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

Hi Kim,

> 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.

When cutting at full load the worst case friction will be the friction due
to 1000lbs load on the fixed nut and 500lbs on the sprung nut. This I agree
with. Unless there is something very wrong with the ballscrews this friction
will be a tiny percentage of the torque needed to overcome the 500lb cutting
load.

Let's do some very rough sums: Assume the ball nuts are 95% efficient. This
means that 5% of the torque is needed to overcome friction losses. With no
cutting load there is 500lbs load on each nut, giving a total of 1000lbs. 5%
of 1000lbs is 50lbs. Therefore the motor needs to produce enough torque to
push the equivalent of 50lbs just to overcome the friction losses in the
ball nuts. This is the friction you feel when turning the screw by hand.

At full cutting load the worst case friction will be the equivalent of 1500
* 5% = 75lbs. The total load that the motor 'sees' will be the torque needed
to produce a thrust of 500lbs + 75 lbs =575 lbs.

With no preload the motor would have to be able to produce a thrust of
525lbs so the preloading only adds about 9% to the required torque. Unless
your motor is already right on the limit of it's capabilities, the extra
load should be insignificant.

Les

> 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
> >