|
|
| Chris: I really appreciate all the work going into this analysis. I keep trying to ascertain if any of this issue is part of the Legacy heat issue where I have to bleed the dump valve after a temp change on the ramp. Is this related in your opinion?
Paul Spruce Creek On 2012-02-13, at 9:23 AM, Chris Zavatson wrote: 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
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
|
|
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
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.
Chris Zavatson
N91CZ
360std
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.
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.
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
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 Michigan |
|
|