Insane Hydraulics

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Ten Steps to Changing Hydraulic Oil

Today I want to continue with the topic of clean oil and show you one of the ways to tackle a "proper oil change", using one of my recent service calls as an example. Consider this as a "how the big boys do it" presentation if you will.

Hydraulic oil change - what can be simpler? Drain the old oil and poor the new one in - it's not rocket science, is it? And I say yes to you, my good Sir! You are one hundred percent correct! Oil hydraulics indeed has nothing to do with rocket science! I mean - this is why it's called "oil hydraulics", and not the... you know... the "rocket science..."

This is my way of saying that there can be more to exchanging oil than simply "drain and fill". Note that I said "can be" - because with hydraulics an answer to any question starts with "it depends..." If it's a throwaway gear pump driven log splitter that runs 100 hours per year - then yes - the oil change is, well, a straightforward change, but as the size, the price and the performance standard of the hydraulic equipment goes up - so does the complexity of all service operations, including such a seemingly simple procedure as the oil change.

And the main reason why such "more intricate" service procedures are needed is, of course, the liability, or, simply put - "who's gonna pay for it if something breaks?" When you are servicing a piece of equipment that costs north of a million bucks, you'd better make sure that all the steps you take are well documented, and that you have hard proof on your hands that your part of intervention went smoothly and followed the accepted industry standards. Especially when an OEM representative is not there to whiteness what you are doing.

Random breakdowns do happen, even to perfectly serviced equipment. However, despite the fact that everybody knows this, any breakdown that happens while the "memory of the last service procedure is fresh" - will be attributed to the said procedure. The reasoning usually starts with -"You were the last one to fiddle with it and now it's broken!" and is followed by "You broke it - you fix it, coincidence or not!"

Now think about it - an oil change is probably the easiest procedure of all the service procedures in the world to blame for a breakdown!

So, let me walk you through the steps of my "bulletproof" process for such situations.


The mine called and asked for help with changing the oil of "that big HPU" because they had the oil tested and the results showed that the acid number was too high, so an executive decision was made to go ahead and exchange it - all the 4000 liters of it.

Of course, the regular maintenance crew were no strangers to changing hydraulic oil. Mines are driven by hydraulics, thank God for that, but since it was also time to verify all of the adjustments, and due to the COVID-19 restrictions the OEM techs could not make it, yours truly was asked to "assist". I am glad I was asked, though. The "operation" went smoothly, and I just want to break the experience down into steps and showcase some of the gear that I've used for years with what I consider to be positive results, in hopes that you may find some of the points useful. Or, maybe, correct me if I did something atrociously bad... It's a win for me in either case.

Step 1 - Preparation

If possible, ask for the clean oil to be delivered to the worksite, and (once again, if possible) to be "parked" in the place where it will no longer be disturbed and where your oil transferring gear can reach it, before starting the work.

Firstly - this handles the logistics. Whenever I deal with big warehouses - I always run into delays. and the bigger the company, the worse is the bureaucracy. There's nothing worse than spending hours chasing supplies. And if you say something like: "But surely a big company will have all the supplies prepared for you before the scheduled work should begin, right"? - I will reply: "Yeah... Right..."

Secondly - you get to transfer the oil from a container that has "sat quietly" for a while, which is a good thing because if you are careful enough to not be deliberately sucking oil from the farthest corners at the very bottom, you will leave the heaviest particles behind. Believe me - you never know what will be inside your "new" oil, and depending on its origin, the number of large particles in it can vary from "none" to "plenty", so an oil container that has rested for a while is always better than the one that has just taken an agitated ride on top of the forks of a multi-function loader.

In my case, the containers were four 1000 liter IBCs, and they were thoughtfully delivered to the site a couple of days before the intervention started - I couldn't possibly have hoped for a better deal!

Step 2 - Drain the old oil and inspect the bottom layer left in the tank

This one is, actually, as simple as it sounds, but there are a few points worth mentioning. First - if the old oil looks really bad, you should keep a sample of it - just in case. Second - how easy it is to drain the old oil will greatly depend on the concrete system design, so make sure you know it and think it through before opening "the tap". Third - more often than not a thin layer of oil will stay at the bottom of the tank after you've drained it, and it is always a good idea to inspect this bottom layer, and, once again document and keep whatever "treasures" you may find. And fourth - don't forget that some actuators may still contain a significant amount of work fluid, so if the condition of the oil that you are replacing can be described as "remarkably sad", draining the actuators might be not a bad idea.

The mine crew had most of the oil drained from the tank when I arrived, but there was still about an inch of oil left at the bottom of the tank, so I put my "clean suit" on, placed myself inside the tank, did the bottom layer investigation, and then and used a "special tool" (a squeegee improvised from a broomstick and a strip of thick rubbery plastic) to "usher the oil towards the exit".

Step 3 - Inspect the "insides" of the tank

This step is pretty self-explanatory. Tanks, especially steel ones - need to be inspected for rust spots, peeling paint, varnish deposit - etc.

In my case, the tank was in a very good condition, almost pristine. Props to the Rexroth guys who built it - the paint job inside the tank was outstanding. I did find some condensation corrosion on the "ceiling" - but nothing too alarming.

It is curious that condensation still happens even though the tank is sealed and employs large desiccant air breathers.

Step 4 - Inspect the "insides" of the filters

If there is a single place that can tell the most about the health of a hydraulic system - then it's the return filter.

I always like inspecting the bottom of the return filter housing for something "interesting". You would be surprised to hear about the things I've found.

In this case, everything was "clean as a whistle".

Step 5 - Corect and clean

This is the "do you damned job!" bit, where you correct all of the issues and entertain yourself with the possibly worst and yet most inevitable part of this job - cleaning everything that needs cleaning. Look, how happy this fellow looks, for some reason. I blame it on the toxic fumes...

