Section 15.8

Bearings & Seals

Ball and roller main bearings, bearing sumps, and labyrinth and carbon seals.

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Main ideas

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.

Solved examples

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.

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