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There are many types of valves, such as selector, pressure reducing, sequence, check, restrictor, and relief. While the basic function for each type of valve is similar, the design and construction may be very different. Examples of these valves are discussed in the following text. SELECTOR VALVES Selector valves are used in a hydraulic system to direct the flow of fluid. A selector valve directs fluid under system pressure to the desired working port of an actuating unit (double-acting), and, at the same time, directs return fluid from the opposite working port of the actuating unit to the reservoir. Some aircraft maintenance instruction manuals (MIMs) refer to selector valves as control valves. It is true that selector valves may be placed in this classification, but you should understand that all control valves are not selector valves. In the strict sense of the term, a selector valve is one that is engaged at the will of the pilot or copilot for the purpose of directing fluid to the desired actuating unit. This is not true of all control valves. Selector valves may be located in the pilot’s compartment and be directly engaged manually through mechanical linkage, or they maybe located in some part of the aircraft and be engaged by remote control. Remote-controlled selector valves are generally solenoid operated. The typical four-way selector valve has four ports—a pressure port, a return port, and two cylinder (or working) ports. The pressure port is connected to the main pressure line from the power pump, the return port is connected to the reservoir return line, and the two cylinder ports are connected to opposite working ports of the actuating unit. Three general types of selector valves are discussed in this chapter. They are the poppet, slide, and solenoid-operated valves. Practically all selector valves currently in use come under one of these three general types. Poppet-Type Selector Valve Poppet-type selector valves are manufactured in both the balanced and unbalanced design. An unbalanced poppet selector valve offers unequal working areas on the poppets. The larger area of the poppet is in contact with the working lines of the system; consequently, when excessive pressure exists within the working lines due to thermal expansion, the poppet will open. This action allows the excessive pressurized fluid to flow into the pressure line, where it is relieved by the main system relief valve. The balanced poppet selector valve has equal poppet areas. The poppets will remain in the selected position during thermal expansion of working line fluid. For this reason, thermal relief valves are installed in working lines that incorporate balanced poppet selector valves. Figure 8-7 shows a typical four-port poppet selector valve. This is a manually operated valve, and consists of a group of conventional spring-loaded poppets. The poppets are enclosed in a common housing and interconnected by passageways to direct the flow of fluid in the desired direction. Figure 8-7.—Poppet-type selector valve. 8-9

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