the lobes, which unseat one pressure poppet andone return/exhaust poppet (fig. 6-31). The valveis now in the working position. Fluid underpressure, entering the pressure port, flows throughthe vertical fluid passages in both pressure poppetsseats. Since only one pressure poppet, IN (2), isunseated by the cam lobe, the fluid flows past theopen poppet to the inside of the poppet seat. Fromthere it flows through the diagonal passages, outone cylinder port, C2, and to the actuating unit.Return fluid from the actuating unit enters theother cylinder port, C1. It then flows through thecorresponding fluid passage, past the unseatedreturn poppet, OUT (1), through the vertical fluidpassages, and out the return/exhaust port. Whenthe camshaft is rotated in the opposite directionto the neutral position, the two poppets seat andthe flow stops. When the camshaft is furtherrotated in this direction until the stop pins hits,the opposite pressure and return poppets areunseated. This reverses the flow in the workinglines, causing the actuating unit to move in theopposite direction.Rotary Spool ValveFour-way directional control valves of thistype are frequently used as pilot valves to directflow to and from other valves (fig. 6-32). Fluidis directed from one source of supply through therotary valve to another directional control valve,where it positions the valve to direct flow fromanother source to one side of an actuating unit.Fluid from the other end of the main valve flowsthrough a return line, through the rotary valveto the return or exhaust port.The principal parts of a rotary spool direc-tional control valve are shown in figure 6-22.Figure 6-31.—Working view of a poppet-type, four-waydirectional control valve.Figure 6-32.—Sliding spool valve controlled by a rotary spoolvalve.Figure 6-33 shows the operation of a rotary spoolvalve. Views A and C show the valve in a positionto deliver fluid to another valve, while view Bshows the valve in the neutral position, with allpassages through the valve blocked.Rotary spool valves can be operated manually,electrically, or by fluid pressure.Sliding Spool ValveThe sliding spool four-way directional controlvalve is similar in operation to the two-wayvalve previously described in this chapter. It issimple in its principle of operation and is themost durable and trouble-free of all four-waydirectional control valves.The valve described in the following para-graphs is a manually operated type. The sameprinciple is used in many remotely controlleddirectional control valves.The valve (fig. 6-34) consists of a valve bodycontaining four fluid ports—pressure (P),Figure 6-33.—Operation of a rotary spool, four-waydirectional control valve.6-22
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