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

Hi Kim,

> 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.

With no cutting load there is a load of 500lbs on each nut. The important
thing to remember is that the load on each nut is in opposite directions
therefore the torque generated by the preload on one nut is cancelled out by
the other nut.
If we now take the case of full cutting load on the unsprung nut: there is a
load of 1000lbs on the unsprung nut and 500 lbs on the sprung one. The force
that the motor has to generate is the difference between these two values.

In the other direction there is a force of 500lbs on the sprung nut and 0lbs
on the unsprung one. The difference is still 500lbs.

Adding preload does slightly increase the friction in the nuts but this is a
prettly small loss and probably not worth worrying about.

Les

>
> 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 ?
>
>