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Power-Driven Pumps

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valve (B). Thus, a pressure stroke is produced with each stroke of the pump handle. Hand pumps are examined frequently for leakage, general condition, and efficiency in operation. To check the operation of a hand pump, the following procedure is recommended: 1. Connect a direct-reading hydraulic pressure gauge into the emergency hand pump pressure line. 2. Insert and lock the hand pump handle in the pump actuating socket. 3. Select an appropriate subsystem to operate, and place its selector valve in an operating position. 4. Actuate the hand pump handle until the unit being operated has completed its movement. Check the pressure gauge for a drop in system pressure. NOTE: Air in emergency systems will cause the pump handle to spring rapidly to the other end of the stroke. 5. If a pressure drop is indicated, check the system for leakage before removing the pump for repair or replacement. 6. Observe the hand pump handle for piston creep, which indicates that the pump should be removed for repair or replacement. Removal, replacement, and operational check of hand pumps should correspond to the procedures recommended in the specific MIM. Power-Driven Pumps As previously mentioned, power pumps are generally driven by the aircraft engine, but may also be electric-motor driven. Power pumps are classified according to the type of pumping action used, and may be either the gear type or piston type. Power pumps may be further classified as constant displacement or variable displacement. A constant displacement pump is one that displaces or delivers a constant fluid output for any rotational speed. For example, a pump might be designed to deliver 3 gallons of fluid per minute at a speed of 2,800 revolutions per minute. As long as it runs at that speed, it will continue to deliver at that rate, regardless of the pressure in the system. For this reason, when the constant displacement pump is used in a system, a pressure regulator or unloading valve must also be incorporated. The pressure regulator valve will maintain a set pressure in the system by diverting excess pump flow back to the reservoir. The unloading valve will divert all pump flow back to the reservoir when the preset system pressure is reached. This condition remains in effect until further demand is placed on the system. A variable displacement pump has a fluid output that varies to meet the demand of the system. For example, a pump might be designed to maintain system pressure at 3,000 psi by varying its fluid output from 0 to 7 gallons per minute. When this type of pump is used, no external pressure regulator or unloading valve is needed. This function is incorporated in the pump and controls the pumping action by maintaining a variable volume, at near constant pressure, to meet the hydraulic system demands. GEAR-TYPE PUMP.—A gear-type pump consists of two meshed gears that revolve in a housing (fig. 7- 14). The drive gear in the installation is turned by a drive shaft that engages an electric motor. The clearance between the gear teeth as they mesh and between the teeth and pump housing is very small. The inlet port is connected to the reservoir line, and the outlet port is connected to the pressure line. In the illustration, the drive gear is turning in a counterclockwise direction, and the driven (idle) gear is turning in a clockwise direction. As the teeth pass the inlet port, fluid is trapped between the teeth and the housing. This fluid is carried around the housing to the outlet port. As the teeth mesh again, the fluid Figure 7-14.—Gear-type power pump. 7-14



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