Re: Bent Nook ballscrews ?????

Kim,

I'm confused... Here's what I understand so far:

You have some motors which some members of the group told you were
too small, saying you wouldn't get good performance using them... But
you ARE getting good performance from them, and you attribute it to
your anti-backlash nut design, which is of a non-compliant 2 nut
spacer type. And you have this design in operation (production
setting) on both ground and rolled ballscrews. ?

Also you post that the required torque from a given motor is FAR in
excess of what many of us have experienced first hand in our own
machines. This seems to be in conflict with the first part above???

So please help me understand how the motors which were too small in
the first place (according to those who believe-perhaps erroneously-
that the preload only adds a slight drag to the assembly) have great
performance, since by your stated calcs they "should be" even worse?!?

If you're onto something, I'll go ahead and listen... Maybe we can
get back to something which will help us all build better machines.
So how does this all fit together to give you great performance with
small motors? or cheaper screws?

Thank you in advance,

Ballendo

P.S. FWIW, loading ballnuts with oversize balls to create a negative
clearance IS something the ball screw and nut mfrs. HAVE been doing
for decades...

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Kim Lux <lux@d...> wrote:
> Comments below.
>
>
> 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
> > >
> >
> >
> > Addresses:
> > FAQ: http://www.ktmarketing.com/faq.html
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> Kim Lux <lux@d...>