quite small, it is apparent that the reliability and BYPASS RELIEF efficiency of the entire system depends upon ade- VALVE (50 PSI) quate filtering.
Filters may be located within the reservoir, in the pressure line, in the return line, or in any other location where the designer of the system IN -→ OUT decides that they are needed to safeguard the hydraulic system against impurities.
There are many models and styles of filters.
Their position in the aircraft and design requirements determine their shape and size.
Most filters used in modern aircraft are of the inline type. The inline filter assembly is comprised of three basic units: head assembly, bowl, - BODY and element. The head assembly is that part which is secured to the aircraft structure and connecting lines. Within the head there is a bypass FILTER ELEMENT valve which routes the hydraulic fluid directly from the inlet to the outlet port if the filter element becomes clogged with foreign matter. The bowl is the housing which holds the element to the filter head and is that part which is removed when element removal is required. FILTER The element may be either a micronic, porous BOWL metal, or magnetic type. The micronic element is made of a specially treated paper and is normally thrown away when removed. The porous metal FIGURE 8-3. Hydraulic filter, micronic type.
and magnetic filter elements are designed to be cleaned by various methods and replaced in the system.
Micronic Type Filters A typical micronic type filter is shown in figure
8-3. This filter utilizes an element made of specially treated paper which is formed in vertical convolutions (wrinkles). An internal spring holds 100 MICRONS HUMAN HAIR IS ABOUT the elements in shape. DIAMETER The micronic element is designed to prevent the • passage of solids greater than 10 microns (0.000394 40- MICRON IO-MICRON 5- MICRON inch) in size (figure 8-4). In the event that the PARTICLE PARTICLE PARTICLE filter element becomes clogged, the spring loaded relief valve in the filter head will bypass the fluid after a differential pressure of 50 p.s.i. has been 25,400 MICRONS • 1 INCH built up.
Hydraulic fluid enters the filter through the inlet port in the filter body and flows around the element inside the bowl. Filtering takes place as the fluid passes through the element into the hollow core, leaving the foreign material on the outside of the element.
Maintenance of Filters Maintenance of filters is relatively easy. It mainly involves cleaning the filter and element or cleaning the filter and replacing the element.
FIGURE 8-4. Enlargement of small particles.
Filters using the micronic-type element should have the element replaced periodically according to applicable instructions. Since reservoir filters are of the micronic type, they must also be periodically changed or cleaned. Filters using other than 314
the micronic-type element, cleaning the filter and element is usually all that is necessary. However, the element should be inspected very closely to insure that it is completely undamaged. The methods and materials used in cleaning all filters are too numerous to mention. Consult the manufacturer's instructions for this information.
Some hydraulic filters have been equipped with an indicator pin that will visually indicate a clogged element. When this pin protrudes from the filter housing, the element should be removed and cleaned; also, the fluid downstream of the filter should be checked for contamination and flushed if required. All remaining filters should be checked for contamination and cleaned (if required) to determine the cause of contamination.
BASIC HYDRAULIC SYSTEM Regardless of its function and design, every hydraulic system has a minimum number of basic components in addition to a means through which the fluid is transmitted.
Hand Pump System Figure 8-5 shows a basic hydraulic system.
The first of the basic components, the reservoir, stores the supply of hydraulic fluid for operation of the system. It replenishes the system fluid when needed, provides room for thermal expansion, and in some systems provides a means for bleeding air from the system.
A pump is necessary to create a flow of fluid.
The pump shown in figure 8-5 is hand operated;
however, aircraft systems are, in most instances equipped with engine-driven or electric motordriven pumps.
The selector valve is used to direct the flow of fluid. These valves are normally actuated by solenoids or manually operated, either directly or indirectly through use of mechanical linkage. An actuating cylinder converts fluid pressure into useful work by linear or reciprocating mechanical motion, whereas a motor converts fluid pressure into useful work by rotary mechanical motion.
The flow of hydraulic fluid can be traced from the reservoir through the pump to the selector valve in figure 8-5. With the selector valve in the position shown, the hydraulic fluid flows through the selector valve to the right-hand end of the actuating cylinder. Fluid pressure then forces the piston to the left, and at the same time the fluid which is on the left side of the piston (figure 8-5) is forced out, up through the selector valve, and back to the reservoir through the return line.