Use your imagination, everything goes here - from brooms, lint rollers, rags, and shop towels to sophisticated purpose-built cleaning equipment. Two things I want to mention here - make sure your towels don't leave half of their fibers behind and remember that a vacuum cleaner is your best friend, so don't forget to bring it along.

Step 6 - Close the tank

Another simple and straightforward step with a couple of "honorable mentions".

If the tank is big enough for you to have worked inside - make sure you leave no tools or cleaning supplies behind. I like to do the inspection myself, and then always ask somebody else to re-inspect it. We always find stuff.

Make sure you know where your rechargeable work light is before you close the inspection cover, and then look at it after you've closed the tank and register this moment in your head. Ask the person who inspected the tank with you to look at it as well.

This is absolutely necessary, because if you misplace or lose it, and then only discover that it's missing after you've finished the commissioning - you'll be wondering if it's still inside the tank for the rest of your life!

Step 7 - Replace filters

Make sure that you document the elements that you use. Only use high-quality stuff, seriously. And go for fiber. Cellulose elements are for suckers.

Step 8 - Fill the oil in

If the tank is not yet equipped with a proper filing coupling - install one. New oil should only enter a hydraulic system through the system's return filter.

As for the actual oil transfer - a bigger transfer pump is not necessarily a good thing. I am not too fond of high flow oil carts. Higher speed means worse filtration, and since an oil transfer is a "single pass deal" you need all the filtration you can get. Plus, since you will most surely be transferring oil that is cold, a higher flow will mean a higher risk of cracking the return filter's by-pass valve, which defeats the purpose of transferring the oil through the filter in the first place!

This small and portable puppy here has been my trusted sidekick for the last seven years. It can only dish out two and a half gallons per minute, but it has never let me down. It has removable suction and pressure elements and is surprisingly cheap. And in case you're wondering - yes, I used it for this job as well - I prepared all of the hoses and left the night shift to take care of the transfer. A little extreme? Maybe it is, I agree, but it did the job, and you are about to see how well in the next step!

Step 9 - Flush the oil and register its cleanliness level

It's time for the high-end tools now. If it's a "double Ex" hydraulic system (i.e. Extensive and Expensive) - you should not start it after an oil change without at least hard logging the oil's cleanliness code (you know, the above-mentioned liability thing and all...). I prefer doing the oil flushing whenever possible before starting a system. Almost always there's a way to do it with nothing but an external filter bank because it is common for big hydraulic power packs to have some sort of a cooling/filtering circuit where you can introduce the filter bank and your particle counter and operate this circuit alone till you run the ISO codes into the ground.

Two important points here. The filters and the target cleanliness level, Let's do one at a time.

Flushing Filters

This is my field-tested filter bank, and I really like this arrangement:

It's composed of two parallel FPB34s from UFI, which means that I can go up to 420 bar if I want to (watch out, closed loops!), and I get to use UFI's fiberglass "cans" with the beta above 1000, plus the parallel connection allows me to easily go up to 400 l/min (100 gal/min) with the tightest 5 µm elements and up to a 1000 l/min with the 12 µm. I have only used the 12s since we always have them in stock. The fact that the filters are inserted in the cage makes the transportation, "deployment" and servicing a lot easier, and the rotating Ts allow me to orient the connecting hoises in any direction, which is very convenient, especially when you are using 2'' "flexible" hoses. I also drilled and tapped holes in the Ts to connect a pressure gauge and a particle counter.

Target cleanliness level

This is another important question. When is the oil clean enough? Isn't it just a matter of checking the pump's catalog? Well, this is a tricky one. This system is running two Rexroth A4VSG500s and two A4VSG355s. If we check the respective catalog, we find the following quote:

"A cleanliness level of at least 20/18/15 is to be maintained according to ISO 4406".

OK, let's check out what other brands have to say:

Parker Gold Cup series - 20/17/14 or better.

Eaton closed loop series 1 - 18/13 (a two-digit code, essentially standing for the last two digits of the three-digit code).

Danfoss Series 90 closed-loop pump - 22/18/13

Having doubts yet? My advice - don't sweat too much about these numbers. You can (and should) do better. Modern filters will easily put the numbers at least a couple of digits below what manufacturers recommend - and once your FCU has printed out those paper slips - no one will dare say that the fluid is out of spec.

Let's get back to our HPU for a concrete example. The unit had a 375 l/min cooling circuit that served nicely for the flushing!

I used a Hydac FCU 2210-1 (four channels, 4, 6, 14, and 21 µm) to monitor the oil cleanliness.

After the pump ran for an hour - I got a steady 16/10/7, which meant that the transfer operation went well. Then I left the pump running overnight, and the next morning I was greeted with a 14/10/7! Which meant it was time for the final step:

Step 10 - run the system, and sample the oil at a key point

After all the "preliminaries" (checking the accumulators' pre-charge, making sure there are no air pockets in suction lines, etc...) I installed the FCU at the outlet of one of the charge pumps and started the HPU. The key pressures were in check, so I went back to checking the oil - and it read 13/11/08 and after a couple of hours of work stabilized at 12/10/07!

The quality of a hydraulic system is defined by the filtration, and the Rexroth guys spared no expense. So I was not surprised to see the numbers go down. Also - the system had zero suction strainers! (I have a strong opinion on suction line filtration, you know).

That's it with the steps. At this point the job is done, and you a free to combine all of the pics you took and the data you collected into a nice looking report.

So - as you can see, an oil change can be an exiting multi-day experience.

Of course, this is just an example of how I did it that one time, but I strongly believe that sharing experiences is important, especially in this field - we all want to get better at what we do, right?