Bleed air Electrical actuation Anti-icing wing Off Bleed air system Isolation valve Isolation valve Anti-icing empennage Off On Mercury thermostat (closes light circuit on rising temperature) Air valve Temperature bulb Pneumatically-actuated, Thermal switch (opens solenoid-controlled valve valve control circuit (opens when solenoid is energized) on risíng temperature) FIGURE 7-12. Schematic of a typical theral anti-icing system.
bellows are located at strategic positions to absorb any distortion or expansion of the ducting which may occur due to temperature variations.
The joined sections of ducting are hermetically sealed by sealing rings. These seals are fitted into annular recesses in the duct joint faces. When installing a section of duct, make certain that the seal bears evenly against, and is compressed by, the adjacent joint's flange.
When specified, the ducts should be pressure tested at the pressure recommended by the manufacturer of the aircraft concerned. Pressure testing is particularly important with pressurized aircraft, since a leak in the ducting may result in the inability to maintain cabin pressure. However, pressure tests are more often made to detect defects in the duct which would permit the escape of heated air.
The rate of leakage at a given pressure should not exceed that recommended in the aircraft maintenance or service manual.
Air leaks can often be detected audibly, and are sometimes revealed by holes in the lagging or thermal insulation material. However, if difficulty arises in locating leaks, a soap and water solution may be used.
All ducting should be inspected for security, general condition, or distortion. Lagging or insulating blankets must be checked for security and must be free of flammable fluids such as oil or hydraulic
GROUND DEICING OF AIRCRAFT The presence of ice on an aircraft may be the result of direct precipitation, formation of frost on integral fuel tanks after prolonged fight at high altitude, or accumulations on the landing gear following taxiing through snow or slush.
Any deposits of ice, snow, or frost on the external surfaces of an aircraft may drastically affect its performance. This may be due to reduced aerodynamic lift and increased aerodynamic drag resulting from the disturbed airflow over the airfoil surfaces, or it may be due to the weight of the deposit over the whole aircraft. The operation of an aircraft may also be seriously affected by the freezing of moisture in controls, hinges, valves, microswitches, or by the ingestion of ice into the engine.
When aircraft are hangared to melt snow or frost, any melted snow or ice may freeze again if the aircraft is subsequently moved into subzero temperatures. Any measures taken to remove frozen deposits while the aircraft is on the ground must also prevent the possible re-freezing of the liquid.
Frost Removal Frost deposits can be removed by placing the aircraft in a warm hangar or by using a frost remover or deicing fluid. These fluids normally contain ethylene glycol and isopropyl alcohol and can be applied either by spray or by hand. It should be applied within 2 hrs. of fight.
Deicing fluids may adversely affect windows or the exterior finish of the aircraft. Therefore, only the type of fluid recommended by the aircraft manufacturer should be used.
Removing Ice and Snow Deposits Probably the most dificult deposit to deal with is deep, wet snow when ambient temperatures are slightly above the freezing point. This type of deposit should be removed with a brush or squeegee.
Use care to avoid damage to antennas, vents, stall warning devices, vortex generators, etc., which may be concealed by the snow.
Light, dry snow in subzero temperatures should be blown off whenever possible; the use of hot air is not recommended, since this would melt the snow, which would then freeze and require further treat- Moderate or heavy ice and residual snow deposits should be removed with a deicing fluid. No attempt should be made to remove ice deposits or break an ice bond by force.
After completion of deicing operations, inspect the aircraft to ensure that its condition is satisfactory for flight. All external surfaces should be examined for signs of residual snow or ice, particularly in the vicinity of control gaps and hinges.
Check the drain and pressure sensing ports for obstructions. When it becomes necessary to physically remove a layer of snow, all protrusions and vents should be examined for signs of damage.
Control surfaces should be moved to ascertain that they have full and free movement. The landing gear mechanism, doors, and bay, and wheel brakes should be inspected for snow or ice deposits and the operation of uplocks and microswitches checked.
Snow or ice can enter turbine engine intakes and 299
freeze in the compressor. If the compressor cannot be turned by hand for this reason, hot air should be blown through the engine until the rotating parts are free.
WINDSHIELD ICING CONTROL SYSTEMS In order to keep window areas free of ice, frost, etc., window anti-icing, deicing, defogging, and demisting systems are used. The systems vary according to the type of aircraft and its manufacturer.
Some windshields are built with double panels having a space between, which will allow the circulation of heated air between the surfaces to control icing and fogging. Others use windshield wipers and anti-icing fluid which is sprayed on.
