Principles of Aircraft Engine Lubrication
By Harold Tucker
July / August 1998
At this year's Professional Aviation Maintenance Association's annual meeting which took place in Kansas City, Harold Tucker, Lubricants Technical Director for Phillips 66 Company offered a presentation to maintenance personnel from around the country on some of the basics of aviation engine oil. The following is an overview of that presentation:
What major functions do lubricants perform in aviation engines?
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.
What are the benefits of using a lubricant that cleans the engine?
All aviation oils clean. When we say an aviation oil cleans, we think of removing sludge, varnishes, and grunge accumulations in the oil pan, on plugs, or in the screen. However, when a lubricant keeps your airplane engine clean, it also means a clean ring belt area and better control of the combustion process. When those rings are able to move freely, your engine operates at higher efficiencies, has better ring seal, produces less blow-by, and consumes less oil.
A dirty ring belt restrains the movement of the rings within the grooves and they can't seal. This may create pressure between the ring face and the cylinder wall — leading to wear, scarring or scuffing.
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.
How does oil seal an aviation engine?
Aviation oil not only provides a seal between the rings and cylinder walls, but also helps seal the gasketed areas and the rubber or synthetic seals for the crankshaft. When oil washes around those areas, it helps retain a seal. Thus, aviation oil must be of a blend or formulation that is compatible with the seal materials so that the seal itself lasts longer.
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.
If that's true, why do engines wear out?
Your biggest problems are on surfaces where there is no oil. That usually happens after an engine has been sitting for a while.
You need the right viscosity and the right velocity between moving parts to keep oil where it needs to be. Think about what happens inside your engine whenever you do something like a cold start. If it's very cold when you fire up your engine, there is maximum velocity between metal parts and maximum oil viscosity. The oil isn't going to provide good hydrodynamic lubrication until the engine warms up.
With bearings, the clearances are so close and so contained that they will sometimes keep a good lubricating film on that bearing for years.
In elastohydrodynamic lubrication, an oil can act like a solid — as in areas of very fast, extreme force, such as where the rocker arm contacts the valve stem. The contact happens so quick that the oil can't get out of the way. When engine parts hit that fast, the oil literally acts like a solid. Elastohydrodynamic lubrication provides effective protection for the instant it's needed. The oil acts as a shock absorber, and hence, exhibits elastohydrodynamic properties.
What does viscosity have to do with lubrication? All of these lubrication types — the mixed film, dynamic, hydrodynamic, and elastohydrodynamic, all relate to and depend on oil viscosity. Viscosity is a measure of a fluid's resistance to flow. All fluids flow better when they are warm — cold oil is thick, but thins and flows better as it gets hot.
Oil viscosity is more important in an aviation engine than in an automobile engine. The fewer additives in the oil, the more dependent it is on its viscometrics (viscosity properties). Straight, untreated base oil can be limited in its lubrication without supplemental additives. Aviation oil will assist in boundary or mixed film lubrication, detergency and other lubrication aspects.
Ash cannot be added to aviation piston engine oils. Regulations prohibit the use of ash-bearing detergents and anti-wear, zinc-dithio-phosphate that are used in automotive or diesel truck engine oils because they may cause pre-ignition or detonation in an aircraft engine.
What is an oil's viscosity index?
While viscosity is an oil's internal resistance to flow, its viscosity index is simply its resistance to changing flow characteristics due to changes in temperature. If an oil's viscosity changes very little, despite significant temperature changes, the oil has a high viscosity index.
Viscosity index is an arbitrary numbering system. Higher numbers mean an oil's viscosity changes little with temperature, and lower numbers means it changes more. Single grade oils typically have a viscosity index of 90 to 110.
Multi-grade oils, with a viscosity index of 150 or higher, can tolerate extreme temperature changes and better retain their viscosity characteristics. Some automatic transmission fluid is so multi-graded that it may have a viscosity index of 200. Multi-grade oils are common in applications such as aviation oil, automatic transmission fluid, power steering fluid, gear oil, and hydraulic fluids.
