forces set up within the cylinder, since the forcesthere are equal. As indicated by the arrows againstthe pistons of the spool, the same pressure actson equal areas on their inside surfaces. In theclosed position, one of the pistons of the spoolsimply blocks the inlet port, thus preventing flowthrough the valve.A number of features common to most slidingspool valves are shown in figure 6-23. The smallports at either end of the valve housing providea path for any fluid that leaks past the spool toflow to the reservoir. This prevents pressure frombuilding up against the ends of the pistons, whichwould hinder the movement of the spool. Whenspool valves become worn, they may lose balancebecause of greater leakage on one side of the spoolthan on the other. In that event, the spool wouldtend to stick when it is moved back and forth.Small grooves are therefore machined around thesliding surface of the piston; and in hydraulicvalves, leaking liquid will encircle the pistons andkeep the contacting surfaces lubricated andcentered.THREE-WAY VALVESThree-way valves contain a pressure port, acylinder port, and a return or exhaust port. Thethree-way directional control valve is designed tooperate an actuating unit in one direction; itpermits either the load on the actuating unit ora spring to return the unit to its original position.Cam-Operated Three-Way ValvesFigure 6-28 shows the operation of a cam-operated, three-way, poppet-type directionalcontrol valve. View A shows fluid under pressureforcing the piston outward against a load. Theupper poppet (2) is unseated by the inside cam(5), permitting fluid to flow from the line (3) intothe cylinder to actuate the piston. The lowerpoppet (1) is seated, sealing off the flow into thereturn line (4). As the force of the pressurized fluidextends the piston rod, it also compresses thespring in the cylinder.View B shows the valve with the controlhandle turned to the opposite position. In thisposition, the upper poppet (2) is seated, blockingthe flow of fluid from the pressure line (3). Thelower poppet (1) is unseated by the outside cam(6). This releases the pressure in the cylinder andallows the spring to expand, which forces thepiston rod to retract. The fluid from the cylinderflows through the control valve and out the returnFigure 6-28.—Three-way, poppet-type directional controlvalve (cam-operated).6-19
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