When the selector valve is moved to the opposite position, the fluid from the pump flows to the left Reservoir Pressure line Hand pump Selector valve in "down" position Selector valve in "up" position
- Working lines Retum line OI Actuating unit FreURE 8-5. Basic hydraulic system with hand pump.
side of the actuating cylinder, thus reversing the process. Movement of the piston can be stopped at any time by moving the selector valve to neutral.
In this position, all four ports are closed and pressure is trapped in both working lines.
Power Driven Pump System Figure 8-6 shows a basic system with the addition of a power-driven pump and filter, pressure regulator, accumulator, pressure gage, reliet valve, and two check valves. The function of each of these components is described in the following paragraphs.
The filter removes foreign particles from the hydraulic fluid, preventing dust, grit, or other undesirable matter from entering the system.
The pressure regulator unloads or relieves the power-driven pump when the desired pressure in the system is reached. Thus, it is often referred to as an unloading valve. When one of the actuating units is being operated and pressure in the line between the pump and selector valve builds up to the desired point, a valve in the pressure regulator automatically opens and fluid is bypassed back to the reservoir. This bypass line is shown in figure 8-6 leading from the pressure regulator to the return line.
Many hydraulic systems do not use a pressure regulator, but have other means of unloading the 315
pump and maintaining the desired pressure in the system. These methods are described in this chapter.
The accumulator (figure 8-6) serves a twofold purpose: (1) It acts as a cushion or shock absorber by maintaining an even pressure in the system, and
(2) It stores enough fluid under pressure to provide for emergency operation of certain actuating units. Accumulators are designed with a compressed air chamber which is separated from the fuid by a flexible diaphragm or movable piston.
The pressure gage (figure 8-6) indicates the amount of hydraulic pressure in the system.
The relief valve is a safety valve installed in the system to bypass fluid through the valve back to the reservoir in case excessive pressure is built up in the system. Air The check valves allow the flow of fluid in one Fluid direction only. Check valves are installed at various points in the lines of all aircraft hydraulic systems. In figure 8-4, one check valve prevents power-pump pressure from entering the hand-pump line; the other prevents hand-pump pressure from being directed to the accumulator.
The units of a typical hydraulic system used most commonly are discussed in detail in the following paragraphs. Not all models or types are included, but examples of typical components are used in all cases.
RESERVOIRS There is a tendency to envision a reservoir as an individual component; however, this is not always true. There are two types of reservoirs and they are:
(1) In-Line this type has its own housing, is complete within itself, and is connected 1. Reservoir 7. Hand pump with other components in a system by tub- 2. Power pump 8. Pressure gage ing or hose.
(2) Integral-this type has no housing of its 3. Filter 9. Relief valve own but is merely a space set aside within 4. Pressure regulator 10. Selector valve some major component to hold a supply of 5: Accumulator 11. Actuating unit operational fluid. A familiar example of this type is the reserve fluid space found 6. Check valves within most automobile brake master cylinders. FIGURE 8-6. Basic hydraulic system with power pump and other hydraulic components.
In an in-line reservoir (figure 8-7), a space is provided in the reservoir, above the normal level of the fluid, for fluid expansion and the escape of a dipstick or a glass sight gage by which fluid level entrapped air. Reservoirs are never intentionally can be conveniently and accurately checked.
filled to the top with fluid. Most reservoirs are designed so the rim of the filler neck is somewhat below the top of the reservoir to prevent over filling during servicing. Most reservoirs are equipped with Reservoirs are either vented to the atmosphere or closed to the atmosphere and pressurized. In vented reservoirs, atmospheric pressure and gravity are the forces which cause fluid to flow from the reservoir 316
Filler neck, Connection for of hydraulic system, by using an air pressure regucap, and fastener vent line or lator.
