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HYDRAULIC PUMP (PISTON TYPE)
The function of the hydraulic system power pump is to change mechanical horsepower to hydraulic horsepower. Reciprocating Piston Variable Displacement Pumps attain volumetric efficiencies of up to 98% and they can maintain pressures from 1500 to 6000psi. They can achieve overall efficiencies of up to 92% and can move fluid volumes up to 50 gallons per minute. The Reciprocating Piston Pump, which will be described here, has a maximum delivery rate of approximately 37-1/2 gallons per minute at 3750 rpm and 3000psi. The pump is a two stage, variable displacement, pressure compensated unit. The pump is capable of operating in two modes, normal and depressurized.
The pump first stage, is an inlet boost impeller that provides approximately 35psi pressure rise for the incoming fluid to ensure that pump cavitation does not occur. The pump first stage is not normally required, but permits pump operation at full flow with pump inlet port pressures as low as 5psi.
The piston and the piston shoe. Note the hole at center of the shoe for the hydraulic balance
Other view of the same piston where the ball endof the piston is visible into the shoe
The second stage pumping mechanism consists of a revolving cylinder barrel containing nine pistons. The cylinder barrel is supported and driven by the internal drive shaft which is supported at the drive end by a ball bearing in the pump mounting flange and at the first stage end by a needle bearing in the pump adapter block. The pistons are supported in the cylinder block and connected to the rotating cam plate by hydraulically balanced piston shoes. Oil at system pressure is admitted through holes in the piston and piston shoe to an undercut area in the face of the shoe. The pressure applied to the undercut area, which is slightly less than the piston area, balances the forces so that the shoes and cam plate are supported on an oil film at all times. The axial thrust of the cylinder barrel is also hydraulically balanced against the adapter block. The rotating cam plate causes the pistons to reciprocate as the cylinder barrel revolves. The cam plate is supported in the cylinder yoke and restrained in the yoke by the hold-down plate. The stroke of the pistons is varied by changing the angle of inclination of the yoke and the came plate, thereby changing the displacement of the pump. The angle of inclination of the yoke, in turn, is controlled by the compensator actuator piston and compensator valve.
The cylinder barrel
The cylinder barrel with a piston
As the pumping mechanism revolves, reservoir pressurized fluid flows through the pump inlet port, through the first stage impeller, and through porting in the adapter block to the cylinder barrel. As the cylinder barrel revolves the pistons are first partially withdrawn from the cylinders by the inclination of the yoke allowing fluid at the first stage discharge pressure to fill the cylinder bore. As the cylinder barrel continues to rotate, the inlet port to the cylinder is closed and the inclination of the yoke forces the pistons into the cylinder bores discharging high- pressure fluid through porting in the adapter block to the pump outlet port.
The compensator valve, through the compensator actuator piston, regulates the fluid volume delivered in accordance with the demand of the system, thereby maintaining system pressure within a predetermined pressure range. System pressure is directed to the compensator valve, which is held in the closed position by an adjustable spring load. When system pressure reaches 3000psi, the valve cracks open to admit fluid at reduced pressure to the cylinder of the actuator piston. As system pressure increases above 3000psi the compensator valve continues to open until at 300 (+/-50)psi the valve is full open. When the valve is first cracked the pressure reduction through the valve is high. As system pressure increases causing the valve to open wider, the pressure reduction through the valve decreases until the valve is fully open causing approximately 800psi to act on the actuator piston. The pressure discharged from the compensator valve loads the actuator piston causing the piston to move against the rate spring. The movement of the actuator piston is proportional to the pressure imposed by the valve. As the actuator piston moves against the rate spring, it decreases the angle of inclination on the yoke, thereby reducing the stroke length of the pumping pistons, reducing the fluid volume pumped, and maintaining the system pressure within the desired range. As system volume demands increase, system pressure will decrease causing the compensator valve to move toward the closed position and reduce the pressure supplied to the actuator piston. As the pressure supplied to the actuator piston decreases, the rate spring moves the piston and the yoke to a greater angle of inclination. When the yoke is moved to a greater inclination the stroke of the pumping pistons is increased, causing an increase in pump output flow rate.
In the blocked mode of the pump operation the solenoid valve is energized to the open position, thereby porting pump pressure to the back of the blocking valve poppet to close off the outlet port from the external hydraulic system. Simultaneously outlet pressure is applied to the compensator piston, which forces the piston to move the spool of the compensator valve to a point beyond normal metering position. The compensator valve becomes inoperable and connects the actuator piston directly to pump outlet pressure. With pump outlet pressure of approximately 800psi applied to the actuator piston, the piston will move against the rate spring and move the yoke to a position normal to the pump drive shaft so that the pressure outlet port cannot exceed 800psi. In this condition the pump is depressurized.
The following formulae may be used to determine the volumetric output of a piston pump, the pump horsepower, the pump’s volumetric efficiency and overall efficiency :
Reference : Boeing Aircraft Maintenance Manuals
Engine driven Hydraulic pump on engine accessory gear box