Aircraft Hydraulic Pumps: Aircraft Hydraulic System Pump Types

Aircraft Hydraulic Pumps

Aircraft Hydraulic Systems

Smaller aircraft can be flown by hand because of the relatively low flight control surface loads. Early aircraft used hydraulic systems for their brake systems. Pilots could no longer move the control surfaces by hand as aircraft got larger and flying faster, therefore hydraulic power boost systems were developed. Even though the pilot still operates the flying controls using a cable or push rod, power boost devices help the pilot overcome strong control forces.

Landing gear, flaps, and brakes are all operated and controlled by hydraulic systems in aircraft. Larger aircraft heavily rely on these systems for their thrust reversers, spoilers, and flight controls. These hydraulic systems in aircraft are dependable because they employ hydraulic fluid, which is almost incompressible, capable of transmitting high pressures, lighter, and more resilient than pneumatic systems.

Aviation hydraulic systems are capable of performing well under challenging in-flight circumstances. They are required for the proper operation of all flight-essential parts, including the landing gear, flight control surfaces, and brake systems. It is necessary for aircraft hydraulic components to be lightweight and simple to install and maintain. Additionally, fluid dynamics make hydraulics virtually 100% efficient, with little fluid loss from friction.

A power supply system and fly-by-wire flight control are features found in many modern aircraft. The flight control servos receive the pilot input electrically. There are no push rods or cables employed. The most recent development of the hydraulic system is small power packs. By getting rid of hydraulic lines and a lot of hydraulic fluid, they lighten their load. Some manufacturers are replacing hydraulic systems with electrically operated ones in their aircraft. First aircraft with more electrical systems than hydraulic systems was the Boeing 787.

Aircraft Hydraulic Pumps

All aircraft hydraulic systems feature one or more power-driven pumps, and when the engine-driven pump is not working, an extra manual pump may be present. The main source of energy is provided by power-driven pumps, which can be either air, electric, or engine-driven. Electrical motor pumps are typically installed for usage during ground activities or in emergency situations. A ram air turbine (RAT) can be used by some aircraft to produce hydraulic power.

Hand Pumps

Some older aircraft use the hydraulic hand pump to operate their hydraulic subsystems, and some contemporary aircraft use it as a backup device. In general, hand pumps are installed for testing purposes and for usage in an emergency. To service the reservoirs from a single refilling station, hand pumps are also provided. The likelihood of fluid contamination entering the system is decreased by the single refilling station.

Hand pumps come in single action, double action, and rotary varieties. In a single action hand pump, the fluid is drawn into the pump on one stroke and pumped out on the following one. As a result of its inefficiency, it is rarely utilised in the aircraft.

On each handle stroke, double-action hand pumps generate pressure and fluid flow. The double-action hand pump is made up primarily of a housing with two ports and a cylinder bore, a piston, two check valves that are spring-loaded, and an operational handle. Between the two chambers of the piston cylinder bore, an O-ring on the piston prevents leakage. Leaks between the piston rod and housing are prevented by an O-ring in a slot on the end of the pump housing.

The pressure in the chamber to the piston's left is reduced as the piston is shifted to the right. The ball check valve on the inlet port opens, allowing hydraulic fluid to enter the chamber. In addition, as the piston moves to the right, the piston ball check valve is pressed up against its seat. The output port forces fluid in the chamber to the right of the piston out into the hydraulic system. The intake port ball check valve seats when the piston is shifted to the left. The piston ball check valve is forced from its seat as a result of increased pressure in the chamber to the piston's left. The piston transmits fluid from the left chamber to the right chamber. Due to the displacement the piston rod causes, the volume in the chamber to the right of the piston is less than that in the chamber to the left. The extra fluid is driven out of the outlet port into the hydraulic system as it moves from the larger left chamber into the smaller right chamber.

Additionally, a rotary hand pump can be used. While the handle is moving, it continuously generating output.

Power-Driven Pumps

Current aircraft have a large number of variable supply, compensator-controlled hydraulic pumps. Pumps with constant delivery are also in use. Both types of pumps operate according to the same principles. Power transfer units (PTU), pumps driven by RATs, air-driven power pumps, electrically driven power pumps, and engine-driven power pumps are all used in modern aircraft. The Airbus A380, for instance, has two hydraulic systems, eight engine-driven pumps, and three electrically powered pumps. The Boeing 777 has three hydraulic systems, including a hydraulic pump motor powered by the RAT, two engine-driven pumps, four electrical pumps, two air-driven pumps, and four electrical pumps.

Classification of Pumps

Positive displacement and nonpositive displacement classifications are applicable to all pumps. Positive displacement pumps are the most common type utilised in hydraulic systems. Continuous flow is produced using a nonpositive displacement pump. Although it does not have a strong internal seal against slippage, this causes its output to vary significantly as pressure changes. Nonpositive-displacement pumps include centrifugal and impeller pumps. The pressure would increase and the output of a nonpositive-displacement pump would drop to zero if the output port were blocked. Slippage within the pump would cause flow to stop even if the pumping part would continue to move. Slippage in a positive displacement pump is very little in comparison to the volumetric output flow of the pump. The pressure would instantly rise to the point where the pump pressure relief valve would open if the output port were blocked.