pressurizing line Reservoirs that are pressurized with hyrdaulic fluid (figure 8-8) are constructed somewhat differ- Normal fluid level ently from reservoirs pressurized with air. A flexible, coated-fabric bag, called a "bellowfram" or dia- Glass sight phragm, is attached to the reservoir head. The bag gage hangs inside a metal barrel to form a fluid con- Finger tainer. The bottom of the diaphragm rests on a strainer large piston. Attached to the large piston is an indicator rod. The other end of the indicator rod is machined to form a small piston which is exposed to fluid pressure from the hydraulic pump. This pressure forces the small piston upward, causing the larger piston to move upward, thus producing a for return Connection reservoir pressure of approximately 30 to 32 p.s.i.
pipe line in normal operation. If the internal pressure should Fin exceed 46 p.s.l., the reservoir relief valve will open allowing fluid to escape through the drilled head of the valve retainer. This type of reservoir must be completely filled with hydraulic fluid and have all Connection the air bled from it.
for emergency Connection system pump for main system pump Reservoir Components FIGURE 8-7. Reservoir, "in-line". Baffles and/or fins are incorporated in most reservoirs to keep the fluid within the reservoir from into the pump intake. On many aircraft, atmos- - Bleed valve pheric pressure is the principal force causing fluid aircraft, atmospheric pressure becomes too low to to flow to the pump intake. However, for some valve Bleed supply the pump with adequate fluid, and the reser- Fluid pressure voirs must be pressurized. from system Filler cap There are several methods of pressurizing a reservoir. Some systems use air pressure directly from the aircraft cabin pressurization system; or from the engine compressor in the case of turbine-powered aircraft. Another method used is an aspirator Bleeder hole or venturi-tee. In other systems an additional hy- Rod draulic pump is installed in the supply line at the Drain hole reservoir outlet to supply fluid under pressure to the main hydraulic pump. Retainer Pressurizing with air is accomplished by forcing air into the reservoir above the level of the fluid. In Bellowfram most cases, the initial source of the air pressure is (fabric side) Piston the aircraft engine from which it is bled. Usually, air coming directly from the engine is at a pressure Barrel Drain plug Drain stud of approximately 100 p.s.i. This pressure is reduced FICURE 8-8.. Hydraulic reservoir pressurized with hydrauto between 5 and 15 p.s.i., depending upon the type lic fluid.
having random movement such as vortexing (swir]- ing) and surging. These conditions can cause fluid to foam and air to enter the pump along with the fluid.
Many reservoirs incorporate strainers in the filler neck to prevent the entry of foreign matter during servicing. These strainers are made of fine mesh screening and are usually referred to as finger strainers because of their shape. Finger strainers should never be removed or punctured as a means of speeding up the pouring of fluid into the reservoir.
Some reservoirs incorporate filter elements. They may be used either to filter air before it enters the reservoir or to filter fluid before it leaves the reservoir. A vent filter element, when used, is located in the upper part of the reservoir, above the fluid level. A fuid filter element, when used, is located at or near the bottom of the reservoir. Fluid, as it returns to the reservoir, surrounds the filter element and fows through the wall of the element. This leaves any fluid contaminant on the outside of the filter element.
Reservoirs with filter elements incorporate a bypass valve normally held closed by a spring. The bypass valve ensures that the pump will not be starved of fluid if the filter element becomes clogged. A clogged filter causes a partial vacuum to develop and the spring-loaded bypass valve opens.
The filter element most commonly used in reservoirs is the micronic type. These filter elements are made of treated cellulose formed into accordion-like pleats. The pleats expose the fluid to the maximum amount of filter surface within a given amount of space. These micronic elements are capable of removing small particles of contamination.
Some aircraft have emergency hydraulic systems that take over if main systems fail. In many such systems, the pumps of both systems obtain fluid from a single reservoir. Under such circumstances a supply of fluid for the emergency pump is ensured by drawing the hydraulic fluid from the bottom of the reservoir. The main system draws its fluid through a standpipe located at a higher level. With this arrangement, adequate fluid is left for operation of the emergency system should the main system's fluid supply become depleted.