One of the more common methods for controlling ice formation and fog on modern aircraft windows is the use of an electrical heating element built into the window. When this method is used with pressurized aircraft, a layer of tempered glass gives strength to withstand pressurization. A layer of transparent conductive material (stannic oxide) is the heating element and a layer of transparent vinyl plastic adds a nonshattering quality to the window.
The vinyl and glass plies (figure 7-13) are bonded by the application of pressure and heat. The bond is achieved without the use of a cement as vinyl has a natural affinity for glass. The conductive coating dissipates static electricity from the windshield in addition to providing the heating element.
- Tempered glass ply, 3 places Vinyl ply, 2 places Conductive coating on the inner face of the outer glass ply FIcURE 7-13. Section through a laminated windshield.
On some aircraft, thermal electric switches automatically turn the system on when the air temperature is low enough for icing or frosting to occur.
The system may stay on all the time during such temperatures, or on some aircraft it may operate with a pulsating on-and-off pattern. Thermal overheat switches automatically turn the systems off in case of an overheating condition which could damage the transparent area.
An electrically heated windshield system includes:
(1) Windshield autotransformers and heat control relays.
(2) Heat control toggle switches.
(3) Indicating lights.
(4) Windshield control units.
(5) Temperature-sensing elements (thermistors) laminated in the panel.
A typical system is shown in figure 7-14. The system receives power from the 115 v. a.c. buses through the windshield heat control circuit breakers. When the windshield heat control switch is set to "high," 115 v., 400 Hz a.c. is supplied to the left and right amplifiers in the windshield control unit.
The windshield heat control relay is energized, thereby applying 200 v., 400 Hz a.c. to the windshield heat autotransformers. These transformers provide 218 v. a.c. power to the windshield heating current bus bars through the windshield control unit relays. The sensing element in each windshield has a positive temperature coefficient of resistance and forms one leg of a bridge circuit. When the windshield temperature is above calibrated value, the sensing element will have a higher resistance value than that needed to balance the bridge. This decreases the flow of current through the amplifiers and the relays of the control unit are de-energized.
As the temperature of the windshield drops, the resistance value of the sensing elements also drops and the current through the amplifiers will again reach sufficient magnitude to operate the relays in the control unit, thus energizing the windshield heaters.
When the windshield heat control switch is set to "low," 115 v., 400 Hz a.c. is supplied to the left and right amplifiers in the windshield control unit and to the windshield heat autotransformers. In this condition, the transformers provide 121 v. a.c.
power to the windshield heating current bus bars through the windshield control unit relays. The sensing elements in the windshield operate in the same manner as described for high-heat operation to maintain proper windshield temperature control.
The temperature control unit contains two hermetically sealed relays and two three-stage electronic amplifiers. The unit is calibrated to maintain a windshield temperature of 40° - 49° C.
(105° - 120° F.). The sensing element in euch windshield panel has a positive temperature coeficient of resistance and forms one leg of a bridge which controls the flow of current in its associated ampli- 300
(238V) R1 Right windshield (228V) R2 heat transformer (218V) R3 Right windshield (200V) L2 heat Com LJ K2 PIOA Right windshield Windshield heat 115V 400~ Amplifier (right) control relay 1ỌV AC - Right windshield heat • Output R3
• Sensing element Input • 115V 400~ 2 115V R2 Left windshield heat CR2 115V 400~ Left windshield heat Pion Com LI 115V 400~ (200v) L2 (218V) R3 Left windshield heat transformer (228V) R2 Left windshield (238V) R1 Amplifier (left) 10V AC •
• Output Sensing element Input Windshield heat control High Windshield heat = 1150 O TA control switch CR2 Off 115V. 400~ Low FIGURE 7-14. Windshield. temperature control circuit.
fier. The final stage of the amplifier controls the hermetically sealed relay which provides a.c. power to the windshield heating current bus bars. When the windshield temperature is above calibrated value, the sensing elements will have a higher resistance value than that needed to balance the bridge.
This decreases the flow of current through the amplifiers and the relays of the control unit are de-energized. As the temperature of the windshield drops, the resistance value of the sensing elements also drops and the current through the amplifiers will again reach sufficient magnitude to operate the relays in the control unit, thus energizing the circuit.
There are several problems associated with electrically heated windshields. These include delamination, scratches, arcing, and discoloration.
Delamination (separation of the plies) although undesirable, is not harmful structurally provided it is within the limits established by the aircraft manufacturer, and is not in an area where it affects the optical qualities of the panel.
