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).

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.

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.