CHAPTER 6VALVESIt is all but impossible to design a practicalfluid power system without some means ofcontrolling the volume and pressure of the fluidand directing the flow of fluid to the operatingunits. This is accomplished by the incorporationof different types of valves. A valve is defined asany device by which the flow of fluid may bestarted, stopped, or regulated by a movable partthat opens or obstructs passage. As appliedin fluid power systems, valves are used forcontrolling the flow, the pressure, and thedirection of the fluid flow.Valves must be accurate in the control of fluidflow and pressure and the sequence of operation.Leakage between the valve element and the valveseat is reduced to a negligible quantity byprecision-machined surfaces, resulting in carefullycontrolled clearances. This is one of the veryimportant reasons for minimizing contaminationin fluid power systems. Contamination causesvalves to stick, plugs small orifices, and causesabrasions of the valve seating surfaces, whichresults in leakage between the valve element andvalve seat when the valve is in the closed position.Any of these can result in inefficient operationor complete stoppage of the equipment.Valves may be controlled manually, electri-cally, pneumatically, mechanically, hydraulically,or by combinations of two or more of thesemethods. Factors that determine the method ofcontrol include the purpose of the valve, thedesign and purpose of the system, the location ofthe valve within the system, and the availabilityof the source of power.The different types of valves used in fluidpower systems, their classification, and theirapplication are discussed in this chapter.CLASSIFICATIONSValves are classified according to their use:flow control, pressure control, and directionalcontrol. Some valves have multiple functions thatfall into more than one classification.FLOW CONTROL VALVESFlow control valves are used to regulate theflow of fluids in fluid-power systems. Control offlow in fluid-power systems is important becausethe rate of movement of fluid-powered machinesdepends on the rate of flow of the pressurizedfluid. These valves may be manually, hydrau-lically, electrically, or pneumatically operated.Some of the different types of flow controlvalves are discussed in the following paragraphs.BALL VALVESBall valves, as the name implies, are stopvalves that use a ball to stop or start a flow offluid. The ball, shown in figure 6-1, performs theFigure 6-1.—Typical ball valve.6-1
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