Re: Acme screws and pre-load nuts was:Re:Re:greetings

> Date: Mon, 18 Sep 2000 06:55:56 -0000
> From: ballendo@...

> Dan Dowell asks,
> >Here I go again. Whats a preload nut? Would like to see one.
> >THANKS Dan
>
> Dan,

<art snipped>

> 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.
>
> In practice, the two nuts are "keyed" to each other, But only one
> connects to the machine.
>
> Ballendo
>
> P.S. Nowadays there are more elegant(and effective) means than the
> spring shown here(cams, for one). Look up KERK and Ballscrews and
> actuators to see examples of these. Belleville washers are commonly
> used in BALLSCREW nut assemblies.

I love this list! Someone asks a question and while I'm stuttering away
here four other folk post better answers than I can. To quote the singer
Ray Charles, in at least a half dozen songs, "y'all help me now!"

Since the canyon walls of acme thread are steeper than regular thread you
can force the nuts out against them without increasing the friction
losses as much as you would with common threads. This is what the spring
or Belville washer does in B's diagram. The left nut is held against
the left wall of the thread and the right against the right wall. The
increased friction of the nut is proportional to the spring tension used.

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.

Some on the list have tried another approach. They have cast plastic nuts
from one of the slippery compounds, like Moglice. One of these people
could better respond about the effectiveness/problems they have
encountered.

Ray