Does this make sense? was Re: Bent Nook ballscrews ?????

Kim,

I believe that John is correct, and you are misunderstanding some
things... To begin with, let's assume a perfect thread in the
ballscrew and ballnut. Now let's put a non-compliant spacer between
the two nuts which is exactly the correct length to place the balls
into a zero clearance relationship with the threads of the nut and
screw.(which we can do in this example due to the perfect threads)

This totally non backlash system will have the ability to withstand
axial forces up to the material deformation level of the nut and
screw threadform(s) and balls. And MORE importantly, it will add
NOTHING to the rotational force required to turn the screw.

Now in the real world, threadforms are not perfect, and spacers of
exact length are not often possible, except with extremely accurate
screws which will ALWAYS be of at least a ground precision type.

So instead of adding zero to the rotational torque required, a SMALL
additional force is required. This allows the balls to be preloaded,
which means they have LESS than zero clearance. (They are deformed
slightly, intentionally.) The threadform of the screw and nut is also
deformed by this preloading. It is this deformation which increases
the rotational force required over the perfect theoretical ideal
described above. And remember, that had NO additional torque required
up to the material deformation limit.

The degree to which the torque required to turn the screw increases
is based on the quality and consistency of the ball size, threadform,
and lead(pitch). The balls need to be preloaded to the worst case
variation in the combination of these three items.

Therefore, when modern, high accuracy machines are made, they use
ground screws so that a lower level of preload is required (resulting
in a non-backlash system with added motor torque required along the
lines of what Jon E said). Our commonly used rolled screws WILL
absorb more rotational force to deal with the variations. But you
will find that this comes nowhere near what you have proposed...

I have personally used this deformation capability to my benefit
(following Dan Mauchs' lead) by buying a SINGLE ballnut, and
reloading it with slightly bigger balls to make it non-backlash.
Carefully done, and carefully broken-in, this technique works well
for machines which can tolerate the overall load capability of a
single circuit/path nut. This technique also takes advantage of the
fact that by using a single nut, the variation dealt with at any
given point is less than it would likely be with the two-nut approach.

Hope this helps,

Ballendo

P.S. Certainly there are MANY high quality machines out there making
parts with the belleville washer type dual nut anti-backlash setup;
as this was the defacto standard in the industry until the wider
availability of cheaper, better quality thread grinding machines made
ground screws more common, and more accurate (for a given price).
Which made the use of non-compliant spacers more common on high
quality machines made today.

P.P.S. Part of the calculations necessary to use ballscrews properly
is to choose a size which allows the material of the nut, screw, and
balls to withstand the necessary loads/forces... In other words, if
your load requirements are high you use a larger ball size(or more
balls/circuits) to handle the load without exceeding the elastic
limit of the material. (Also compression strength of the screw, but
that is not pertinent to this specific discussion.)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Kim Lux <lux@d...> wrote:
>
> Comments below.
>
> On Mon, 2003-08-11 at 22:24, Jon Elson wrote:
> > Kim Lux wrote:
> >
> > >A substantial amount of force would have to be greater than the
pushin
> > >capacity of the motor so that there was never a force that would
> > >displace the nuts against each other. If the spring is weaker
than the
> > >pushing force of the motor, then it will displace the backlash
under a
> > >heavy cut.
> > >
> > >As soon as the nuts are adjusted for the maximum force the motor
is
> > >capable of moving, there isn't any force left to move the
cutter.
> > >
> > >
> > No, you're confusing drag with linear thrust.
>
> No, I'm pretty sure I'm not. Lets say we have an axis that needs,
on
> occasion, 500 lbs of cutting force. In order to ensure that we
never
> had backlash, we'd have to put a preload and the backlash nut of 500
> lbs, to keep the backlash zero in the presence of our load. That
then
> puts the load on the ballscrew at 1000 lbs, ie 500lbs for the
cutting
> load and 500 lbs for the backlash.
>
> > If the system is designed to
> > apply 1000 Lbs of linear thrust, you want somewhat over 1000 Lbs
> > of preload force between the two ballnuts.
>
> Yes. Thus the total load on that ballscrew is 2000 lbs. It takes
twice
> the motor to turn the preloaded system.
>
> > On a good ground screw,
> > 1000 Lbs of preload will cause a very mild drag, maybe 10 Oz-In
> > when new, and decreasing as the screw and nuts wear in a little.
>
> Where do you get your information from ? A 3/4" 5 TPI ballscrew
has a
> force to torque ratio of about 31 lbs force per in pound of torque.
> 1000lbs/31 inlbs/lb = 32ish inlbs of torque or 516 oz in of torque,
just
> to turn the backlash preload on the nuts.
>
> > You don't have to adjust the nuts until they bind up. That could
be
> > 5000 - 10,000 Lbs of preload, and will quickly destroy the balls.
>
> HUH ?
>
>
> >
> > Jon
> >
> >
> > Addresses:
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> Kim Lux <lux@d...>