Insane Hydraulics
Bold and audacious blog about fluid power
As I was looking through the pictures on my phone, I realized that I forgot to tell the rest of the story of the grader from the article about two steering valves with equal displacements but very different steering speeds. Better late than never, right?
Let me start from where we left off. The steering pump of the grader was toast, and there was no way for me to repair it. Even the body got damaged, so replacing the pump was pretty much my only option. It was the Eaton 70422-RDU legacy Cessna unit (38 cc, 115 bar compensator) with a special feature - compact configuration, which allowed it to be mounted very close to the engine - to be driven by a small helical gear tightly mounted on its tapered shaft.
When you are looking for a replacement hydraulic pump, having the term "legacy" in its description is bad, having "legacy" plus "special feature" is worse, and having "legacy" + "special feature" + "tapered shaft" is a catastrophe, because if you don't "strike" the exact model rusting in someone's stock, you'll never find a replacement that fits!
We did find the right pump, and we did order it, and it was a complete disaster because the pump that arrived turned out to be completely different from what we neeeded, so we sent it back, and looked, and looked some more, and we didn't manage to find anything with a sensible delivery term. And the machine had to be fixed ASAP!
Luckily, getting out of trouble is a specialty of mine, and after the mechanic confirmed that the pump was powering exclusively the closed-center steering valve, a decision was made to turn this closed-center pressure-source circuit into the classic open-center by replacing the piston pump with a gear pump and the closed-center steering valve with an open-center one (26 cm³ GR3 pump and an OSPC250ON).
I grabbed a GR3 bearing support (Borelli 25404-6) with the 1:8 tapered shaft and had a local machinist lathe it down and make a thin SAE-B front plate to go in front of it - and thus got a pump that a) could perfectly fit in the tight spot it was supposed to go and b) had the correct bearing-supported shaft - so no modifications to the driving gear were necessary.
Unfortunately, I didn't take any pictures of this "masterpiece" - but it's not that important, all you need to know is that a pressure source was replaced with an open loop.
The mechanic installed the new pump and the steering valve and called me saying that the steering was working perfectly fine. What a relief! But, of course, this is not the end of the story.
The man called back a couple of days later and said:
- "Look, man, I got news about the grader..."
- "Good news? "
- "Well...." (not the answer you want to hear when you are asking if the news is good) "I kind of missed that the pump was also "pressure sourcing" the brakes, and now... well... now you need to turn the steering wheel to the end of travel to charge them accumulators, and it is impossible to work like that! Can you come up with something to make it work? "
Challenge accepted! I did just say "getting out of trouble" was my middle name, didn't I?
If you work in the same business as I do, I'm sure you know that construction clients don't want "pretty" - they want "fixed", and they want "fast" so whichever solution I might come up with had to be functional and immediate. Not more, not less. And reliable too, of course, because you know... brakes. And by the way, I totally forgot to mention that the grader had a classic industrial positive braking system, which used hydro-pneumatic accumulators to store "braking energy" - very common in heavy industrial vehicles.
The original 115-bar pressure-source system did a great job keeping the braking accumulators charged and making sure the power steering worked at the same time, but it was gone now. So - how would I get the 100 bar into the braking system without compromising steering in the new open-center system?
An inline 100-bar sequence valve? It would work, but it would also fry the oil.
A simple accumulator charging valve for the brakes? It would work, but it would also be shutting the power steering every time it diverted the flow to charge the brake accumulators - and I needed to make sure that both the brakes and the steering worked at all times.
In that case... how about a flow divider and an accumulator charging valve? The regulated flow goes to the brakes, the excess goes to the steering, and when the brakes are charged, the regulated flow joins the excess and all flow goes to the steering. This... actually doesn't sound too bad, especially given the fact that we do stock flow dividers and unloading valves with differential unload/reload, and so, with the plan devised, I went to the shelves to get supplies:
Here you have an "economic" three-way flow control valve (VRF C3C/C/34 from HPT), the good old VMSP-78 (pilot-operated unloading cartridge valve), the respective manifold (OC1009118- R901100747), and a random check valve. And how do you build a hydraulic circuit when you are short on time? You make your best impression of Keanu Reeve's voice and say: "Fittings. Lots of fittings!" And then you end up with something like this:
The most amazing thing is that this abomination, which took about fifteen minutes to design, build, and test, worked. And it has been working for almost half a year so far, problem-free! Unfortunately, I didn't assist the actual machine, but the mechanic told me that it sounded like normal factory-made open-center braking valves - occasionally making a "z-z-z-z-ummmm" when the accumulators would need a bump in charge, with steering operational all the time and no overheating/control issues whatsoever. He didn't even need either the check valve or an additional accumulator - he managed to tap into the original brake valve and use the existing ones.
But I still want to add a couple of comments.
First of all - the leakage. Such a circuit will always have leakage through the orifice of the unloading valve logic element when it is open. It is definitely not a problem for this case, with the steering relief set to 115 bar, and the fact that most of the time it actually works with much lower pressure, but the orifice is not that tiny:
It is sub-mm (and I was too lazy to measure it), but I can tell it's not too small, so were such a circuit to work with the max admissible port 2 pressure for this cartridge valve (210 bar) permanently - I would measure the leakage flow to decide if this loss is acceptable or not.
Second - as always, when you devise a system with a cartridge valve, you look up its function and design. Luckily, Rexroth (Oil Control) engineers responsible for publishing technical literature didn't go to the same school as engineers from Sun Hydraulics, and their catalog does include a nice cutaway view of the unloading cartridge:
This is not a "normal" unloading valve one would use in a two-stage log splitter. This one has a built-in 15% hysteresis, which means that after unloading, it re-loads at 85% of the set pressure, which is essential for accumulator circuits. Without the hysteresis it would most surely work as a relief valve, never truly unloading the pump, and I actually have seen technicians commit the mistake of using a normal unloading valve for accumulator-charging circuits and then wonder why their pump is stuck at max pressure.
It is all in the cutaway view - before the pilot poppet is lifted, the cartridge is essentially a piloted relief valve, and the larger piston (piloted by port 3) does nothing - it is retracted because the pressure above it (coming from port 1 through the orifice in the logic element) is higher than in port 3 (due to the pressure drop of the check valve). However, as soon as the poppet lifts from the seat and the pressure above the larger piston drops - it "jumps" up and starts "helping" the poppet with its larger area, and the more the poppet opens, the more the larger piston "helps". It is, in essence, a by-stable switch, and I find its principle of operation pretty neat. The simplified symbol definitely does not do this valve true justice.
So, there you have it - we saved a grader, we learned a valve, and we even used a John Wick reference! I consider this a total win!