I did a project for a garden tractor manufacturer several years ago.  A guy
would get his shiney red tractor home, mowing the lawn showing off his new
toy when one of the blade spindles would freeze up.  The manufacturer
thought they had an assembly problem on the spindles which each had 2 ball
bearings, and asked if we could design a test fixture to detect faulty
assemblies using vibration analysis.

To test our claims that we could reliably detect faulty assemblies, they
built some assemblies with known problems and sent a lot of 20 to us.  I
think 18 of the 20 showed problems, mostly with defects in the bearings.
The manufacturer sputtered a little when I explained the results, because
they had intentionally built only 10 bad assemblies.  It looked like bad
bearings were probably the cause of the failures rather than faulty assembly
methods, so the next step was to examine the bearings they used.

Each spindle used 2 bearings of different sizes.  It was arranged to send us
10 of each size.  A few days later, a shoebox-sized box arrived. It looked
like the bearings were rejects swept up from the floor.  The bearings were
loose and not wrapped at all, rattling  around loose in the box with a few
sheets of newspaper.  I called my contact and told him he needed to send a
new lot of bearings.  Someone must have mis-understood and sent a box of
junk bearings.  Bearings, being precision devices, should have been wrapped
to prevent dirt from contaminating the grease and should have been padded to
prevent damage from rattling around loose.  The answer astonished me: "This
is how we receive them from China.  They come in a large box packed just
like this."  My analysis was pretty easy and nearly instantaneous "These
bearings are crap.  We don't need any testing to tell you that."

We received another shipment of "premium" bearings, also from China.  These
came in plastic tubes, 10 in a tube.  Not perfect but at least these didn't
look like floor-sweepings.  We tested the bearings at a facility used to
testing noise-quiet bearings for use in submarine machinery.  As the testing
we had to invent new word for how bad the bearings were.  Listening the
vibration data taken while turning the bearings, one some the bearings, you
could actually hear one or two of the balls bouncing around because they
were undersized.  This was later verified by physical measurements.  Of the
10 bearings of of each size, I think 2 were rated acceptable in one size and
3 in the other.  With 70 - 80% of bearings being bad, the chances of getting
a good spindle assembly were pretty slim!

The manufacturer changed to a well-known brand of bearings, and the need for
a spindle-assembly test station disappeared.


Jon
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