PART ONE. FUNDAMENTALS OF POWER PLANTS

CHAPTER 1

BASIC REQUIREMENTS

Aircraft power plants must meet exacting requirements for dependability and endurance. Many difficult engineering problems have been overcome in an effort to satisfy these requirements with further advances being made each day. Requirements imposed on aircraft power plants in an effort to obtain engines suitable for aircraft include--

• Reliability.
• Durability.
• Compactness.
• Low weight per horsepower.
• High specific power output.
• Reasonable cost.
• High thermal efficiency.
• Freedom from vibration.
• Ease of maintenance.
• Operating flexibility.

DEFINITION OF TERMS

Reliability

Reliability is the most important fundamental power plant requirement. In the air each working part, no matter how small is important. Only by careful attention to the smallest detail can aircraft power plant manufacturers and mechanics assure power plant reliability.

Durability

Durability is the measure of reliable engine life. The durability realized by an engine depends largely on the type or condition of operation. Intelligent application of operation and maintenance procedures results in greatly improved power plant durability.

Compactness

Compactness is essential to power plant design in order to lower parasitic drag and to attain higher speeds.

The overall form an engine takes is determined to a high degree by the compactness required. The degree of compactness that may be achieved is limited by the physical requirements of the engine. For example, compactness is limited on radial air-cooled engines duet to the frontal area required for sufficient cooling of engine cylinders.

Low Weight per Horsepower

Minimum weight per horsepower (HP) is a primary requirement in aeronautics. The weight of a power plant must be kept as low as possible. This allows the aircraft to carry a large useful load with a satisfactory margin of safety in proportion to gross weight. The larger modern aircraft reciprocating engines have attained a horsepower-to-weight ratio of 1 horsepower to 1 pound weight. Gas turbine engines currently used by the Army have a greater horsepower-to-weight ratio. A good example is the T-55-L-712, which develops 4500 shaft horsepower (SHP) and weighs 750 pounds (dry) (6.0 HP per pound).

High Specific Power Output

Power output is based on engine size, RPM, and weight for the fuel-air mixture. Size and RPM are limited in the reciprocating engine. Therefore, an increase in the effective working pressure in the cylinders is one of the most valuable ways to increase the specific power output. Greater pressure increases are possible by supercharging the engine (comprising the mixture before it enters the cylinders). The limiting factors in increasing cylinder pressure are resistance of the fuel to detonation and the maximum allowable cylinder pressure.

Reasonable Cost

Despite perfection of design and quality of workmanship, no power plant will be desirable if it is too costly in a competitive market. A primary factor determining the usefulness of an engine is its cost. Because of the raw materials and the great number of man-hours involved, complex designs drive up the cost. The most satisfactory design is generally the simplest that will meet requirements.

High Thermal Efficiency

Thermal efficiency is a measure of the losses suffered in converting heat energy in the fuel into mechanical work; it is the ratio of the heat developed into useful work to the heating value of the fuel. High thermal efficiency, therefore, means high fuel economy-something of great importance in aircraft engines. The less fuel required for a military mission, the greater the military load that can be carried and the lower the fuel cost.

Freedom From Vibration

A power plant that is free from vibration is important in the light, somewhat flexible aircraft structure since severe engine vibration will in some cases reduce the life of certain structural parts. The need for freedom from vibration is met usually by using a large number of cylinders to offset the vibration torque delivered by the individual cylinders. Counterweights are installed on crankshafts to balance rotating masses. These are usually hinged to provide dynamic damping of vibration which results from power impulses and to counteract undesirable torsional or twisting vibration. Also, flexible engine mount isolators are used to permit certain movements of the power plant that are harmful to aircraft structures.

Ease of Maintenance

The requirement of ease of maintenance is especially important to military operations in the field. Simplicity of design and use of standard parts, when possible, assist in keeping maintenance at a low level.

Operating Flexibility

Flexibility is the ability of a power plant to run smoothly and provide the desired performance at all speeds from idling to maximum power output. The wide range of operating requirements demanded of aircraft engines presents difficulties rarely encountered in other power plant fields. In addition to the requirement of unfailing reliability, the engine must operate in widely varying positions, altitudes, and atmospheric conditions.

