Hydraulic valves, used in conjunction with actuators, help make hydraulics unique in its control of force, torque and motion. Valves govern the direction, pressure and flow of hydraulic fluid, enabling smooth, safe control of actuators.
A valve’s purpose may be as simple as relieving pressure to protect your pump and actuator or as complex as electronically controlling both pressure and flow with a proportional valve. A valve circuit may contain a single lever valve or an extensive complexity using a dozen valves per function, as seen in custom manifolds.
Directional control valves
The directional control valve comes available in myriad configurations and is named appropriately to its primary function, which is to control the path of fluid flow in some way. Directional control valves manage fluid by blocking, diverting, directing or dumping. Their complexity varies immensely (just like their cost), as does the integration method. Valve construction runs the gamut from cartridge valves to monoblock valves or subplate mounted valves to inline valves.
The operation of a directional valve depends on the industry application in which they are typically applied. For example, directional control valves for log splitters represent the economical and straightforward end of the spectrum. In contrast, servovalves controlling flight simulators perform well at the precise yet expensive niche. You’ll find valves to operate every possible combination of pressure and flow, although extreme combinations of simultaneous pressure and flow are rare.
The most basic directional valve is the check valve; it allows flow into one work port and blocks flow coming back through the opposite work port. Alternatively, directional valves with complex construction are also common, such as with the pilot-operated valve, which uses a small valve to control a larger one. A standard solenoid spool valve has one directly operated component (the spool) that controls fluid direction when it shifts. However, as flow increases, the force upon the spool also increases, and these flow forces can prevent a spool from actuating, as is most often the case with direct-acting 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 is nearly limitless.
Directional valves are often described by the number of “ways” fluid can travel through them and also by the positions available into which the valve may shift. The ways are equal to the number of work ports, so a 4-way directional valve contains Pressure, Tank and A and 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. Very few valves offer more than three positions, although the snowplow float valve is one such animal. (See sidebar for more on valves.)
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, D05 and so on. Also common to mobile and industrial markets are cartridge valves installed into manifold blocks3. Cartridge valve manufacturers offer many unique products and allow high levels of creativity with limitless available valve combinations. Inline valves offer a standalone combination of valve with a ported body, which must be plumbed as a separate component not directly interfaced with any other manifold or body.
In its most concise description, a pressure valve offers designers an option to limit pressure. Most pressure valves use spring-energized poppets pushed against a seat with some form of adjustment screw to modify the spring’s pretension. Pressure valves often use a simple ball and spring configuration or spools for high flow circuits. Relief valve 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 slowly open, proportionally bleeding off fluid to limit pressure.
A relief valve limits maximum pressure for either the entire system or a sub-circuit, with the lowest pressure parallel valve opening first. It is critical to understand that pressure takes the path of least resistance. Therefore, selecting a pressure valve downstream with a lower setting than the main system relief valve will see all pump flow dump through the valve with the lower setting, thereby leaving no hydraulic energy source for other actuators.
Most other pressure valves are based on the relief valve’s simple spring-loaded ball or poppet construction. Sequence, counterbalance, and brake valves are variations of the relief valve but with added utility or functionality, such as reverse flow check valves or integrated pilot operation. The pressure-reducing valve differs from the other pressure valves because it limits pressure downstream rather than upstream. Reducing valves are used in applications where sub-circuit pressures need to be lower without sacrificing any pressure performance in the rest of the system.
Sequence valves operate much as their name suggests — they are pressure valves that remain closed until upstream pressure overcomes the valve’s spring setting. At this point, it simply opens to pass flow to a downstream subcircuit. Sequence valves may be specified with a reverse flow check valve that allows the flow to bypass in the reverse direction. Sequence valves, or any valve with pressure at all work ports simultaneously, should contain a method to drain the spring chamber of trapped pressure. Without a drain, pressure becomes trapped in the spring chamber, which is additive to the spring valves. The result is a valve that opens later than intended or not at all.
Counterbalance and brake valves are motion control valves designed to safely limit and control loads with overrun potential. Essentially any cylinder that may pull or drop under load, or any motor with constant tension, should use a motion control valve. Installed on the actuator port where load-induced pressure occurs, these are essentially relief valves that require extensively more than load pressure to open directly. More commonly, they take a pilot signal from the opposing work port to open the counterbalance or brake valve using a fraction of the load-induced pressure. This method is safer and more efficient and has the effect of limiting the actuator to a velocity dictated by pump flow rather than load-induced acceleration.
Pressure reducing valves remain open until downstream pressure rises above the valve’s setting, an effect much different from other pressure valves, which remain closed until cracked open by pressure. Pressure reducing valves may reduce pressure on either work port or the primary pressure port and also perform best when their spring chambers can drain to tank. They operate by modulating incoming flow rate, which in turn reduces downstream pressure.
Flow control valves
Flow control valves control or limit flow by one of various methods. They are often just a needle valve, which is just a variable restriction, adjusted by a screw or knob much like pressure valves, to restrict the cross-sectional area to reduce flow. When installed with reverse flow check valves, we change the name from needle valve to flow control. Flow control valves can sometimes have multiple ports, such as a priority flow control. They provide controlled, fixed flow to one part of the circuit (sometimes at the sacrifice of another part), but only if input flow is high enough for its priority demand.
Flow controls are (ideally) pressure compensated, allowing 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 component, often added to other valves in a circuit to provide flow rate accuracy independent of load, such as with an electronic proportional valve.
Proportional valves are considered both flow and directional valves to meter flow and control the direction in which flow is metered. Proportional valves use pulse-width modulation to vary current while they maintain voltage. Varying the current modifies the force of the magnetic field and subsequently 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 limit current, but it is inefficient and cannot provide a PWM controller’s benefits.
An electronic valve controller can provide adjustable minimum and maximum settings. A minimum current value is needed to move the spool past its “dead zone” 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 of 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 allow the spool to stick inside the body. 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.