nuckolls.bob(at)aeroelect
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 Posted: Sun Feb 15, 2009 3:34 pm Post subject: Starting currents rusting the crankcase? |
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At 03:41 PM 2/15/2009, you wrote:
| Quote: | My A&P mechanic told me to bolt the 2ga wire right next to the
starter. This asures the best ground dring the highest amp loading
on this circuit. Also, in some spam machines, it has been reported
that electrogalvanic corrosion was caused by forcing starter motor
current to run through the block, etc. to a ground wire on the rear
of the engine.
Bob, comments from your experience?
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There's nothing wrong with getting grounded "close"
to the starter but if there's a handier fat bolt
I'm not sure I'd wrap more wire around the engine
just to snuggle up to the starter. The voltage drop differences
between the "worst" location on the crankcase and
the "best" are very tiny and perhaps even difficult to measure.
I've heard this "galvanic" thing tossed around in
a host of conversations that spoke to every possible
malady from rotting the skins at the rivets and overlaps
to control surface hinges becoming detached. About
25 years ago, somebody posted a thing on the bulletin
boards at Beech warning against the use of lead pencils
to mark on aluminum sheet. The writer alluded to some
inspectors squawk for a surface defect where a lead
pencil was used to circle the area of interest on the
skin. The story went that evil, metal-munching bits
of graphite went to work on the aluminum and the whole
disk fell out.
I hadn't seen that little poster in many years and
brought it up during some conversation with a processes
and materials wiennie . . . he smiled and allowed as
how the event didn't happen.
Intuitively it makes a good tale but practically,
you just can't get (1) enough voltage drop across such
a low resistance joint in a crankcase for (2) sufficient
time supported by (3) the necessary moisture to make
such an event happen.
Now, there HAVE been instances here currents circulating
through engine parts have caused serious and relatively
rapid damage. A shorted armature winding in a turbine
engine starter-generator can cause circulating currents
to flow in the generator's quill shaft and through the
bearing balls that support the drive spline.
This is a relatively low voltage (under 1v in some cases)
but lots of current that causes the balls and races
to become pitted. Bearing failure soon follows.
Modern S-G designs add insulators between the
quill shaft and armature shaft to prevent
recurrences. But between non-moving, bolted-up
faces of a crankcase? Don't think so.
Bob . . .
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