Re: How does one adjust Ballscrews on a Bridgeport

Jon,

Yes, the bearing pair is ~1.2" wide and lives on a step ~1.1" wide, so
all the force applied on the nut at the end of the ballscrew goes
through the spacers into preloading the facing bearings inner races. I
looked at a set of factory drawings, and the setup is essentially the
same as Bridgeport used on a manual machine.

I didn't know the Boss series had ballscrews fixed on both ends...
must be interesting to control stresses due to CTE mismatch, not to
mention getting a stable rotational setup for a ballnut. Definitely
makes for larger potential load carrying capacity, with the load
shared on both ends.

On the second point, I can't see bearing wear causing this, since the
preload is still not causing the inner races to come into contact. If
they were worn, I would expect the inner races to contact, which could
cause insufficient preload (and symptoms like we see) or outright slop
that would look like backlash.

The interesting thing is that with the torque applied, the amount of
'springy' rotation does not seem to change in proportion. The 0.025"
slop is only estimated based on the 1/8 -1/10 rotation on a 0.200"
lead ballscrew. When I look at the inner race springing in response to
the applied rotation, it looks more like 0.005" - 0.01", although my
eye has not been calibrated lately. Gotta figure a way to look more
closely at that. I looked hard at the yoke; I didn't analyse that
beyond rule of thumb, but my intuition tells me it is very unlikely to
be flexing (it is built like a brick) like I'm observing.

I know this can be resolved; I will post whatever I can learn from
SWI. I may make something to experiment with preload and check for a
preload to spring range correlation more in depth (it is already
clearly not 1:1). I know we're missing something, but I'm very damn
careful with this stuff, and whatever it is isn't obvious - yet.
Famous last words...

> Are you positive that the outer bearing's inner race is free to

slide toward

> the other bearing? Are you sure the keeper that presses on the inner
> race is not ALSO pressing on the end of the leadscrew? IF it hits the
> end of the leadscrew, it can't maintain force on the inner race.

This would

> mean the length from the wider step for the inner bearing to the
> place where it steps down near the outer bearing is too long. A spacer
> with an ID equal to the bearing ID might fix it. Without seeing the
> whole setup, all I have to go on is how Bridgeport did it.
>
> Jon