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Hi Chris,
We would all like to get at the root cause of the
problem but, as yet, that hasn't happened. Likely because it's too much work to
track down the actual cause and fix it. The questionable pump has to be removed
and forensically examined by someone who knows what they're doing. You have
offered to do the examination but the questionable pump still has to be removed
and shipped to the examination and while it's away, the aircraft is useless in
the hangar.
So far we only have a good plausible theory missing
only demonstratable proof. The problem is infrequent enough that most people
"take their chances" and figure that the dump valve will solve the
problem.
However, for the problem in question, the dump
valve does not work. If the spool & poppet valve comes off it's end
position, for whatever reason, and traps the fluid volume in the lines and the
hydraulic system sees a large temperature rise, the pressure in both the HI and
LO side can exceed the pressure switch settings rendering the pump
inoperable. With the pressure excessively high in both the HI and LO
side, activating the dump valve has no effect.
The way I see it there are three ways to fix the
problem,
1) Test the pump under the offending conditions and
if it fails, apply the suggested repairs and test it again until it passes
operational test. This means removing the pump and testing in a controlled
environment while the aircraft is laid up.
2) Install a three port dump valve with the
third line going back to the pump's fluid reservoir so when actuated both HI and
LO sides dump back to the pump instead of simply cross connecting the HI and LO
sides. This still requires pilot intervention if/when the problem
occurs.
3) Shameless plug - - install my safe module on top
of your pressure switches, a 30 minute operation. This module monitors both
pressure switches and if both come open (high pressure) then activate the pump
in the direction currently selected by the gear switch forcing the spool & poppet valves back into the required position
and relieving the offending pressure in the lines. This monitoring is a
continuous operation on the ground and in the air and does not require any pilot
intervention.
Wolfgang
----- Original Message -----
Sent: Monday, February 13, 2012 9:23
AM
Subject: Re: [LML] Re: 320/360 Hydraulic
Pressure Switch
Wolfgang,
I would like nothing better
than to get at the root cause for those pumps exhibitng a system
lock-up.
We can show by
analysis and test that when both poppets are allowed to close, the hydraulic
system will lock up on the ground when heated.
We can also show by
analysis and test that if the spool
remains in its last commanded position that a lock up cannot
occur.
We have measured the force
it takes to move a spool on a number of different pumps
and configurations and foud the force far exceeds what can be
expected by airframe vibration.
It seems to me one should
examine in detail any pump that acts differently than the general fleet.
If I owned one hundred identical aircraft and one exhibited low
fuel pressure, I would not redesign the fuel system for the entire
fleet in response. Rather, I would examine why the one
was different. Likewise, when a pump allows the hydraulic system to lock-up, the proper
thing to do would be to examine that pump in great detail and determine how it
differs from the rest. I have a standing offer on the table to
examine and test any pump that exhibits this behavior. While I
have examined pumps for other reasons, I have not yet had the opportunity
to look at one that has been known to a cause a lock up. I
am really curious to see what the differences are and if they have any
implications for the rest of the Lancair fleet.
There is no need to further
test a pump with return springs. We know what can
happen. Note 'can happen'. The system will not lock up every
time, in fact, it will be very infrequent. Several conditions must be
met. While the return springs, at 26 lb/in, are strong enough to
get the spool moving towards center, they do not always make it all the
way. After all, spring force is a function of deflection. Near
center, they run out of steam. On occasion, I had to add some vibration to
lock both poppets.
The vibration levels
required to move a spool without a spring are enormous. Those levels would likely
destroy the motor assembly, not to mention the pressure switches, avionics and
so on.
Until we can examine a
mis-behaving pump and compare certain parameters, the root cause for that pump
will indeed be unknown. Fortunately, we know a great deal about how
properly behaving pumps operate and what configurations will cause
problems. Hypothesizing about a failure mode without being willing to
examine the pump itself is rather pointless.
Chris Zavatson
N91CZ
360std
From: Wolfgang
<Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Friday, February 10, 2012 7:26
AM
Subject: [LML] Re: 320/360
Hydraulic Pressure Switch
If it takes that much force to move the spindle against the O-rings then
those springs must be pretty darn strong. I find that dificult to
accept.
Still, that port with the added spring, I believe it's a return port for
excess hydraulic fluid that can be used with a three port dump valve.
. . . but why the spring if it's plugged anyway ?
If the ultimate answer is to remove the "extra" springs, then let's
demonstrate that by testing a pump with the springs under pressure, heat, AND
simulated engine vibration. Short of that, this failure mode remains a
question.
