FLUID POWER HANDBOOK
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HYDRAULIC
VALVES
HYDRAULIC
valves, used in conjunction with 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 control of actuators. Valve use can be as simple as a relief valve to protect your pump and actuator. Conversely, the complexity of a hydraulic circuit can be extensive, using a dozen valves per function as can be seen in manifolds.
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 gamut 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, and these forces can prevent a spool from actuating, as is most often with electric coils. 56
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By using a small pilot valve to control the movement of the larger, mainstage 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 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 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.” 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 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.
PRESSURE CONTROLS A pressure valve is any component designed to limit pressure. Most pressure valves are based on a poppet being pushed against a seat with an adjustable spring, although pressure valves can be a simple ball and spring configuration or use spools 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
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