Engine Bearings: Purpose and Function

Engine Bearings

• Engine main bearings are located along the length of the rotor shaft and are critical for support.
• The number of bearings required is determined by the length and weight of the rotor shaft.
• Ball bearings support both axial (thrust) and radial (centrifugal) loads, while roller bearings only support radial loads.
• A minimum of one deep-groove ball bearing and one straight roller bearing are required to support one shaft.

Main Bearing Configuration

• The main bearings are typically a combination of ball and roller bearings.
• This configuration allows for engine flexibility during operation and temperature changes.

Disadvantages of Ball and Roller Bearings

• Both are vulnerable to damage from foreign matter.
• Both can fail without warning, making proper lubrication and sealing essential.

Bearing Housing and Sealing

• The main engine bearings are housed in a bearing support or housing.
• The support forms a chamber that separates the bearing from the engine core cavity.
• Bearing seals keep lubricating oil and oil mist from entering the engine core.

Labyrinth Seals

• Labyrinth seals are commonly used for main bearing chamber sealing.
• They have no contacting parts and prevent oil leakage by controlling opposing air leakage into the chamber.
• Air cooling of the engine bearing chambers is not normally necessary, but can be achieved through a double-skinned bearing housing with cooling air fed into the intermediate space.

Labyrinth Seal Variations

• Labyrinth air seal: used to prevent oil leakage and consists of a finned rotating member with a static bore lined with a soft abradable material or a high-temperature honeycomb structure.
• Labyrinth air/oil seal: uses a rotating annulus of oil to prevent heat build-up due to seal rub caused by shaft flexing.
• Thread type labyrinth seal: forms a helical path similar to a screw thread, allowing for a metered amount of compressor air to flow into the bearing chamber.

Other Types of Seals

• Hydraulic seals: used to seal a bearing chamber, but do not allow a controlled flow of air to traverse across the seal.
• Ring seals: used for bearing chamber sealing, except in hot areas where oil degradation due to heat would lead to ring seizure.
• Carbon seals: consist of a static ring of carbon that constantly rubs against a collar on a rotating shaft, relying on a high degree of contact to prevent oil or air leakage.
• Brush seals: circumferential seals made of high-temperature nickel-cobalt alloy bristles, used in large or small gas turbine secondary and tertiary flow gas paths.