Arcing in a windshield panel usually indicates that there is a breakdown in the conductive coating.
Where chips or minute surface cracks are formed in the glass plies, simultaneous release of surface compression and internal tension stresses in the highly tempered glass can result in the edges of the crack and the conductive coating parting slightly. Arcing is produced where the current jumps this gap, particularly where these cracks are parallel to the window bus bars. Where arcing exists, there is invariably a certain amount of local overheating which, depending upon its severity and location, can cause further damage to the panel. Arcing in the vicinity of a temperature-sensing element is a particular problem since it could upset the heat control system.
Electrically heated windshields are transparent to directly transmitted light but they have a distinctive color when viewed by reflected light. This color will vary from light blue, yellow tints, and light pink depending upon the manufacturer of the window panel. Normally, discoloration is not a problem unless it affects the optical qualities.
Windshield scratches are more prevalent on the outer glass ply where the windshield wipers are indirectly the cause of this problem. Any grit trapped by a wiper blade can convert it into an extremely effective glass cutter when the wiper is set in motion. The best solution to scratches on the windshield is a preventive one; clean the windshield wiper blades as frequently as possible. Incidentally, windshield wipers should never be operated on a dry panel, since this increases the chances of damaging the surface.
Assuming that visibility is not adversely affected, scratches or nicks in the glass plies are allowed within the limitations set forth in the appropriate service or maintenance manuals. Attempting to improve visibility by polishing out nicks and scratches is not recommended. This is because of the unpredictable nature of the residual stress concentrations set up during the manufacture of tempered glass.
Tempered glass is stronger than ordinary annealed glass due to the compression stresses in the glass surface which have to be overcome before failure can occur from tension stresses in the core. Polishing away any appreciable surface layer can destroy this balance of internal stresses and can even result in immediate failure of the glass.
Determining the depth of scratches has always presented some difficulty. An optical micrometer can be used for this purpose. It is essentially a microscope supported on small legs rather than the more familiar solid base mounting. When focused on any particular spot, the focal length of the lens (distance from the lens to the object) can be read from a micrometer scale on the barrel of the instrument.
The depth of a scratch or fissure in a windshield panel, for example, can thus be determined by ob taining focal length readings to the surface of the glass and to the bottom of the scratch. The difference between these two readings gives the depth of the scratch. The optical micrometer can be used on flat, convex, and concave surfaces of panels whether they are installed on the aircraft or not.
Window Defrost System The window defrost system directs heated air from the cabin heating system (or from an auxiliary heater, depending on the aircraft) to the pilot's and copilot's windshield and side windows by means of a series of ducts and outlets. In warm weather when heated air is not needed for defrosting, the system can be used to defog the windows.
This is done by blowing ambient air on the windows using the blowers.
Windshield and Carburetor Alcohol Deicing Systems An alcohol deicing system is provided on some aircraft to remove ice from the windshield and the carburetor. Figure 7-15 illustrates a typical two-engine system in which three deicing pumps (one for each carburetor and one for the windshield) are used. Fluid from the alcohol supply tank is con- 302
trolled by a solenoid valve which is energized when any of the alcohol pumps are on. Alcohol flow from the solenoid valve is filtered and directed to the alcohol pumps and distributed through a system of plumbing lines to the carburetors and windshield.
Left carburetor Windshield alcohol alcohol pump pump Anti-ice fluid Right carburetor solenoid valve alcohol pump 300 50W 5A
d.c. bus FreURE 7-15. Carburetor and windshield deicing system.
Toggle switches control the operation of the carburetor alcohol pumps. When the switches are placed in the "on" position, the alcohol pumps are turned on and the solenoid-operated alcohol shutoff valve is opened. Operation of the windshield deicer pump and the solenoid-operated alcohol shutoff valve is controlled by a rheostat-type switch, located in the pilot's station. When the rheostat is moved away from the "off" position, the shutoff valve is opened and the alcohol pump will pump fluid to the windshield at the rate selected by the rheostat.
When the theostat is returned to the "off" position, the shutoff valve closes and the pump stops operating.
Pitot Tube Anti-Icing To prevent the formation of ice over the opening in the pitot tube, a built-in electric heating element is provided. A switch, located in the cockpit, controls power to the heater. Use caution when ground checking the pitot tube since the heater must not be operated for long periods unless the aircraft is in flight. Additional information concerning pitot tubes is found in Chapter 2 of this handbook, Aircraft Instrument Systems.
