Insane Hydraulics
Bold and audacious blog about fluid power
Today, I am going back to the topic of overcenter/counterbalance valves, and I want to talk about the three plus one overcenter valve types that every tech must know - the standard, the partially-balanced, the fully-balanced, and the two-stage. There are more types (all will be discussed in due time), but for now, I want to talk about these four. This information is basic (albeit served "IH style"), but being a good hydraulic tech is all about knowing the basics, is it not? So I'm positive it will be useful to some of you, my fellow techs.
We all know, of course, that overcenter valves exist to provide control and safety. Control would be making sure the load does not overrun our oil when we lower it and then stays put when we want to hold it, and safety would be essentially making sure nothing falls when a hose bursts and that a cylinder does not turn into a balloon under an "especially optimistic" load. It's a given, so let's start from here with
Here are some examples of what I call a standard overcenter valve, which is essentially a pilot-assisted relief valve coupled to a by-pass check:
These are the Eaton's 1CE30, the Rexroth's A-VBSO-SE-78 (08.35.20 - X - Y - Z), and the Parker's E2*1 cartridges. The equivalent from my favorite SUN would be the good old CBCA, but despite the SUN's excellent technical articles and very informative data sheets, their cutaway views suck big time - they are very pixelated - and sometimes it is really hard to figure out "what is going on in there." This is not the best of ways to present information, in my opinion, but then again, omitting detailed cutaway views from catalogs seems to be the universal trend these days - even the cartridge from the example above - the 1CE30 - used to have a beautiful cutaway view in the Eaton's catalog, and now if you look at the datasheet provided by Danfoss - all you get is the external view and a couple of dimensions. Maybe it is an old-generation vs new-generation thing? I don't know, really, but when I choose a cartridge valve I want to know exactly what's inside!
But I digress (again), my apologies! So - the standard counterbalance valves provide the control and the safety, but they are very sensitive to the back pressure by design. The spring chamber is vented to the exhaust port (and as you can see - the Eaton's symbol does not depict this, which is a mistake), and the pressure inside the exhaust port acts over the pilot area plus the relieving area, greatly increasing the valves' setting.
Such a valve could never be used with a closed center DCV with individual port relief valves, because its relieving function would be essentially in series with the port valves, which would make no sense. So - only open-center spools for these guys!
Some very clever engineers thought of that too and decided that one should still be able to enjoy all the benefits of an overcenter valve in a closed-enter DCV system and allow for the port relief valves to provide the individual anti-spike protection, and so they came out with the ingenious partially-balanced overcenter valve, in which the poppet is designed in such a way that it exposes equal opposing areas to the pressure in the spring chamber (the exhaust port):
These are the Eaton 1CER30 and Rexroth A-VBSO-SE-CC-78 08.39.27 - X - Y - Z. Eaton, once again, got the symbol totally wrong, but the one provided by Rexroth represents the vale's function perfectly! It is interesting to note the two different approaches the engineers took to achieve the same result. I prefer the Eaton's design. Partially-balanced overcenter valves are better, because they can be used with closed-center spools with port anti-spike valves, provided that their setting is lower than the setting of the relieves. They also have all of their parts closed off from the outside, which means they are very protected from the environment, but even though their relief function is more or less independent from the back pressure, the pilot-assisted part is not, which is why they can still be subject to instabilities caused by fluctuations of the back pressure.
Naturally, the most logical solution to remove the back pressure effects altogether would be connecting the back-pressure-sensitive area to a separate drain or the atmosphere - and this is exactly how the fully-balanced valves were born (Eaton got the symbol right this time!):
Here you see the Eaton's 1CEB30, the Rexroth's A-VBSO-SE-CCAP-78, the Parker's E6*1 (atmospheric) and E9*1 (external drain), and I would love to show you the SUN's CW**s (drain) or CA**s (atmospheric), but, as I already mentioned, their cutaways "leave much to be desired".
Once again we can appreciate the different approaches engineers took to venting the valves. (I would love to know where the vent hole in the Parker's cartridge is located, though). The fact that the valves are referenced to a stable pressure, be it the atmosphere or an external drain, means that the back pressure has no effect on them and, of course, that they can be used with closed-center DCVs with port relief valves, which is very cool. In my experience - atmospherically referenced cartridge valves don't fare too well in "mining conditions" - moisture and corrosion always find a way in. Whenever I get to open one of these, the atmospheric part is always full of "goo." This doesn't mean they don't work, though. They do, and pretty well, but I would advise keeping them wrapped with densyl tape at all times.
If you know and can tell these three types apart - you are already a good tech, but there's one more type I would like to show you, the so-called
A rather weird-looking symbol, don't you agree? This is an Eaton's "invention" - the 1CEL30 in this example. As you can see, there are two springs, a hollowed pilot piston, and a separate poppet in what looks like a partially-balanced design. I never used these valves, but I managed to find two different cutaways (one is from the catalog page, and the other is from their very informative article on overcenter vales from 2009).
I believe that the function of such a valve is better seen if you start from this cutaway - as you can see, the hollow pilot piston affects the outer spring and has no effect on the inner spring. This means that the inner spring plus the poppet constitute an adjustable counterbalance valve (and this is, probably, the only time where I would differentiate between the terms "counterbalance" and "overcenter", because in such a configuration you indeed have a relief valve that is counter-balancing a load), and the outer spring, in combination with the pilot piston (and the poppet), constitute the piloted overcenter valve. You can actually call this valve a counter-balanced overcenter valve. How about that for a term?!!
But why on earth would one ever require such a "Frankenstein?" Well - it's all about stable and controlled movement of big things "spoiled" with stored energy. As soon as hydraulic valves began to be used with large cylinders moving extensive structures, it was discovered that a volume of pressurized oil, sitting behind an overcenter valve, can store quite a punch when a cylinder is moved to the end of travel and then pressurized to the system's max pressure. Depending on how a machine is built, the energy can be stored not only due to the compressibility of oil but also due to the elasticity of the machine's structure, and then a situation is created where the overcenter valve is diligently securing the loaded "pressure bomb" by its zero leak property, but since the stored pressure is relatively close to its setting, the valve requires very little pilot pressure to open, and as soon as the operator activates the respective lever, a momentary runaway condition is created, which in its turn, can create a (possibly entertaining) self-sustaining oscillation of whatever it is that is moving (especially when this "whatever" is rocking on pneumatic tires).
This design allows the counter-balancing poppet to keep the flow from the actuator "in check" even when the pilot piston is lifted. This provides more control and stability, but this also robs the actuator of some force in the downward direction because the counter-balancing relief is always engaged. To improve on this, another version of this valve was created (the 1CEL30 shown above), in which the central poppet is a little bit wider at the top, which means that it now has a small differential area over which the pilot pressure can act to lift the poppet. The catalog states that this area's ratio is 0.4 - so you essentially get a factory-set overcenter valve with a normal ratio (set by shims and the spring constant), and an adjustable overcenter valve with a ridiculously low ratio of less than one. The very strange symbol actually depicts this idea pretty well. I would love to try setting one of these! Eaton states these are "ideal for most severe traditionally unstable applications". They may be on to something with this design!
This gives me an idea - if I ever come across a boom that's "acting out" with a normal overcenter valve - I'll make sure to try and install a counterbalancing relief downstream just to see what happens - if the oscillations go away - it's going to be a pretty cool solution!