Double Action Hand Pumps The double-action hydraulic hand pump is used in some older aircraft and in a few newer systems Inlet Outlet port port Piston O-ring -O-ring Check valve Piston rod Operating Outlet handle
• Inlet port FIcURE 8-9. Double-action hand pump.
as a backup unit. Double-action hand pumps produce fluid flow and pressure on each stroke of the handle.
The double-action hand pump (figure 8-9) consists essentially of a housing which has a cylinder bore and two ports, a piston, two spring-loaded check valves, and an operating handle. An O-ring on the piston seals against leakage between the two chambers of the piston cylinder bore. An O-ring in a groove in the end of the pump housing seals against leakage between the piston rod and housing.
Power-Driven Pumps Many of the power-driven hydraulic pumps of current aircraft are of variable-delivery, compensator-controlled type. There are some constantdelivery pumps in use. Principles of operation are the same for both types of pumps. Because of its relative simplicity and ease of understanding, the constant-delivery pump is used to describe the principles of operation of power-driven pumps.
Constant-Delivery Pump A constant-delivery pump, regardless of pump
r.p.m., forces a fixed or unvarying quantity of fluid through the outlet port during each revolution of the pump. Constant-delivery pumps are sometimes called constant-volume or fixed-delivery pumps.
They deliver a fixed quantity of fluid per revolution, regardless of the pressure demands. Since the constant-delivery pump provides a fixed quantity of fluid during each revolution of the pump, the quantity of fluid delivered per minute will depend upon 318
pump r.p.m. When a constant-delivery pump is used in a hydraulic system in which the pressure must be kept at a constant value, a pressure regulator is required.
Variable-Delivery Pump A variable-delivery pump has a fluid output that is varied to meet the pressure demands of the system by varying its fluid output. The pump output is changed automatically by a pump compensator within the pump.
Pumping Mechanisms Various types of pumping mechanisms are used in hydraulic pumps, such as gears, gerotors, vanes, and pistons. The piston-type mechanism is commonly used in power-driven pumps because of its durability and capability to develop high pressure.
In 3,000 p.s.i. hydraulic systems, piston-type pumps are nearly always used.
Gear Type Pump A gear-type power pump (figure 8-10) consists of two meshed gears that revolve in a housing. The driving gear is driven by the aircraft engine or some other power unit. The driven gear meshes with, and is driven by, the driving gear. Clearance between the teeth as they mesh, and between the teeth and the housing, is very small. The inlet port of the pump is connected to the reservoir, and the outlet port is connected to the pressure line. When the driving gear turns in a counterclockwise direction, as shown in figure 8-10, it turns the driven gear in a clockwise direction. As the gear teeth pass the inlet port, fluid is trapped between the gear teeth and the housing, and is then carried around the housing to the outlet port.
Gerotor Type Pump A gerotor-type power pump (figure 8-11) consists essentially of a housing containing an eccentric-shaped stationary liner, an internal gear rotor having five wide teeth of short height, a spur driving gear having four narrow teeth, and a pump cover which contains two crescent-shaped openings.
One opening extends into an inlet port, and the other extends into an outlet port. The pump cover as shown in figure 8-11 has its mating face turned up to clearly show the crescent-shaped openings.
When the cover is turned over and properly installed on the pump housing, it will have its inlet port on the left and the outlet port on the right.
During the operation of the pump, the gears turn To outlet port Driving gear Driven gear To inlet port FICURE 8-10. Gear-type power pump.
Cover Spur driving gear Liner
• Outlet port Inlet port Internal gear rotor FIcURE 8-11. Gerotor-type power pump.
clockwise. As the pockets on the left side of the pump move from a lowermost position toward a topmost position, the pockets increase in size (figure 8-11) resulting in the production of a partial vacuum within these pockets. As the pockets open at the inlet port, fluid is drawn into them. As these same pockets (now full of fluid) rotate over to the right side of the pump, moving from the topmost position toward the lowermost position, they decrease in size. This results in the fluid being expelled from the pockets through the outlet port.