Wolfgang
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Wolfgang,
The ball and spring go in the port labeled "Cavity "E"
Back-Pressure". This is the pipe plug that sits in between the two
output ports.
You might consider securing
the dump valve with a secondary locking feature. It too relies on
friction alone to stay in position. Seriously, given the force it
takes to move the spool, if mine were not staying put, I would want
to know why.
From previous measurements:
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.
The 013-90 is specified for pumps with back pressure circuits
such as we have.
Chris
Zavatson
N91CZ
360std
From: Wolfgang
<Wolfgang@MiCom.net>
To:
lml@lancaironline.net
Sent: Monday, February 6,
2012 9:24 AM
Subject: [LML] Fw: [LML] Re: 320/360
Hydraulic Pressure Switch
I see the centering springs but what about the added spring, eyelet
and ball ? Where does that fit into the schematic ?
I still don't like the idea of friction being the only thing
keeping spindle at it's end point. Sure, removing the springs will help
but a positive system to pressure the spindle back into it's required
position automatically as needed still seems prudent.
Wolfgang
----- Original Message -----
Sent: Sunday, February 05, 2012 8:34 AM
Subject: Re: [LML] Re: 320/360 Hydraulic Pressure
Switch
Several folks have been asking privately about the spool centering
springs mentioned below. This link contains both schematics and
photos showing how to revert a new pump back to the
original 320/360 functionality.
http://www.n91cz.com/Hydraulics/Pump-Unlock.pdf
Chris Zavatson
N91CZ
360std
From: Chris Zavatson
<chris_zavatson@yahoo.com>
To:
lml@lancaironline.net
Sent: Wednesday, February
1, 2012 12:46 PM
Subject: [LML] Re: 320/360
Hydraulic Pressure Switch
Lorn,
Had the temperature been 90 or 100 degrees your pressure would have
been even higher. In some of my hydraulics videos, you
will see the 2,000 psi gauge pegged on occasion during thermal
heating tests.
http://www.n91cz.com/Hydraulics/Lancair_Hydraulics.htm
Your predicament of having to crack a hydraulic line is
exactly what can happen if the spool centers after pump shut down.
I find that a completely unacceptable design. Imagine taking off
into IMC and the gear won't come up. I had the opportunity to test
out a pump in this configuration last summer. Just as you
experienced, I was able to reproduce a complete lock up.
http://www.n91cz.com/Hydraulics/NewPumpLockUp_0001.wmv - hi
res
http://www.n91cz.com/Hydraulics/NewPumpLockUp_0002.wmv - low
res
After discussions with Lancair and Parker, I reverted the
configuration back to the original
320/360 config and all is good now. Only one side can ever be
pressurized at a time. Unfortunately due to employee turn-over, no
one currently at Parker or Lancair knows why the centering springs were
added. After reviewing the 320/360 system both agreed that a
system lock up is possible using replacement pumps currently sold
(108AMS32-CZZ-3V-14-08-Y). Parker sent me parts to revert three
pumps back to the original configuration.
The TRVs in our pumps are the lowest available -fixed setting of
2,500 psi. This may be too high to protect some of our
components. I have run them up to 2,000 psi.
In flight, I have seen 1,800 psi, descending from 18k into the southwest
deserts.
Chris Zavatson
N91CZ
360std
From: Lorn H Olsen
<lorn@dynacomm.us>
To:
lml@lancaironline.net
Sent: Wednesday, February
1, 2012 6:33 AM
Subject: [LML] Re: 320/360
Hydraulic Pressure Switch
Again,My
Oildyne internal pressure switch came (set by Oildyne) at 1,500 PSI. How
do I know? I parked my plane on the ramp at Newport News, Va in the
evening. The temperature was 40°F. 3 days later, when I was going to
take off. The temperature was 80°F. I looked at my pressure gauges. Both
the up and down were at 1,500 PSI. I had to crack a nut in a hydraulic
line to release the pressure. The gear would not go up or down, at this
pressure. The emergency dump valve would not release, at this pressure.
The pump would not run at all, at this pressure.Now, I have
Wolfgang's high pressure adapter in the system. It runs the pump
whenever the pressure is high on both the up and the down
side.There is no speculation here. Just the facts,
mam.--Lorn H. 'Feathers' Olsen, MAA, ASMEL, ASES,
Comm, InstDynaComm, Corp., 248-345-0500, mailto:lorn@dynacomm.usLNC2,
FB90/92, O-320-D1F, 1,800 hrs, N31161, Y47, SE
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