Insane Hydraulics

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Four Way Directional Control Valves or "Which Way Will the Oil Go?"

Imagine this:

Your next-door neighbor built a very simple log splitter from a 4-way 3-position spring-return monoblock directional control valve he found online real cheap and a hydraulic cylinder from an old outrigger he bought at a scrapyard. The valve is not equipped with the log-splitter kit yet (it's in the plans), so the blade return is manual, and the system is powered by a small tractor. Not bad, right?!!

So, one Sunday morning, his hand gets caught in between the two logs he tries to split at the same time. He stops just in time before any damage is done, but now - he's stuck and can't reach the control valve, so he hollers for help. Lucky for him you are watering the daisies in your back-yard and rush to help as soon as you hear the cries "My hand! My beautiful hand!..."

You jump over the fence and find your friend with his hand stuck in between the logs, and it is up to you to save him, however now you have to make the choice - which way to move the lever? You can trace the hoses from the DCV to the jack, but how do you know which of the ports will get the pressure? Sounds a lot like a Saw movie, doesn't it? Move the lever the wrong way - and your friend ends up in the ER... C'mon now, Mr. Hydraulic Technician, make your choice! Surely you know your way about directional control valves!

Seriously now, the imaginary predicament aside - I believe that it would be pretty cool to be able to look at a lever-operated DCV and tell which of the ports will pressurize when you move the lever, and although it's impossible if you are not familiar with the valve, I want to share a "mental trick" of sorts, that makes remembering "where the oil will go" a lot easier at least for the valves that you normally work with.

If you consider any mobile directional control valve with manual control - be it a simple monoblock or an advanced pre- or post-compensated model - the general gallery arrangement inside the directional control part is always the same and can be written down as:


Of course, when you deal with post compensation, the flow has to "wrap around" the spool, but still - the basic order remains the same - you have the pressure in the middle, the two work ports on the sides, and the tank galleries on the outside.

I know, I know - most DCVs are more than simple four-way valves, in fact, most monoblock spools could (and probably should) be called six-way valves, but we are concentrating on the four-way part of our spool - i.e. - the part that distributes oil to the work ports A and B. That's all we need.

So yes - if you leave out the rest of the valve, and consider only the gallery layout of our four-way section (the T-A-P-B-T), you will see that there are two ways you can devise a directional spool to distribute the oil, and the difference between them will define whether the P is connected to A or B when the spool is shifted in the same direction.

I gave names to the two spool variants - I call them "Move Pressure" and "Move Line", and you can see the spool types in this diagram.

Allow me to explain my secret "mnemonics technique":

First of all, I "mark" in my head that it is the notch in the spool that "moves" the oil (the notch being the part of the spool that's machined down). Then I look at a centered spool and see where the work port and the pressure galleries are. If the "notch" is in the "P" - it is the "Move Pressure" type, if the notch is in the work port - it is the "Move Line" type.

Once again, very easy to remember - the notch is in the P - "Move Pressure", the notch is in the line - "Move Line".

And then I tell myself - the pressure is always in the middle, between the work ports. If it is the "Move Pressure" spool - when I move it, I move the Pressure towards a work port, which means that If I, say, push the spool in, I am moving (pushing) the pressure (which is in the middle, between the ports, remember?) towards the opposite line (the port that is further away), and if I pull the spool towards me, I am "pulling" the pressure towards the port that is closest to me.

And in the case of the "Move Line" spool - again, I know that the pressure is in the middle, but now I am moving the Line towards the Pressure, so if I am pushing the spool, I am imagining that I am moving the work ports, and since the P is in the middle, I am pushing the work port that is closest to me to the center - i.e. it is this port that will become P. And if I pull on the spool - I am moving the Line (towards P), and it is the line that is farther away that connects to the P.

So - if you need to determine which way the oil will go when you shift the lever of a directional control valve all you need to know is whether it is of the "Move Pressure" or "Move Line" type, and then look at the spool shifting mechanism and see which way the spool is shifting when you move the lever. The rest - you already know!

Now a fair question - but how can you tell from looking at a directional control vale what type of the spool it has? And I am afraid, there's no answer to his question... From the outside, you definitely can't.

However, it is much easier to remember "Move Pressure" or "Move Line" than anything else. For example - I know that all the monoblock DCVs that we stock, without exception, are of the "Move Line" type. I just know it and I don't need to think about it. And if I come about an unknown monoblock valve and look inside the work ports when the spool is centered - if I see the notches (machined down parts of the spool) - I know it's the "Move Line" type.

It isn't that hard to have a look at the models that you normally work with, assign them a type, and never again wonder "which way the oil will go".

Now, why the hell is this knowledge important? I mean - aside from the situation when you have a foot stuck in a hydraulic press and moving the lever the right way means the difference between you going dancing tonight or never dancing again?

To this I respond that this knowledge is just like any knowledge - it doesn't weight on you when you carry it around, plus it is a good mental exercise and a way to look into how DCVs work. Believe it or not, but a lot of mechanics who deal with hydraulics all the time, and install hundreds of directional control valves, have very little idea of how the spool works inside, and are surprised to hear that their beloved 4-way 3-position valve uses a 6/3 spool.

Here are some more examples:

Walvoil SD-11, monoblock valve, side pressure pass-through, "Move Line" type.
Danfoss PVG32 - proportional pre-compensated, "Move Line" type.
Danfoss PVG100 - proportional, post-compensated, "Move Line" type.
Danfoss PVG128, proportional pre-compensated, "Move Pressure" type.
Parker L90LS, proportional pre-compensated, "Move Pressure" type.
Salami VDM6, monoblock, central pressure pass-through, "Move Line" type

So, which one do you like better - "Move Pressure" or "Move Line"?