Re: Bent Nook ballscrews ?????

Here is another way to look at it.
Think of a Pogo stick this is a stick
with a spring under the foot pedals used
to hop around with.
Well the spring has a certain load rating
the force that presses against the footpedal
at all times - the preload.
Lets assume this is 100 pounds.
When nobody is on the Pogo the pressure against
the ground would be zero + weight of pogo.
If you have a 50 pound child trying to use it
the ground force would be 50 pounds also but
this would not be enough weight to compress the
100 pound preload so backlash free operation is
ensured with this load.
Now assume a 200 pound teenager is gone use the Pogo
as soon as he got on the spring would fully depress
and the weight on the ground would be 200 pound.
The full travel distance of the spring Backlash would
be taken up due to the load condition.
I hope this makes it clear that preload dose not subtract
from the load rating of the screw assembly it is just
a minimum load that is allways present and is overcome
once the load exceeds this value.
As far as additional tourque is concerned here we have
steel balls rolling on a casehardened steel screw kind
of like the steel wheel of a railroad running on steel
tracks there is a reason they went for this in the
old days wich is very little roll resistence.
Tousands of tons of weight can be pushed around easily
by some tiny locomotive.
A few hundred pounds of preload adds very little on the
tourque requirement.
Hope this helps :-)

--- 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:
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> Kim Lux <lux@d...>