"Ned Simmons" wrote in message
...
On Sun, 29 Nov 2009 16:58:29 -0500, "Ed Huntress"
wrote:


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




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.

Not quite. Unless very lightly loaded, all bearings will eventually
fatigue and fail by spalling. Very light loads will cause problems
related to skidding of the balls.



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.

This doesn't make sense to me. If a high ABEC class bearing will
outlast a lower class bearing under ideal conditions, what's the
mechanism that will cause it to fail sooner in a less than ideal
installation?

--
Ned Simmons

There's nothing much to "crush." A given amount of displacement of one race
relative to the other can produce a substantially higher preload in a
higher-class bearing. The difference may be slight, but small variations in
the percentage of yield strength that a bearing is subject to will produce
large variations in its fatigue life -- in other words, the time it takes
for the bearing balls or the race to spall.

--
Ed Huntress