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

On Tue, 2003-08-12 at 11:38, Les Newell wrote:
> Hi Kim,
>
> > It depends which direction the cut is going. In one cutting direction
> > there is going to be a load of 500 pounds from the cut AND 500lbs from
> > the preload, for a total of 1000 pounds of load on the ballscrew.
>
> This is correct for the fixed nut. However you have forgotten about the
> preload nut. It still has a load of 500lbs in the opposite direction. The
> ballscrew BETWEEN the two nuts will have a load of 1000lbs but the load on
> the rest of the screw will be the difference between the two loads (500
> lbs).

I knew this was going to come up !

In one direction, there are 2 nuts exerting a force of 500lbs. One nut
is getting 500 pounds for the cut. The other nut has 500 pounds from
the spring. The total axial load on the ball screw is 500 lbs from one
nut and 500 pounds from the other nut. Total load is 1000 pounds, thus
the torque requirement.


> Think of two people pulling on a rope(the ballscrew). They are the nuts.
> They both pull the same amount so they stand still. This is the preload. Now
> someone pulls on one of the loose ends of the rope (he is the motor). No
> matter how hard the two people are pulling on the rope it is easy for the
> third person to pull the other two over because all their effort is going
> into the rope and the ground between them (the assembly that holds the
> nuts).

I agree that both people need to pull on the ends of the rope and that
the tension in the rope is 500 lbs. The ballscrews are like the hands
pulling on the rope. One set of hands has the friction to pull 500
pounds. The other set of hands has the friction to pull 500 pounds.
The amount of friction on the rope (that the servo has to turn) is 1000
pounds.


> > HUH ? Am I missing something here ? 500 pounds of axial preload on a
> > ballnut is a very considerable load, especially for people using smaller
> > motors.
>
> The amount of preload does not affect the motor because the motor does not
> 'see' the preload.
>
> Les
>
>
>
> > Comments below.
> >
> > On Tue, 2003-08-12 at 09:13, Les Newell wrote:
> > > 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.
> >
> > It depends which direction the cut is going. In one cutting direction
> > there is going to be a load of 500 pounds from the cut AND 500lbs from
> > the preload, for a total of 1000 pounds of load on the ballscrew.
> >
> >
> > > 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 one direction it is. In the other direction it is the sum of the
> > two.
> >
> > > 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.
> >
> > HUH ? Am I missing something here ? 500 pounds of axial preload on a
> > ballnut is a very considerable load, especially for people using smaller
> > motors.
> >
> >
> > > Les
> >
>
>
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Kim Lux <lux@...>