Heating elements should be checked for functioning by ensuring that the pitot head begins to warm up when power is applied. If an ammeter or loadmeter is installed in the circuit, the heater operation can be verified by noting the current consumption when the heater is turned on.
WATER AND TOILET DRAIN HEATERS Heaters are provided for toilet drain lines, water lines, drain masts, and waste water drains when they are located in an area that is subjected to feezing temperatures in flight. The types of heaters used are integrally heated hoses, ribbon, blanket, or patch heaters that wrap around the lines, and gasket heaters (see figure 7-16). Thermostats are provided in heater circuits where excessive heating is undesirable or to reduce power consumption. The heaters have a low voltage output and continuous operation will not cause overheating.
RAIN ELIMINATING SYSTEMS When rain forms on a windshield during flight, it becomes a hazard and must be eliminated. To provide a clear windshield, rain is eliminated by wiping it off or blowing it off. A third method of rain removal involves chemical rain repellants. Rain is blown from the windshield of some aircraft by air from jet nozzles located beneath the windshield. On other aircraft, windshield wipers are used to eliminate the rain. The windshield wipers of an aircraft accomplish the same function as those of an automobile. In each instance, rubber blades wipe across the windshield to remove rain and slushy ice.
Electrical Windshield Wiper Systems In an electrical windshield wiper system, the wiper blades are driven by an electric motor (s) which receive (s) power from the aircraft's electrical system. On some aircraft the pilot's and copilot's windshield wipers are operated by separate systems to ensure that clear vision is maintained through one of the windows should one system fail.
Figure 7-17 shows a typical electrical windshield wiper installation. An electrically operated wiper is installed on each windshield panel. Each wiper is driven by a motor-converter assembly. The converters change the rotary motion of the motor to reciprocating motion to operate the wiper arms. A shaft protruding from the assembly provides an attachment for the wiper arm.
• Blanket type heater Gasket heater Typical integrally heated hose Ribbon heater Heated drain mast Waterline heater patch type FIGURE 7-16. Typical water and drain line heaters.
The windshield wiper is controlled by setting the wiper control switch to the desired wiper speed.
When the "high" position is selected (figure 7-18), relays 1 and 2 are energized. With both relays energized, field 1 and field 2 are energized in parallel. The circuit is completed and the motors operate at an approximate speed of 250 strokes per minute.
When the "low" position is selected, relay 1 is energized. This causes field 1 and field 2 to be energized in series. The motor then operates at approximately 160 strokes per minute. Setting the switch to the "off" position, allows the relay contacts to return to their normal positions. However, the wiper motor will continue to run until the wiper arm reaches the "park" position. When both relays are open and the park switch is closed, the excitation to the motor is reversed. This causes the wiper to move off the lower edge of the windshield, opening the cam-operated park switch. This de-energizes the motor and releases the brake solenoid applying the brake. This ensures that the motor will not coast and re-close the park switch.
Hydraulic Windshield Wiper Systems Hydraulic windshield wipers are driven by pressure from the aircraft's main hydraulic system. Figure 7-19 shows the components making up a representative hydraulic windshield wiper system.
WASHER WIPER
PILOT'S OVERHEAD PANEL
- Wiper blade Wiper arm Motor converter
• Parked position FICURE 7-17. Electrical windshield wiper system.
The speed control valve is used to start, stop, and fow of fluid to the control unit, and therefore the control the operating speed of the windshield wip- speed of the windshield wipers.
ers. The speed control valve is a type of variable The control unit directs the flow of hydraulic restrictor. Turning the handle of this valve counter- fluid to the wiper actuators and returns fluid disclockwise increases the size of the fluid opening, the charged from the actuators to the main hydraulic 305
Field High Field 2 Windshield control module | Relay 1 Relay 2 High Low Left windshield wiper motor FIcURE 7-18. Windshield wiper circuit diagram.
Actuator of aero Speed control valve From main hydraulic Control Unit Retur line system FIcURE 7-19. Hydraulic windshield wiper schematic.
system. The control unit also alternates the direction of hydraulic fluid flow to each of the two wiper actuators. The wiper actuators convert hydraulic energy into reciprocating motion to drive the wiper arms back and forth.
Figure 7-20 shows the construction and the plumbing of the actuators. Notice that each actuator consists of a two-port housing, a piston rack, and a pinion gear. The teeth of the pinion gear mesh with those on the piston rack. Thus, whenever pressurized fluid enters the actuator and moves the piston rack, the pinion gear is rotated a fraction of a turn.
Since the pinion gear connects through a shaft to the wiper blade, the blade rotates through an arc.
