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Hydraulic Valves: Directional, Proportional, and Servo

For  decades, hydraulic valves have operated in more or less the same simple way: they flipped back and forth, opening first one line, and then another. Normally, one pipe was the ‘do nothing’ line, and one was the ‘do something’ line. But as our understanding of hydraulic systems increased, our desire to manipulate them in more precise manners piqued — and in response, different kinds of valves developed. Here’s a brief summary of the three major types of hydraulic valve.


Directional Valves

Also known as ‘switching valves,’ or more colloquially, ‘bang-bang valves’ because of the noise they made when switching, these are the valves described above. Directional valves have evolved over the past decades, however, from a simple left-right decision to a single valve that can contain several different outputs. Many also moderate the speed of the hydraulic fluid by altering the aperture through which it can flow.


Directional valves are useful, but simple: every change in direction, flow, or pressure requires its own directional valve, making even modestly complex hydraulic circuits enormous in size and expensive to produce.


Proportional Valves

Proportional hydraulic valves use solenoids to allow the valve to take any desired position between and including ‘closed,’ ‘left output,’ and ‘right output.’  This means they can adjust the flow to any proportion between the two outputs. This gives the ability to put speed, flow, and directional controls all on a single valve, dramatically reducing the space taken up by a complex circuit.


In addition, the ability of a proportional valve to adjust the speed of a circuit anywhere between ‘stop’ and ‘full power’ means that a single source and a single hydraulic pump can be used to power a wide variety of hydraulic devices, even if they require entirely different flow speeds and/or pressures to operate. That meant not only were circuits smaller, but there were fewer circuits necessary for any given set of jobs.

Servo Valves

Servo valves aren’t new — they’ve been around since the 1940s — but they’re rare, because they’re expensive. They operate using a combination of input pressure from the hydraulic line and electronic controls to create a valve that doesn’t ‘bang-bang’ — it moves smoothly and accurately. The net effect is that a servo valve lasts a very long time, responds quickly to controls, and has low hysteresis compared to the other two valve types.


Hydraulic Cylinders: When Hysteresis Isn’t Funny

There’s a common phenomenon in literally 100% of hydraulic systems that simply must be taken into account, and it has the misfortune of bearing a name that makes it sound like something we should all be grinning at: hysteresis.


What Is Hysteresis?

Simply put, every single kind of hydraulic cylinder, hydraulic valve, or hydraulic end-device (winch, servo arm, whatever it may be) that has at least two directions of movement suffers from the same problem. That being, the pressure required to move them from one position and the pressure required to move them back aren’t the same pressure.


Let’s take the simplest example: a valve designed to open when pressure gets too high — a ‘release valve.’  As the pressure behind the release valve builds, it will remain steady until that pressure reaches a certain point — say, 500 psi — at which point the valve opens. As the pressure decreases, however, the valve does not then close at 500 psi; it will only close when the pressure hits a lower point — say, 480 psi. The difference between those two pressures is called the ‘hysteresis’ of the valve, and it’s most often given as a percentage — 480 being 96% of 500, we would say the hysteresis of the valve is 4%.


Hysteresis is also rate-dependent, so for example, bringing a given hydraulic cylinder slowly from one pressure to another will result in less hysteresis than if you just crank the control all the way over to the other side in one swift action.


Why is Hysteresis a Problem?

Hysteresis is a problem primarily because of the way in which the human mind thinks — we expect, for example, that if we put all of the settings on a given hydraulic circuit to the same positions they were in last time, we’ll get the same result. But because of hysteresis, the result you get as you increase the pressure to get to Point X can differ significantly from the result you get if you reach Point X by decreasing the pressure — and the results can vary even more if you increase and/or decrease the pressure quickly rather than gently.


The end result is that confident operators can do everything they are ‘supposed to’ in order to achieve a specific end result, and end up missing that end result by enough to cause a disaster on the job site. Minimizing hysteresis — and constantly monitoring it — are critical goals for every job site that relies on hydraulic circuits, especially when lives could be endangered by an error.