Insane Hydraulics
Bold and audacious blog about fluid power
They say that smart people learn from other's mistakes. If this is true, then I must be pretty dumb, given that 99% of what I learned in hydraulics came from my own mistakes, inevitably. This also makes you - the reader of this blog - a very smart person, because you are about to go through yet another mistake of mine, and I sincerely hope that you'll learn something new.
This mistake is well-documented in a multi-page hydraulic system evaluation report, written in English and Portuguese, with high-resolution pictures and detailed hydraulic schematics (which makes it even more embarrassing, not that I think about it). There's my full name and contact information at the end, and anybody who's got the copy (and I reckon quite a lot of people do now) can read it and say "Wa-a-a-ait a minute! This guy got it all wrong, he did!" And I'll have to admit that they're right.
In my defense - only partially right, because other points that I mention do stand, but the fact remains - one of the technical things that I point out is downright incorrect, and it stemmed from my incomplete understanding of the operation of a certain type of counter-balance valve, something that I realized long after the report was delivered both to the owner and the OEM of the equipment in question. What can I say? An engineer somewhere must have had a good laugh about it, and laughter is never a bad thing, is it?
In any case, my "bottom is covered " now, for if someone mentions this to me in the future - I'll be able to say something like: "You're totally right, man, I did screw up on that one! Big time! I even wrote about it in my blog the other day!" And who reads technical reports anyway? People who sign checks don't, and the very few people who do read them don't sign checks, so I guess it's a no-harm-done situation no matter how you look at it.
Still, the technical detail that I am about to describe is important and interesting, and I've seen even experienced techs make the same mistaken assumption that I made. Even though I am not at liberty to disclose the details of that case, I'll do my best to explain the tech stuff.
So, the counter-balance valve (a.k.a. motion-control or over-center) - in its classic and most used form is something like this:
Here you have an actuator, a counter-balance valve (or a pair of valves), most often combined with a check valve in a single cartridge, and an open-center directional valve.
Almost all hydraulic techs begin their motion-control valve "journey" with this configuration when they are explained that the counter-balance valve is essentially a relief valve with a piloted setting override, and then they are also taught that the open-center directional valve is required to ensure the correct relief-valve function of the over-center valve, to protect the actuator from over-pressure (be it load-related or thermal).
There's nothing wrong with this explanation - but very often that's the only information that is fed to apprentice techs by the "shop gurus". Any further "motion-stability-related" details are never mentioned or even touched on. I guess self-education is assumed to happen at a point, but if it "doesn't happen", a tech is left with the "assumption chain" that goes like this:
"actuator overload protection" => "counter-balance valve relief function" => "open center DCV required"
Not wrong, but incomplete.
As you probably already guessed - that's exactly how I learned the counter-balance valve operation, and this is exactly why, for a very long time, I thought that counter-balance valves could only be applied with open-center directional control valves, and if, for some reason, they were to be applied with a closed-center directional valve, their relief function would (very obviously) be defeated, which would be a bad thing, because then the actuator would not be protected against overload or have adequate thermal relief, and then, in my "rookie head", using a closed-center DCV equipped with anti-spike relief valves would be an even bigger "blasphemy" because we would be investing in a more expensive component that would never operate properly because we would be putting the over-center valve's relief in series with the line's relief valve - which would sky-rocket the cracking pressure to "explosive" levels, so a circuit like this made zero sense to me:
This was exactly what I encountered in the hydraulic circuit that I was so scrupulosity evaluating for the report that I mentioned at the beginning of this post. A DCV (Danfoss PVG32) with a closed-center spool, a couple of anti-shock vales, and an actuator equipped with directly mounted over-center valves. "Insane!" - thought I to myself, and then ran into the maintenance office, grabbed the lead engineer by the shoulders, looked him in the eye, and shook him vigorously, shouting: " The counter-balance valves have their relief function defeated, man! DEFEATED!!!".
I may have made up the shoulder-shaking part for the sake of "poetic exaggeration" (a good information retention trick), but I did put in black and white that there were three actuators with counter-balance valves that were combined with closed-center DCV spools and therefore had their relief function defeated, which left the actuators unprotected against pressure spikes, even with the anti-shock valves present in each section of the DCV.
It was that statement that was wrong - "... unprotected against pressure spikes...". And what makes it even worse - I, quite thoroughly, documented all the components in the report, and even included the hydraulic diagrams, which actually showed that the anti-spike protection of this hydraulic system was not compromised at all!
If you have already figured this one out - good for you! You don't need to read any further, But if you think that this is strange or even downright impossible - you are about to hear something cool!
Let's take one step back. Why did I think the anti-spike protection would not work? Because of the closed-center spool and the back pressure on the counter-balance valve, obviously! This is how a typical direct-acting counter-balance valve is built:
Any back pressure in the valve port will increase its setting by the factor of its pilot ratio plus one. And even if it would somehow open, it would still have to crack the anti-shock vale for this opening to matter, and since we all know how series-connected relief vales behave, we can imagine that the pressure spike would blow the actuator up long before any relieving action could occur. So, duh! Them open center spools exist for a reason!
This reasoning is correct, but what if you had a system that would require you to have back pressure? For example, if it were a large boom and you would need an asymmetrical restrictive spool to be able to move it without oscillations. The always-changing and pretty big back pressure would screw your counter-balance valve setting up big time! So what would be the solution then? Very simple - you would use the so-called fully-balanced counter-balance valve, in which the back pressure does not affect its setting because the spring chamber is isolated from the circuit and is vented to the atmosphere, like so:
Eaton's 1CEB (the cutaway view above) or SUN's CACG are good examples of such valves. But what does this mean to the relief function then? Well - the fact that such a valve has an atmospherically referenced spring chamber means that it is no longer a relief valve, but rather a sequence valve (this is the point where you imagine me shaking you by the shoulders, shouting "Sequence valve, I tell you!!!"), which in its turn means that after it's open - it doesn't add a pressure drop, which in its turn means that the anti-spike valves that sit above the "prohibited closed-center spool" can do their job just fine now!
How cool is that!? I didn't pay attention neither to the fact that the system designer used fully balanced atmospherically referenced valves for all the actuators, nor to the fact that all the symbols in the respective hydraulic diagrams were actually representing the valves correctly by adding a small triangle next to the spring, like so:
So, to conclude - the fact that a counter-balance vale was applied with a closed-center spool does not mean the actuator is left with no protection. If it's a fully-balanced valve the protection can be still provided by the DCV's anti-shock valves.
On another note - about them asymmetrical spools. They can do wonders for motion control of "boom-like" loads. PVG 32s have these, in case you didn't know. If you need further details - look up part numbers 11146795, 11146797, and 11151486 in this catalog. A note from my experience - what Danfoss calls "open/closed neutral position" spools appear to be "very closed neutral position" in real life. I haven't seen many of these, but all that I "physically witnessed" had nothing even remotely resembling "open" in neutral, like at all. Go figure... Maybe I got a prototype series of the spools or something? I even contacted Danfoss on this, and, obviously, got no reply. Still good to know that they exist and work pretty well, but I would use carefully chosen anti-spike valves with these spools no matter what the catalog tells me about their "open-center-like nature".
The final and obvious lesson is - pay attention to symbol details in the hydraulic diagrams and don't rush your reports!