nuckollsr(at)cox.net
Guest
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 Posted: Wed Oct 04, 2006 6:01 am Post subject: RBM/Stancor/White-Rogers Contactors |
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At 05:24 AM 10/4/2006 -0700, you wrote:
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I have a suggestion for the reasons a manufacturer wants the relay
mounted in a certain orientation, based on experience doing similar
things. It has mostly to do with water: Generally, you want to
mount the part in a way that water has the least chance of getting
inside, so the cap is made like a roof so the water runs off.
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In this case, the mfgr has suggested "cap down" . . . but
yes, water may have been a driving consideration.
| Quote: | Also,
you want to have the internals arranged in a way that a drop of water
inside has the least chance of causing a corrosion failure. That
might be to have the electrical connections to the coil at the top -
or the contacts could be at the top if they are more vulnerable.
Reasons like this make far more sense than G-load resistance.
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Sure. Of course, the guys who designed this thing have been
dead for decades. The guys who wrote/modified the data
sheets over the years are not accessible to us. So the best
we can do is apply whatever understanding we can to the
question.
I've encountered several failure modes on sophisticated
products where the present powers-that-be go to great
lengths to obfuscate reality. Unlike engineers who are
interested in exercising and improving on the best-we-know-
how-to-do, others will exert no small effort to avoid
confessing to less-than-the-best. It's a lawyer thing.
It may be that the orientation suggestion on the data
sheet was added after years of field experience where
water became a consideration. That might have been
during the era of floating cowl issues on Cessnas
back in the 70's . . . but I doubt it. For every airplane
that carries an RBM/Stancor/White-Rogers contactor,
there's hundreds of boats, fork lifts, etc. that use
them too. Bottom line is that we'll never know the
original thinking or intent.
The alternative is due diligence for reverse
engineering. Disassemble a design looking for the
simple-ideas that both support a hypothesis or
explain a failure. In any case, if the task is done
well, it's not uncommon for the consumer of a legacy
product to end up knowing more about that product than the
manufacturer! That happens all too often in aviation
(and I suspect elsewhere).
To recap I think we can agree that
(1) cap-down orientation improves probability of
survival for cool splash ingestion of water (water
pools away from vitals). Hmmmm . . . wonder about
adding a drain hole in a down-facing cap!
(2) cap-down increases gravity aided, contact spreading
velocity for an improvement of contact service life.
Any other thoughts out there?
Bob . . .
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