Asking “what is a hydraulic valve?” is like asking “how long is a piece of string?” The variations available to the hydraulic designer are absolutely astounding, not only because of the myriad types of valves on the market, but also because of the numerous manufacturers making them.
The most basic description of a valve is a mechanical device that opens and closes, most often to control the flow of fluid—liquid or air. Valves exist in nearly every industry, from automobile engines to the foundries that cast the engine’s valves; yes, there are valves on the machines that make valves.
This article isn’t about the poppet valves in your 1999 Civic SI VTEC. This article is about hydraulic valves. Hydraulic valves are unique because they must be capable of withstanding 3,000 psi or more of fluid pressure, which require them to be manufactured from strong (and often heavy) steel and iron. Their construction must be such that hydraulic pressure is entirely contained, yet able to function smoothly and accurately, without being prevented from functioning because of the high forces imposed by that pressurized fluid.
So a hydraulic valve is just a device that opens and closes to allow the flow that will move actuators and loads. It sounds simple, but there are various techniques used in hydraulics to allow this to occur. Valves can be mechanically operated (by handle, knob or cam), electric solenoid-operated, or pilot-operated (air or hydraulic pressure actuates the valve). Some valves use the pressure of the circuit’s fluid to actuate themselves, like with relief valves. Valves can also be actuated with cables, levers, plungers, torque motors and so forth.
There are nearly as many types of hydraulic valves as there are ways to actuate them. You have solenoid valves, flow control valves and pressure control valves as the three primary groups of valves, but each of those also have their own sub-species. Solenoid valves can be poppets or spool valves, and either of those can be electro-proportional or servo-controlled. Flow control valves can be hydrostats (also known as pressure-compensators) or simply needle valves, and can be used to meter-in or meter-out fluid.
Pressure control valves are the most varied of the three primary groups. They open and close, just as other valves, but these are more dynamic, with linear rise and fall of performance, based on the pressure acting upon them. Most pressure valves (like relief valves, sequence valves, counterbalance valves, and so forth) are normally closed, meaning that it takes a rise in pressure to open them. However, the pressure-reducing valve is the only one that closes when pressure rises above a set point.
There is a dizzying array of hydraulic valves available, and each one could warrant their own page. It’s hard to simplify a vast subject and answer “what is a hydraulic valve,” but as long as you understand they’re devices that open and close to control the pressure, flow and direction of fluid in a hydraulic system, then you’ve got the basics down.