Friday, April 27, 2012

Hydraulic Reservoirs, Filters, Pumps, Accumulators, And Motors


A means of storing hydraulic fluid and minimizing contamination is necessary to any aircraft hydraulic system. These functions are performed by reservoirs and filters. The component which causes fluid flow in a hydraulic system--the heart of any hydraulic system--can be a hand pump, power-driven pump, accumulator, or any combination of the three. Finally, a means of converting hydraulic pressure to mechanical rotation is sometimes necessary, and this is accomplished by a hydraulic motor.


The hydraulic reservoir is a container for holding the fluid required to supply the system, including a reserve to cover any losses from minor leakage and evaporation. The reservoir can be designed to provide space for fluid expansion, permit air entrained in the fluid to escape, and to help cool the fluid. Figure 1-1 shows two typical reservoirs. Compare the two reservoirs item by item and, except for the filters and bypass valve, notice the similarities.

Filling reservoirs to the top during servicing leaves no space for expansion. Most reservoirs are designed with the rim at the filler neck below the top of the reservoir to prevent overfilling. Some means of checking the fluid level is usually provided on a reservoir. This may be a glass or plastic sight gage, a tube, or a dipstick. Hydraulic reservoirs are either vented to the atmosphere or closed to the atmosphere and pressurized. A description of each type follows.

Vented Reservoir

A vented reservoir is one that is open to atmospheric pressure through a vent line. Because atmospheric pressure and gravity are the forces which cause the fluid to flow to the pump, a vented reservoir is mounted at the highest point in the hydraulic system. Air is drawn into and exhausted from the reservoir through a vent line. A filter is usually installed in the vent line to prevent foreign material from being taken into the system.

Pressurized Reservoir

A pressurized reservoir is sealed from the atmosphere. This reservoir is pressurized either by engine bleed air or by hydraulic pressure produced within the hydraulic system itself. Pressurized reservoirs are used on aircraft intended for high altitude flight, where atmospheric pressure is not enough to cause fluid flow to the pump.

In reservoirs pressurized by engine bleed air, the amount of air pressure is determined by an air pressure regulator--usually 10 to 15 pounds per square inch (psi) gage. An example of a hydraulically pressurized reservoir used in the CH-47 hydraulic system is shown in Figure 1-2.

This reservoir, or tank as it is referred to by Boeing-Vertol, is constructed of a metal housing with two internal pistons, one fixed and the other a floating piston which slides along a central tube. Attached to the floating piston is a larger tube that projects through the forward end of the tank and is calibrated to indicate FULL and REFILL fluid levels for ramp-up and ramp-down positions.

Typical Hydraulic Reservoirs
Figure 1-1. Typical Hydraulic Reservoirs.

Hydraulic fluid at 3,000 psi flows into the central tube as shown in Figure 1-2, passes through two outlet holes, and applies pressure at the piston area between the two tubes. Because the smaller piston has a .5-square-inch (sq in) exposed surface and the floating piston has a 30-sq-in exposed surface, the 3,000-psi pressure acting upon the smaller forward area produces an opposing pressure of 50 psi on the return fluid stored at the rear of the piston.

Additional Reservoir Components

Many reservoirs, as shown in Figure 1-1, are constructed with baffles or fins to keep the fluid from swirling and foaming. Foaming can cause air to become entrained in the system.

Filters are incorporated in some reservoirs to filter the fluid before it leaves the reservoir.
A bypass valve is used to ensure that the pump does not starve if the filter becomes clogged.
A standpipe is used in a reservoir which supplies a normal and an emergency system. The main system draws its fluid from the standpipe, which is located at a higher elevation. This ensures an adequate fluid supply to the secondary system if the main system fails.

Hydraulic Reservoir Pressurized With Hydraulic Fluid
Figure 1-2. Hydraulic Reservoir Pressurized With Hydraulic Fluid.


Contamination of hydraulic fluid is one of the common causes of hydraulic system troubles. Installing filter units in the pressure and return lines of a hydraulic system allows contamination to be removed from the fluid before it reaches the various operating components. Filters of this type are referred to as line filters.

Line Filter Construction

A typical line filter is shown in Figure 1-3. It has two major parts--the filter case, or bowl, and the filter head. The bowl holds the head that screws into it. The head has an inlet port, outlet port, and relief valve. Normal fluid flow is through the inlet port, around the outside of the element, through the element to the inner chamber, and out through the outlet port. The bypass valve lets the fluid bypass the filter element if it becomes clogged.

