lml@lancaironline.net Mailing List Archive

Chris,

You are on the right track. Lowering the low side PRV is worth giving a shot. Without looking more closely, I can't say if that would make your system more prone to lock-up during the extension cycle. For the current pump design I advocate raising the low side pressure. I am not sure how many of the old style pumps are still in use today. Perhaps someone with one can chime in regarding settings and any operational issues. The LB circuit was introduced specifically to eliminate what you running into.

Chris Zavatson

N91CZ

360std

www.N91CZ.com

From: Christopher Skelt <cskelt@earthlink.net>
To: lml@lancaironline.net
Sent: Tuesday, April 2, 2013 7:05 PM
Subject: [LML] Re: LNC2 Old Pump Hydraulics Grief

Chris,

The pump is 108AA19-AL-3VT with a date stamp 10/89 and presumably predates the alphabet soup in your article. The spool is symmetrical. The dip stick through hole is also the filler for the metal cylindrical reservoir. Does the absence of a stand pipe mean the return is via the low side PRV during retract?

I’ll screw up the shut-off on the high side. The low side pressure is building up over 1000 psi before the pump stops during retraction. Can I also reduce the low side PRV operating pressure to limit the low side pressure buildup? I see no reason why it should be significantly more than the shutoff pressure on the low side pressure switch.

Thank you for the reply—I wonder what folks did before the www.

Regards, Chris.

From: Chris Zavatson

Sent: Monday, April 01, 2013 10:21 AM

To: lml@lancaironline.net

Subject: Re: [LML] LNC2 Old Pump Hydraulics Grief

Chris,

The behavior you describe indicates the lack of a functioning back pressure circuit. Does your pump part number have an "LB" in the number? If instead it has an "LL", the pump pre-dates the LB back pressure circuit. If it is an "LB" pump, your spool is in backwards and just needs to be flipped around.

Unfortunately, the photo hints at an older pump. I see only one TRV and the dip-stick through hole is drilled out. Also, there is no stand pipe for the return to reservoir.

If it is indeed and older pump without a back pressure circuit (and they are not retrofittable according to Parker), you'll need to raise the operating pressure of the high side circuit to perhaps 1,500 psi. Right now your net working pressure on the high side is reduced to: High side settings minus low side PRV setting. -not enough to get the gear up cleanly.

The ball a spring you removed are for the thermal relief.

Chris Zavatson

N91CZ

360std

http://www.n91cz.net/

From: Christopher Skelt <cskelt@earthlink.net>
To: lml@lancaironline.net
Sent: Monday, April 1, 2013 4:08 AM
Subject: [LML] LNC2 Old Pump Hydraulics Grief

This is for the aficionados of old hydraulic pumps. Mine is from 1989 and I’m having trouble with the retract cycle. Since my post several months ago I have installed pressure gauges, rebuilt the nose leg actuator, sequence valve and gear door actuator, and serviced the pump with the kit from Lancair. Same problems, but at least the pressure gauges help diagnose.

On the retract cycle pressure builds up on the low side as well as the high side and the pump stops until the low side pressure drops, the pump restarts, raises the gear a bit further, stops, and so on. Once the gear is up, the pressure on the low side drops off and high side pressure is maintained. These are symptoms of a reversed spool valve, but this pump’s is symmetrical with a single o-ring.

Lowering is OK, and pressure is maintained in the down and locked position, so apparently no internal leaks.

I guessed the return line was blocked, or the ball valve sticky. See the attached photos. The spring is very stiff and takes about 15 lb to close it about 0.1 inches. With a port diameter of less than 1/10 inch, this translates into about 2000 psi to overcome that force. This doesn’t look like it’s offering only 100 psi resistance and it seems beefier than the example on Chris Zavatson’s figure 14 on N91cz.com... Can anyone confirm that the spring looks right or wrong—maybe Friday afternoon at the factory? And where can I get a replacement return line spring and ball?

The photo of the pump shows the return valve location on the left.

Thanks in advance.

Regards, Chris.

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