"Ned Simmons" wrote in message
news
On Mon, 30 Nov 2009 09:59:17 -0500, "Ed Huntress"
wrote:
"Ned Simmons" wrote in message
. ..
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.
Right. But I understood you to say that the imperfections on the
contact surfaces of a low class bearing will cause it to fail earlier
than a high class bearing when they are both properly mounted. On the
other hand, you also seem to be saying that those imperfections are
*protecting* the low class bearing in a poor mounting.
Right. Both. Local overloads are what cause a lower-class bearing to fail
sooner than a high-class bearing, given a good mounting. But the presence of
anomalous bumps and so on are also what give it some "crush" room.
A low-grade bearing will fail sooner than a high-class one in a good
mounting. It will fail even sooner in a bad mounting. Depending on the
nature of the "bad" mounting, however, it can last longer than a high-class
bearing in the bad mounting.
Some of the anomalies in a lower-class bearing don't result in premature
spalling of the balls or races; they may just displace locally. That's the
"crush" room.
If you imagine looking at only a very small patch on the bearing race,
there's no way to tell whether the bearing is mounted properly or not,
all you can determine is the contact pressure as a ball passes. If
there are imperfections in the surface of the low class bearing's
race, there will be local peaks in the contact stress, regardless of
the how the bearing is mounted.
Right. But even when they fail, the spalling may not spread around. If the
load on these "points" is very high, they may not spall at all: the points
will just displace plastically, or fracture off from local overload. Then
some of the preload is relieved; the bearing runs loose; but the mean load
on the bearings is reduced.
A high-class bearing, poorly mounted, will subject the balls and/or spots on
the races to continual overload at certain points in their rotation. There's
nothing much to "crush," so nothing will relieve the overload; they'll spall
sooner, depending again on how the mounting is "bad," than the lower-class
bearings.
You know, all of this is from memory, based on explanations derived from
practice and perhaps from theory as well, passed along to me by bearing
specialists many years ago. I had the job of dealing with lubrication issues
when I was at _American Machinist_, and in those days, that meant listening
to some really boring stuff in interviews. g The guys at Timken were
great; they spent hours explaining things to me about bearings in the real
world. That's where I got all this stuff. My memory for these things usually
is Ok, but I'm reconstructing it.
In general, the idea that good bearings can go to pot quicker than
poorer-class bearings in a bad setup, with a poor mounting, is clear in my
memory.
Re the temperature compensation business, I was looking thru some of
my references and found this:
http://tinyurl.com/ya7hvm4
There's about a half page missing, but the jist of it is there.
Well, right through page 542, there's most of the story. I didn't follow the
relative radii between balls and races in detail, but I see the picture.
Finite-element analysis and bench-testing a prototype sound like a good
idea. g I've never touched thermal FIA so I don't know how predictable the
bearing and spindle temperatures are, but Slocum does identify some problems
that have to be tested.
Very interesting, Ned. Thanks.
--
Ed Huntress