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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