Wolfgang,
Yes, I have considered this. The spool with O-ring is a very tight
fight. While I have not tried to measure the force to move it, I would
estimate something over 50 g's to move it (I'll measure this the next time I
have a pump opened up) Could it be that a pump somehow ended up mounted
such that it was subjected to some resonant frequency? Seems like a
remote possibilty given all the variables and the mass of the pump.
How about wear and tear? Perhaps
sitting dry and idle for ten years would do it. A missing O-ring or a
spool that doesn't utilize an O-ring would allow the spool to move very
easily. Parker does make spools that don't even have O-rings. Did
these end up getting installed by chance?
Based on reports from the LML, the behavior
seems to be rather binary. On the one hand we have planes with over 1000
hours and more than ten years of service that have never once had the spool
move off the poppet. On the other hand, we have planes with circuits
that lock up right out of the box (put into service).
This leads me to believe there is a
difference in configuration. The easiest way to verify this is to simply
open up a pump that is known to lock up hi and low circuits
simultaneously.
While I have opened up several pumps (primarily to flip around spool
valves), I have not yet worked on one that had this lock up issue. I am
optimistic the difference will be quite obvious upon inspection.
Chris Zavatson
N91CZ
360std
Sent from my iPad
Chris, have you considered what effect airframe
vibration has on the position of the spool in the pump and the problem in
general ?
Wolfgang
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I would like to extend the offer to the whole group. I
think the entire community would benefit from understanding why some
pumps are allowing the system to lock up. If anyone has a pump
that exhibits this behavior and could part with it for a week, I am
willing to check it out on the test stand and examine the internal
configuration.
Chris Zavatson
N91CZ
360std
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