stuck proportional cartrige valves

[subsearobot]

We are commissioning a mobile vehicle. We have 14 proportional open-center spool valves. these cartridge valves control both gear motors and pistons.

We filter with 6micron abs filters. We are using 32 wt hydraulic oil.

From the beginning, the valves were behaving a bit oddly. when starting up, at full pressure (3300psi) there would be no response to a command. we would reduce pressure to about 1000psi, and valves would begin to work fine. then pressure could be increased to operating pressure, and the valves would continue to function normally.

then we got called off on another project for a week. on returning, we found that ALL of the valves were all stuck in the center position. turning system pressure down did nothing now. ambient temperature had dropped since beginning testing (the shop was maybe 45deg F).

We pulled the valves, and examined them. pressing on the spools, I could feel resistance beyond normal spring force. Pressing harder, the spools would slowly begin to move- it felt like pushing the spool thru some sticky goo, or rolling a particle past the gap between the spool/housing. then after the spools moved maybe 1/4 of the full range, it freed up. then it felt normal. we checked the valves on our test bench, and they function fine after "clearing" them.

has anyone experienced anything like this? Any input would be greatly appreciated.

 

[hydromech]

If the system is contaminated, the debris will be pushed into the valves, getting between the mating parts and locking valve.

Having a filter in line will only catch particles that move through the filter.

You need to get the valves cleaned and fit flushing blocks and run the pump to shift the debris.

Check the cleanliness of the system before you fit the valves.

 

[subsearobot]

Dirt was my first thought as well, but after a review, I began thinking there might be another cause. If it was debris, wouldn't the valves fail interdependently? also, wouldn't some valves stick in the open position? these valves all fail after the system is de-energised, and all stick in the center position. they do not fail while in operation. I certainly reserve the right to be wrong about the dirty oil theory, however!

we flushed the system with flushing valves before installing the suspect valves to remove any debur chips left in the tubing from the build. we fill the system with 10 micron filtered oil.

one thing i notice is that there is a decent amount of air in the pressure side, which does not blow thru. not sure how it does not blow thru to the tank, but it does not. I am looking into that. might be a dead-end in a manifold with too many spare functions.

if the gear pump is intaking aerated oil, could adiabatic compression of bubbles lead to localized oil varnishing that we might not pick up with our temperature sensor? the sensor is located just after the pressure relief valve (used to set system pressure, so full power goes thru this valve when the system is idle). We have not seen temperatures greater than 190F, and that for only minutes at a time.

my boss thinks the problem is due to a check valve at the pump output...

 

[hydtools]

Can you attach a schematic?

Ted

 

[subsearobot]

here is the main system.

 

[hydtools]

Are the spring cavities wet or dry?

After 'clearing' and working on the bench do the valves continue to work properly when reinstalled in the system?

Ted

 

[subsearobot]

ted,
the spring cavities are wet.

after clearing them, there have been a couple of lock-ups on the bench- froze in the closed condition, no response to the coils magnetic command. cleared these again, and then functionality returned. We also put different valves into the system, and saw a couple of them lock up (closed position again). this seemed to disprove a batch manufacturing problem, as the initial frozen valves were from a different production of valves (replacements were from our older stock, as opposed to the valves specifically ordered for this project).

as a side note, we have tested the control side; we see a magnetic field from the coils, and current draw is consistent between working and stuck valves.

 

[hydtools]

Sounds like the spring cavaties are not venting to allow fluid to move to permit the spool to shift.

Ted

 

[chief]

Flush,filter,clean and vent your system to at least NAS 8 and select a lighter hydraulic oil down to 10W. Activate valves whilst circulating to clear possible debris.

Offshore Engineering&Design

 

[MikeHalloran]

I think you've got seawater in your oil.
Look for leaks in the accumulator/reservoirs.



Mike Halloran
Pembroke Pines, FL, USA

 

[IceStationZebra]

Not sure if this applies but mind the installation torque. Depending on the design, the cartridges can get distorted if torqued too high.

ISZ

 

[subsearobot]

Thanks for the thoughts, fellas.

we have flushed, and filtered several times, with no apparent effect. I have looked at the fluid and valves with a 50X microscope, nothing apparent.

We have a pressurized 'reservoir' (we call them compensators).
I am a bit concerned about water in the oil from oil that has sat in a humid environment for some time. not too concerned though: in the past an identical system sucked a significant water into the system to the point that the oil looked like Mayonnaise. the valves were fine, client flushed the system, and valves continue to function 2 years later.

we install valves with a torque wrench. i will get the guys to check that it is operating correctly...

we are sending some stuck valves back to manufacturer for evaluation. I will update if anything interesting comes of it.

 

[subsearobot]

Manufacturer disassembled, washed and filtered the results onto a 'slide' (filter disk).

they were satisfied it was a contamination issue. I examined the contaminants with our microscope, and I see a lot of junk including tiny hairs, fibers and what looks like quite a bit of aluminum shavings. The aluminum is from our hard anodized aluminum manifolds. the shavings have remnants of the black anodize on one side, and are shiny on the other. the sizes are approx less than .0005" to maybe .002". they were probably galled from the threads and mating surfaces due to installation??

I still don't understand how they got past the filter, but am fairly confident that this is what caused the problems. We have changed the filter and had a successful test run on new valves today. I hope this is the solution.

 

[hydtools]

Thanks for the followup feedback.

Depends on where the filter is relative to the source of debris. Also filters with a bypass valve may allow bypass to occur on cold system start up if the filter does not have a flow rating enough in excess of normal system flow.

Ted

 

[subsearobot]

Final follow-up:
The all of the valves locked up again after a "cold" (45degF) night. Of course, we were due to deliver the following week.

Our hydraulic systems are closed loop, where the reservoir, and thus all tank ports, is pressurized to ~15psi. We were using a well known hydraulic oil, 32 weight. Turns out that with that little pressure, some varnished oil and likely a little residual dirt, the apparent viscosity increased to above the valves ability to shift. when we removed a stuck valve and warmed it, pop- the spool would shift.

We flushed, switched to a synthetic oil (32 wt), and the valves all function properly now, even in the "cold".