Heating unit Thermometer Liquid bath Oil Container Cork 60 c.c. - Reservoir FIGURE 8-1. Saybolt viscosimeter.
Chemical Stability Chemical stability is another property which is exceedingly important in selecting a hydraulic liquid. It is the liquid's ability to resist oxidation and deterioration for long periods. All liquids tend to undergo unfavorable chemical changes under severe operating conditions. This is the case, for example, when a system operates for a considerable period of time at high temperatures.
Excessive temperatures have a great effect on the life of a liquid. It should be noted that the temperature of the liquid in the reservoir of an operating hydraulic system does not always represent a true state of operating conditions. Localized hot spots occur on bearings, gear teeth, or at the point where liquid under pressure is forced through a small orifice. Continuous passage of a liquid through these points may produce local temperatures high enough to carbonize or sludge the liquid, yet the liquid in the reservoir may not indicate an excessively high temperature. Liquids with a high viscosity have a greater resistance to heat than light or low viscosity liquids which have been derived from the same source. The average hydraulic liquid has a low viscosity. Fortunately, there is a wide choice of liquids available for use within the viscosity range required of hydraulic liquids.
Liquids may break down if exposed to air, water.
salt, or other impurities, especially if they are in constant motion or subject to heat, Some metals, such as zinc, lead, brass, and copper, have an undesirable chemical reaction on certain liquids.
These chemical processes result in the formation of sludge, gums, and carbon or other deposits which clog openings, cause valves and pistons to stick or leak, and give poor lubrication to moving parts. As soon as small amounts of sludge or other deposits are formed, the rate of formation generally increases more rapidly. As they are formed, certain changes in the physical and chemical properties of the liquid take place. The liquid usually becomes darker in color, higher in viscosity, and acids are formed.
Flash Point Flash point is the temperature at which a liquid gives off vapor in sufficient quantity to ignite momentarily or flash when a flame is applied. A high flash point is desirable for hydraulic liquids because it indicates good resistance to combustion and a low degree of evaporation at normal temperatures.
Fire Point Fire point is the temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame. Like flash point, a high fire point is required of desirable hydraulic liquids.
TYPES OF HYDRAULIC FLUIDS To assure proper system operation and to avoid damage to non-metallic components of the hydraulic system, the correct fluid must be used.
When adding fluid to a system, use the type specified in the aircraft manufacturer's maintenance manual or on the instruction plate affixed to the reservoir or unit being serviced.
There are three types of hydraulic fluids currently being used in civil aircraft.
Vegetable Base Hydraulic Fluid Vegetable base hydraulic fluid (MIL-H-7644) is composed essentially of caster oil and alcohol:
It has a pungent alcoholic odor and is generally dyed blue. Although it has a similar composition to automotive type hydraulic fluid, it is not interchangeable. This fuid is used primarily in older type aircraft. Natural rubber seals are used with vegetable base hydraulic fluid. If it is contaminated with petroleum base or phosphate ester base fluids, the seals will swell, break down and block the system. This type fluid is flammable.
Mineral Base Hydraulic Fluid Mineral base hydraulic fluid (MIL-H-5606) is processed from petroleum. It has an odor similar to penetrating oil and is dyed red. Synthetic rubber seals are used with petroleum base fluids. Do not mix with vegetable base or phosphate ester base hydraulic fluids. This type fluid is flammable and turboprop aircraft.
operating factor.
Intermixing of Fluids Auids.
usually requires longer than just momentary exposure; and spills that are wiped up with soap and water do not harm most of these materials.
Paints which are Skydrol ® resistant include epoxies and polyurethanes. Today polyurethanes are the standard of the aircraft industry because of their ability to keep a bright, shiny finish for long periods of time and for the ease with which they can be removed.
Skydrol ® is a registered trademark of Monsanto Company. Skydrol ® fluid is compatible with natural fibers and with a number of synthetics, including nylon and polyester, which are used extensively in most aircraft.
Petroleum oil hydraulic system seals of neoprene or Buna-N are not compatible with Skydrol B and must be replaced with seals of butyl rubber or ethylene-propylene elastomers. These seals are readily available from any suppliers.
Health and Handling Skydrol ® fuid does not present any particular health hazard in its recommended use. Skydrol ® fluid has a very low order of toxicity when taken orally or applied to the skin in liquid form. It causes pain on contact with eye tissue, but animal studies and human experience indicate Skydrol ® fluid causes no permanent damage. First aid treatment for eye contact includes flushing the eyes immediately with large volumes of water and the application of any anesthetic eye solution. It pain persists, the individual should be referred to a physician.
In mist or fog form, Skydrol ® is quite irritating to nasal or respiratory passages and gen erally produces coughing and sneezing. Such irritation does not persist following cessation of exposure.
Silicone ointments, rubber gloves, and careful washing procedures should be utilized to avoid excessive repeated contact with Skydrol ® order to avoid solvent effect on skin.
