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

I've worked many years with this type of repetitious conversation, between
Ivy-League Engineers Fired up on Expresso and Laymen in the field working
with the hardware for 30 years. I came to the conclusion many years ago
that its really a waste of precious time and talent.

In the company I work for they Layed off over 4500 people.. Take a guess
what type of Engineers and Technicians they kept...

----- Original Message -----
From: "Kim Lux" <lux@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, August 12, 2003 8:00 AM
Subject: Re: [CAD_CAM_EDM_DRO] Does this make sense? was Re: Bent
Nookballscrews ?????


> 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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> > > >
> > > > OFF Topic POSTS: General Machining
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> > > >
> > > > 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
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> > > > bill
> > > > List Mom
> > > > List Owner
> > > >
> > > >
> > > >
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> > > --
> > > Kim Lux <lux@d...>
> >
> >
> > Addresses:
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> > FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
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> >
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timg@...
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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
> > List Owner
> >
> >
> >
> > Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
> --
> Kim Lux <lux@...>
>
> Addresses:
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> Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
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timg@...
> Moderator: pentam@... indigo_red@... [Moderators]
> URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
> 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
> List Owner
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
>