----- Original Message -----

From: Chris Zavatson

To: lml@lancaironline.net

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

www.N91CZ.net

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

www.N91CZ.net

Sent from my iPad

On Apr 7, 2011, at 11:53 AM, "Wolfgang" <Wolfgang@MiCom.net> wrote:

Chris, have you considered what effect airframe vibration has on the position of the spool in the pump and the problem in general ?

 

Wolfgang

From: Chris <chris_zavatson@yahoo.com> Sender: <marv@lancaironline.net> Subject: Re: [LML] Re: LNC2 Gear Hydraulics Date: Wed, 06 Apr 2011 07:38:35 -0400 To: lml@lancaironline.net

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 www.N91CZ.net