Notice that one line from the control unit connects to port No. 1 of actuator A, while the other line connects to port No. 4 of actuator B. Notice, toc that a line connects ports No. 2 and No. 3 of th..
actuators.
Turning on the speed control valve allows fluid to flow from the main hydraulic system into the control unit, which directs pressure first into one line and then into the other. When line No. 1 is placed under pressure, fluid flows into port No. 1 and into the chamber at the left of actuator A. This drives the piston rack to the right, causing the pinion and wiper blade to rotate through a counterclockwise arc. As the piston rack moves to the right, it forces fluid in the right chamber of actuator A to move out port No. 2, through the connecting line to port No. 3 and into actuator B. This causes the piston rack in actuator B to move to the right, causing its pinion and wiper blade to rotate counterclockwise.
As the piston rack moves to the right, it forces fluid in the right chamber of actuator B to move out of 306
012N96 Actuator-A Actuator-B Ma Line 1 Line 2 Code Pressure Return FIGURE 7-20. Windshield wiper actuators.
port No. 4, into line No. 2, through the control unit, and into the hydraulic system return line.
When line No. 2 is pressurized by fluid from the control unit, the fow of fluid and the action of the actuators are reversed.
Pneumatic Rain Removal Systems Windshield wipers characteristically have two basic problem areas. One is the tendency of the slipstream aerodynamic forces to reduce the wiper blade loading pressure on the window, causing ineffective wiping or streaking. The other is in achieving fast enough wiper oscillation to keep up with high rain impingement rates during heavy rain falls. As a result, most aircraft wiper systems fail to provide satisfactory vision in heavy rain.
With the advent of turbine-powered aircraft, the pneumatic rain removal system became feasible.
This method uses high pressure, high temperature engine compressor bleed air which is blown across the windshields (figure 7-21). The air blast forms a barrier that prevents raindrops from striking the windshield surface.
Windshield Rain Repellant When water is poured onto clean glass, it spreads out evenly. Even when the glass is held at a steep angle or subjected to air velocity, the glass remains wetted by a thin film of water.
However, when glass is treated with certain chemicals, a transparent film is formed which causes the water to behave very much like mercury on glass. The water draws up into beads which cover only a portion of the glass and the area between beads is dry. The water is readily removed from the glass.
This principle lends itself quite naturally to removing rain from aircraft windshields. The high velocity slipstream continually removes the water beads, leaving a large part of the window dry.
Jet blast rain removal system -Fairing Plenum -Duct assembly From bleed air system FIcURE 7-21. Typical pneumatic rain removal system.
A rain repellant system permits application of the chemical repellant by a switch or push button in the cockpit. The proper amount of repellant is applied regardless of how long the switch is held. The repellant is marketed in pressurized disposable cans which screw into the aircraft system and provide the propelling force for application. Actuating the control switch opens an electrically-operated solenoid valve which allows repellant to flow to the discharge nozzles. The liquid repellant is squirted 307
onto the exterior of the windshield and uses the rain itself as the carrying agent to distribute the chemicals over the windshield surface.
The rain repellant system should not be operated on dry windows because heavy undiluted repellant will restrict window visibility. Should the system be operated inadvertently, do not operate the windshield wipers or rain clearing system as this tends to increase smearing. Also the rain repellant residues caused by application in dry weather or very light rain can cause staining or minor corrosion of the aircraft skin. To prevent this, any concentrated repellant or residue should be removed by a thorough fresh-water rinse at the earliest opportunity.
After application, the repellant film slowly deteriorates with continuing rain impingement. This makes periodic re-application necessary. The length of time between applications depends upon rain intensity, the type of repellant used, and whether windshield wipers are in use.
MAINTENANCE OF RAIN ELIMINATING SYSTEMS Windshield Wiper Systems Maintenance performed on windshield wiper systems consists of operational checks, adjustments, and troubleshooting.
An operational check should be performed whenever a system component is replaced or whenever the system is suspected of not working properly.
During the check, make sure that the windshield area covered by the wipers is free of foreign matter and is kept wet with water.
Adjustment of a windshield wiper system consists of adjusting the wiper blade tension, the angle at which the blade sweeps across the windshield, and proper parking of the wiper blades. Figure 7-22 illustrates the adjustment points on a typical wiper blade installation.
One adjustment is that of the tie rod length. The tie rod length adjustment nut is shown in figure
7-22. The tie rod connects the wiper blade holder to a pivot bolt next to the drive shaft. With the drive arm and the tie rod connected to the wiper blade holder, a parallelogram linkage is formed between the wiper blade holder and the wiper converter. This linkage permits the wiper blade to remain parallel to the windshield posts during its travel from one side of the windshield to the other.
