valve, around the main valve spring, and downto the return line.There is also a narrow passage (piston passage)through the main valve piston. This passageconnects the high-pressure line to the valvechamber.The pilot valve is a small, ball-type, spring-loaded check valve, which connects the top of thepassage from the valve chamber with the passagethrough the main valve stem. The pilot valve isthe control unit of the relief valve because thepressure at which the relief valve will opendepends on the tension of the pilot valve spring.The pilot valve spring tension is adjusted byturning the adjusting screw so that the ball willunseat when system pressure reaches the presetlimit.Fluid at line pressure flows through thenarrow piston passage to fill the chamber.Because the line and the chamber are connected,the pressure in both are equal. The top andbottom of the main piston have equal areas;therefore, the hydraulic forces acting upwardand downward are equal, and there is no tendencyfor the piston to move in either direction.The only other force acting on the main valveis that of the main valve spring, which holds itclosed.When the pressure in the high-pressure lineincreases to the point at which the pilot valveis set, the ball unseats (fig. 6-13, view B).This opens the valve chamber through thevalve stem passage to the low-pressure returnline. Fluid immediately begins to flow out of thechamber, much faster than it can flow throughthe narrow piston passage. As a result thechamber pressure immediately drops, and thepilot valve begins to close again, restrictingthe outward flow of fluid. Chamber pressuretherefore increases, the valve opens, and the cyclerepeats.So far, the only part of the valve that hasmoved appreciably is the pilot, which functionsjust like any other simple spring-loaded reliefvalve. Because of the small size of the pistonpassage, there is a severe limit on the amountof overpressure protection the pilot can providethe system. All the pilot valve can do is limitfluid pressure in the valve chamber above themain piston to a preset maximum pressure,by allowing excess fluid to flow through thepiston passage, through the stem passage, andinto the return line. When pressure in the systemincreases to a value that is above the flow capacityof the pilot valve, the main valve opens,permitting excess fluid to flow directly to thereturn line. This is accomplished in the followingmanner.As system pressure increases, the upward forceon the main piston overcomes the downwardforce, which consists of the tension of the mainpiston spring and the pressure of the fluid in thevalve chamber (fig. 6-13, view C). The piston thenrises, unseating the stem, and allows the fluid toflow from the system pressure line directly intothe return line. This causes system pressure todecrease rapidly, since the main valve is designedto handle the complete output of the pump. Whenthe pressure returns to normal, the pilot springforces the ball onto the seat. Pressures are equalabove and below the main piston, and the mainspring forces the valve to seat.As you can see, the compound valve over-comes the greatest limitation of a simple reliefvalve by limiting the flow through the pilot valveto the quantity it can satisfactorily handle. Thislimits the pressure above the main valve andenables the main line pressure to open the mainvalve. In this way, the system is relieved when anoverload exists.PRESSURE REGULATORSPressure regulators, often referred to asunloading valves, are used in fluid power systemsto regulate pressure. In pneumatic systems, thevalve, commonly referred to as a pressureregulator, simply reduces pressure. This type ofvalve is discussed later in this chapter underpressure-reducing valves. In hydraulic systems thepressure regulator is used to unload the pump andto maintain and regulate system pressure at thedesired values. All hydraulic systems do notrequire pressure regulators. The open-centersystem (discussed in chapter 12) does not requirea pressure regulator. Many systems are equippedwith variable-displacement pumps (discussed inchapter 4), which contain a pressure-regulatingdevice.Pressure regulators are made in a variety oftypes and by various manufacturers; however, the6-9
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