Aviation Lubrication Techniques
Lubricants are used to reduce friction and wear, whether it's in an aviation engine or the wheel bearing on a car.
Other major functions of a lubricant include cleaning, cooling and sealing, in addition to helping fight corrosion and rust in the engine.
Airplanes that are used infrequently especially need the corrosion and rust protection that good aviation lubricants can provide. Unused aircraft have a high potential for rust and corrosion, among other downtime problems. The more frequently and consistently an airplane is flown, the easier it is to properly maintain and lubricate.
Air-cooled aircraft engines rely on their oil for cooling far more than water-cooled automotive engines. Automotive oil typically accounts for about 40 percent of the engine's cooling capacity. In aviation engines, the oil must carry off a greater percentage of the engine's heat.
Oil is a heat-transfer medium which flows through the crankcase and oil coolers, and dissipates the heat from moving parts, thus constantly cooling engine bearings and piston rings.
Without the cooling oil film on a cylinder wall, the rings wouldn't have a good heat transfer path. This can lead to melting, galling, or scarring problems. Oil also cools the valve springs and the whole valve train.
What about the job we think of first when we think of oil — lubrication?
Lubricating properties are among the most important physical characteristics of aviation oil. Proper lubrication requires a strong enough and thick enough oil film between moving parts to keep friction and wear to a minimum.
Oil properties can include boundary or mixed film, dynamic, hydrodynamic, and elastohydrodynamic forms.
Boundary or mixed film lubrication is found in the upper cylinder area in the outer boundary of an aircraft engine. This is the most remote engine area to lubricate because the oil rings scrape most of the oil film off the cylinder walls before it reaches the upper cylinder. However, there must be a residual amount of lubrication in the upper cylinder to protect the engine on startup. Also, if an engine has been sitting idle for a month, some lifters have been pressed against cam faces and loaded under maximum spring pressure. Most of the oil has been squeezed out of that junction. When the engine is fired up, it takes a while to get oil to all those surfaces again. So, for that crucial moment, you need good boundary or mixed film strength at those critical boundary areas. Oil film retention is not as critical on startup in cam and crank journal areas.
Dynamic lubrication is produced through the pressure generated by an oil pump and this pressure provides an adequate flow of oil to the lubrication system. Hydrodynamic lubrication is like water skiing — it provides a smooth surface for any moving part to ride on and prevents any direct contact between moving parts.
Hydrodynamic lubrication is full-film lubrication that keeps moving parts from contacting one another.
In true hydrodynamic lubrication, as with water skiing, contact pressure is much lower and is spread over a large surface area. A constant supply of oil is required between the parts for hydro-dynamic lubrication.
When everything is operating properly in an aircraft engine, there is a constant lubricating film between any parts that might rub together. Any wear that the lubricant flow itself could cause is so slight that it would take several lifetimes to wear out a component — like a river wearing away the rocks.
Other major functions of a lubricant include cleaning, cooling and sealing, in addition to helping fight corrosion and rust in the engine.
Airplanes that are used infrequently especially need the corrosion and rust protection that good aviation lubricants can provide. Unused aircraft have a high potential for rust and corrosion, among other downtime problems. The more frequently and consistently an airplane is flown, the easier it is to properly maintain and lubricate.
How do aviation lubricants keep an engine cool?
Air-cooled aircraft engines rely on their oil for cooling far more than water-cooled automotive engines. Automotive oil typically accounts for about 40 percent of the engine's cooling capacity. In aviation engines, the oil must carry off a greater percentage of the engine's heat.
Oil is a heat-transfer medium which flows through the crankcase and oil coolers, and dissipates the heat from moving parts, thus constantly cooling engine bearings and piston rings.
Without the cooling oil film on a cylinder wall, the rings wouldn't have a good heat transfer path. This can lead to melting, galling, or scarring problems. Oil also cools the valve springs and the whole valve train.
What about the job we think of first when we think of oil — lubrication?
Lubricating properties are among the most important physical characteristics of aviation oil. Proper lubrication requires a strong enough and thick enough oil film between moving parts to keep friction and wear to a minimum.
Oil properties can include boundary or mixed film, dynamic, hydrodynamic, and elastohydrodynamic forms.
Boundary or mixed film lubrication is found in the upper cylinder area in the outer boundary of an aircraft engine. This is the most remote engine area to lubricate because the oil rings scrape most of the oil film off the cylinder walls before it reaches the upper cylinder. However, there must be a residual amount of lubrication in the upper cylinder to protect the engine on startup. Also, if an engine has been sitting idle for a month, some lifters have been pressed against cam faces and loaded under maximum spring pressure. Most of the oil has been squeezed out of that junction. When the engine is fired up, it takes a while to get oil to all those surfaces again. So, for that crucial moment, you need good boundary or mixed film strength at those critical boundary areas. Oil film retention is not as critical on startup in cam and crank journal areas.
Dynamic lubrication is produced through the pressure generated by an oil pump and this pressure provides an adequate flow of oil to the lubrication system. Hydrodynamic lubrication is like water skiing — it provides a smooth surface for any moving part to ride on and prevents any direct contact between moving parts.
Hydrodynamic lubrication is full-film lubrication that keeps moving parts from contacting one another.
In true hydrodynamic lubrication, as with water skiing, contact pressure is much lower and is spread over a large surface area. A constant supply of oil is required between the parts for hydro-dynamic lubrication.
When everything is operating properly in an aircraft engine, there is a constant lubricating film between any parts that might rub together. Any wear that the lubricant flow itself could cause is so slight that it would take several lifetimes to wear out a component — like a river wearing away the rocks.
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