basic operating principles of all regulators aresimilar to the one illustrated in figure 6-14.A regulator is open when it is directing fluidunder pressure into the system (fig. 6-14, view A).In the closed position (fig. 6-14, view B), the fluidin the part of the system beyond the regulator istrapped at the desired pressure, and the fluid fromthe pump is bypassed into the return line and backto the reservoir. To prevent constant opening andclosing (chatter), the regulator is designed to openat a pressure somewhat lower than the closingpressure. This difference is known as differentialor operating range. For example, assume that apressure regulator is set to open when the systempressure drops below 600 psi, and close when thepressure rises above 800 psi. The differential oroperating range is 200 psi.Referring to figure 6-14, assume that thepiston has an area of 1 square inch, the pilot valvehas a cross-sectional area of one-fourth squareinch, and the piston spring provides 600 poundsof force pushing the piston down. When thepressure in the system is less than 600 psi, fluidfrom the pump will enter the inlet port, flow tothe top of the regulator, and then to the pilotvalve. When the pressure of the fluid at the inletincreases to the point where the force it createsagainst the front of the check valve exceeds theforce created against the back of the check valveby system pressure and the check valve spring, thecheck valve opens. This allows fluid to flow intothe system and to the bottom of the regulatoragainst the piston. When the force created by thesystem pressure exceeds the force exerted by thespring, the piston moves up, causing the pilotvalve to unseat. Since the fluid will take the pathof least resistance, it will pass through theregulator and back to the reservoir through thereturn line.When the fluid from the pump is suddenlyallowed a free path to return, the pressure on theinput side of the check valve drops and the checkvalve closes. The fluid in the system is thentrapped under pressure. This fluid will remainpressurized until a power unit is actuated, or untilpressure is slowly lost through normal internalleakage within the system.When the system pressure decreases to a pointslightly below 600 psi, the spring forces the pistondown and closes the pilot valve. When the pilotvalve is closed, the fluid cannot flow directly tothe return line. This causes the pressure to increasein the line between the pump and the regulator.This pressure opens the check valve, causing thefluid to enter the system.In summary, when the system pressuredecreases a certain amount, the pressure regulatorwill open, sending fluid to the system. When thesystem pressure increases sufficiently, theregulator will close, allowing the fluid from thepump to flow through the regulator and back tothe reservoir. The pressure regulator takes the loadoff of the pump and regulates system pressure.Figure 6-14.—Hydraulic pressure regulator.6-10
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