"Ned Simmons" wrote in message
...
On Tue, 24 Nov 2009 20:38:12 -0500, "Ed Huntress"
wrote:


In good lathe designs, I believe you'll find that the typical setup is to
have a pair of preloaded bearings facing each other at the spindle-nose
end.
In classic designs, these were angular tapered-roller bearings in larger
lathes, and angular-contact ball bearings in smaller ones. All of your
Z-axis location is accomplished with these head-end bearings. Then the
tail
end of the spindle was held in a single- or double-row bearing that
allowed
linear movement -- either straight rollers, or ball bearings that allow
the
spindle to move.

It doesn't take much heat to make the spindle grow substantially, which
will
either overload your bearings or unload the preload on the head-end
bearings. Also, this lathe is no wimp. It would handle much higher loads
than my SB-10L and would be roughly the same size, although a little wider
and probably a little shorter (mine has the 54" bed).

Hardinge uses a single pair of angular contact bearings, one bearing
at either end of the spindle, in the HLVH headstock -- a 3000RPM
spindle with 25 millionths runout.

Hmm. Is that right? I thought Hardinges had the classic two-bearing-front,
floating rear setup. But I don't know for sure. Maybe Gunner would know.

In any case, they use Class 9 bearings in their top-of-the-line, and those
spindles run exceptionally cool. They aren't a good example to follow for
any ordinary lathe. You'd never be able to duplicate the performance of that
spindle.

The preload is controlled by a pair
of spacers that introduce an offset between the inner and outer races.
The drive belt is outboard of the rear bearing.

One problem with tapered roller bearings, depending on how fussy you
are, is that the runout specs on standard bearings is pretty bad
compared to run of the mill ball bearings. And precision grade roller
bearings are horribly expensive and can be difficult to source.

--
Ned Simmons