FUELS

Requirements

An engine fuel must be tailored to an engine and vice versa since there must be enough quantities of fuel available to the engine. Some significant properties of aviation fuels are discussed below.

Heat Energy Content or Net Heating Value. The energy content or heating value of a fuel is expressed in heat units (British thermal units [BTUs]). A fuel satisfactory for aircraft engines must have a high heat energy content per unit weight. A high heat energy content causes the weight of fuel carried to be lower than a low heat energy content. Then more of the load-carrying capacity is available for the payload Aviation gasoline and JP fuels are very desirable from this standpoint. The heat energy content for aviation gasoline is about 18,700 BTUs/pound, and for JP fuels about 18,200 BTUs/pound. The various alcohols, which have maximum energy content of about 12,000 BTUs/pound, do possess some other desirable characteristics as an internal combustion engine fuel.

Volatility. A volatile liquid is one capable of readily changing from a liquid to a vapor when heated or when contacting a gas into which it can evaporate. Since liquid fuels must be in a vaporous state to burn volatility is an important property to consider when choosing a suitable fuel for an aircraft engine. Volatility determines the starting accelerating vapor-locking and distribution characteristics of the fuel. Gasoline and JP fuels are very satisfactory because they can be blended during the refining process to give the desired characteristics. Because of the nature of constant pressure combustion in gas turbine engines a highly volatile fuel is not necessary. JP fuels are of rather low volatility while aviation gasoline is highly volatile. Comparing a highly volatile fuel like aviation gasoline to a less volatile one like JP fuel the following effects become apparent. The highly volatile fuel--

• Starts easier in cold temperatures.
• Has a slightly better combustion efficiency.
• Leaves less deposit in the combustion chamber and on the turbine blades.
• Is a greater fire hazard.
• Creates a greater danger of vapor lock of the fuel system.
• Has high evaporation losses through the breather of the fuel tank at high altitudes.

NOTE: The last two difficulties are practically nonexistent with fuels having low volatility.

Stability. The fuels used in aircraft engines must be stable. Because aviation fuels are sometimes stored for long periods, they must not deposit sediment. The gums that are normally formed are insoluble in gasoline and JP fuels and may cause restrictions in fuel strainers and liners. Aviation fuel must also retain its original properties during storage.

Purity. Aviation fuel must be free from water, dirt, and sulfur. Small amounts of water will not usually cause any difficulty because water can be removed from the fuel system by draining. Large amounts of this impurity, however, can cause complete engine failure. It is very important that corrosive sulfur be eliminated from fuel. The sulfur content of fuel may form corrosive acids when brought in contact with the water vapor formed in the combustion process.

Flash Point. The flash point is the lowest temperature at which fuel will vaporize enough to form a combustible mixture of fuel vapor and air above the fuel. It is found by heating a quantity of fuel in a special container while passing a flame above the liquid to ignite the vapor. A distinct hash of flame occurs when the flash point temperature has been reached.

Fire Point. The fire point is the temperature which must be reached before enough vapors will rise to produce a continuous flame above the liquid fuel. It is obtained in much the same manner as the flash point.

Reid Vapor Pressure. Reid vapor pressure is the approximate vapor pressure exerted by a fuel when heated to 100%. This is important because it is used to determine when a fuel will create a vapor lock.

Specific Gravity. Specific gravity is the ratio of the density (weight) of a substance (fuel) compared to that of an equal amount of water at 60^o F. Specific gravity is expressed in terms of degrees API. Pure water has a specific gravity of 10. Liquids heavier than water have a number less than 10. Liquids lighter than water have a number greater than 10. An example is JP-4, whose specific gravity in degrees API is 57. The American Petroleum Institute (API) has chosen pure water by which to measure the specific gravity of fuels.

NOTE: Both flash and fire points give a relative measure of the safety properties of fuel a high flash point denotes that a high temperature must be reached before dangerous handling conditions are encountered. The minimum flash point permitted in a fuel is usually written into the specifications.

Grades

Turbine fuels are high-quality fuels covering the general heavy gasoline and kerosene boding range. They do not contain dyes or tetraethyl lead.

