Machine Design

Pressure Valves

Valves are essential elements for controlling fluid system performance.

Virtually every fluid-power system requires some type of valve. In a hydraulic system, valves may control pressure, flow to an actuator, or quantity of flow permitted past a given point. Pneumatic valves are similar in design and operation to their hydraulic counterparts, although they may differ in construction.

Trends in the valve industry today include miniaturization of traditional designs and space-saving stackable valves. Compatibility with electronic controls means enhanced valve performance. Manufacturers are also turning to plastics to cut weight while improving lubricity and corrosion resistance. And advanced ceramics are being used for longer life and contamination resistance.

The primary concern in fluid-power circuits is to control either pressure level or the rate of flow. In theory, most flow-control valves could be used to control pressure. If orifice size, supply flow, and fluid viscosity are fixed, pressure remains constant; if any one of the three varies, pressure varies. Typically, however, such valves produce only the crudest kind of pressure control. For more accurate control, several types of pressure-control valves have been developed; they are categorized by the function to be performed. Counterbalance valves resist movement or balance the load being held by a cylinder or motor. These valves, by controlling pressure, provide excellent dynamic control. If they must hold the load for long periods, experts recommend that they be supplemented with a pilot check valve, which has better static-holding capabilities. Diverting valves (known in the mobile industry as "sequence valves") establish flow priorities within a circuit by using a pressure-actuated three-way valve with controls. The valve assembly directs pressurized fluid to a primary port until a predetermined pressure level is reached. At this value, flow is diverted from the primary outlet to a secondary outlet. Flow continues at the secondary as long as the pressure in the primary outlet is maintained. Pressures can be cascaded through several steps if needed. Sequence valves are used to determine the sequence of machine operations by sensing pressures other than maximum. These normally closed valves permit flow between inlet and output ports when the pressure reaches preset levels, and can be fitted with free-flow checks to permit flow in the opposite direction.

Many sequence valves have two or more spools or poppets that must be actuated before flow can pass through the valve. Typically, a signal shifts the control spool, ensuring that a certain minimum pressure has been developed in one part of the circuit before fluid can pass through another part. Reducing valves can limit pressure levels by restricting flow through a portion of a hydraulic system. In a normally open two-port unit, the reducing valve receives the signal from its low-pressure outlet. The valve is often biased by a spring or weight that may be supplemented with a pilot mechanism.

Reducing valves are used with suitable orifices to provide uniform pressure drop in flow-control valves. A check valve can be included for uncontrolled return flow through the valve, or pilot pressure can be used to hold the spool in the open positive to permit free return flow through the valve. Unloading valves provide free passage through a low-pressure area when a signal is applied to a pilot connection. In a typical application, unloading valves can be arranged to accept a signal from an accumulator. At a predetermined pressure, when the accumulator is charged to the specified level, the pump unloads to tank. The unloading pressure of this type of valve is commonly determined by a spring-loaded spool; the spring can be adjusted to vary unloading pressure. Alternatively, the valves can be controlled by application of a pilot pressure to hold the valve closed at pressures higher than that provided for by the spring. Safety valves pop open to avoid or eliminate abnormally high pressure peaks. They are designed strictly for fast action rather than pressure modulation, and they may well be subject to such problems as noise and chatter. They are typically nonadjustable, or have the pressure setting protected from tampering.

Essentially, safety valves perform the function of "fuses" in the system. In lieu of one, devices called hydraulic fuses also can be used. These quasi-valves use a disc or similar device that fractures at a preset pressure. They do not reset automatically and must be manually repaired after fracturing. Relief valves do the same job as safety valves, but they also smoothly and continuously modulate flow to keep pressure from exceeding a preset level. A relief valve is normally closed until the pressure level approaches a preset value. As system pressure rises, relief flow through a properly sized valve increases until the entire pump output passes through the valve. When system pressure drops, the valve closes smoothly and quietly.

Relief valves are available with simple direct actuation or with piloted operation. In addition, some electrically modulated relief valves perform an almost servo function to instantly modulate system pressure over a wide range of electrically signaled values.

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