Typical Line Filter Assembly
Figure 1-3. Typical Line Filter Assembly

Types of Filter Elements

The most common filtering element used on Army aircraft is the micronic type. It is a disposable unit made of treated cellulose and is formed into accordion pleats, as shown in Figure 1-3. Most filter elements are capable of removing all contaminants larger than 10 to 25 microns (1 micron equals 0.00004 inch).

Another type is the cuno filter element. It has a stack of closely spaced disks shaped like spoked wheels.
The hydraulic fluid is filtered as it passes between the disks.


The heart of any hydraulic system is the pump which converts mechanical energy into hydraulic energy. The source of mechanical energy may be an electric motor, the engine, or the operator's muscle.
Pumps powered by muscle are called hand pumps. They are used in emergencies as backups for power pumps and for ground checks of the hydraulic system. The double-action hand pump produces fluid flow with every stroke and is the only type used on Army aircraft.

Handle to the Right. The double-action hand pump, shown in Figure 1-4, consists of a cylinder piston with built-in check valve, piston rod, operating handle, and a check valve built into the inlet port. As the handle is moved to the right, the piston and rod also move to the right. On this stroke, the inlet check valve opens as a result of the partial vacuum caused by the movement of the piston, allowing fluid to be drawn into the left chamber. At the same time, the inner check valve closes. As the piston moves to the right, the fluid in the right chamber is forced out into the system.

Double-Action Hand Pump
Figure 1-4. Double-Action Hand Pump.

Handle to the Left. When the handle is moved to the left, the piston and rod assembly also move to the left. The inlet check valve now closes, preventing the fluid in the left chamber from returning to the reservoir. At the same time, the pistonhead check valve opens, allowing the fluid to enter the right chamber.

Fluid Into the System. The pump produces pressure on both strokes because of the difference in volume between the right and left chambers. The piston rod takes up a good share of the space in the right chamber. Therefore, the excess fluid is forced out of the pump and into the hydraulic system, creating fluid pressure.


Power-driven pumps receive their driving force from an external power source, such as the aircraft engine. This force is converted into energy in the form of fluid pressure. The four basic types of power-driven hydraulic pumps are gear, vane, diaphragm, and piston. Of these, the piston type is most commonly found in Army aircraft. The reason for this is that it operates more efficiently at higher pressures and has a longer life than any of the others. Piston pumps are further categorized as either constant delivery or variable delivery.

Pumps are coupled to their driving units by a short, splined coupling shaft, commonly called a drive coupling. As shown in Figure 1-5, the shaft is designed with a weakened center section called a shear section, with just enough strength to run the pump under normal circumstances. Should some trouble develop within the pump causing it to turn unusually hard, the shear section will break. This prevents damage to the pump or driving unit.

Pump Drive Coupling
Figure 1-5. Pump Drive Coupling.

Constant-delivery piston pumps deliver a given quantity of fluid per revolution of the drive coupling, regardless of pressure demands. The quantity of fluid delivered per minute depends on pump revolutions per minute (rpm). In a system requiring constant pressure, this type of pump must be used with a pressure regulator. The two types of constant-delivery piston pumps used in Army aircraft are the angular and cam.

Angular Piston Pump Construction

The basic components of an angular piston pump are shown in Figure 1-6. They are--

(1) A rotating group consisting of a coupling shaft, universal link, connecting rods, pistons, and cylinder block.

(2) A stationary group consisting of the valve plate and the pump case or housing.

The cylinder bores lie parallel to, and are evenly spaced around, the pump axis. For this reason, a piston pump is often referred to as an axial piston pump.

Packings on seals are not required to control piston-to-bore leakage. This is controlled entirely by close machining and accurate fit between piston and bore. The clearance is only enough to allow for lubrication by the hydraulic fluid and slight expansion when the parts become heated. Pistons are individually fitted to their bores during manufacture and must not be changed from pump to pump or bore to bore.

Pump Operation

As the coupling shaft is turned by the pump power source, the pistons and cylinder block turn along with it because they are interconnected. The angle that exists between the cylinder block and coupling shaft causes the pistons to move back and forth in their respective cylinder bores as the coupling is turned:

• During the first half of a revolution of the pump, a cylinder is aligned with the inlet port in the valve plate. At this time the piston is moving away from the valve plate and drawing hydraulic fluid into the cylinder. During the second half of the revolution, the cylinder is lining up with the outlet port in the valve plate. At this time, the piston is moving toward the valve plate, thus causing fluid previously drawn into the cylinder to be forced out through the outlet port.

