Insane Hydraulics

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How Can Two Steering Valves With Equal Displacements Produce Very Different Steering Speeds?

Today's post is hard to classify with a single label because it's "all over the place": I'll start by showing you a unique, or better - record-breaking - swash-plate wear (which should go into the "kaboom" section), then I will (indirectly) call the assumption that the displacement of a single-rotor steering valve is what determines the steering speed a myth (which is supposed to go into the "myths" section), and then I will explain why it is so and demonstrate a very cool steering technology, kindly provided by Mr. Eaton (belongs to "let's talk hydraulics" - the place I ended up putting it in). As usual, all of this will be framed in a troubleshooting story (which should be published in the "battlefield").

So, let us start with the record-breaking wear of a swash-plate. This swash-plate:

Full-blown 14 millimeters (more than half an inch) of wear. And the most notorious thing is that the pump (a legacy 38 cc Cessna) was still (kind of) working when it was removed from the machine, and the rotary group parts were still in one piece! On the point of breaking, of course, but still entirely recognizable. I call this a miracle, and I want all pump designers to carefully look at the pistons and note that a steel piston shoe is an awesome longevity hack for an axial piston pump.

But what does this have to do with the steering speed I just mentioned? Wait till you hear the story of the steering circuit that it powers. Wait... Not powers - powered...

So, a client complained that the steering valve of one of his older graders broke, and he wanted a replacement. We and I suppose every other hydraulic business on the planet, deal with stuff like this every day. The steering seized the spool valve assembly and began "power-steering on its own". It was a 120 cc Char-Lynn® closed-center non-reaction unit that worked in a constant pressure system, "pumped up" by the afore-mentioned Cessna at a modest 120 bar. The valve didn't even have a relief valve in the P port - only the anti-shocks and, of course, the regular anti-cavitation checks. Just look at it - do you see anything special about it? Me neither!

I opened it up, and didn't find much damage, aside from the busted thrust bearing and the stuck spool valve:

I then managed to get the spool valve assembly unstuck, found a matching thrust bearing in my "emergency second-hand parts reserve" (which every decent hydraulic shop has, even when they tell you they don't), gave the steering a quick run on our test bench, and told the client that he should give this valve a second chance before getting a new one because she passed the bench test with flying colors. No charge.

The man took the steering valve back to the grader, installed it, tried it out, and called me saying that it was all good now! And then called half an hour later saying that it's stuck again... So, I wondered if he checked the hydraulic system for possible contamination sources, like damaged cylinders or pumps, and he said that it was, apparently, all good. I didn't expect any other answer, but I did ask, didn't I? I guess someone will be selling a steering valve today after all.

So, what do you do when you need a 120 cc Eaton closed-center non-reaction steering valve and you don't have a 120 cc Eaton closed-center non-reaction steering valve? You replace it with a 120 cc __(insert the steering vale brand that your company represents)__ closed-center non-reaction steering valve. And that's exactly what I did - opting for the Danfoss OSPC-125 (part number 150-0179), which we conveniently had in stock. Specs-wise it was supposed to be the most direct replacement possible, the units even looked the same:

The man left the shop with a new steering valve and called me a couple of hours later saying that the steering was working again, but it felt slower than it used to be. Hmm... That's kind of strange, isn't it? Of all things, a new 125 cc unit should be marginally faster than an old 120 cc one, should it not? But as I was thinking about what it could be, the man called again and said that "this one seized as well", and then when the mechanic did some digging, and opened the pump that was feeding the steering, he discovered the spectacular damage that I showed you at the beginning of this article.

OK, so now I knew where the small particle contamination that was causing the very sensitive spool valves of steering units to get stuck was coming from, but I still had no explanation as to why the new steering unit of the same size performed slower under the same conditions (very bad conditions, yes, but still the same).

I was missing something, but I knew that I would figure it out soon because both of the steering valves were on their way to our shop. And, of course, when I looked at the parts "with a seeing eye", and then read the catalog, I quickly found out the reason why the two equally sized steering valves had such a different dynamic behavior, and before I tell you what it is, let me prove that these valves, indeed have equal displacements:

When you stack the gears, you can see that they have an almost identical profile, and their height is also virtually the same. You can actually calculate the displacement of the gerotor with the dimensions of the internal gear if you plug the D (external diameter), d (internal diameter), and the gear width in the gear pump displacement calculator, and then multiply the result by seven. Why 7? There is a very detailed explanation in my article on the orbital principle, but if you don't want to go through it - click here - the link will take you directly to the animated drawing - start the animation, and count how many revolutions a 6-point gear makes when the shaft makes a complete turn. The dimensions for the Eaton are 53.0, 38.6, and 16.5, and the Danfoss: 53.3, 38.5, and 16.2, which gives us the theoretical values of 119.6 cm³ and 121 cm³ - so, yes, they have equal displacements all right!

It turns out that very clever engineers from Eaton came up with an ingenious design that can make a steering valve, fed by a constant pressure source (or a load sensing circuit) much faster than a "normal" steering valve, for which, indeed, the amount of oil that it "pumps through" is defined by the rotor displacement and the steering wheel speed. They even gave it the coolest name "Q-Amp®". Steering units that have this feature, have additional variable orifices that connect the P gallery with the target work port when the steering wheel is turned, bypassing the gerotor! This is genius! All it takes is a couple of tiny holes and thin slots in the timing grooves of the spool valve:

If you have trouble appointing functions to the multiple orifices and slots in the spool valve of a steering vale, here's a series of posts that can help you become an "orbital steering guru":

A Non-Reaction Orbital Steering Valve With... Reaction?!! - Part 1
A Non-Reaction Orbital Steering Valve With... Reaction?!! - Part 2
Orbital Hydraulic Motor Principle Explained
How a Danfoss Reaction Orbital Steering Valve Works
Orbital Steering Valves With Static Load Sensing

What's even more fascinating about this "invention" is the fact that the emergency (no pump supply) steering in such a valve performs as usual (which essentially means that you get the benefits of much more expensive double-displacement steering in a single-gerotor valve), and also that you can configure the variable orifices asymmetrically, and then connect the valve to an unequal-area steering cylinder, and get approximately the same amount of turns of the steering wheel in both directions! I've never seen such an application in real life, but I find the idea extremely cool! (Google for "Q-Amp Flow Amplification for Load Sensing Circuits" for more details).

Now, catalogs are great, but we are real people and we like real-life tests. Is this "fancy" steering really that much different from the good old classic orbital steering? I knew you were going to say something like that! This is why I shot a short demonstration (after I cleaned and re-assembled both of the steering valves) so you can tell for yourself if it is different or not:

So, yes - equal gerotor displacements, very different steering speeds, just as promised!