15.8–15.10 Bearings and seals, lubricants and fuels, lubrication systems

Scope

• 15.8 Bearings and seals — B1 only, level 2: construction and operation.
• 15.9 Lubricants and fuels — A 1, B1 2: properties and specifications of standard, alternate and drop-in fuels; lubricant properties and specifications; fuel additives; safety precautions.
• 15.10 Lubrication systems — A 1, B1 2: operation, layout and components.

Main shaft bearings

Bearing	Load carried	Notes
Ball bearing	Axial (thrust) and radial	One per shaft — the location (thrust) bearing that fixes the shaft axially and transmits the rotor's net axial thrust to the casing
Roller bearing	Radial only	Allows the shaft to expand axially with temperature; used at the other support points
Plain (journal) bearings	Radial	Rare in main shafts; used in gearboxes

• Bearings sit in bearing chambers (sumps) sealed from the gas path; supplied by oil jets.
• Squeeze-film (oil-film) damped bearings: a thin oil film between the outer race and its housing damps shaft vibration and reduces loads transmitted to the casing.
• Rolling-element bearings give low starting friction, tolerate short oil interruptions, and fail progressively (detected by chip detectors, oil analysis, vibration).
• Inspection: spalling, brinelling (static overload or vibration while stationary — false brinelling), corrosion, overheating (discolouration), skidding.

Seals

Seal	Principle	Use
Labyrinth	Series of fins/knife edges with small clearance against a smooth or abradable land; each restriction drops pressure. Pressurised with air so leakage flows into the bearing chamber, keeping oil in	Bearing chambers, inter-stage gas path sealing
Carbon seal	Spring-loaded carbon ring rubbing against a hard, polished face; very low leakage	Bearing chambers, accessory drives
Brush seal	Densely packed fine wire bristles in contact with the shaft — flexible, tolerant of shaft movement	Modern engines, air sealing
Ring seals	Piston-ring-type metal rings	Some bearing housings
Hydraulic (oil) seals	Rotating oil film/water-wheel seals between shafts	Inter-shaft locations

Key fact: A labyrinth seal does not touch — it relies on air pressure to stop oil escaping. Low air pressure (e.g. at idle or with excessive clearance) can cause oil leakage, smoke or high oil consumption.