Insane Hydraulics

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A Couple of Things That Can Throw a Tech Off Guard

How do we get fooled? We get fooled when we recognize a situation as "standard" because it looks like something we've seen countless times before, and then it turns out to be that one percent of cases when our intuition was actually completely wrong. And you know what? Such moments are awesome. Awesome and precious, because we learn something new from them every time!

So, today I want to review an evaluation report that unintentionally (and luckily for us) documented a couple of such "counter-intuitive hydraulic tricks" that can "ambush" a technician.

Imagine a very basic system - a hydraulic arm with a couple of double-acting cylinders and a rake at the end, used for automatic cleaning of water channel trash racks. Here's a picture, just give you an idea of what I'm talking about:

The system operated fine, but when the original rake was upgraded to a heavier gripper rake - it pushed the arm to the very limit of its lifting capacity and would cause it to "get stuck" with a heavier algae load or with a stronger current pushing the loaded rake against the grill. Long story short - a hydraulic cylinder-operated arm required a "force boost", and a hydraulic technician was requested to study the system and report on the possible solutions.

The (very professionally made) report mentioned two possibilities - increasing the system pressure and replacing the existing lifting cylinders with "fatter" ones. These suggestions, of course, were one hundred percent correct, but there were a couple of points in the report that immediately stood out to me as "classic tech traps", and I would like to show you where exactly the tech "got caught" by his standard thinking.

Let us start with the system pressure increase. The boom cylinders operated at 100 bar, and it was determined (hopefully with the use of the Tube Pressure Calculator, and you can read more on how to determine the max working pressure of a cylinder tube here) that for the given tube size and operational conditions it was safe to increase the working pressure to 150 bar. So far so good - this sounds like a "free" 50% boost! Then, since the HPU was using a Rexroth PV7 10-14 pump, which is a variable displacement vane pump, that has a max. operating pressure of 160 bar and is equipped with a max displacement limiter - it was determined that with the 3 KW electric motor, it would be possible to increase the working pressure to the required 150 bar safely by limiting the pump flow to about 7-8 l/min. So... What's wrong with this picture? How did the tech get caught here?

If you just whipped out your calculator and saw that 8 l/min at 150 bar was 2 KW, and were about to say that he could be aiming for a bigger flow - you would be wrong. That's not it. The seemingly lower flow recommendation was actually OK because the PV7 10-14 has a leakage of almost 3 l/min at zero stroke and P max, and you actually need to account for that power loss as well, along with the inevitable mechanical losses - so the 7-8 l/min target flow was perfectly fine. The "counterintuitive" error, or better - omission - is elsewhere. Any ideas?

If you have no ideas - I will tell you. Look at the whole picture. Where is this equipment located? It's a freshwater channel. And such places are notorious for mandatorily requiring environmentally acceptable hydraulic fluids! And, depending on the type of the "green" hydraulic fluid - a safe maximum pressure for a given pump can be much lower than the nominal pressure stated for the "normal" mineral oil. Let us consult the official catalog for the PV7.

It says:

"...Hydraulic fluid for use at up to 160 bar (nominal pressure): Mineral oil HLP according to DIN 51524, part 2..."

And then it says:

"..Special hydraulic fluids up to operating pressure p max = 100 bar: HETG and HEES hydraulic fluids according to VDMA 24 568, HFD-U according to ISO 12922..."

Aha! So the tech should have inquired about the hydraulic fluid used in the system, and the (otherwise absolutely correct) recommendation for increasing the pressure to 150 bar should have been given with a note that this would only be recommended if the system used mineral oil. We are so used to working with the "good stuff" in our tanks, that we forget that sometimes it is not the case...

Let us take a couple of moments to "decipher" some of the catalog "gibberish" from above, shall we?

What is HLP?

HLP, according to DIN (Deutsches Institut für Normung) 51524-2 (standard for pressure fluids) - is a mineral-based hydraulic oil with anti-wear, anti-oxidation, and corrosion-protection additives. In other words - "normal" hydraulic oil.

A HEES fluid is a synthetic hydraulic fluid based on esters. "HE" stands for Hydraulic Environmental, and "ES" for Ester Synthetic. There are so-called saturated and unsaturated esters, that can be used as bases for bio-degradable hydraulic fluids, but a proper HEES will always be based on the saturated one. Many manufacturers will tell you that you can operate your system with such a fluid just as you would with the classic HLP oil, provided that you keep the temperature under control, use high-quality fiber filter elements, and make sure there's no water ingress - but these fluids are crazy expensive, which is why you'll almost never come across them.

What is HETG hydraulic fluid?

The "TG" stands for "Triglycerides" (basically - naturally occurring fats). In essence - it's vegetable oil (most of the time - rapeseed oil) with some additives, And it is, probably, the worst stuff that you can put in your hydraulic system! HETG oils are way cheaper than HEES fluids, and so whenever an end-user is forced to use a bio-degradable hydraulic fluid, he will choose HETG over HESS nine times out of ten! No, cross that, ninety-nine times out of a hundred! In my opinion, HETG is only good as a mediocre paint stripper. But that's just me - I am a gen-X-er, so I'm not environmental "by design".

What is that VDMA thing then?

The abbreviation VDMA stands for Verband Deutscher Maschinen und Anlagenbau e.V., which means German Machinery and Equipment Manufacturers Association, and, in case you wonder, the letters e.V in the end mean something like "registered association" in Germany. The 24568 is an old technical standard that defines the requirements for HE.. fluids. There's a much more modern ISO 15380, which, basically, specifies the same, but lots of folks still mention VDMA in their catalogs. Is it a case of "copy/paste catalog engineering"? I don't know. I don't really care either.

What is an HFDU hydraulic fluid?

HFDU is a water-free fire-resistant fluid, whcih can employ a variety of bases: esters (synthetic and natural), glycol, and even triglycerides and mineral oils (once again - I would steer clear from anything that is triglyceride-based!). Obviously. any lubricant that uses a synthetic base is expensive.

Hew.... I got carried away for a minute with them abbreviations now, didn't I? There's still one more "counterintuitive trap" that I wanted to highlight, so let us get back to it.

In the second part of the report, the tech proposed the application of lifting cylinders with the same travel and rod size, but with a bigger tube ID, as an alternative to the pressure increase. Mechanically it was possible, and. of course, this approach is entirely correct as well. For example - using a tube with an ID of 110 instead of the original 100 would give a 20% increase in linear force (95/78.5 = 1.21), and an ID of 120 would secure a 44% increase (113/78.5 = 1.44)! There was, however, a very interesting phrase in the report:

" of the possible challenges in applying the cylinders with bigger volume would be the need to address the increased fluid level shift in the hydraulic tank during the operation...".

Sounds logical, doesn't it? When you install a bigger cylinder you need more oil to make it move, don't you? So... Where's the promised catch?

The promised catch lies in the fact that the new cylinder would indeed house a tube with a larger ID, but it would maintain the same rod diameter and, obviously, the same travel, which means that the volume differential between the cylinder sides would remain the same!

When you need to calculate how much oil a normal double-acting differential cylinder will consume from the tank - all you need to do it to calculate the volume of the rod (for a given stroke)! Surely - the total amount of oil in the system would increase, and the speed of the cylinder would decrease - but the level shift in the tank between the open and closed cylinder positions would remain unchanged, which means that there was no issue to address in the first place!

So, there you have it - a couple of things that can catch a tech off guard - biodegradable hydraulic fluid, and a cylinder liner upgrade. Who knew?!