Vane Type Pump The vane-type power pump (figure 8-12) consists of a housing containing four vanes (blades), a hollow steel rotor with slots for the vanes, and a cou- 319
pling to turn the rotor. The rotor is positioned off Flanged center within the sleeve, The vanes, which are mounting base mounted in the slots in the rotor, together with the • Bypass port for rotor, divide the bore of the sleeve into four sec. Pump cooling and lubrication tions. As the rotor turns, each section, in turn, drive Inlet shaft passes one point where its volume is at a minimum, (exposed port and another point where its volume is at a maxi- end) mum. The volume gradually increases from mini- Drain and gradually decreases from maximum to minimum mum to maximum during one-half of a revolution, port Cooling fins • Outlet port during the second half of the revolution. As the volume of a given section is increasing, that section FIcURE 8-13. Typical piston-type hydraulic pump.
is connected to the pump inlet port through a slot the increase in volume of the section, fluid is drawn in the sleeve. Since a partial vacuum is produced by into the section through the pump inlet port and the the pump drive shaft by a drive coupling (figure Torque from the driving unit is transmitted to slot in the sleeve. As the rotor turns through the second half of the revolution, and the volume of the given section is decreasing, fluid is displaced out of the section, through the slot in the sleeve, through set of male splines on both ends. The splines on one the splines on the other end engage with female 8-14). The drive coupling is a short shaft with a end engage with female splines in a driving gear;
the outlet port, and out of the pump.
Sleeve Shear section Vanes of drive coupling Rotor FIGURE 8-14. Pump drive coupling.
Inlet port Outlet port Piston Piston FIGURE 8-12. Vane-type power pump.
Piston that are applicable to all piston-type hydraulic Piston Type Pump The common features of design and operation Cylinder block Piston pumps are described in the following paragraphs.
Piston-type power-driven pumps have flanged mounting bases for the purpose of mounting the pumps on the accessory drive cases of aircraft engines and transmissions. A pump drive shaft, which turns the mechanism, extends through the pump housing slightly beyond the mounting base (figure 8-13). FIcURF 8-15. Axial-piston pump mechanism.
splines in the pump drive shaft. Pump drive couplings are designed to serve as safety devices. The shear section of the drive coupling, located midway between the two sets of splines, is smaller in diameter than the splines. If the pump becomes unusually hard to turn or becomes jammed, this section will shear, preventing damage to the pump or driving unit.
The basic pumping mechanism of piston-type pumps (figure 8-15) consists of a multiple-bore cylinder block, a piston for each bore, and a valving arrangement for each bore. The purpose of the valving arrangement is to let fluid into and out of the bores as the pump operates. The cylinder bores lie parallel to and symmetrically around the pump axis. The term "axial-piston pump" is often used in
- Valve plate Cylinder bearing pin Drive shaft plate Universal link Piston A Seal
• Cylinder block
• Valve plate shaft Coupling 00000000 Radial bearing
• Inlet port Radial bearing Thrust bearing - Outlet port Piston B Drain port FIcURE 8-16. Typical angular-type pump.
referring to pumps of this arrangement. All aircraft axial-piston pumps have an odd number of pistons (5, 7, 9, 11, etc.).
Angular Type Piston Pump A typical angular-type pump is shown in figure
8-16. The angular housing of the pump causes a corresponding angle to exist between the cylinder block and the drive shaft plate to which the pistons are attached. It is this angular configuration of the pump that causes the pistons to stroke as the pump shaft is turned.
When the pump is operated, all parts within the pump (except the outer races of the bearings which support the drive shaft, the cylinder bearing pin on which the cylinder block turns, and the oil seal) turn together as a rotating group. Because of the angle between the drive shaft and the cylinder block, at one point of rotation of the rotating group a minimum distance exists between the top of the cylinder block and the upper face of the drive shaft plate. At a point of rotation 180° away, the distance between the top of the cylinder block and the upper face of the drive shaft plate is at a maximum.
At any given moment of operation, three of the pistons will be moving away from the top face of the cylinder block, producing a partial vacuum in the bores in which these pistons operate. Fluid wil be drawn into these bores at this time.
Movement of the pistons when drawing in and expelling fluid is overlapping in nature, and results in a practically non-pulsating discharge of fluid from the pump.
