Principles of aircraft engine lubrication

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...

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.

Final thoughts
•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.......

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.

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