Aircraft Bearings and Lubrication

Comprehensive flashcards covering the introduction, testing, cleaning, lubrication, and defects of aircraft bearings based on lecture notes.

Axial/Thrust loads

Forces that act along the axis of the shaft on the bearings.

Radial/Journal loads

Forces that act perpendicular to the axis of the shaft on the bearings.

Sealed or shielded bearings

Types of bearings designed to prevent the ingress of foreign particles, preventing Foreign Object Damage (FOD) or surface contamination.

Backlash

Relative to bearings, it is the free play or excessive movement in the system that may indicate wear.

Ultrasonic baths

A final cleaning method for bearings using high-frequency sound passed through a solvent bath to remove stubborn particles.

Base oil

The primary component of a lubricant, making up $90\%$ of its volume, such as vegetable oils, synthetic hydrogenated esters, silicones, or fluorocarbons.

Additives

Substances that make up less than $10\%$ of a lubricant to deliver reduced friction, increased viscosity, and resistance to corrosion, oxidation, and ageing.

Grease

A lubricant suitable for low rotational speeds or oscillating functions, common in airframe applications.

Oil

A lubricant substance more suitable for high speeds where grease would generate high temperatures.

Labyrinth seals

An oil retaining device used to prevent the loss of lubricant.

Zerk Grease Nipple

A fitting provided for some open bearings so that grease can be replenished via a grease gun.

Hydrodynamic lubrication

A type of lubrication where load-carrying surfaces are separated by a thick film of lubricant, and viscosity is the key factor in reducing friction.

Hydrostatic lubrication

A lubrication method using air and oil forced into the bearing area at high pressure to separate surfaces without requiring relative motion.

Boundary lubrication

Lubrication where the oil film is only a few molecules thick and the lubricity of the oil is more important than its viscosity.

Solid Film lubrication

Paint-like coatings of fine lubricating pigments (like graphite or molybdenum disulfide) blended with a binder and applied by spraying, dipping, or brush.

True Brinelling

Indentations in the bearing cup raceways caused by excessive impact or static overload greater than the material's elastic limit.

False Brinelling

Hollow spots resembling dents caused by vibration and swaying at contact points while the bearing is in a static state.

Burnishing

The smoothing of original surface roughness in bearing races as a result of being mechanically rolled over during normal running.

Overheating

A defect causing discolouration of rings, balls, and cages from gold to blue, often resulting from excessive operating temperatures or improper lubrication.

Galling

Metal transfer resulting from a welding or tearing action when a rolling element skids against its race.

Spalling

The breakdown of surfaces due to cyclic loading, resulting in metal fatigue and the flaking of small metal particles from the ringway.

Wear

The reduction of dimensions and roughening of surfaces caused by the ingress of dirt or hard foreign objects acting as a lapping compound.

Corrosion

A chemical attack on bearing materials, mainly caused by moisture, appearing as black pitting or reddish-brown rust.

Electrical Current Damage

Dark brown or greyish black fluting (corrugation) or craters in raceways caused by electric current passing through the contact surfaces.