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...

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.

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