• Fluid is constantly being drawn into and expelled out of the pump as it turns. This provides a multiple overlap of the individual spurts of fluid forced from the cylinders and results in delivery of a smooth, non-pulsating flow of fluid from the pump.

Cam-Piston Pumps

A cam is used to cause the stroking of the pistons in a cam-piston pump. Two variations are used: in one the cam rotates and the cylinder block is stationary, and in the other the cam is stationary and the cylinder block rotates. Both cam-piston pumps are described below:

Typical Angular Piston Pump
Figure 1-6. Typical Angular Piston Pump.

• Rotating-cam pump

The rotating-cam pump is the one most commonly used in Army aviation. As the cam turns in a rotating-cam pump (Figure 1-7), its high and low points pass alternately and in turn under each piston. It pushes the piston further into its bore, causing fluid to be expelled from the bore. When the falling face of the cam comes under a piston, the piston's return spring pulls the piston down in its bore. This causes fluid to be drawn into the bore.

Each bore has a check valve that opens to allow fluid to be expelled from the bore by the piston's movement. These valves are closed by spring pressure during inlet strokes of the pistons. This fluid is drawn into the bores only through the central inlet passages. The bores only through the central inlet passages. The movement of the pistons in drawing in and expelling fluid is overlapping, resulting in a non-pulsating fluid flow.

Typical Rotating-Cam Piston Pump
Figure 1-7. Typical Rotating-Cam Piston Pump

• Stationary-cam pump

The operation and construction of a stationary-cam pump are identical to that of the rotating cam except that the cylinder block turns, not the cam. The stationary-cam pump is not used on the Army's OV-1, AH-1G, and UH-1C.


A variable-delivery piston pump automatically and instantly varies the amount of fluid delivered to the pressure circuit of a hydraulic system to meet varying system demands. This is accomplished by using a compensator, which is an integral part of the pump. The compensator is sensitive to the amount of pressure present in the pump and in the hydraulic system pressure circuit. When the circuit pressure rises, the compensator causes the pump output to decrease.

Conversely, when circuit pressure drops, the compensator causes pump output to increase. There are two ways of varying output--demand principle (cam) and stroke-reduction principle (angular).

Demand Principle

The demand principle (Figure 1-8) is based on varying pump output to fill the system's changing demands by making the piston stroke effective in varying degrees.

Variable-Delivery Demand-Principle Cam Pump
Figure 1-8. Variable-Delivery Demand-Principle Cam Pump.

The pistons are designed with large hollow centers. The centers are intersected by cross-drilled relief holes that open into the pump case. Each piston is equipped with a movable sleeve, which can block the relief holes. When these holes are not blocked, fluid displaced by the pistons is discharged through the relief holes into the pump case, instead of past the pump check valves and out the outlet port.

When full fluid flow is required, the sleeves are positioned to block the relief holes for the entire length of piston stroke. When zero flow is required, the sleeves are positioned not to block the flow during any portion of the piston stroke. For requirements between zero and full flow, the relief holes are uncovered or blocked accordingly.

The sleeves are moved into their required positions by a device called a pump compensator piston. The sleeves and compensator piston are interconnected by means of a spider. Fluid pressure for the compensator piston is obtained from the discharge port (system pressure) through a control orifice.

Stroke-Reduction Principle

The stroke-reduction principle (Figure 1-9) is based on varying the angle of the cylinder block in an angular pump. This controls the length of the piston's stroke and thus the volume per stroke.

The cylinder block angle change is achieved by using a yoke that swivels around a pivot pin called a pintle. The angle is automatically controlled by using a compensator assembly consisting of a pressure-control valve, pressure-control piston, and mechanical linkage that is connected to the yoke.

As system pressure increases, the pilot valve opens a passageway allowing fluid to act on the control piston. The piston moves, compressing its spring, and through mechanical linkage moves the yoke toward the zero flow (zero angle) position. As system pressure decreases, the pressure is relieved on the piston, and its spring moves the pump into the full flow position.


The purpose of a hydraulic accumulator is to store hydraulic fluid under pressure. It may be used to—:

• Dampen hydraulic shocks which may develop when pressure surges occur in hydraulic systems.

• Add to the output of a pump during peak load operation of the system, making it possible to use a pump of much smaller capacity than would otherwise be required.

• Absorb the increases in fluid volume caused by increases in temperature.

