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Chris,
You are right, looking at the drawing from Oildyne (I posted some time back) I agree that the low pressure poppet needs to remain open and it does so since the pressurized side keeps the spool forced up against the poppet by overcomming the spring force. If the high side leaks to zero then the spool will center but only then. So it could be argued why bother with the extra springs at all. Oildyne's response to that was that it prevents sticking of the spool. I might reconsider and remove the springs if I notice the system missbehaving, so far it has not,. My fallback right now is the three way dump valve.
What took me here in the first place was that I was getting pressurization on the low side that was either due to a bad bypass valve, expansion or leakage across the spool, which was not cracking the relief valve (I subsequently removed that valve which allows all fluid to vent to the tank immediatly on the low side) There is no need for this valve since the low side is not providing a retaining function
Tom
From: Chris <chris_zavatson@yahoo.com>
To: lml@lancaironline.net
Sent: Mon, April 18, 2011 8:56:21 PM
Subject: [LML] Re: Gear operation fix for the LNC2
Tom,
Please be carefull with system changes. I still sounds like you have designed in the very condition we seek to avoid. The spool must keep the poppet valve open else you seal off the side that needs to remain at zero pressure. Wolfgang's circuit will undo this undesirable condition once the "zero side" has been pressurized enough to close the pressure switch. Until then however, the net working pressure of the system is reduced (net=higher side pressure minus lower side pressure). It also adds wear and tear by requiring additional pump cycles to restore the unlocked condition. Best to keep the spool where it was last commanded, the way the pump was designed to do. The symmetric spool avoids installation errors - a good thing (I would really like to examine schematics and drawings to look for any unintended consequences). Pressure gauges should be considered a must-have.
If your pump now has some new
way of keeping the non-working side from becoming pressurized, please elaborate. In all seriousness, we just want to make sure you don't end up in one of those you-tube videos.
Chris Zavatson
N91CZ
360std
Sent from my iPad
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.
<GearFix2s.jpg>
<GearSw1s.jpg>
----- 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.
Wolfgang
----- 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.
Chris Zavatson
N91CZ
360std
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.
Wolfgang
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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