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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