How can an oil's viscosity index be improved?
Viscosity index can be increased by adding viscosity modifiers, or viscosity index improvers, to base oils. Several types of polymers are used to change the viscosity index of aviation oils.
Viscosity modifiers are available in different molecular weights, so oil formulators can select those with the most desirable performance and cost characteristics.
What other oil characteristics can be changed with additives?
Some additives help the oil, while others protect engine components.
Dispersants, flow-improvers, anti-foam, anti-rust, anti-corrosion, and oxidation inhibitors can all be found in aviation oils, as can some ashless, anti-wear additives.
Dispersants isolate minute particles to prevent sludge and deposit formation. Ashless dispersants in aviation oil are important because they encapsulate these very small particles of contamination and keep them from clumping and getting big enough to cause internal problems such as contributing to deposits or sludge, oil thickening, and oil screen restrictions.
Flow improvers help prevent wax crystal formations and slow viscosity increases that occur when oil gets cold. Sometimes you can improve the pour point of an oil significantly for a cold engine or cold starts by adding a little flow improver or flow modifier.
Anti-foam additives allow small bubbles in oil to burst, preventing excessive foam formation. Reducing foam improves oil cooling and lubrication. If an oil is foaming, it can't adhere to an engine's surface and can't cool as effectively.
Oxidation inhibitors reduce reactions of oxygen with oil molecules and thereby minimize engine deposits.
Rust and corrosion inhibitors help protect the metal engine components from corrosive contaminants introduced by typical engine operation. Oxidation inhibitors, as the name implies, tend to fortify the oil against oxidation.
•Change your aircraft engine oil frequently, based on manufacturer recommendations.
•Fly your plane monthly to reduce the effects of rust and corrosion on engine components. That doesn't mean starting-up and idling the engine for 10 minutes. You must fly the plane to allow the moisture to dissipate.
•Use only approved aviation oils in aircraft engines.
Grease is the word
Although not in the same category as aviation oils, greases are derivatives of oils that provide the same types of protection to other parts of the aircraft. The following information on grease is from the Sky Ranch Engineering Manual by John Schwaner (916) 421-7672.
Greases are thickened oils that seal, protect, cushion, and provide long service life. Greases are often referred to by the type of thickener used. Calcium (lime) is the original type of thickener, but is becoming less popular. It has high water resistance but poor high-temperature performance.
Lithium thickeners are used in Aeroshell Grease 7 (MIL-G-23827D) and Aeroshell Grease 17 (MIL-G-21164D). These have high melting points ("drop out") and adequate water resistance. Inorganic gels, as used in Aeroshell Grease 22(MIL-G-81322D), AeroShell Grease 5 (MIL-G-3545C) and AeroShell Grease 16 (MIL-G-5760), offer superior high temperature performance over lithium or calcuim thickeners.
Inorganic gel does not melt and the grease does not soften at high temperatures. The high temperature point of the grease is often governed by the flash point of the oil portion. These greases burn, rather than melt, if subjected to excessive temperatures. Clay-based greases (bentonite) are sometimes used in high temperature greases.
The type of oil that makes up the grease can either be synthetic or mineral oil. AeroShell Grease 7, 16, and 17, are all synthetic oil greases. AeroShell Grease 5 is the most common mineral oil grease. It is not good practice to mix a synthetic oil grease with a mineral oil grease. AeroShell Grease 5 and 22 are both used as a wheel bearing grease. AeroShell Grease 22, an inorganic gel synthetic grease, has superior high and low temperature performance and is specified in higher-performance aircraft wheel bearings. AeroShell Grease 5, a mineral oil grease, is also used in wheel bearings. AeroShell Grease 5 offers superior water and corrosion resistance.
