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70 G’s? That’s an awful lot
of force. Wouldn’t your pump be experiencing that as well? I think my
pump at least 10 lbs. Isn’t that like 700lbs? That’s pretty
scary, maybe I should beef up my mount a bit.
If there is that much vibration in a
typical airplane, I wonder if I should leave my laptop at home. I’m not
sure I want it experiencing those kinds of vibrations. That would really mess
up my HD.
Kevin
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Wolfgang
Sent: Friday, April 15, 2011 11:11
AM
To: lml@lancaironline.net
Subject: [LML] Re: LNC2 Gear
Hydraulics
From a design point of view, I still have a big concern
about relying on the friction from an O-ring to keep the landing gear
functioning properly. Vibration levels can exceed 70 G's. I would be much
happier with a spring loaded ball detent or something similar.
That's why I came up with the gear fix module.
----- Original Message -----
Sent: Wednesday, April
13, 2011 8:25 AM
Subject: Re: [LML] Re: LNC2
Gear Hydraulics
A symmetric back-pressure circuit and spool will eliminate
the possibility of installing the spool backwards as happened with pumps
manufactured in about the '96 through '98 time frame. The spool must
still remain in place after the pump shuts down to prevent both high and low
circuits from locking up simultaneously.
Using o-ring 013-70 requires 30 g's to move the spool.
Using o-ring 013-90 requires 70 g's to move the spool.
This includes the return force generated by the poppet springs (410462).
The difference between the two o-rings is durometer. I
have been using the softer one without any problems. The stiffer o-ring
(intended for pumps with back pressure circuits) should leave no doubt. The
first thing to do with a pump that has a history of locking up both sides is to
measure the return force for the spool and make sure it has an o-ring
installed.
From: Wolfgang
<Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Mon, April 11, 2011 4:56:57
AM
Subject: [LML] Re: LNC2 Gear
Hydraulics
When I first got into this problem with the hydraulic pumps
last year, I had a few conversations with the product manager of the Oildyne
108 pump. There have been many changes in it's production life and one of them
was to make the spool symmetrical and have the relief circuit the same both
ways.
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, have you considered what effect airframe vibration
has on the position of the spool in the pump and the problem in general ?
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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.
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