Re: [CAD_CAM_EDM_DRO] Affordable ball screws

At 12:54 AM 6/22/02 +0000, you wrote:

>That is very different from a bearing that may stop and reverse direction,
>or just move very slowly, and still needs lube protection. the closest thing
>in an IC engine is the wrist pin and connecting rod top end, which don't
>have anything close to continuous rotation. I know that there are some
>tricks related to microfinishing of the pin that help in this area.

You've put your finger on it but perhaps not as intended. It boils down
like this: 2S motors, which while under power always have a downward load
on wrist pin, cannot remain effective oil films in that zone. Hence they
always have needle bearing small ends. 4S motors all have plain bearings at
small ends, because oil film can re-form during load reversals. The point
is that the oil film forms again in an instant as load reverses, and the
secondary point is that even with those small and fairly slow motions at
the pin, the oil film there is indeed hydrostatic.

So the idea that oil fims would not form in a plain nut/screw combo because
it may reverse is 180 deg out. Also way out is the idea that these combos
will not serve hydrostaticaly under a steady oil supply, when in fact they
serve hydrostatically under just being dosed with an oilcan now and then.
In short, if you have oil and two parts moving relatively in it, a
hydrostatic film always and inevitably forms and there's nothing you can do
about it at least up to pressures on order of 2-3000 PSI.

>Hydrostatic bearings require very thin layers of the lubricant fluid and some
>other tricks to make them stable.

Really, once you're above boundary layer thickness, there are no
hydrostatic bearings.

> A standard car crankshaft bearing would
>probably just jump to one side, or maybe rattle, if you tried to make it work
>as a hydrostatic bearing, and keep the bearing flying over the journal at
>zero speed. This is the condition that the bearings of a machine tool axis
>works under, however, slow or no movement, but it has to be stable and
>resist cutting forces.

Point is, no bearing in motion is really hydrostatic. But let's not ignore
film strength alone, which in most oils is still good for extreme pressures.

At 01:04 AM 6/22/02 +0000, you wrote:

>few thousandths of an inch would be the way to go. You'd need to
>have a bunch of tiny holes along the flank of the nut's threads, and let the
>oil squirt out from the major diameter all the way along the nut. Just
>taking a standard nut and pumping oil into the middle, a la a car crank
>bearing, is not going to provide the type of Bernoulli lift forces you need

Not to put too fine a point on my pencil, if oil is inside the bearing at
all, almost any degree of motion makes it hydrodynamic.