One of the major differences between the wideboiling and kerosene types is the fuel volatility. JP-4 fuels have a wider boding range with an initial boiling point considerably below that of kerosene. As a group these fuels have lower specific gravities than kerosene types. Wide-boil-range fuels have Reid vapor pressures of 2 to 3 pounds and flash points below room temperature. Kerosene-type fuels have Reid vapor pressures of less than 0.5 pound and flash points higher than l00^o F (38^o C). Wide-boiling-range fuels generally have lower freezing points than kerosene fuels.

The fuel authorized for Army aircraft gas turbine engines is JP-4. The letters "JP" stand for jet propulsion; the number 4 indicates fuel grade.

Military specification MIL-T-5624 covers JP-4, JP-5, and JP-8 fuels. Jet A, Jet Al, and Jet B are commercial fuels which conform to the American Society for Tinting Materials specification ASTM-D-1655.

Jet B is a JP-4 type fuel; its freezing point is -56° F (-49^o C) instead of -72^o F (-58^o C) for JP-4.

JP-5, Jet 4 and Jet A-1 are kerosene-type fuels. ASTM Jet A and A-1 differ primarily in their fuel freezing points. Jet A is considered suitable down to fuel temperatures of -36^o F (-38^o C); Jet A-1, to -54^o F (48^o C).

JP-4 is a fuel consisting of approximately 65 percent gasoline and 35 percent light petroleum distillate, with rigidly specified properties. JP-4 is currently the Army standard fuel for turbine engines.

JP-5 is a specially refined kerosene having a minimum flash point of 140^o F and a freezing point of -51^o F (-46° C).

JP-8 is a specially refined kerosene with a minimum flash point of 110^o F and a freezing point of -54° F (-48° C). This fuel is being classified as a total replacement fuel for all of NATO. It will replace all fuels currently used in military equipment from generators to tanks to aircraft and even to trucks. This classification will ease logistics in combat. Having only one fuel for all equipment prevents accidentally mixing or using the wrong fuels. To date testing of JP-8 is proceeding well with the total single-fuel concept on its way to full fielding.

JP fuels vary from water white to light yellow; color coding, however, does not apply to these fuels.

Additives in JP fuels include oxidation and corrosion inhibitors, metal deactivators, and icing inhibitors. Icing inhibitors also function as biocides to kill microbes in aircraft fuel systems.

Should mixing of JP fuels become necessary, there is no need to drain the aircraft fuel system before adding the new fuel. Due to the different specific gravities of these fuels, mixing them will affect the turbine engine's performance. Be sure to consult appropriate technical manuals for additional information and procedures.

When changing to a fuel with a different specific gravity, externally adjusted fuel controls and fuel flow dividers on some engines may require retrimming or readjustment for optimum performance.

COMBUSTION

Mixing fuel with air and burning it would seem to be a very simple process, but this apparent simplicity is deceptive. Problems encountered are with distribution knock, ignition timing, and so forth. In an internal combustion engine, the combustion process is the rather rapid reaction between fuel and oxygen. This process liberates the potential energy contained in the fuel supplied to the engine. In a gas turbine engine atmospheric air is taken in and compressed; fuel is then burned in the compressed air, which then expands through a turbine that drives a compressor.

The combustion problem would not be so great if weight and space were not so important in aircraft gas turbine engines. Without such limitation the air supply for the compressors could be divided. A portion suitable to the desired power output could be burned at approximately the chemically correct fuel-air ratio in a low-velocity combustion chamber. Design of the combustion chamber is such that less than a third of the total volume of air entering the chamber is permitted to mix with the fuel. The excess air bypasses the fuel nozzles and is used to cool the hot surfaces and to mix with and cool the burned gases before they enter the turbines.

Temperature Limitations

Gas turbine engines produce work in proportion to the amount of heat released internally. Most of this heat is obtained by burning fuel although some heat originates when air is compressed in the compressor. Low fuel-air ratio is required to keep the temperature of the gases delivered to the turbine down to a value which the turbine wheel can tolerate. With present materials of construction, the highly stressed blades or buckets of the turbine wheel cannot stand a temperature of more than about 1500^o F (815^o C); therefore, the gases at entrance to the turbine vanes cannot exceed about 1600^o F (898^o C) for more than brief periods.

