Mechanical Engineer Looking for Instruction in Hydraulic Valve Repair


Thread Starter

Manolis Pairis

Hello friends

As my title says, i am looking for someone to learn me everything i need to know about hydraulic valves in order to be ready to repair them.

I work in a plastic company in Greece, and we have more than 50 proportional hydraulic valves crash per year. They stop working for any reason (filter, coils, boards, etc.), and we send them to USA for repair which costs money and time.

So i decided to read and learn about this object. i am mechanical engineer with some basic knowledge about valves and circuits. In my factory we have mostly moog-parker-rexroth valves. So if any of you think that he can help and share his knowledge with seminar or guides he is more than welcome to mail me.

My mail is [email protected]

Of courses all expenses are covered by us.

Thank you

When you send these hydraulic valves off for repair do you receive a report to tell you what the problem(s) with them were?

Most problems with hydraulic valves are the result of poor hydraulic oil quality. Do you have the hydraulic oil(s) tested regularly, or have you ever had the hydraulic oil(s) tested?

Do you replace the hydraulic oil(s) regularly?

I would be asking the repair facility(s) what problems or issues they found with the valves. I also would be looking very closely at the hydraulic oil systems on the equipments--testing the oils regularly, and even considering flushing the equipment and replacing the oil with fresh, clean oil. I would think the manufacturers would have recommendations for testing and flushing and replacement, as well as specifications for oil quality which would be necessary to order the proper testing as well as for analyzing the test results.


I am glad that you answered CSA. I also have sent mail in your site.

To your questions, i think most of the problems are on coils, but i will verify this for sure!

I use hp46 nuto oil, and it is appropriate for our valves. But to be honest they don't refresh it, they just refill the lost oil with new. My problem is The cost it occurs from all these factors, the cost of shipping to USA the repair, costs plus the time! So on first stage i prefer to invest on my business and to repair the valves inside.

I tried to fix some valves with some basic knowledge i have, but i failed. I opened some moogs valves and cleaned it all inside, changed the filter, and finally i tried to measure the coil voltage-power-ampere. I did all these, and then i compared the results from coil with the results from a new moog valve. They were almost same.
After all the valve didn't work!

As a result i am looking for someone to learn me how to measure all parameters correct, and where/which spare parts can i find/use!

CSA imagine that we have more than 200 moog-parker valves which don't work and they preferred to buy new instead of fixing them!

Thank you in advance for your help.

Dave Ferguson

As a bystander let's be honest there are probably a couple reasons.....oil contamination issues or misapplication.

So it is hard to say what the issues are but it IS NOT normal to have that number of failures.

Do you do regular oil analysis? Is there proper electrical application? Source power and noise issues?

We rarely had issues and have 1000s of rexroth applications from solenoid configurations to proportional valves.

There is a lot to it though and not something to "dabble" in part time. We had an entire dedicated Mechatronics group dedicated just to our hydraulic systems. A supervisor, and automation tech (instrumentation) and a mechanical tech made up the group and did extensive training.

Dave Ferguson
Control Systems Engineer
I have to hope that there wasn't misapplication of these valves to this level. Maybe on some machines, but, hopefully not on all--or even this many!!! (YIKES!!!)

I have seen oil reservoirs re-filled without cleaning the top of the reservoir/tank--and in some cases the tops were FILTHY (either with dirt/dust or shavings from the process, or both). What a colossal waste of money, because the dirt is going to get into the reservoir at some point, and it's going to cause issues with the valves.

If it's mostly the coils of the electro-hydraulic valves, then it could be problems with the amount of current being applied to the coils. And, that could be caused by excessive current(s) being applied because the hydraulic circuits were plugged or choked with varnish or contaminants. Again oil testing would seem to be indicated to really get to the bottom of this situation (the "root cause"). Or it could be temperature-related (coils not rated for the temperatures being experienced in these applications--too hot, OR too cold!).

I have to say: taking apart electro-hydraulic valves and putting them back together again without the proper equipment is just begging for trouble and failure. Most electro-hydraulic servo-valves I have worked on require oil at a specific temperature and pressure and with an accurate means of measuring flow-rate to test and/or calibrate. Now, I know that some hydraulic valve manufacturers sell this equipment--but I also know that it's very expensive and requires training and maintenance--two things in short supply at many plants these days.

There MUST be a high profit margin in the plastic being made to afford this level of replacement. And, if not then someone hasn't been properly analyzing the costs using their spreadsheet program and proper data. OR, it's possible they have the proper data in their spreadsheet and it's economically feasible to repair/replace under the present circumstances because the profit margin allows it. (But, just think how much more profit could be made, OR, how the price of the goods could be lowered which might lead to additional sales and higher profits on increased sales!) But, in reality--SOMEONE has determined the present course of action is economically feasible for the plant.

There are LOTS of facilities and companies that refurbish electro-hydraulic valves outside the United States. It might be that the facility being used in the US gives a better warranty or something like that, or maybe their price (including the cost of shipping to and from) is better than other other firms in other countries. (US manufacturing CAN be competitive in world markets!)

