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

On Tue, 2003-08-12 at 11:39, Jon Elson wrote:

> Kim Lux 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 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.
> >
> Yes, but this load is ONLY between the two ball nuts!

Yes, that load is between the two ballnuts.

> >
> >
> >
> >> 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.


> No, the motor only is loaded by the FRICTION caused by the force between the
> two nuts! That friction is amazingly small for ballscrews - a tiny fraction
> of a percent of the real forces.

There is where you are wrong. Do this: set up 2 ballnuts with a 500
pound spring between them. Do they freewheel on the ballscrew or do
they stick ? (They will stick because of the friction of the 500 pound
load.) Now add a 500 pound cutting load. The total load on the ball
screw is 1000 pounds between the the two ballnuts, 500 pounds from the
cutting and another 500 pounds from the preload.

> >
> > 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.
> >
> >
> What you are missing is that the force applied by the two nuts is in
> OPPOSITE directions, therefore it cancels out! All that is left is the tiny
> friction of the rolling elements.

No, friction does not cancel out. There is a force moving a distance
and thus there is friction.

>
> >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 ?
> >
> >
> If you apply excessive preload to the nuts of a ballscrew, it will
> damage the
> entire ballscrew system. But, the first components to break down are the
> balls, because they have so little surface area, they wear the quickest.
>
> The hard preload systems actually stretch the screw in the zone between the
> two ball tracks, and extremely small variations in screw pitch will cause
> huge variations in preload force. So, the ballscrew, itself, becomes a
> spring,
> although one with a very high spring rate.

As I explained in a previous email, this has not proven to be a
problem. I too thought it might be, but the ballscrew pitch is even
enough that the variance is very, very small. We can adjust the nuts
for zero backlash and have the nuts freewheel down the entire
ballscrew.

>
> Jon
>
>
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--
Kim Lux <lux@...>