Fuel-Air Ratio

The fuel-air ratio is so low in gas turbine engines that if fuel and air were uniformly mixed, the mixture would not ignite. Complete combustion of both fuel and air is obtained with isooctane at a fuel-air ratio of 0.066. This is the theoretical value which would produce complete combustion if given sufficient time to reach equilibrium. In practice, complete combustion of either fuel or air requires an excess of one or the other. Thus, if all of the air is to be burned excess fuel must be present and a fuel-air ratio of about 0.080 is required. Control of power output is largely determined by means of fuel-air ratio. Increasing fuel-air ratio increases the quantity of air and the temperature at which it is discharged at the exhaust jet pipe.

Rich-Mixture Blowout

Combustion efficiency in a well-designed combustion chamber using the most favorable fuel may be as high as 98 percent at sea level. On the other hand, it may fall to as low as 40 percent at extremely high altitudes with a badly designed combustion chamber and unsuitable fuel As combustion efficiency is reduced, a point is reached when the turbine does not develop enough power to drive the compressor. Increasing the fuel supply in order to maintain or increase engine speed may not result in increased engine RPM, and the unburned fuel may extinguish the flame. This is known as rich-mixture blowout.

Lean-Mixture Die-Out

In contrast to rich-mixture blowout, lean-mixture die-out occurs when the mixture is too lean to bum under conditions of efficient combustion. When combustion efficiency is very low, the mixture may be ignited from an external source (such as an igniter plug). It will then either be extinguished when the spark ceases or may bum so slowly that the flame is carried out through the turbine. Lean-mixture die-out can also occur when the fuel supply is reduced in order to decrease engine speed.

Rich mixture blowout and lean mixture die-out have been virtually eliminated through the refinement of fuel delivery systems.

LUBRICATION

Lubrication is a very important part of power plant operation. An engine allowed to operate without lubrications certain to fail. Lubrication not only combats friction but also acts as a cooling agent.

WARNING Never mix reciprocating engine oils and gas turbine engine oils; they are not compatible. Mixing them causes engine failure.

The primary purpose of a lubricant is to reduce friction between moving parts. Because liquid lubricants (oils) can be circulated readily, they are used universally in aircraft engines. In theory, fluid lubrication is based on actual separation of the surfaces so that no metal-to-metal contact occurs. As long as the oil film remains unbroken, metallic fiction is replaced by the internal fluid friction of the lubricant. Under ideal conditions friction and wear are held to a minimum. In addition to reducing friction, the oil film acts as a cushion between metal parts. This cushioning effect is particularly important for such parts as reciprocating engine crankshaft and connecting rods, which are subject to shock loading. As oil circulates through the engine, it absorbs heat from the parts. Pistons and cylinder walls in reciprocating engines especially depend on oil for cooling. Oil also aids in forming a seal between the piston and cylinder wall to prevent gas leaks from the combustion chamber. Oils also reduce abrasive wear by picking up foreign particles and carrying them to a filter to be removed.

Friction Reduction

Lubricating oil decreases friction by preventing metal-to-metal contact at bearing points throughout the engine. Separating mating surfaces of moving parts by a thin film of oil changes dry or solid friction to fluid friction. The result is less heat generated in the moving parts and decreased wear on the parts.

Cushioning Effect

Lubricating oil cushions bearing surfaces by absorbing the shock between them.

Cooling

It has been noted that reducing friction results in less heat being generated. Also, as oil is circulated through bearings and splashed on various engine parts, it absorbs a great amount of heat. Lubrication is particularly important in reciprocating engines to cool the piston and cylinder. An efficient lubrication system will absorb as much as 10 percent of the total heat content of fuel consumed by the engine. By carrying away this heat, the oil flow reduces operating temperatures of internal parts not directly cooled by the engine cooling system.

Sealing Action

Oil helps seal mating surfaces in the engine, and the film of oil on various surfaces is an effective pressure seal. In reciprocating engines the oil film between the cylinder wall and piston and piston rings is important in retaining the high gas pressure in the cylinder.

Cleaning Effect

Oil cleans the engine by picking up carbon and other foreign particles as it passes through and around engine parts. It carries these particles through the system to a strainer where they are filtered from the oil.

