Tom Kendall wrote:

I was investigating a failure of a large oil-fed journal bearing at work
today, and it struck me there may be similarities with the smaller, high
speed journals used in automotive engines (yes, even rotaries!).  The
journal overheated and wiped (cause still unknown), and was even noticed and
monitored at the time.  An embedded thermocouple alarmed on high temperature
while the damage was being done, but unfortunately, the people monitoring
the condition did not recognize the bearing really was hot because the
temperature coming out of the bearing remained cool.  They just figgered it
was a bad thermocouple (note to self: when thermocouples go bad, they don't
just drift... they fail high or low, depending on the failure mode).

But I digress.  The babbit in the journal got too hot.  Some of the
literature I was reviewing indicated that cooling oil to typical journal
bearings needs to be less than 200 deg F to ensure against softening the
babbit in the bearings.  I suppose that babbit can come in different alloys
of tin and lead (and whatever), and probably has different softening
temperatures.  Anybody got a feel for top end recommended oil temps for
typical automotive journals?  Are these bearings of a higher temp alloy?

I also was reading of journal seizing problems stemming from high RPM
operation.  It appears that (in at least the industrial sized shafts), when
shaft speeds exceed 4,000 RPM there is frequently a problem with the shaft
expanding faster than the journal and taking up all of the oil gap
clearance.  The fix is a pre-warming of the system or a warm-up run at lower
RPMs.

Since aircraft engines run well below this speed, and automotive engines
geared down for aircraft use generally run higher, I was wondering if there
was any real-world experience with this kind of problem in auto-conversion
engines.  Seems like a candidate for problems during takeoff and climb out
when there has been little or no pre-warming.  Up here in Wisconsin in the
winter I can't get the oil temp off the bottom peg in a C172 until about 400
ft AGL and climbing.  How about it guys?

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