Aerospace Hydraulic System Description.


The Axial Piston, Electrically Depressurized, Pressure Controlled, Variable
Displacement Hydraulic Pump Assemblies, Models PV3--240--10, PV3--240--10A,
PV3--240--10C, and PV3--240--10D are engine--driven, inline, pressure--compensated
type units. When employed in an hydraulic system, the pumps provide a low--pulsating
flow of fluid in varying volume and within a predetermined pressure range. The pumps
are capable of operating in either a normal operating pressure range or in a depressurized
range, depending on system demands (see Fig. 1).
The pumps consist of several major components which are enclosed within a mounting
flange, housing adapter block, and valve block and control subassembly.
Mounting flange (1, Fig. 2), located at the coupling shaft end of the pump, is an aluminum
alloy machined casting. Its function is to provide attachment of the pump to the aircraft
engine accessory drive pad. A portion of the hydraulic seal between pump and aircraft
engine is provided by a seal mating ring (2) and retainer (3) which are located in the
mounting flange. Machined recesses in the mounting flange are provided to accept drive
shaft bearing (4) and control spring (5).
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Housing (6) contains the pumping mechanism components. These components are
cylinder block (7), nine piston and shoe subassemblies (8), yoke (9), and drive shaft (10).
The drive shaft is supported in the mounting flange by drive shaft bearing (4) and in the
adapter block (11) by roller bearing (12). The cylinder block (7) is coupled with the drive
shaft by internal and external splines. The pumping action is generated by the piston and
shoe subassemblies within the cylinder block and is dependent upon the displacement
angle of the yoke.
Yoke (9) is supported in the mounting flange and swivels through an arc of 0 to 1730’ on
two yoke pintle bearings (15). The angle of the yoke is controlled by controlled fluid
pressure acting on actuator piston (16). Control spring (5) moves the yoke to the
maximum angle (1730’)when outlet fluid pressure is less than previously adjusted value.
Shoe bearing plate (17) supports the piston and shoe subassemblies which are retained
in the yoke by hold--down plate (18), retainer (19), and screws (20).
Wafer plate (13), located between cylinder block (7) and adapter block (11), provides the
valving action to direct fluid flow to and away from the cylinder block.
Case relief valve (21) in the adapter block, permits case fluid to pass directly into the
pump inlet in the event of case pressure surge. In this manner, the housing is protected
from possible rupture.
Impeller (22) is coupled to the drive shaft by impeller key (23). Incorporation of the
impeller permits pump full flow operation with an inlet pressure as low as 5 psig (35 kPa,
absolute).
The valve block and inserts subassembly (24) includes the solenoid--operated
depressurizing valve (26), and outlet blocking valve (27). The inlet, outlet and case drain
ports are an integral part of the valve block and controls subassembly. The inlet and outlet
ports provide the connection between the hydraulic system and the pump. The case
drain port provides connection between the hydraulic system reservoir and the pump.
Four seepage drain ports are provided, three of which are normally plugged. Seepage
drain from the fourth shall not be returned to the hydraulic reservoir.
Adapter block (11) contains the pressure compensator valve (25), case relief valve (21),
and case fill and bleed valve (32). The pressure compensator valve (25) consists
principally of a spring--loaded pilot valve operating within a bushing and an adjusting
screw which controls the preload on the spring--loaded pilot valve. The preload
adjustment determines the system pressure at which the pump begins to reduce its
yoke angle of displacement. Auxiliary piston (28) and control valve (29) activate the pilot
valve when the solenoid--operated depressurizing valve (26) is energized. The case
fill and bleed valve (32) minimizes trapped air pockets in the pump casewhich could contribute
to dry starts.
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1. Mounting Flange    2. Mating Ring
3. Retainer                4. Drive Shaft Bearing
5. Control Spring       6. Housing
7. Cylinder Block       8. Piston and Shoe Subassembly
9. Yoke                     10. Drive Shaft
11. Adapter Block Subassembly
12. Roller Bearing     13. Wafer Plate
14.  Spring                15. Pintle Bearings
16. Actuator Piston    17. Shoe Bearing Plate
18. Hold Down Plate  19. Retainer
20. Screws                21. Case Relief Valve
22. Impeller              23. Impeller Key
24. Valve Block and Inserts Subassembly
25. Pressure Compensator Valve
26. Depressurizing Valve
27. Outlet Blocking Valve
28. Auxiliary Piston    29. Control Valve
30. Shaft Seal            31. Coupling Shaft
32. Case Fill and Bleed Valve
_________________________________________

The solenoid--operated depressurizing valve (26) enables the pump to operate in the
normal pressure range or in the depressurized pressure range. The normal pressure
range is 3025 psi (20 857 kPa) and the depressurized pressure range is approximately
1200 psi (8 274 kPa). When the pump is operating as a depressurized unit, a fluid flow of
about 1.5 gpm (5.7 liters/min) is generated by the unit. This flow provides lubrication and
cooling for the pump components. This fluid returns to the reservoir via the case drain
port.
Outlet blocking valve (27) is used in conjunction with the solenoid--operated
depressurizing valve (26) to isolate the pump from the rest of the system during periods
of depressurization, or in event of system failure. The outlet blocking valve will close
when the solenoid--operated depressurizing valve is energized and pump internal
pressure is directed to the back side of the blocking valve piston. The blocking valve will
reopen only after the solenoid--operated depressurizing valve (26) is de--energized.
Shaft seal (30) and other packings prevent external leakage. Shaft seal (30) is located in
mounting flange (1). Coupling shaft (31) connects the pump drive shaft to the engine accessory
drive. The coupling shaft incorporates a shear section to protect the pump
against sudden overload and to protect the engine accessory drive in the event of pump
failure.
The pump identification plate, located on the housing, contains areas for the following
items: model, part numbers, serial number, pump displacement (cu.in/rev), maximum
drive speed (rpm), maximum operating pressure (psi) and an inspection stamp.
Adjacent to the identification plate is a fluid plate which identifies the pump hydraulic fluid
as BMS--3--11 (Alkyl Phosphate Ester Base). The instruction plate, also located on the
housing, alerts personnel that the unit shall be filled with hydraulic fluid before starting.
The rotation plate, located on the mounting flange, indicates that the pump shall be
driven in a clockwise direction, as viewed from the coupling shaft end of the pump.



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