"Ned Simmons" wrote in message
...
On Sat, 28 Nov 2009 23:08:15 -0500, "Ed Huntress"
wrote:



The HLVH layout is extreme, but some space between the front bearing
pair is not unusual. I'm looking at a cross section of a 10EE
headstock and it appears the spacers are about 2-1/2" long. The
support at the tail is an unspaced pair of angular contact bearings.
Top speed of an EE is about 1000RPM higher than an HLVH.

The bearings at the nose of a Bridgeport spindle are separated perhaps
1-1/2". In this case there's a single deep groove bearing at the top
of the quill. A BP spindle running at top speed gets much hotter than
an HLVH.

Grinder spindles typically have the bearings pairs mounted directly
back-to-back.

--
Ned Simmons

That generally agrees with what I've seen, although I haven't had any
spindles apart for a few decades. Thirty degrees F produces about 0.001
in.
of growth in about 5 inches of length.

One thing that we've neglected here is that this is, at least
potentially, a self compensating system. As the spindle warms up and
expands the preload drops, reducing the heat generated in the bearing.
The problem is coming up with a design that will settle at a
reasonable equilibrium under the normal range of operating speeds and
loadings. It seems Hardinge has been able to do this.


That shouldn't be a problem for
ordinary bearings, which are less that perfect all around; there's a
little
room for elastic compression.

As it's been explained to me, the problem becomes more critical as the
bearing class goes up. The Class 9 bearings in a Hardinge HLVH must be
very
touchy in terms of the growth they'll tolerate.

I've never heard that and find it hard to swallow. If a lower class
bearing has imperfections that allow it to deflect more easily, that
implies there are areas of high stress that would be more sensitive to
damage.

There are. That's why they don't last as long if both types are properly
applied. Unless it's overloaded, a Class 9 bearing will run until hell
freezes over, while a lesser bearing will eventually spall and fail. That
assumes that they aren't abused and brinelled, or otherwise damaged.

I can see where a bearing with more accurate geometry might be
stiffer as a result of better stress distribution...

Yes.

but I'd expect that
would make it more robust, not less.

It will last longer in proper service. It also is more susceptible to
overloading from thermal growth, misalignment, etc. If you're going to use
Class 9, everything in the setup had better be perfect. If it is, it will
outlast a lesser-quality bearing.

--
Ed Huntress