Re: [CAD_CAM_EDM_DRO] Does this make sense? was Re: Bent Nook ballscrews ?????

On Tue, 2003-08-12 at 08:26, ballendo wrote:

> 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)

OK.

> 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.

OK

> 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.

Sure. Lets assume that we've got our ballnut adjusted to be but snug on
the most divergent 3" of the thread. Thus, there will be areas on the
screw where the backlash nut is loose. See comments below for what
happens in the real world.

> 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.

The degree of torque required to turn the ballscrew is defined by an
equation. There is a direct mathematical relationship between the axial
force on the ballscrew and the torque required to turn it. This
relationship is governed by friction, thread pitch, etc. The preload
put on the ballnuts is one of these axial forces.

> 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.

NO. THIS IS WRONG. The mathematical equation describes the
relationship between axial loads and torque. Preload is an axial load,
pure and simple. Torque = axial load x constant, pure and simple.

> But you
> will find that this comes nowhere near what you have proposed...

1000 pounds of axial load, whether from preload or from cutting load
takes 30ish inlbs of torque, no matter how you slice it.

> 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.

I know nothing about this technique and can't comment.

> 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.

First of all, if this worked then the ball screw manufacturers would be
doing it. Secondly, are you sure it is zero backlash ? Have you
measured it with dial indicators with enough load to induce backlash ?
We've measured our zero backlash setup and it is truly zero backlash,
even under loads, with a 0.0001" dial indicator.

With the two ballnut preloaded system, some people are worried about the
ballnuts jamming on sections of the ballscrew with a narrower or wider
pitch. In practices, this hasn't been a problem. I suspect that there
is very, very little pitch variation in a rolled or ground ballscrew (we
have both) but what drives the error spec the ballscrew manufacturers
quote is the gross error in the lead over a distance. Ie when they
quote 5 TPI or 60 TPfoot, we really get 60 threads in 12.004" instead of
12.000 inches. If one runs a tightened zero backlash ballnut assembly
over a length of ballscrew, there are no tight spots and we've been
unable to detect any backlash anywhere on the screw.

> 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.

Our two ballnut system is a version of the non compliant spacer setup.

> 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.)

Agreed. Why are you stating this ?

>
>
>
>
>
>
>
> --- 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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>
>
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>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
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--
Kim Lux <lux@...>