Wiper blade Tie rod • Tie rod length adjustment nut Drive arm Pressure adjustment Pivot bolt nut Mounting plate FIcURE 7-22. Adjustment of windshield wiper components.
The length of the tie rod may be adjusted to vary the angle at which the wiper blade sweeps across the windshield.
Another adjustment is that which is required for proper parking of the windshield wiper blades.
When they are not operating, the wiper blades should move to a position where they will not interfere with vision. If the wipers do not park as they should, the cam which actuates the microswitch on the converter can be adjusted.
The other adjustment to be made is that of the windshield wiper spring tension. To make the adjustment, place a lightweight spring scale under the drive arm at its point of attachment to the wiper blade, and lift the scale up at a 90° angle to the drive, to a point at which the blade is just ready to leave the glass. (If tension is properly adjusted, the spring scale should indicate between 5 and 6 Ibs.)
If the scale reading does not fall within this limit, adjust the pressure adjustment nut shown in figure 7-22 until the proper tension is indicated on the scale.
Pneumatic (Jet Blast) Systems Maintenance of a jet blast system includes the replacement of defective components, the checking (by hand) of duct-to-valve connections for leakage, and an operational checkout.
CHAPTER 8
HYDRAULIC AND PNEUMATIC POWER SYSTEMS
AIRCRAFT HYDRAULIC SYSTEMS The word hydraulics is based on the Greek word for water, and originally meant the study of the physical behavior of water at rest and in motion.
Today the meaning has been expanded to include the physical behavior of all liquids, including hy- Hydraulic systems are not new to aviation. Early aircraft had hydraulic brake systems. As aircraft became more sophisticated newer systems with hydraulic power were developed.
Although some aircraft manufacturers make greater use of hydraulic systems than others, the hydraulic system of the average modern aircraft performs many functions. Among the units commonly operated by hydraulic systems are landing gear, wing flaps, speed and wheel brakes, and flight control surfaces.
Hydraulic systems have many advantages as a power source for operating various aircraft units.
Hydraulic systems combine the advantages of light weight, ease of installation, simplification of inspection, and minimum maintenance requirements. Hydraulic operations are also almost 100% efficient, with only a negligible loss due to fluid friction.
All hydraulic systems are essentially the same, regardless of their function. Regardless of application, each hydraulic system has a minimum number of components, and some type of hydraulic fluid.
HYDRAULIC FLUID Hydraulic system liquids are used primarily to transmit and distribute forces to various units to be actuated. Liquids are able to do this because they are almost incompressible. Pascal's Law states that pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part. Thus, if a number of passages exist in a system, pressure can be distributed through all of them by means of the liquid.
Manufacturers of hydraulic devices usually specify the type of liquid best suited for use with their equipment, in view of the working conditions, the service required, temperatures expected inside and outside the systems, pressures the liquid must withstand, the possibilities of corrosion, and other conditions that must be considered.
If incompressibility and fluidity were the only qualities required, any liquid not too thick might be used in a hydraulic system. But a satisfactory liquid for a particular installation must possess a number of other properties. Some of the properties and characteristics that must be considered when selecting a satisfactory liquid for a particular system are discussed in the following paragraphs.
Viscosity One of the most important properties of any hydraulic fluid is its viscosity. Viscosity is internal resistance to flow. A liquid such as gasoline flows easily (has a low viscosity) while a liquid such as tar flows slowly (has a high viscosity). Viscosity increases with temperature decreases.
A satisfactory liquid for a given hydraulic system must have enough body to give a good seal at pumps, valves, and pistons; but it must not be so thick that it offers resistance to flow, leading to power loss and higher operating temperatures.
These factors will add to the load and to excessive wear of parts. A fluid that is too thin will also lead to rapid wear of moving parts, or of parts which have heavy loads.
The viscosity of a liquid is measured with a viscosimeter or viscometer. There are several types, but the instrument most often used by engineers in the U.S. is the Saybolt universal viscosimeter (figure 8-1). This instrument measures the number of seconds it takes for a fixed quantity of liquid (60 cc. (cubic centimeters)) to flow through a small orifice of standard length and diameter at a specific temperature. This time of flow is taken in seconds, and the viscosity reading is expressed as SSU (seconds, Saybolt universal). For example, a certain liquid might have a viscosity of 80 SSU at 130° F.