Cam Type Pump Cam-type pumps (figure 8-17) utilize a cam to cause stroking of the pistons. There are two variations of cam-type pumps; one in which the cam turns and the cylinder block is stationary, and the other in which the cam is stationary and the cylinder block rotates.
As an example of the manner in which the pistons of a cam-type pump are caused to stroke, the operation of a rotating cam-type pump is described Oil seal - — Internal splines Check valve • Intake — Drive shaft and cam Drainage - recess Outlet Drain Springs Piston Pressure port annulus FIGURE 8-17. Typical cam-type pump.
as follows: As the cam turns, its high and low Pressure Relief Valves points pass alternately and, in turn, under each A pressure relief valve is used to limit the amount piston. When the rising ramp of the cam passes of pressure being exerted on a confined liquid. This under a piston, it pushes the piston further into its is necessary to prevent failure of components or bore, causing fluid to be expelled from the bore. rupture of hydraulic lines under excessive pres- When the falling ramp of the cam comes under a sures. The pressure relief valve is, in effect, a sys piston, the piston's return spring extends the piston sem safety valve.
outward of its bore. This causes fluid to be drawn The design of pressure relief valves incorporates into the bore. Because the movement of the pistons adjustable spring-loaded valves. They are installed when drawing in and expelling fluid is overlapping in such a manner as to discharge fluid from the in nature, the discharge of fluid from the cam-type pressure line into a reservoir return line when the pump is practically non-pulsating. pressure exceeds the predetermined maximum for Each bore has a check valve that opens to allow which the valve is adjusted. Various makes and fluid to be expelled from the bore by movement of designs of pressure relief valves are in use, but, in the piston. These valves close during the inlet general, they all employ a spring-loaded valving strokes of the pistons. Because of this, inlet fluid device operated by hydraulic pressure and spring can be drawn into the bores only through the cen- tension. Pressure relief valves are adjusted by intral inlet passage. creasing or decreasing the tension on the spring to PRESSURE REGULATION determine the pressure required to open the valve.
Hydraulic pressure must be regulated in order to use it to perform the desired tasks. Pressure regulating systems will always use three elemental devices; a pressure relief valve, a pressure regulator and a pressure gage.
Two general forms of pressure relief valves, the two-port and the four-port, are illustrated in figure
8-18.
Pressure relief valves may be classified as to their type of construction or uses in the system. However, Pressure adjusting Pressure adjustment screw Adjusting screw cap Compression spring Compression Return spring port Return port-
• Return!
port Ball seat sleeve Pressure port -
- Pressure port Pressure port - Ball stud Four-port pressure relief valve Two-port pressure relief valve FIcURE 8-18. Pressure relief valves.
the general purpose and operation of all pressure relief valves are the same. The basic difference in construction of pressure relief valves is in the design of the valving. The most common types of valve are:
(1) Ball type. In pressure relief valves with a ball-type valving device, the ball rests on a contoured seat. Pressure acting on the bottom of the ball pushes it off its seat, allowing the fluid to bypass.
(2) Sleeve type. In pressure relief valves with a sleeve-type valving device, the ball remains stationary and a sleeve-type seat is moved up by the fluid pressure. This al lows the fluid to bypass between the ball and the sliding sleeve-type seat.
(3) Poppet type. In pressure relief valves with a poppet-type valving device, a coneshaped poppet may have any of several design configurations; however, it is basically a cone and seat machined at matched angles to prevent leakage. As the pressure rises to its predetermined setting, the poppet is lifted off its seat, as in the ball-type device. This allows the fluid to pass through the opening created and out the return port.
Pressure relief valves cannot be used as pressure regulators in large hydraulic systems that depend upon engine-driven pumps for the primary source of pressure because the pump is constantly under load, and the energy expended in holding the pressure relief valve off its seat is changed into heat.
This heat is transferred to the fluid and in turn to the packing rings causing them to deteriorate rapidly. Pressure relief valves, however, may be used as pressure regulators in small, low-pressure systems or when the pump is electrically driven and is used intermittently. Pressure relief valves may be used as:
(1) System relief valve, The most common use of the pressure relief valve is as a safety device against the possible failure of a pump compensator or other pressure regulating device. All hydraulic systems which have hydraulic pumps incorporate pressure relief valves as safety devices.
