B.  
Main Circuit Operation

 (1)  
On one half of each drive shaft (and cylinder block) revolution, the pistons move away from the valve face of the cylinder block. The volume that expands in the cylinder block bores is filled by fluid flow from the impeller. The fluid comes from the reservoir through the inlet port of the valve block, and the inlet side of the impeller. On the other half of each drive shaft revolution, the pistons move toward the valve face of the cylinder block. This movement forces the fluid out of the cylinder block bores, through the open ports of the outlet blocking valve, and the outlet port of the valve block to the hydraulic system.

 (2)  
Each of the nine pistons completes this cycle on each revolution of the drive shaft, giving a continuous, non-pulsating flow of fluid to the system. The impeller increases the inlet fluid pressure approximately 20 psi to make sure that sufficient pressure is available at the cylinder block face to prevent cavitation on the inlet stroke of each piston.

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 29-11-05
 ALL  ú ú 01 Page 4 ú Dec 10/88
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 C. Control Circuit Operation
 (1)  
In the normal operating pressure range, resistance to fluid flow in the hydraulic system causes pressure to increase on the outlet side of the pump (and cylinder blocks). The larger this resistance is to flow, the larger will be the pressure buildup. This pressure rise is transmitted by the fluid through flow passages in the adapter block to the pressure compensator valve. Pressure in excess of the spring setting of this valve forces it to open and let fluid flow under pressure to the actuator piston. The actuator piston will then change the yoke angle to decrease the reciprocating action of the pistons in the cylinder block bores. If the pump flow meets the system demands, the yoke will stay in a minimum pumping position, just sufficiently off of the zero stroke position to make allowance for volumetric losses.

 (2)  
When necessary, the pump can be operated in a depressurized pressure range. This range may be used as protection for the pump if the system fails. If the solenoid-operated-depressurizing valve is energized, part of the pump output flow will turn to the top of the blocking valve and to the control piston on the end of the pressure compensator. The blocking valve will close against the outlet pressure because it is hydraulically balanced and the pilot valve is pushed farther than its normal position. Thus the blocking valve will not operate, and the actuator piston is connected directly to the control pressure.

 (3)  
The pump will decrease to zero flow at approximately 800 psig (5516 kPa). This is the pressure necessary on the actuator piston to reduce the yoke angle to zero when the outlet and control pressures are balanced.

 D.
Lubrication Circuit Operation

 (1)  
When the unit is installed in the circuit, the case is fully filled with hydraulic fluid and the case drain port is connected to the resrvoir. Leakage around the pistons increases the volume of fluid in the housing and causes the fluid to flow out of the case drain port. Flow of the fluid in the housing gives lubrication and cooling for the moving part of the hydraulic pump. The head of fluid at the drain line gives a positive case pressure to keep air out of the system. This flow of fluid is the lubrication circuit.

 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 29-11-05
 ALL  ú ú 01 Page 5 ú Dec 10/88
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 ENGINE-DRIVEN HYDRAULIC PUMP - MAINTENANCE PRACTICES
____________________________________________________
 1.  General
_______
 A.  This procedure has these tasks:
 (1)  
Engine-driven pump (EDP) Dispatch Equipment Installation.

 (a)  
This task removes an EDP which does not operate and installs a device to seal the gear shaft and a cover on the opening in the EDP pad.

 (2)  
Deactivated engine-driven pump installation.

 (a)  
　
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