Constant-Displacement Pumps

Regardless of the pump's rotations per minute, a constant-displacement pump propels a fixed or constant volume of fluid out the outlet port with each revolution. Pumps that have a constant displacement are also known as constant-volume or constant-delivery pumps. Regardless of the pressure requirements, they deliver a constant amount of fluid per revolution. The amount of fluid delivered per minute relies on the number of pump rotations per minute since a constant-delivery pump delivers a specific amount of fluid during each revolution of the pump. A pressure regulator is necessary when a constant-displacement pump is used in a hydraulic system where the pressure must be maintained at a constant value.

Gear-Type Power Pump

A constant-displacement pump is one that has a gear design. It is made up of two meshing gears that spin inside a casing. The aircraft engine or another power source powers the driving gear. The driving gear drives and meshes with the driven gear. There is very little space between the teeth when they mesh and between the teeth and the housing. The reservoir is connected to the pump's inlet port, and the pressure line is attached to the pump's outlet port.

The driven gear also rotates when the driving gear does. The teeth catch the fluid as it passes the intake, and it continues around the housing before leaving via the outlet.

Gerotor Pump

An internal gear rotor with seven wide teeth of low height, a spur driving gear with six thin teeth, a pump cover with two crescent-shaped openings, and an eccentric-shaped stationary liner make up the basic components of a gerotor-type power pump. Both openings extend into ports: one into an inlet and the other into an outlet. The gears work together to rotate the pump in a clockwise direction. The pockets between the gears on the left side of the pump grow in size as they move from a lowermost position to a topmost position, creating a partial vacuum between these pockets. Fluid is attracted into the pockets because they increase as they pass over the input port crescent. The size of these similar pockets decreases as they rotate to the right side of the pump and move from the uppermost position to the lowermost position. The fluid is subsequently forced out of the pockets through the output port crescent as a result.

Piston Pump

Both constant-displacement and variable-displacement piston pumps are available. The next paragraphs provide an explanation of the basic design and functional characteristics that all piston-type hydraulic pumps share. Pumps of the piston type with flanged mounting bases can be mounted on the accessory drive cases of aviation engines. The mechanism is turned by a pump drive shaft that extends slightly past the mounting base through the pump housing. A drive coupling transmits the driving unit's torque to the pump drive shaft. Short shaft with male splines on both ends makes up the drive coupling. The splines on either end interact with female splines in a driving gear on one end, and the pump drive shaft on the other. Pump drive couplings are made to act as security measures. The drive coupling's shear section, which sits halfway between the two sets of splines, has a smaller diameter than the splines. This portion shears if the pump becomes abnormally difficult to spin or becomes jammed, protecting the pump and driving unit. A multiple-bore cylinder block, a piston for each bore, and a valve plate with inlet and output slots make up the basic pumping mechanism of piston-type pumps. As the pump runs, fluid can enter and exit the bores through the valve plate slots. The cylinder bores are symmetrical and parallel to the pump axis. An odd number of pistons are present in every aircraft axial-piston pump.

Bent Axis Piston Pump

In a conventional constant-displacement axial-type pump, the angular housing of the pump creates a similar angle between the cylinder block and the drive shaft plate to which the pistons are attached. The pump's angular design is what causes the pistons to move when the pump shaft is cranked. All components inside the pump rotate collectively as a rotating group when the pump is in operation, with the exception of the oil seal, the cylinder bearing pin, and the outer races of the bearings that support the drive shaft. There is a minimum gap between the top of the cylinder block and the upper face of the drive shaft plate at one point of rotation of the rotating group. The space between the top of the cylinder block and the upper face of the drive shaft plate is at its closest position due to the tilted housing at a 180° angle. Three of the pistons are always moving away from the top face of the cylinder block while the engine is running, creating a partial vacuum in the bores where these pistons are located. Fluid is sucked into these bores at this moment because it happens over the input port. Three distinct pistons are travelling toward the block's upper face on the opposite side of the cylinder block. These pistons force fluid out of the pump as the revolving group passes over the outlet port as a result of this. The pump output is essentially nonpulsating due to the overlapping, continuous, and quick motion of the pistons.

Inline Piston Pump

The swash plate design, in which a cylinder block is rotated by the drive shaft, is the most basic sort of axial piston pump. Through piston shoes and a retraction ring, pistons fitted to cylinder block bores are joined so that the shoes bear against an angled swash plate. The pistons reciprocate as a result of the piston shoes following the swash plate as the block rotates. In order enable the pistons to pass through the inlet as they are pulled out and the exit as they are shoved back in, the ports in the valve plate are positioned in such a way. These pumps' displacement is controlled by the size, quantity, and length of their pistons, whose stroke length vary with the swash plate angle.