Greases are separated by their usage. A low-speed, high pressure gear requires different grease characteristics than a high-speed, roller bearing grease. High pressure sliding surfaces require extreme pressure additives such as Molybdemum Disulfide. These "Moly" greases form a solid-film lubricant. Low or moderate pressure sliding surfaces may require a grease that will not evaporate, prevent water wash off, and prevent corrosion.
Moly is not desired in roller bearings because of its coating property. Roller bearings require a clean grease that has excellent thermal stability. Grease in a roller bearing will be pushed from the race by the action of the balls. If grease is too thin or melts, the race fills with grease, causing churning of the grease and adding friction and heat to the bearing. If the grease is too thick or dries out, the grease will be displaced to the side and therefore perform no lubricating action. Grease of the proper thickness will come in contact with the side of the ball as it passes and impart a thin film of oil onto the ball. High-speed ball bearing greases should be kept clean. Five gallon pails of grease are subject to dirt contamination because of the length of time it takes to use up that much grease.
Ball bearing greases
MIL-PRF-81322E (AeroShell Grease 22, Royco 22CF, Mobilgrease 28)
A synthetic inorganic gel grease, used in low or high temperature applications. Wide temperature performance makes it a preferred grease in jet aircraft.
MIL-G-3545C (AeroShell Grease 5, Royco 45)
A mineral oil-based, inorganic gel grease, the most common wheel bearing grease used. Not as temperature stable as MIL-PRF-81322E, but it has superior water resistance at high temperature.
MIL-G-25760A (AeroShell Grease 16, Royco 25)
A synthetic inorganic gel grease with the similar temperature applications as MIL-PRF-81322E. It has moderate water resistance (between MIL-PRF-81322E and MIL-G-3545), but has superior oxidation and corrosion resistance. Used in amphibious wheel bearings.
MIL-G-25537C (AeroShell Grease 14)
A calcium-based mineral oil grease with excellent anti-fretting and oxidation protection. It is used where ball bearings are subject to static vibration that may cause fretting and corrosion. It is used in helicopter main and tail rotor bearings.
Mobil Aviation Grease SHC 100 (No mil-spec)
Mobil Aviation Grease SHC 100's synthetic base oil, combined with selected additives, provide outstanding protection against wear, rust, corrosion, and high-temperature degradation. It is recommended for aviation applications which need a lubricant that can perform normal functions, yet go far beyond that in terms of high and low temperatures, long-life performance. It is particularly suitable for the lubrication of commercial aircraft wheel bearings.
General purpose grease
MIL-G-23827B (AeroShell Grease 7, Royco 27)
A Microgel® grease (AeroShell Grease 7) and a lithium soap (Royco 27), synthetic grease with a broad temperature range (-100 to 250 F). It has low evaporation loss, moderate-load wear index (lower than the moly-based greases), relatively poor water resistance but excellent corrosion resistance. This is a good, all-purpose airframe grease.
MIL-G-81827A (Royco 22MS)
An inorganic gel, molybdenum disulfide synthetic grease with a higher temperature range. It has the greatest load carrying ability of any of the listed greases. It has better water resistance than MIL-G-23827 or MIL-G-21164. Oxidation and evaporation rate are greater than MIL-G-23827. Used where high water-resistance, high temperature, and high load carrying is required.
MIL-G-7711A or MIL-G-24139 (AeroShell Grease 6)
An inorganic gel mineral oil grease with superior water-resistance than for other listed greases. It is used as a general purpose airframe grease where water-resistance and corrosion prevention is important. It is also available with molybdenum disulfide under Royco 11MS part number. Used with high load, slow moving sliding surfaces, such as landing gear bogie pivot assemblies, where water and corrosion resistance are required.
MIL-G-21164D (AeroShell Grease 17, Royco 64)
A Microgel® grease (AeroShell Grease 17) and a lithium soap (Royco 64) synthetic oil molybdenum disulfide grease. It is the same as MIL-G-23827 but contains moly. It is used in jet aircraft where parts are exposed to low temperatures. It is not as good as Royco 11MS in water-resistance and load carrying ability.