I would LOVE to go to Greece (it's on my bucket-list of places to go before I die!), but, really I think there are more knowledgeable resources closer than I who could better help with the problem(s). I personally don't believe in trying to refurbish electro-hydraulic valves "in the field." Have you tried working with the company that is presently refurbishing your valves to see if they can help with analyzing the problem(s) and coming-up with a solution? Many refurbishers offer such services, and if they don't they might be able to recommend a company that does.

Please write back to let us know how you fare in resolving the problems!
When you have repeated failures like that, you have to look for a common denominator.

It can be incomming power or power surges if the electronics are failing, over heated enclosures, etc.

If the valves are failing mechanically, what is their MTBF (mean or average time to fail sort of thing per valve) rating for the pressures and cycle times you run?

If electronically, is it the coil drivers?

Tons of possibilities.

In one recent case totally unrelated to yours, the power supplies were blowing up.

Reason: they were made to EU voltage of 400 VAC 50Hz and sales claimed they could easily handle 440-460 VAC 60VAC common here...and they could, but a line check showed that the plant was running on 500-515 VAC!

The water cooled supplies could handle that, but not the units purchased....believe they got the supplier on board to replace the older units that keep failing...

Manolis Pairis

Thank you all for your answers,they are really helpfull!

I sent oils for testing to verify their condition.

About the voltages I haven’t done anything yet, but as i know (based in old measurements) the voltage varies from 395-408V. Maybe i must cross check that again!

Personally i think for a reason i don’ t know that the valves deregulated. As i said to previous post with my basic circuit knowledge, i measured the voltages-amperes-ohms in a stopped valve and then i did the same in a repaired one. They are almost the same! Both valves respond to electrical pulses so that means the burned one finally isn’t burned. The coil seems to respond, but from this point i can’t do anything to fix it, because i don’t know how. Also one more think i don’t know is where to search for repair kits. for example if coil is burned, where can i find a new one? Moog seems not to reply, as i already sent mails to them!

I really want to thank all of you for answering and for the time you spent. If anyone thinks that can help and teach me how to repair specific valves (3-4 types moog parker rexroth), i can cover all expenses for you to come here or i can come to your place!

Thank you again my friends!

Manolis Pairis

Hello and thank you once again for your help.

I sent the oil for tests and i am waiting the feedback. I understand your words about oil clearness and after test result i will program a refilling oil plan for my machines.

The advantage of my situation in repairing valves is that i have few and specific valve codes to all my machines, so i need to learn only these and not all the range of hydraulic valves.

Here are some questions:

1. Lets talk about my most used valve tyoe MOOG G631 series.

I opened it very carefully and cleaned it. also i replaced the filter and i closed it. Next day mechanics put it on a machine and they said that it doesn't work (no movement), and they sent it back to me.

When i opened it, i gave drift to the coil and it made movement (i gave from 5-10V) so i think the coil worked. (it wasn't burned).

Then i measured a ohm and amperes from a good working moog and compare them with mine, they were almost the same (0,306ohm).

Now there is a part in a coil which manage the axis movement with springs. I didn't know what to do with it so i left is as it was.

Can anyone explain me what i didn't do? or what else i have to do? I know that repairing isn't a so simple clean up job, but compared to new moog results, coils shown to work normally and all parts inside the moog were cleaned. What else?

2. On the moog g631 valve drawing, it shows us all the oil channels, and i noticed that i don't have access to some of them (in order to clean them) so maybe the problem is there.

If you go to page 5 you can see the valve drawing. I don't have access to the pilot stage filter on the left bottom (i opened the screw under it but i can't see anything).

Also channels T and B are blocked with pressed balls, which i don't know if i must open and how open them.

Does all these have any relation with the valve problem?

Thank you!
Your valve is the "flapper-nozzle" design.

The milliamp control signal causes the flapper portion of the valve to control the pressure to the actuating piston, basically a mechanical amplifier.

The flapper-nozzle is very sensitive to build up and must be kept clean.

So the solenoid will function on the bench. But with build up in the flapper nozzle, the valve cannot provide adequate pressure/flow to the spool-piece (acts as an internal sliding valve).

Manolis Pairis

Hello and Happy new year!

If anyone has some time and can help with my questions above please give me a hand!

Thank you!
Get lab tests for the fluid purity, particulates. otherwise you solve the problem only by high maintenance and replacement costs.


>If anyone has some time and can help with my questions above
>please give me a hand!

Please give me a call or email. I can help with both repairing and also ***LIMITED ** teaching. As others have stated, you really shouldn't take these apart in the field. They need to be calibrated on a dedicated bench. If you install a non calibrated valve on the machine you could easily injure people and or equipment or kill someone. Your valves are the flapper type & if your torque motor isn't perfectly centered, once you give it oil, its going to shoot open or closed before you can even attempt to correct. Not to over state it, but it really does take a couple years to get good at these. There are some things you "could"do in the field.. but don't over reach or you're asking for big trouble.