• Act as a source of fluid pressure for starting aircraft auxiliary power units (APUs).

• Assist in emergency operations.

Variable Stroke-Reduction Pump
Figure 1-9. Variable Stroke-Reduction Pump.

Accumulators are divided into types according to the means used to separate the air fluid chambers; these are the diaphragm, bladder, and piston accumulators.

Diaphragm Accumulator

The diaphragm accumulator consists of two hollow, hemispherical metal sections bolted together at the center. Notice in Figure 1-10 that one of the halves has a fitting to attach the unit to the hydraulic system; the other half is equipped with an air valve for charging the unit with compressed air or nitrogen. Mounted between the two halves is a synthetic rubber diaphragm that divides the accumulator into two sections. The accumulator is initially charged with air through the air valve to a pressure of approximately 50 percent of the hydraulic system pressure. This initial air charge forces the diaphragm upward against the inner surface of the upper section of the accumulator.

Diaphragm Accumulator.
Figure 1-10. Diaphragm Accumulator.

When fluid pressure increases above the initial air charge, fluid is forced into the upper chamber through the system pressure port, pushing the diaphragm down and further compressing the air in the bottom chamber. Under peak load, the air pressure in the lower chamber forces fluid back into the hydraulic system to maintain operating pressure. Also, if the power pump fails, the compressed air forces a limited amount of pressurized fluid into the system.

Bladder Accumulator

The bladder accumulator operates on the same principle and for the same purpose as the diaphragm accumulator but varies in construction, as shown in Figure 1-11. The unit is a one-piece metal sphere with a fluid pressure inlet at the top and an opening at the bottom for inserting the bladder. A large screw-type plug at the bottom of the accumulator is a retainer for the bladder that also seals the unit. A high-pressure air valve is also incorporated in the retainer plug. Fluid enters through the system pressure port. As fluid pressure increases above the initial air charge of the accumulator, it forces the bladder downward against the air
Bladder Accumulator
Figure 1-11. Bladder Accumulator.

charge, filling the upper chamber with fluid pressure. The broken lines in Figure 1-11 indicate the approximate position of the bladder at the time of the initial air charge.

Piston Accumulator

The piston accumulator serves the same purpose and operates by the same principles as do the diaphragm and bladder accumulators. As shown in Figure 1-12, the unit consists of a cylinder and piston assembly with ports on each end. Fluid pressure from the system enters the left port, forcing the piston down against the initial air charge in the right chamber of the cylinder. A high-pressure air valve is located at the right port for charging the unit. A drilled passage from the fluid side of the piston to the outside of the piston provides lubrication between the cylinder walls and the piston.

Piston Accumulator
Figure 1-12. Piston Accumulator.


Hydraulic motors are installed in hydraulic systems to use hydraulic pressure in obtaining powered rotation. A hydraulic motor does just the opposite of what a power-driven pump does. A pump receives rotative force from an engine or other driving unit and converts it into hydraulic pressure. A hydraulic motor receives hydraulic fluid pressure and converts it into rotative force.

Figure 1-13 shows a typical hydraulic motor. The two main ports through which fluid pressure is received and return fluid is discharged are marked A and B, respectively. The motor has a cylinder block-and-piston assembly in which the bores and pistons are in axial arrangement, the same as in a hydraulic pump. Hydraulic motors can be instantly started, stopped, or reversed under any degree of load; they can be stalled by overload without damage. The direction of rotation of a hydraulic motor can be changed by reversing the flow of fluid into the ports of the motor.

Typical Hydraulic Motor
Figure 1-13. Typical Hydraulic Motor.


The basic components of any hydraulic system are reservoirs, filters, and pumps (hand or power-driven). The reservoir holds the fluid supply for the system and helps cool the fluid. Filters are used to ensure that no contamination reaches the components in a hydraulic system. The pleated micronic filter is the most common.

The pump converts mechanical energy to fluid flow. The most common power-driven pump is the piston pump. In all but the simplest hydraulic systems, variable-delivery pumps are used. A variable-delivery pump delivers only the amount of fluid demanded by the system. This is accomplished through the use of a compensator.

Depending on the type of aircraft, hydraulic accumulators and hydraulic motors can also be found in the system. Accumulators are used primarily to supply pressure for starting auxiliary power units and emergency hydraulic pressure. Hydraulic motors perform a variety of functions, including raising and lowering cargo doors, operating rescue hoists, and positioning wing flaps.


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