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Question:
If the spool valve is centered, aren’t
both the high and low sealed off from the reservoir? I would think you would want
one side to stay open? I still trying to get my head around this. If both
sides are sealed, it seems to me that after flying at 17,000 or 18,000 feet,
then descending into a warm air mass could pressurize the low side.
I thought the purpose of the spool
remaining on one side or the other was to keep the low side open so the
pressure switch doesn’t open?
None of this is making any sense to me…
Thanks
Kevin
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of TOM GARDINER
Sent: Monday, April 18, 2011 10:10
AM
To: lml@lancaironline.net
Subject: [LML] Re: Gear operation
fix for the LNC2
Folks,
As I posted some time back, there is
a fix for the spool getting stuck in one position or the other. Oildyne
offers additional springs and a symmetric spool (not the original symmetric
spool) The alternative spool is ported on both sides to allow fluid to
move freely to tank when pushing excess blind end fluid back to tank.
(The other side being ported does not really matter) The springs which
are quite heavy will certainly center the spool and take it off the poppets and
bypass relief valve after the pump is de-energized. Oildyne actually sent
me the spool and spring upgrade free of charge (no guarantee that would happen
for everybody on the forum!). With the new self centering spool and 3 way
dump valve I am now 100% confident I will not have a gear actuation failure -
seen too many u-tube videos of various types belly landing over a stupid
hydraulic or electrical failure.
Tip: add pressure gauges to your
system if you have not already done so. You will learn a tonne about your
system
Tom
826Ct LNC2 360
From: Wolfgang
<Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Sun, April 17, 2011 10:13:18
AM
Subject: [LML] Gear operation fix
for the LNC2
Here are some details on the Gear Fix Module - - - - -
There is a potential for the landing gear to fail to operate
in the LNC2.
The spool in the hydraulic pump can come off it's end
position and seal the fluid volume in the lines. When the system incurs a large
temperature rise, the trapped fluid expands raising the pressure in the lines,
tripping both pressure switches open and making the hydraulic pump
inoperable.
A module to fix this gear operation failure in the LNC2
using the standard hydraulic system is now available. It is installed across
the pressure switches and monitors their operation. If both switches
are tripped open by high pressure, the pump is made to run in the
direction set by the gear switch re-seating the spool in it's end
position restoring normal pump operation and relieving pressure in the
offending side. This happens automatically without pilot intervention.
The module is 2.5" x 1.5" x .75", weighs
2.3 oz. and has two pairs of leads that connect directly to each pressure
switch with 1/4" spade terminals. No other wiring is necessary. Price is
$250.00 each. Simple installation instructions included.
----- Original Message -----
Sent: Saturday, April
16, 2011 8:24 PM
Subject: Re: [LML] Re: LNC2
Gear Hydraulics
Wolfgang,
What is your gear fix module?
Bryan
On Fri, Apr 15, 2011 at 10:11 AM, Wolfgang <Wolfgang@micom.net> wrote:
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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