LUBRICATING OIL REQUIREMENTS

The conditions which the engine operates under determine the requirements for lubricating oil. Conditions like temperature, contact pressure, and type and rate of motion vary so much that one lubricant cannot provide ideal lubrication for all components. Using a lubricating oil with all the desirable properties in degrees will provide satisfactory results. TB 55-9150-200-24 specifies engine oils for use in Army aircraft. Some desirable lubricating oil qualities are--

• Viscosity.
• Antifriction ability.
• Cooling ability.
• Chemical stability.

Viscosity

The degree of resistance of an oil flow at a specified temperature indicates its viscosity. An oil that flows slowly is described as a viscous oil or an oil of high viscosity. An oil that flows readily is said to possess a low viscosity. The viscosity of all oils is affected by temperature. As the temperature increases, oil becomes thinner. The rate at which an oil resists viscosity changes through a given temperature range is called its viscosity index The viscosity of aircraft engine oil is fairly high because of high operating temperatures high bearing pressures, and relatively large clearances inside an aircraft engine. Since aircraft engines are also subjected to a wide range of temperatures, an oil with a high viscosity index is required.

Antifriction

The theory of fluid lubrication is based on the actual separation of metallic surfaces by an oil film. Lubricants should have high antifriction characteristics to reduce frictional resistance of the moving parts and high antiwear properties to resist the wearing action that occurs during engine operation.

Cooling Ability

Lubricating oil should have maximum cooling ability in order to absorb as much heat as possible from all lubricated surfaces.

Chemical Stability

The extreme operating conditions and high dollar value of aircraft engines make it necessary to use lubricating oil of the very best quality. The following chemical transformations can occur in a lubricating oil and make it unfit for service.

Acidity. Acidity in oils is dangerous chiefly when high temperatures and moisture are present as is the case in aircraft engines. The results of a high acid content are corrosion of metal and the formation of sludges, emulsions, and deposits in the oil system.

Oxidation. All lubricating oils tend to oxidize when in contact with air. The compounds formed by oxidation are undesirable and harmful since they are generally of a gummy or acid character.

Sulfur. Sulfur may occur in lubricating oil as free sulfur or as sulfonates. Free sulfur may be present through careless or improper refining or by actual addition. Free sulfur is corrosive in nature and impairs the stability of the oil. The presence of sulfonates indicates overtreatment with sulfuric acid or inadequate washing of the oil to free it of chemicals during the refining process. Sulfonates are strong emulsifying agents that tend to promote sludge formation in the oil system. Lubricating oils containing high percentages of sulfur oxidize more easily than those with low sulfur content.

Carbon Residue. Petroleum lubricating oils are complex mixtures of hydrocarbons that vary widely in their physical and chemical properties. Owing to these difference some oils may vaporize under atmospheric conditions without leaving any appreciable residues. Other oils leave a nonvolatile carbon residue upon vaporization. This carbon residue is the result of a partial breakdown of the oil by heat, which is caused by destructive distillation of the oil without air entering into the reaction. Many parts in the engine operate at a temperature high enough to cause this reaction and to form carbon deposits. These deposits are undesirable as they may restrict passages. In reciprocating engines they may cause sticking piston rings and sticking valves.

Moisture. Corrosion of bearing metals is almost always due to moisture in the oil. Although it is possible for corrosion to occur from acidity, it is unlikely unless moisture is also present. Therefore, to prevent corrosion, it is important to eliminate moisture from the oil as much as possible. This is especially true at high temperatures because they increase the rate of corrosion.

Because of the accumulation of these harmful substances common practice is to drain the entire lubrication system at regular intervals and refill it with new oil. The time between oil changes varies with each make and model aircraft and engine combination.

WARNING When handling oil used in gas turbine engines, do not allow oil to remain on skin any longer than necessary. It contains a toxic additive that is readily absorbed through skin.

GAS TURBINE ENGINE LUBRICANTS

There are many requirements for turbine engine lubricating oils; but because of the small number of moving parts and the complete absence of reciprocating motion, lubrication problems are less complex in the turbine engine. This, together with the use of ball and roller bearings, requires a less viscous lubricant. The turboprop engine, while using essentially the same type of oil as the turbojet, must use a higher-viscosity oil because of the higher bearing pressures introduced by the highly loaded propeller reduction gearing.

