# Insane Hydraulics

## External-Pilot-Operated Over-Center Valve Calculator

Foreword

I use the terms counterbalance and overcenter interchangeably, and I know that some folks find this incorrect. Please, look at the symbol of the valve in the drawing below - this is the pressure valve I am talking about in this article, and I am sure you know what it is used for.

Calculating the opening pilot pressure for a generic non-balanced externally-piloted overcenter valve is easy if you consider the fact that it is just a relief valve with an external-pilot-adjustable setting (and a check-valve) which, like any respectable relief valve, opens when the pressure in the "business end" is equal to its setting. So, if we attribute the following designators:

• Pp as the pilot pressure we're trying to figure out
• Vp as the valve setting pressure
• Lp as the load-induced pressure
• Bp as the back pressure in the exhaust port
• Vr as the valve's pilot ratio, and
• Cr as the cylinder ratio (bore area/annulus area)

then we can say that the pressure that our valve will be relieving is the load-induced pressure plus the pilot pressure that adds to the load-induced pressure via our actuator (this is exactly where the actuator's area ratio comes into play):

Lp + (Pp x 1/Cr), or Lp + (Pp x Cr), or Lp + Pp (differential-area cylinder pointing up/differential-area cylinder pointing down/equal-area actuator or a hydraulic motor).

And the valve's cracking pressure would be its initial setting plus the back-pressure times valve's ratio plus one minus the pilot pressure times the valve's ratio:

Vp + Bp(Vr + 1) - (Pp x Vr)

Equalling these expressions gives us the elegant formulae that you can see in the interactive drawing below.

The app calculates the opening pilot pressure of a non-balanced external-pilot-operated over-center valve. You can tap or click on squares on the sides of the sliders for finer adjustment of values. Radio buttons below the sliders change the orientation of the cylinder. Use a cylinder ratio of 1 to simulate a hydraulic motor. The rod width is depicted to scale, and you can use this to visualize how counter-intuitive rod thickness may appear for a given area ratio (try setting it to 2:1 and tell me if the rod doesn't look "too thick").

### Overcenter Valve Pilot Pressure Calculator

 Load Pressure, bar Valve Setting, bar Valve Raio Cylinder Ratio Back Pressure, bar

Correct setting of a counterbalance valve when it is mounted in a system is virtually impossible, because the load-induced pressure is always dynamic (and often you can not measure it at all), so monitoring just the pressure in the piloting line is simply not enough. It may even seem that changing the setting of the valve does not affect the pilot pressure at all.

Imagine that you have to set a 1:3 counterbalance valve, mounted on a rod-up 1.5:1 boom cylinder to exactly 230 bar. Not 250, not 200 - but 230. Using the calculator, you can simulate that with a 100 bar load (let us imagine that this is the pressure in the lifting cylinder in its current position), the setting of 230 bar would require 35 bar to lower the cylinder, and, for example, the setting of 200 bar - 27 bar, and 250 - 40 bar. The 8-bar-ish difference in the piloting pressure is already quite small to monitor (and in the order of magnitude comparable to a pressure drop in a long line), but if the load-induced pressure dropped to, say, 80 bar when the cylinder moved (due to dynamic reasons - for example, the boom angle change) - the pressure required to lower the boom with a 230 bar setting would be now 40 bar (the 250 bar valve setting), or if the load pressure jumped to 130 bar - the pilot would drop to 27 bar (the 200 bar valve setting) - so, as you can see, any dynamic change in load pressure would screw your adjustments up big time. Theoretically - if you could monitor both the load-induced pressure and the pressure in the pilot line at the same time, you would be able to set the valve precisely - but this is highly impractical, to say the least.

The only way to set such a valve correctly is to remove it from a system and connect it as a relief valve to a test bench or a manual pump. I, personally, like to visually monitor the exhaust port for leakage when I do that - first - this allows you to detect the cracking pressure exactly, and second - it allows you to detect even the smallest leakage (drops) if it is present.

If a load is constant (for example, it is a horizontal hinged door that has always the same weight), you could set such a valve without removing it from the system by putting the load in a position where it induces the maximum pressure and then lowering the valve's setting to the point the load begins to drop, and then incrementing the setting by a necessary safety margin (possible if you know the valve's pressure per turn characteristic) - but that's about the only time you can get away with it, and of course, this is only possible when you have a single valve. If you were to have two identical cylinders on each side of the said door, each boasting an overcenter valve - the best you could hope would be an approximation of what's required. I don't recommend it (but I still wish you the best of luck if you want to give it a try).

Some "counterbalancing" bullet points:

• Overcenter valves with higher plot ratios are "easier to open", which makes them less stable because the higher pilot ratio pushes the valve closer to the on/off (rather than modulating) regime, which is why lower ratio valves result in less efficient, but more stable systems.
• A non-balanced valve (i.e. the valve that exposes its spring chamber to the exhaust port) is sensitive to back pressure. Very sensitive. For example, a back pressure of 8 bar in a 1:6 (non-balanced) counterbalance valve will produce a 50-bar increase in its relief setting!
• It is important to understand that the load-induced pressure acts to open the valve, which means that if the load pushes it close to the valve's setting, or there's trapped pressure in the cylinder (because it got "charged" with max system pressure when it hit the end of travel) - the pilot pressure required to open the counter-balance valve can be very low, which can be the source of unexpected runaway situations in the beginning of the return motion.
• Never forget that some valves have left adjustments. For example - the good old CBCA from SUN, in which turning the adjustment screw clockwise actually decreases the valves' setting. Just saying...
• Never remove a counterbalance valve from a circuit unless you are 100% sure that there's no trapped pressure behind it and the load that it is supposed to be supporting is secure. Unfortunately, due to the way some hydraulic systems are designed, sometimes this is impossible. Well... Be creative, I guess... All I can say is that I've seen all sorts of "extremely amusing" situations throughout my practice. My favorite is (and will always be) removing a cartridge-type counterbalance vale with an impact wrench! In fact, now that I said it - mounting a cartridge valve with an impact wrench is another "favorite" of mine. Why? Because I like selling valves, that's why! But seriously - a runaway load coupled with an oil shower is a killer combination, quite literally, so do use your brain when you need to remove a counterbalance valve.