Figure 6-15.—InstallationSEQUENCE VALVESof sequence valves.Sequence valves control the sequence ofoperation between two branches in a circuit; thatis, they enable one unit to automatically setanother unit into motion. An example of the useof a sequence valve is in an aircraft landing gearactuating system.In a landing gear actuating system, the landinggear doors must open before the landing gearstarts to extend. Conversely, the landing gear mustbe completely retracted before the doors close. Asequence valve installed in each landing gearactuating line performs this function.A sequence valve is somewhat similar to arelief valve except that, after the set pressure hasbeen reached, the sequence valve diverts the fluidto a second actuator or motor to do work inanother part of the system. Figure 6-15 shows aninstallation of two sequence valves that controlthe sequence of operation of three actuatingcylinders. Fluid is free to flow into cylinder A.The first sequence valve (1) blocks the passage offluid until the piston in cylinder A moves to theend of its stroke. At this time, sequence valve 1opens, allowing fluid to enter cylinder B. Thisaction continues until all three pistons completetheir strokes.There are various types of sequence valves.Some are controlled by pressure and some arecontrolled mechanically.Pressure-Controlled Sequence ValveThe operation of a typical pressure-controlledsequence valve is illustrated in figure 6-16. Theopening pressure is obtained by adjusting thetension of the spring that normally holds thepiston in the closed position. (Note that the toppart of the piston has a larger diameter than thelower part.) Fluid enters the valve through theinlet port, flows around the lower part of thepiston and exits the outlet port, where it flows tothe primary (first) unit to be operated (fig. 6-16,view A). This fluid pressure also acts against thelower surface of the piston.Figure 6-16.—Operation of a pressure-controlled sequence valve.6-11
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