Bearings, Seals and Lubrication
What this domain tests
This domain covers how the rotor is supported, sealed, and lubricated. The exam tests bearing types and load paths, seal selection, oil-system architecture, and the meaning of oil pressure, temperature, and debris indications.
Core rules
- Ball bearings take radial and some thrust; roller bearings take radial loads; thrust bearings take axial loads. Bearing preload affects stiffness and skidding.
- Labyrinth seals control leakage by a series of clearances; carbon seals run against a mating face; brush seals offer low leakage with some compliance.
- A dry-sump system uses a pressure pump and scavenge pumps; oil is cooled, filtered, and monitored for pressure, temperature, and debris.
- Oil does more than lubricate: it cools bearings, carries debris to the chip detector, and provides sealing at some locations.
Worked example
Oil flow is 20 kg/min, cp = 2.0 kJ/(kg.K), and the oil temperature rise is 40 K. Heat removed = mdot x cp x delta T = (20/60) x 2.0 x 40 = 26.7 kW. If flow drops to 15 kg/min with the same heat load, delta T rises to about 53 K. The reasoning: lower flow reduces the heat-transfer coefficient and raises oil temperature, which can trigger an exceedance.
Traps
Assuming all bearings take thrust; confusing seal types; treating oil pressure alone as oil health; ignoring scavenge pump failure; forgetting that oil temperature limits are separate from EGT limits.