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Wednesday, October 23, 2019
The Motion Control Community Forum, sponsored by Control Technology Corporation.
MOOG G631 3605B Valves
Question about repairing moog valves

Hello friends,

I posted a thread some months ago about repairing valves and asking for guidelines. I followed all your directions but not received any complete guide or found someone to teach me. So i decided to experimenting myself.

Most of you already work or own a motion control company, so i need to ask you where do you get your spare parts?

Where do you buy new coils, seals, pilots?
Should i go direct to official moog?
I need only parts for MOOG SERIES G631 & D631

Thank you

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One has to admire your perseverance.

I just put "Moog G631" in my preferred World Wide Web search engine, and came up with this as the top result:

There is some really good information, especially on pages 13 and 14. At the back of the document is a list of spare parts.

Personally, I would not waste my time or the time of any of my colleagues or Customers trying to rebuild electro-hydraulic servo-valves. I WOULD definitely take some time and perform a thorough cleaning of one of the machine's hydraulic oil system, and install the filters recommended in the document, and perform periodic oil quality testing. I would closely track the performance of the electro-hydraulic servo-valve(s) on that machine. I would even start by installing a new servo after cleaning and flushing of the hydraulic system was performed--just to have a good baseline with a new and uncontaminated and non-rebuilt valve to start the observations with.

Moog also sells some kind of field tester, which says can be used to pinpoint electrical or hydraulic problems--might be a good investment, also.

Finally, they have some steps for maintenance and refurbishment in the document.

So, have a read of the document--especially the oil cleanliness section (and take that section to heart!!!)--and think about how you're going to set up a test to see if your efforts are rewarded. In addition to cleaning and flushing one machine's hydraulic system and installing proper filters and a new servo before re-starting AND setting up an oil quality testing procedure (the document says with the proper filters it should be possible to maintain the oil quality well below minimum standards), you might also want to clean, flush and install new oil and filters on a second and third machine. And, set up regular, periodic oil quality testing, and use re-built servos on those machines. You could also continue to use the same oil on another machine and install filtration system, and perform regular, periodic oil quality testing to see if the filters do, indeed, clean up the oil--and result in fewer problems with the servos needed maintenance.

Because I will go to my grave insisting most servo-valve problems are the result of poor oil quality. Pure and simple. Many others can be traced to poor wiring and misapplication (the designer(s) of the machine chose the wrong servo-valve for the purpose). I would love to be proven wrong, but 30+ years of experience in the power generation industry with Moog servo-valves says otherwise. We don't know what kind of environment your machines are in, or how they are maintained. But, keeping the oil clean--even possibly replacing the oil when necessary (based on the reports from regular oil quality sampling and testing performed by a knowledgeable firm with a good reputation) and good record-keeping can be invaluable for improving and maintaining machine (and servo-valve) reliability.

I would buy spare parts from Moog.

Best of luck!

Moog will not be very helpful with parts. They stopped selling parts to anyone but authorized Moog repair shops years ago. The only thing that they will sell you is the disc filter and maybe some o-rings. They feel it is a safety issue.

You should be able to get a copy of the service manual online. This will give you all the o-ring sizes. If you can't find it I can probably dig up a copy for you.

Although the valve operation is fairly straight forward. It must be disassembled in the correct order so as not to damage it. I have seen lots of broken feedback wires from people that thought they could do it themselves.

The most common problem is dirt from contamination. If you have to remove the jets, these are the trickiest to set up again.

If you need coils I have lots of them around from damaged torque motors.
NOt sure how to give my contact info without it getting broadcast. if you want you can go through my website at