Gas turbine engine oil must have high viscosity for good load-carrying ability but must also have viscosity low enough to provide good flow ability. It must also be of low volatility to prevent loss by evaporation at the high altitudes at which the engine operates. In addition, the oil should not foam and should be essentially nondestructive to natural or synthetic rubber seals in the lubricating system. Also, with high-speed antifriction bearings, the formation of carbons or varnishes must be held to a minimum.

The many requirements for lubricating oils are met in the synthetic oils developed specifically for turbine engines. Synthetic oil has two principal advantages over petroleum oil: it tends to deposit less lacquer and coke and evaporates less at high temperatures. Its principal disadvantage is that it tends to blister or remove paint wherever it is spilled. Painted surfaces should be wiped clean with petroleum solvent after spills.

Oil-change intervals for turbine engines vary widely from model to model. They depend on the severity of oil temperature conditions imposed by the specific airframe installation and engine configuration. Follow the applicable manufacturer's instructions.

Synthetic oil for turbine engines is usually supplied in sealed l-quart or l-gallon metal cans. Although this type of container was chosen to minimize contamination, it is often necessary to filter the oil to remove metal slivers, can sealants, and so forth, which may occur when opening the can.

Some oil grades used in turbojet engines may contain oxidation preventives, load-carrying additives and sub-stances that lower the pour point, in addition to synthetic chemical-base materials.

BEARINGS

In Army aircraft bearings are found throughout the power train system from the engine to the rotor or propeller. The failure of anyone of these bearings would place the entire aircraft in jeopardy. It is crucial that they be properly serviced and maintained. In fact, bearings are considered so important that a major air raid was conducted at considerable sacrifice during World War II in an attempt to eliminate one of Germany's principal bearing manufacturing centers. If the Allies could have destroyed Germany's capacity to produce antifriction bearings, its entire aviation effort would have come to an immediate standstill. Today, our economy could not function without bearings.

Bearings have the following functions. They--

• Support the load on a shaft. The load may be a wheel, pulley, or gear, or it may be the turning shaft itself.
• Reduce friction created by turning. This is accomplished both by design and by lubrication and is one of the most important functions of bearings.
• Reduce friction created by thrust. A specially designed bearing is required for this purpose.
• Hold a shaft in rigid alignment. A high-speed rotating shaft has a tendency to "whip" unless adequately supported by bearings.
• Provide a place for adjustment.

Bearings are classified into two broad categories:

• Friction- also commonly known as a plain or babbitt type; it makes a sliding contact with the shaft.
• Antifriction- contains rollers or balls; it makes a rolling contact with the shaft.

The term "antifriction" has long been used to differentiate rolling and sliding bearings. The word is gradually being dropped in deference to the word "rolling" to describe ball and roller bearings. The term "rolling bearings" will be used in this manual to describe all bearings consisting of ball or roller elements that roll between concentric inner and outer rings. The term "plain or journal bearings" will be used to describe two-piece bearings where the two rotating surfaces are sliding with respect to each other.

Rolling Bearings

A brief review of history reveals the long gradual development of rolling bearing. They were developed to reduce friction, thereby increasing work output while reducing energy input. Rolling-type bearings changed sliding friction into rolling motion with a greatly reduced friction level.

Assyrians and Babylonians as far back as 1100 BC used round logs as rollers to move huge monuments and stones. With the rollers placed under the load, it could more easily be pulled. As the rollers came out of the rear of the load, they were carried around to the front and placed in the oncoming path of the load

The wheel was an extremely important development; however, it was based on sliding friction. The use of lubricants did reduce friction and increase load and life expectancy of the wheel. But it was not until the axle rested on balls or rollers that sliding friction was finally changed to rolling motion and the fiction level significantly reduced.

Modern rolling bearing had their origin with the great inventor, Leonardo da Vinci, about 1500 AD. Many of his drawings show the use of balls and rollers and foreshadow such modem technology as surface finish, raceway grooves, and conical pivots.

Rolling bearings are also classified into two types: ball and roller.