(2) Thermal relief valve. The pressure relief valve is used to relieve excessive pressures that may exist due to thermal expansion of the fluid.
Pressure Regulators The term "pressure regulator" is applied to a device used in hydraulic systems that are pressurized by constant-delivery type pumps. One purpose of the pressure regulator is to manage the output of the pump to maintain system operating pressure within a predetermined range. The other purpose is to permit the pump to turn without resistance (termed unloading the pump) at times when pressure in the system is within normal operating range.
The pressure regulator is so located in the system that pump output can get into the system pressure circuit only by passing through the regulator. The combination of a constant-delivery type pump and the pressure regulator is virtually the equivalent of a compensator-controlled, variable-delivery type pump.
Pressure Gage The purpose of this gage is to measure the pressure, in the hydraulic system, used to operate hydraulic units on the aircraft. The gage uses a Bourdon tube and a mechanical arrangement to transmit the tube expansion to the indicator on the face of the gage. A vent in the bottom of the case maintains atmospheric pressure around the Bourdon tube. It also provides a drain for any accumulated moisture. There are several ranges of pressure used in hydraulic systems and gages are calibrated to match the system they accommodate.
Accumulator The accumulator is a steel sphere divided into two chambers by a synthetic rubber diaphragm.
The upper chamber contains fluid at system pressure, while the lower chamber is charged with air.
The function of an accumulator is to:
a. Dampen pressure surges in the hydraulic system caused by actuation of a unit and the effort of the pump to maintain pressure at a preset level.
b. Aid or supplement the power pump when several units are operating at once by supplying extra power from its "accumulated" or stored power.
c. Store power for the limited operation of a hydraulic unit when the pump is not operating.
d. Supply fluid under pressure to compensate for small internal or external (not desired) leaks which would cause the system to cycle continuously by action of the pressure switches continually "kicking in."
Diaphragm Accumulator Diaphragm type accumulators consist of two hollow half-ball metal sections fastened together at the centerline. One of these halves has a fitting for attaching the unit to the system; the other half is equipped with an air valve for charging the unit with compressed air. Mounted between the two halves is a synthetic rubber diaphragm which divides the tank into two compartments. A screen covers the outlet on the fluid side of the accumulator. This prevents a part of the diaphragm from being pushed up into the system pressure port and being damaged. This could happen whenever there is an air charge in the unit and no balancing fluid pressure. In some units, a metal disc attached to the center of the diaphragm is used in place of the screen. (See figure 8-19).
System pressure tainer plug. A round metal disc attached to the top of the bladder prevents air pressure from forcing the bladder out through the pressure port. As fluid pressure rises, it forces the bladder downward against the air charge, filling the upper chamber with fluid pressure. The broken lines in figure 8-20 show the approximate shape of the bladder when the accumulator is charged.
System pressure
- Diaphragm Air valve FIGURE 8-20. Bladder-type accumulator.
Air valve FIcURE 8-19. Diaphragm-type accumulator.
Bladder-Type Accumulators The bladder-type accumulator operates on the same principle as the diaphragm type. It serves the same purpose, but varies in construction. This unit consists of a one-piece metal sphere with a fuid pressure inlet at the top. There is an opening at the bottom for inserting the bladder. A large screw-type plug at the bottom of the accumulator retains the bladder and also seals the unit. The high-pressure air valve is also mounted in the re- Piston-Type Accumulators The piston type accumulator also serves the same purpose and operates much like the diaphragm and bladder accumulators. As shown in figure 8-21 this unit is a cylinder (B) and piston assembly (E) with openings on each end. System fluid pressure enters the top port (A), and forces the piston down against the air charge in the bottom chamber (D). A high-pressure air valve (C) is located at the bottom of the cylinder for servicing the unit. There are two rubber seals (represented by the black dots) which prevent leakage between the two chambers (D and G). A passage (F) is drilled from the fluid side of the piston to the space between the seals. This provides lubrication between the cylinder walls and the piston.