toward the canyon walls. Using these kinds of things, you
> can
> take up some of the backlash without increasing the drag much at all.
> But
> if you set the backlash where the screw has worn in, you will never
> get to
> the ends of the travel where the thread has less wear.

The way it is done is that the nuts are a bronze cylinder with a keyway
in them, and
they fit into a flanged cast iron (I think) cylinder that also is
keyed. There is a large screw
with a very wide head that presses the outer but section toward the
deeper one.
This screw has a smaller screw that locks the setting. So, this does
not cause a
CONSTANT pressure, like the spring would. It just prevents the two nut
sections
from being separated more than some amount. The spring method works
fine with
ball screws, but will cause VERY accelerated wear of not only the nuts,
but the
SCREW too!

Jon