Re: [CAD_CAM_EDM_DRO] Bent Nook ballscrews ?????

Can I jump in here? Something from my statics and dynamics coursework from
way back tells me there is somethign missing in this discussion. Maybe I am
wrong but here goes.
I am not seeing how the backlash preload is directly related to the cutting
forces involved except as follows. IF the preload arrangement is assumed to
be a rigid arrangement, like Kim's and others' designs, with a pair of bolts
between the 2 making it for all intents and purposes a rigid body, then the
cutting force, at any given time, is seen mostly, if not entirely, at the
(1) of the (2) assembled nuts opposite the side of the pushing force. The
other nut is just staying in place against the pushed side of the leadscrew
thread, eliminating backlash. Then during direction changes, or when
chatter can be expected due to slop for example cutting a circle at certain
points around its travel, the load is alternately placed to either nut back
and forth rapidly for a short duration until the load is taken up by the
opposite nut. The bolts and other fasteners transfer all the load with no
problem. Now my point is, how is the preload tension in any way shape or
form related to the applied force at the nut? I would counter that the
tension is only related to that amount which eliminates the slop in the
ball-to-screw arrangement. The added applied forces will not increase or
decrease the backlash IF THE BACKLASH ASSEMBLY IS RIGID.

Now of course, if there is a hole in these comments, I welcome correction.

R. William Kichman, P.E.
Kichman Engineering Associates
103 Old Furnace Road
Cornwall, PA 17016-0643
tel/fax 717/270-0714

----- Original Message -----
From: "Kim Lux" <lux@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, August 12, 2003 9:44 AM
Subject: Re: [CAD_CAM_EDM_DRO] Bent Nook ballscrews ?????


>
> 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 pushing
> > >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
> >
> >
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> --
> Kim Lux <lux@...>
>
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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/
>
>
>