Hydraulic Fluid Contamination Experience has shown that trouble in a hydraulic does not appre- system is inevitable whenever the liquid is allowed to become contaminated. The nature of the trouble, whether a simple malfunction or the complete destruction of a component, depends to some extent on the type of contaminant.
Two general contaminants are:
(1) Abrasives, including such particles as core sand, weld spatter, machining chips, and rust.
(2) Nonabrasives, including those resulting from oil oxidation, and soft particles worn or shredded from seals and other organic components.
Contamination Check Whenever it is suspected that a hydraulic system has become contaminated, or the system has been operated at temperatures in excess of the specified maximum, a check of the system should be made.
The filters in most hydraulic systems are designed to remove most foreign particles that are visible to the naked eye. Hydraulic liquid which appears clean to the naked eye may be contaminated to the point that it is unfit for use.
Thus, visual inspection of the hydraulic liquid does not determine the total amount of contamination in the system. Large particles of impurities in
- To vacuum pump the hydraulic system are indications that one or more components in the system are being subjected to excessive wear. Isolating the defective component requires a systematic process of elimination. Fluid returned to the reservoir may contain impurities from any part of the system. To determine which component is defective, liquid samples should be taken from the reservoir and various other locations in the system.
Samples should be taken in accordance with the applicable manufacturer's instructions for a particular hydraulic system. Some hydraulic systems are equipped with permanently installed bleed valves for taking liquid samples, whereas on other systems, lines must be disconnected to provide a place to take a sample. In either case, while the fluid is being taken, a small amount of pressure should be applied to the system. This ensures that the liquid will flow out of the sampling point and thus prevent Filter paper Gooch crucible Rubber crucible holder Filter flask Polyethylene washing bottle Fluid sample bottle Reference filter patch FICURE 8-2. Contamination test kit.
dirt from entering the hydraulic system. Some contamination test kits have hypodermic syringes for taking samples.
Various test procedures are used to determine the contamination level in hydraulic liquids. The filter patch test provides a reasonable idea of the condition of the fluid. This test consists basically of filtration of a sample of hydraulic system liquid through a special filter paper. This filter paper darkens in degree in relation to the amount of contamination present in the sample, and is compared to a series of standardized filter disks which, by degree of darkening, indicate the various contamination levels. The equipment provided with one type of contamination test kit is illustrated in hgure
8-2.
When using this type of contamination test kit, the liquid samples should be poured through the filter paper, and the test filter paper should be compared with the test patches supplied with the test kit. More expensive test kits have a microscope for making this comparison.
To check liquid for decomposition, pour new hydraulic liquid into a sample bottle of the same size and color as the bottle containing the liquid to be checked. Visually compare the color of the two bottles. Liquid which is decomposed will be darker in color.
At the same time the contamination check is made, it may be necessary to make a chemical test.
This test consists of a viscosity check, a moisture check, and a flash point check. However, since special equipment is required for these checks, the liquid samples must be sent to a laboratory, where a technician will perform the test.
(1) Maintain all tools and the work area (workbenches and test equipment) in a clean, dirt-free condition.
(2) A suitable container should always be provided to receive the hydraulic liquid that is spilled during component removal or disassembly procedures.
(3) Before disconnecting hydraulic lines or fittings, clean the affected area with dry cleaning solvent.
(4) All hydraulic lines and fittings should be capped or plugged immediately after disconnecting.
(5) Before assembly of any hydraulic components, wash all parts in an approved dry cleaning solvent.
(6) After cleaning the parts in the dry cleaning solution, dry the parts thoroughly and lubricate them with the recommended preservative or hydraulic liquid before assembly. Use only clean, lint-free cloths to wipe or dry the component parts.
(7) All seals and gaskets should be replaced during the re-assembly procedure. Use only those seals and gaskets recommended by the manufacturer.
(8) All parts should be connected with care to avoid stripping metal slivers from threaded areas. All fittings and lines should be installed and torqued in accordance with applicable technical instructions.
(9) All hydraulic servicing equipment should be kept clean and in good operating condition.
Contamination Control Filters provide adequate control of the contamination problem during all normal hydraulic system operations. Control of the size and amount of contamination entering the system from any other source is the responsibility of the people who service and maintain the equipment. Therefore, precautions should be taken to minimize contamination during maintenance, repair, and service operations.
Should the system become contaminated, the filter element should be removed and cleaned or replaced.
As an aid in controlling contamination, the following maintenance and servicing procedures should be followed at all times:
FILTERS A filter is a screening or straining device used to clean the hydraulic fluid, thus preventing foreign particles and contaminating substances from remaining in the system. If such objectionable material is not removed, it may cause the entire hydraulic system of the aircraft to tail through the breakdown or malfunctioning of a single unit of the system.
The hydraulic fluid holds in suspension tiny particles of metal that are deposited during the normal wear of selector valves, pumps, and other system components. Such minute particles of metal may injure the units and parts through which they pass if they are not removed by a filter. Since tolerances within the hydraulic system components are 313
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.