Hydraulic valves, in addition to the force density advantage of actuators, are what help make hydraulics unique in its control of force, torque and motion. Valves govern direction, pressure and flow of hydraulic fluid, enabling smooth, safe and controlled use of actuators.
Every machine both requires and uses valves, varying vastly in execution from a few valves to dozens on one machine. Their use can be as simple as a relief valve to protect your pump and actuator, such as the relief valve built into the kick-off valve on a logsplitter. Conversely, the complexity of a hydraulic circuit can be extensive, using a dozen valves per function, as can be seen with manifolds, such as a pilot-operated valve with dual counterbalance valves, dual flow controls, dual post-compensation and load sensing checks etc., all of which can be used in just a single cylinder circuit.
Directional control valves
The directional control valve is available in myriad configurations, and is named as such if its primary function is to somehow control the path of fluid flow. Directional control valves manage fluid by blocking, diverting, directing, or dumping. Their complexity varies immensely (just like their cost), as does the method of integration. Valve construction runs the gambit from cartridge valves to monoblock valves, or subplate mounted valves to inline valves. Their usage depends on the industry in which they are typically applied.
The most basic directional valve is the check valve; it allows flow into one work port, and blocks flow from coming back through the opposite work port. Alternatively, directional valves can be complex, such as with the pilot-operated valve. A standard spool valve has one directly operated component that controls fluid through the valve. However, as flow increases, the force upon the spool also increases (especially as pressure increases as well), and these forces can prevent a spool from actuating, most often with electric coils. By using a small pilot valve to control the movement of the larger, main-stage spool, the size (and flow) of the valve are nearly limitless.
Directional valves are often described by the number of “ways” fluid can travel through them, and also by the positions available to be shifted into. The ways are equal to the number of work ports, so a 4-way valve will have Pressure, Tank and A&B work ports. Positions are equal to the number of positional envelopes. For example, one would describe a double-acting single monoblock valve as “4-way, 3-position,” or simply a “4/3 valve.”
Directional valves are available in monoblock or sectional valves, common to the mobile-hydraulic industry, as well as subplate mounted industrial type valves such as ISO style D03’s, D05’s et al. Also common to both mobile and industrial markets, are cartridge valves installed into manifold blocks. Cartridge valve manufacturers offer many unique products, and allow high levels of creativity with limitless available valve combinations.
A pressure valve is any component designed to in some way limit pressure. Most pressure valves are based on a poppet being pushed against a seat with an adjustable spring, although pressure valves can be simple ball and spring configuration or use spools as well for high flow circuits. Their operation is simple; a spring pushes the poppet against a seat, and when pressure from the system is strong enough to counteract the force of the spring, the valve will open, bleeding off fluid to limit pressure.
A relief valve controls maximum pressure for either the entire system or a sub-circuit of it, the lowest spring pressure of a system being the one to open up first. Most other pressure valves are based on the relief valve’s simple spring-loaded ball or poppet. Sequence valves, counterbalance valves, brake valves, etc., are all forms of relief valves, with added utility or functionality, such as reverse flow checks or pilot operation built in. The pressure-reducing valve differs from the other pressure valves because it limits pressure downstream of itself, rather than upstream. It is used in applications where sub-circuit pressures need to be lower, without sacrificing any performance in the rest of the system.
Flow control valves
Flow control valves are any component designed to control or limit flow in one way or another. They are often just a needle valve, which is just a variable restriction, adjusted by a screw or knob much like pressure valves, to limit the energy potential to create flow. When installed with reverse flow check valves, we change the name to flow control. Flow control valves can sometimes have multiple ports, such as with a priority flow control. They are able to provide controlled, fixed flow to one part of the circuit (sometimes at the sacrifice of another part), as long as input flow is high enough for its priority demand.
Flow controls are ideally pressure compensated, which allows the valve to maintain its set flow regardless of load-induced pressure variances. Pressure compensators are a type of flow control valve available as a single item, often added to other valves in a circuit to provide the same flow setting load-independent accuracy, such as with a proportional valve, for example.
Proportional valves are considered both flow and directional valves, and are able to both meter flow and control the direction flow is metered in. Proportional valves use pulse-width modulation to maintain voltage and control current. Varying the current controls the force of the magnetic field and how far the spool or poppet moves within its body, changing the size of the opening available for fluid to take, which of course limits flow. A simple variable resistor can be used to limit current, but they are inefficient and cannot provide the benefits a PWM controller can.
Firstly, a valve controller can provide adjustable min and max settings. A minimum current value is needed to move the spool past its overlap where it “starts” to flow. Also, a maximum current value prevents too much electric juice from fatiguing the valve and coil when only a couple amps are required to achieve full flow anyway. Additionally, a proper controller and driver provide a dither signal to the valve, which vibrates the spool so that static friction doesn’t hold it up. The spool movement is unnoticeable, but is enough so that when a change in current is required, the spool responds rapidly without overshooting the desired new position.