Bearings & Seals
Ball and roller main bearings, bearing sumps, and labyrinth and carbon seals.
Bearings & seals, summary notes
- The main shafts run in rolling-element bearings: ball bearings locate the shaft axially and take thrust loads, while roller bearings take radial load and allow the shaft to grow in length with thermal expansion.
- Bearings sit in sealed sumps (bearing chambers) fed with oil for lubrication and cooling; the oil also carries away heat and debris monitored by magnetic chip detectors.
- Seals keep oil in the sumps and hot gas/air out. Labyrinth seals use a series of fine knife-edges with a controlled air pressure across them; carbon (face) seals use a spring-loaded carbon ring against a running face for a tighter seal.
- Sumps are pressurised/vented with sealing air bled from the compressor so that leakage is always air-into-oil, keeping oil from escaping into the gas path (which would show as smoke or oil consumption).
- Bearing failure shows as metal on the chip detector, rising vibration, oil temperature or oil consumption; correct pre-load and clean oil are essential.
- ⚠ Exam trap: the BALL bearing takes the axial/thrust load and locates the shaft, while the ROLLER bearing takes radial load and permits axial thermal growth; labyrinth seals rely on a controlled air pressure differential to keep oil in.
Why is one main bearing on a shaft a ball bearing and the other a roller bearing, rather than two of the same?
The shaft must be located axially but also allowed to expand lengthways as it heats. The ball bearing fixes the shaft's axial position and carries thrust; the roller bearing carries radial load while letting the shaft slide axially within it as it grows, using two fixed bearings would over-constrain the shaft and cause it to buckle or bind when hot.
Bearings & seals concept map
Bearings & Seals
Bearings & seals quiz
Bearings & Seals, quiz
1. A labyrinth seal restricts oil or air leakage by:
Presenting a series of restrictions that throttle the flowMaking metal-to-metal contactUsing a rubber lip2. Which main bearing takes the axial (thrust) load and locates the shaft?
Ball bearingRoller bearingPlain bush3. The roller bearing on a shaft:
Takes radial load and allows axial thermal growthLocates the shaft axiallyCarries the thrust load4. A labyrinth seal works by:
Knife-edges with a controlled air pressure across themA rubber O-ring onlyMagnetic force5. A carbon (face) seal uses:
A spring-loaded carbon ring against a running faceTwo knife edgesA ball race6. Bearing sumps are sealed with air so that any leakage is:
Air into oil (keeping oil in the sump)Oil into the gas pathFuel into oil7. Metal particles on a magnetic chip detector indicate:
Possible bearing/gear deteriorationNormal operationLow fuel8. Oil supplied to the bearings also serves to:
Cool them and carry away heat/debrisIgnite the fuelSeal the combustor