minimum test of faulty servo valve 2

[Hansen66]

Hi
I'm having a problem with a hydraulic teststand and have a suspicion that it might be because of a faulty servo valve. As we have plenty of oilsupply and a good cooling system the leakage isnt really a problem. My question now is if I just want to check if the valve is faulty and dont care what exactly is the reason of the faulty valve, what tests should be made as a minimum? I have an idea that a null test and a dynamic test would be enough but as my knowlege in the world of hydraulics is not that great I would like to know your oppenion
(If something like this thread is already out there I'm sorry to repost)

Best regards
Hans H.

 

[Hansen66]

Oh I forgot to mention that the valves used in the stands are MTS 252.24C and mts 252.25C. I dont know if its relevant but now you know

 

[hydromech]

Is the oil clean?

By clean, I mean clean to the required NAS or ISO 4406 standards for servo valves.

Are you getting position feedback from the valve? Does the feedback change in relation to the command signal?

Contamination will cause the spool/solenoid sleeve to stick.

 

[Hansen66]

Hi again and thanks for the fast reply
As fare as I can tell the filters are working and the oil is not contaminated but I haven't used the teststand for a while so it might have been at some point. If this might be the problem a sticky spool would result i an abnormality when compared to a new valve right?
As the problem started after i came back from holiday I have a suspicion that one of two things has happened:

1: Someone have forgot about the cooling system so the system has reached a temperature higher than what the valve is rated for

2: The supply pressure have been to high at some point

It is obvious that these two things could be a reason for the error but I dont know exactly what kind of trouble they would result in and if I can see it from a dynamic test

 

[Hydromechdude]

What pressure does the system operate at? I've seen some sticky valves that did not function properly at operating pressure, but upon lowering the pressure the valve operated normally. This was due to the valve clearances being very tight and at the lower pressure, the clearances were acceptable for proper operation, but at the higher (operating) pressure, the spool would hang up on the sleeve.

The problem came from the wrong process in manufacturing where the sleeve and spool were fitted at a lower pressure than operating.

Is it possible to vary flow rate and pressure? How about disassembling the test stand and checking the flow through the valve in either the open or closed position?

 

[IceStationZebra]

The MTS servo valves are made by Moog. Supposedly they have some special tweaks beyond the normal Moog items, but I have never found out what. These two valves are rated at 10 and 15 gpm at 3000psi.

I don't know if overly high pressure would cause operation like that. But if the oil got too hot it could have caused varnish and deposits. MTS is very particular about the oil, and STONGLY recommends Mobil DTE 25 because over time it has proven resistant to varnish formation.

As far as cleanliness goes you should keep the oil cleaner than an ISO 4406 rating of 16/13/9. It is possible that you have some particle stuck inside the valve.

As far as a check goes, the easiest one is the do a continuity check on the coils. Look for oil in the connector or broken wired in the connector back shell. If you do not have a known good valve to substitute (or a bunch of pressure and flow gauges) than the best thing to do is send it to any number of servo repair shops.

ISZ