Vane Pump

A constant-displacement pump is called the vane-type power pump. It is made consisting of a housing with four blades (vanes), a hollow steel rotor with slots for the vane attachments, and a connection to turn the rotor. Within the sleeve, the rotor is positioned off-center. The rotor and the vanes, which are positioned in the rotor's slots, divide the bore of the sleeve into four portions. Each part of the rotor passes a point where its volume is at a minimum and a maximum as the rotor rotates. During the first half of a revolution, the volume gradually rises from minimum to maximum, and during the second half of the revolution, it gradually falls from maximum to minimum. A slot in the sleeve allows a portion to be joined to the pump's inlet port as its capacity increases. Since the section's volume increase creates a partial vacuum, fluid is sucked into the section through the pump's inlet port and the sleeve's slot. Fluid is expelled out of the section, through the slot in the sleeve aligned with the output port, and out of the pump as the rotor revolves through the second half of the revolution and the volume of the given section is diminishing.

Variable-Displacement Pump

A variable-displacement pump may adjust its fluid output to match the system's required pressure. A pump compensator inside the pump automatically modifies the output. The discussion of a two-stage Vickers variable displacement pump will come next. Before the fluid enters the piston pump, the pump's initial stage consists of a centrifugal pump that increases pressure.

Basic Pumping Operation

A gearbox connects the aircraft's engine to the pump drive shaft, cylinder block, and pistons. Pumping motion is produced by piston shoes that slide on the shoe bearing plate in the yoke assembly while being restricted. The drive shaft's rotating motion is changed into a piston's reciprocating motion because the yoke is at an angle to it.

System inlet pressure pushes fluid through a porting mechanism in the valve plate and into the cylinder bore as the piston starts to leave the cylinder block. During the intake stroke, a piston shoe retention plate and a shoe plate hold the piston shoes in the yoke. The piston shoe keeps following the yoke bearing surface as the drive shaft continues to rotate the cylinder block. The piston starts to enter its bore as a result (i.e., toward the valve block).

The fluid in the bore is first precompressed before being released through the outlet port. The discharge stroke is held in place by discharge pressure, which also supplies the shoe pressure balance and fluid film through an aperture in the piston and shoe subassembly.

Each piston completes one intake and one discharge stroke throughout the pumping cycle described above with each rotation of the driving shaft and cylinder block. Through the valve plate and past the blocking valve, high-pressure fluid is routed out to the pump output. The blocking valve is intended to stay open while the pump is running normally. Internal leakage maintains the pump housing full of liquid for cooling and lubricating rotating parts. Through a case drain port, the leak is brought back into the system. By reducing the pressure to the pump inlet, the case valve relief valve safeguards the pump from high case pressure.

Normal Pumping Mode

The pressure compensator is a spool valve that has an adjustable spring load holding it in the closed position. The spool moves to admit fluid from the pump outlet against the yoke actuator piston when pump outlet pressure (system pressure) reaches the pressure preset (2,850 psi for full flow).

Two bearings inside the pump housing support the yoke. The yoke is retained at its greatest angle with respect to the drive shaft centerline at pump output pressures lower than 2,850 psi by the force of the yoke return spring. Reduced system flow demand results in higher outlet pressure, which cracks the compensator valve open and allows fluid to enter the actuator piston.

The pump yoke is stroked to a smaller angle by this control pressure, which defeats the force of the yoke return spring. The shorter piston stroke and decreased displacement are caused by the yoke's reduced angle.

Pump flow is decreased in proportion to the lower displacement. Only the flow necessary to keep the desired pressure in the system is delivered by the pump. The yoke angle lowers to almost 0 degrees stroke angle when there is no need for flow from the system. The device exclusively pumps its own internal leaks in this mode. Pump displacement therefore diminishes as outlet pressure increases for pump outlet pressures exceeding 2,850 psi. Low system pressures prevent any fluid from entering the actuator piston through the pressure compensator valve, and the pump continues to operate at full displacement, supplying the full flow. The system demand is then used to determine pressure. At a system pressure of 3,025 psi, the unit maintains zero flow.

Depressurized Mode

The electrical depressurization valve (EDV) solenoid valve moves against the spring force when the solenoid valve is activated, and the outlet fluid is connected to the EDV control piston on top of the compensator (depressurizing piston). The compensator spool is forced out of its normal measuring position by the high-pressure fluid. By doing this, the compensator valve is eliminated from the system, and the actuator piston is directly linked to the pump output. The blocking valve spring chamber receives outlet fluid as well, balancing the pressure on both sides of the plunger. The blocking valve closes when the blocking valve spring exerts enough force, cutting off the pump's connection to the external hydraulic system. The pump automatically reduces its output to zero at 1,100 psi, which is the pressure needed to compress the actuator piston and bring the yoke angle close to zero. In a system with many pumps, this depressurization and blocking feature can be used to isolate one pump at a time and test for correct system pressure output. It can also be utilised to lessen the burden on the engine during starting.

Also Read

Resolver (Electrical) | Rotary Electrical Transformer | What is the purpose of a resolver?

Full Authority Digital Engine Control (FADEC) System Description & Operation

Post a Comment