Hydraulic valves control the pressure, direction and flow of hydraulic fluids, which is one of the two cornerstones of how hydraulics effectively control torque, motion and force. In combination with the force density advantage granted to actuators, valves enable hydraulic systems to function, with the valves allowing actuators to be used smoothly and safely.

Machines nearly universally use valves, with simpler machines employing only a single or few valves while complex machines can employ dozens or even hundreds of valves. On the less complicated side, a relief valve can provide protection for a pump an actuator – An example might be a relief valve within the kick-off valve on a piece of machinery like a log splitter. On the other end of the complexity spectrum, valves can be deployed by the dozen for each function, such as in manifolds. An example of high end complexity for valve usage might be a pilot operated valve with due flow controls, load sensing checks, twin counterbalance valves and twin post compensation.

One of the most common types of valves is the directional control valve – so named as a result of its main function being to govern the direction and path of hydraulic fluid. The simplest form of the directional valve is the check valve, which permits the flow of liquid into a single work port while denying the liquid flow in the other direction through the opposing work part. At its greatest level of complexity, the directional valve can feature internal pilots, load sense check valves and drains in the form of pilot operated proportional valves.

Directional valves can be characterized and identified by two categories – First, the positions into which the valve can be shifted, and second, by the number of means by which hydraulic fluid can travel through the valve. For the first part, positions are the number of positional envelopes. For the second, the number of means by which fluid can travel equals the amount of work ports. To give an example, a valve might be a single-monoblack, double acting valve as a 4 way, 3 position valve. Often times, this will be shortened to simply the numbers in succession – in this case, a 4/3 valve.

Directional valves come in a variety of types. In the mobile-hydraulic industry, it’s common to find valves available either in sectional or monoblack valves. It’s also common to find subplate mounted valves for industrial usage such as ISO style D03 valves, D05 valves and others along those lines. For mobile or industrial markets, other valves such as cartridge valves are common, where those valves are put within manifold blocks. Cartridge valves are a highly specialized product, and manufacturers offer a wide range of different products. The combinations for valves are nearly infinite.

The pressure valve has a specific purpose: To control the pressure of hydraulic fluid within a system. The most common structure for a pressure valve is that of a poppet and seat, with the poppet pushing via a spring against the seat. A relief valve, a subset of a pressure valve, controls multiple ways with spring pressure dictating which releases before the others. Relief valves can limit pressure either within a sub-circuit of the main system of the entire system itself. More exotic relief valves include brake valves, sequence valves and counterbalance valves, all of which have additional bells and whistles built in. Yet another type of pressure valve is the pressure-reducing valve, which distinguishes itself from the other members of the family because it operates downstream, while other pressure valves limit pressure upstream. The pressure-reducing valve’s value comes in for situations where the pressure needs to be lower in sub-circuits while the rest of the system maintains optimal pressure levels.

Yet another type of valve is the flow control valve, whose basic purpose is to restrict the flow of hydraulic fluid in one direction or way. A flow control valve at its simplest is a needle valve, and users manipulate it via a knob or screw, similarly to with a pressure valve. However, when a system possesses reverse flow check valves, this system now would be described as having flow control. One prominent feature of many flow control valves is having more than one port. A priority flow control is an example. In the case of a priority flow control, the flow of fluid is kept at a regular rate for one component within the circuit at the expense of other, less important parts.

A flow control valve is able to regulate the flow of fluid even as load-induced pressure varies. A subtype within the flow control valve family is the pressure compensator, which is frequently used a supplement to other valves to give accuracy to fluid flow. One such example would be that of a proportional valve.