(a)  
Resistance to the flow of fluid in the system causes pressure to build up on the outlet side of the hydraulic pump. The greater the resistance to the fluid flow, the greater the pressure buildup. The pressure rise is reflected through the pressure sensing passage in the cap port, which acts on the compensating mechanism. At pressures from 0 to 2700 psig, the adjustable compensator spring holds the compensator valve closed. The pressure sensing port is blocked. Under these conditions the rate piston and spring positions the hanger at the maximum angle of displacement. This permits the pump to deliver a maximum volume of fluid (6.3 gpm).

(b)  
As the pressure exceeds 2700 psig, hydraulic fluid under pressure acts on the head end of the compensator valve, overcomes the force of the compensator spring and positions the compensator valve to permit flow between the pressure sensing port and the stroking piston. The hydraulic fluid under pressure, continues to act on the stroking piston, overcomes the force of the rate piston spring and forces the hanger toward a minimum angle of displacement. At approximately 3025 psig the hanger is at zero angle of displacement. In this position the rotating group continues to rotate, but does not pump fluid because the pistons do not reciprocate. The zero displacement angle is a momentary condition which changes as the hanger moves to a displacement angle sufficient to overcome the volumetric loss due to internal leakage. When the system demand lowers the pressure, the compensator spring closes the compensator valve and fluid behind the stroking piston is ported into the pump housing. The stroking piston then retracts and the spring load on the rate spring moves the hanger to a greater angle and increases the pump volume.

(3)  
The cooling and lubrication circuit comprises the flow path of hydraulic fluid in the pump and motor housing.

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May 15/67  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  29-12-21 Page 3 

(a)  When the unit is installed in the system, the case is completely filled with hydraulic fluid and the case drain port is connected to the system B reservoir. Hydraulic fluid is circulated around the motor rotor and stator and through the hollow rotor shaft, this circulated fluid cools the rotor and stator of the electric motor. Leakage past the pistons, and flow from the compensating mechanism increases the volume of fluid in the pump housing and causes flow out the case drain port. Circulation of fluid in the pump and motor housing provide lubrication and cooling for the moving parts and a head of fluid at the case drain line provides a positive case pressure to keep air out of the system. This flow path constitutes the lubrication and cooling circuit.
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SYSTEM B ELECTRIC MOTOR-DRIVEN HYDRAULIC PUMP - REMOVAL/INSTALLATION
EFFECTIVITY
ABEX Pump Installation
1. General
A.  If electrical power to the pump motor is interrupted during normal operation by the ground fault protection system, the pump should be replaced. If the hydraulic pump is replaced due to mechanical malfunction, replace the pressure and case drain filter elements and flush the lines between the pump and filters prior to replacing the pump (Ref 29-12-0 I/C).
B.  A container will be necessary to catch fluid from the disconnected hydraulic lines. Take precaution to prevent spillage of fluid. Should any fluid spill on the airplane, decontaminate  (Ref Chapter 12, Cleaning and Washing).
C.  The system B hydraulic pumps are identical units, therefore only one maintenance procedure is given.
2.  Equipment and Materials
A.  Fire Resistant Hydraulic Fluid -BMS 3-11 (Ref 20-30-21)
B.  A container to catch hydraulic fluid
3.  Prepare Hydraulic Pump for Removal
A.  Open applicable system B pump circuit breakers on P6-11 and P6-12 panels.
B.  Depressurize hydraulic reservoirs (Ref 29-09-300 MP).
C.  Open tire burst protection screen doors.
D.  Disconnect pump supply line at quick disconnect at bottom of reservoir and drain fluid from line.
4.  Remove Hydraulic Pump (Fig. 401)
A.  Disconnect electrical connector.
B.  Disconnect hydraulic lines at pump and install protective covers.
May 15/77 29-12-21 Page 401
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

F87588
System B Electric Motor-Driven Pump Installation  533 
29-12-21  Figure 401  Feb 15/72 
　
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