Things like not cleaning up oil, and having the hydrocarbons attack the
oxidation between sliprings and brushes, for instance.
And most recently we're told the sparking had to do with paint fumes
attacking the oxidation between brushes and sliprings. This time, we
had to have the sliprings ground?
This is the type of thing that I'm wondering about. How many more of
these type of "cockpit errors" are going to get us?
Yep. Some paint fumes can 'reduce' the oxidation layer. See my other post.
That's like asking how many ways can someone make a mistake. You can break
them down in different categories, mechanical, chemical, electrical. Or
maybe operation, maintenance, environment. Or 'dumb, dumber, dumberer' ;-)
Proper seating when replacing. Proper adjustment of holder position and
spring tension (especially after turning the rings). Too much
vibration/chatter within the brush box because of poor fit. Allowing too
much shaft vibration.
Adequate humidity (too low and the layer formed on the rings will be worn
off, leading to high brush wear rates). Atmospheric contaminants (paint
fumes, fuel-oil, gas, or even lube oil fumes, exotic chemicals).
Over current. Under current. Poorly bonded 'pigtails' (more a
manufacturer's defect). Not connecting all the leads so some current flows
through the brush-box into the brush. Routing the leads where they can
chaff and fray. Not reversing the leads periodically to even the
Don't know what 'CBLF deviations' is supposed to mean.
If this is an AC unit (the brushes are for slip-rings), then I'd check....
Runout of the rings (if bad, check the vibration and tolerance of the shaft
bearings). Once the problem is fixed, you'll have to 'turn' the rings to
get them back into round. Even if round, roughness caused by contact with a
brush holder or other rigging can cause the brush to vibrate and spark.
Same hardness of brush as recommended by manufacturer? Extremely hard
brushes can sometimes spark under normal operation. When replacing, have
they been 'seated' to match the curvature of the rings? Is the face of the
brush all one smooth and shiny surface? If there seem to be two or more
'polished' areas, then it's shifting around. When only part of the brush
face contacts the ring, the current density is high and can cause sparking.
If the face has chips missing around the edges, it may be impacting the ring
and shattering the face.
Brush spring tension (can allow 'chatter') and bursh holder alignment. The
holder must hold the brush consistently and not allow the brush to wobble
back and forth. If the brush moves even slightly, then it never gets
completely 'seated' as it shifts position. Especially if the machine was
Color of the slip ring face Absolutely bare brass/silver is *not* good, too
much friction. Should be a slight brown coating. But too dark can be bad
as well. This sort of thing can be oil contamination (too dark, 'sludge'
buildup on rings). Too shiny, look for atmosphere contaminants like some
paint thinners and solvents in the area. They will 'reduce' the 'brass'
surface and the bare brass does not have the best performance (higher
friction with the brush can cause chatter).
Do you swap the field leads periodically, or perhaps you can't do that with
I could probably go on some more, but these are things that I remember
seeing on a variety of different machines.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.