Ball Bearings. The ball bearing is one of the most common used in aircraft. With relatively minor variations it can be adapted to many different uses. It creates the least amount of friction of any common bearing because the ball itself is the best antifriction rolling device known. The ball maintains point contact with the surface it rolls on and reduces friction to a minimum. It is, therefore, best suited to high-speed applications.

Roller Bearing. The roller bearing makes use of a cylindrical-shaped roller between the fiction surfaces.

Since it is a cylinder, it will make line contact rather than point contact. It is therefore more suited to heavy loads because the weight is distributed over a larger contact area.

Construction Features. Refer to Figure 1-1. The rolling elements in rolling bearings are provided with both an inner track and an outer track on which to roll. These tracks are known as "races." The races form a precision, hardened, and true surface for the balls or rollers to ride in. The balls or rollers are held together and spaced evenly around the bearing by means of a cage or separator. Seals are used on some bearings to keep out dirt and to keep grease in.

Dimensional interchangeability does not necessarily indicate functional interchangeability. Therefore, some bearings may be suited for thrust or axial loads while others are not. Bearing design (size and number of balls, depth and type of groove, width and thickness of races, construction of the separators) will determine the load and speed for which the bearing can be used. Bearings made to take both radial and thrust loads will be increased on one side of the outer race and will usually be stamped "thrust."

Plain Bearings

Plain bearings are classified as split type or solid type.

Split Type. The lower end of an engine connecting rod is a good example of a split bearing. It may be installed and used on a shaft in locations that preclude use of other bearing types. Another example of split bearings is the main bearing mounts supporting the engine crankshaft.

Solid Type. An engine connecting rod also furnishes a good example of a solid friction bearing the piston pin bearing or bushing. Bearings of this type can be used only where it is possible to slip them over the end of the shaft on which they run.

Construction Features. Refer to Figure 1-2. A basic requirement for the plain bearing is that it and the shaft be made of dissimilar metals. A steel bearing surface could never be used with a steel shaft. The bearing metal is always softer than the steel shaft, yet it must be hard enough to provide adequate heat transfer and possess good wear qualities. The metal used in bearing is known as babbitt.

A typical friction bearing contains babbitt metal inserts in the bearing housing. A bearing insert consists of a steel shell on which a layer of babbitt has been applied The shell is actually two half shells that when placed in a split-type bearing housing produce a unit which is both efficient and easily maintained. Bearing inserts can be mass-produced to very close tolerances and require little or no fitting during assembly. The bearing is renewed by merely taking the housing apart, replacing the inserts, and bolting it up again.

Inserts on aviation engines are often of the tri-metal variety. The steel shell will have a layer of silver on its inner surface and a layer of babbitt on top of the silver. This bearing is durable and provides excellent heat transfer. It is typically used for crankshaft and rod end bearing (Figure 1-3).

Bearing inserts must always be secured in the housing; they must never turn with the shaft. Split-type inserts will have a tang on each half which will fit into a notch in the housing. Solid-type inserts (bushings) are pressed into the housing. Either method will preclude the chance of turning in the housing. Inserts that must absorb thrust or axial loads are designed with flanges along the sides to take this side load. Bearings without this side flange must never be subjected to thrust or axial loads.

Teflon-Lined Bearings

A new type of bearing finding increasing use is the Teflon-lined bearing. It offers high reliability and easy maintenance. It is self-lubricating, chemically inert, and shock-resistant; and it has a low coefficient of friction. It is designed as a spherical or as a journal bearing and also as a rod end bearing. Refer to Figures 1-4 and 1-5.

Bearing Loads

The two types of bearing loads are--

• Radial. When a wheel turns, it produces centrifugal force. When this force is 90^o from the shaft, it is known as radial loading.
• Axial/Thrust. When the force or load on a bearing is in a direction parallel to the shaft, it is known as axial/thrust loading.

NOTE: Thrust loads are the same as axial loads. Whenever the term "thrust load" is used throughout this manual, it will also refer to axial loads.

Many times a bearing is subject to a combination of both radial and thrust leak; for example, the engine crankshaft main bearings. The throws on the crankshaft produce radial load while the pull of the propeller produces thrust load. On high-horsepower engines the thrust load produced by the propeller is so great that the prop shaft could not turn unless this load (and the resulting friction) were absorbed by a special thrust bearing in the engine front section.