Bearings.pdf

Engine Bearings Purpose of Engine Bearings Engine main bearings are assigned the critical function of support and are located along the length of the rotor shaft. The number of bearings necessary is determined, in part, by the length and weight of the rotor shaft. For example, since a...

Engine Bearings Purpose of Engine Bearings Engine main bearings are assigned the critical function of support and are located along the length of the rotor shaft. The number of bearings necessary is determined, in part, by the length and weight of the rotor shaft. For example, since a dual-spool axial compressor typically has a greater number of rotating components, it requires more main bearings than a centrifugal compressor. The main bearings of a gas turbine engine are either ball or roller anti-friction types. The minimum number of bearings required to support one shaft is one deep-groove ball bearing and one straight roller bearing. Ball bearings support the main engine rotor for both axial (thrust) and radial (centrifugal) loads. The roller bearings support only the engine rotor’s radial loads. Because of their greater surface contact area than ball bearings, they are positioned to absorb the bulk of the radial loading and to allow for axial growth of the engine during operation. The diagram shows one configuration of bearing positions. Aviation Australia Main bearing locations 2022-08-24 B1-15a Gas Turbine Engine Page 195 of 244 CASA Part 66 - Training Materials Only  Plain bearings are not used as main bearings in turbine engines, as they are in piston engines, because turbines operate at much higher speeds and friction heat build-up is prohibitive. The primary loads acting on main bearings are from the following sources: Weight of the rotating mass (compressor and turbine) magnified many thousands of times by radial g-forces Axial forces from power changes and thrust loading Gyroscopic effect of heavy rotating masses trying to remain in place as the aircraft changes direction Compression and tension loads between the stationary casings and the rotor system caused by thermal expansion Vibrations induced by the airstream, the airframe and the engine itself. 2022-08-24 B1-15a Gas Turbine Engine Page 196 of 244 CASA Part 66 - Training Materials Only  Main Bearings The main bearings support the rotor assemblies and then transfer the various loads through the bearing housings and support struts to the outer cases of the engine, and ultimately into the aircraft mountings. The number of main bearings varies from one engine model to another. One manufacturer might prefer to install three heavy bearings, and another five or six lighter bearings to accommodate the same load factors. The bearings are made up of an inner and outer race, with the cage holding the rollers or balls. This cage keeps the roller or balls aligned between the two races, which support the turning shaft. Straight roller bearings can accept only radial loads and will not support the shaft under thrust (axial loads). The thrust load is carried by the ball bearing, which can carry radial and thrust loads. Each engine configuration differs, but a common method is using one ball bearing and one or more roller bearings per spool shaft. Roller bearings offer a larger bearing contact surface than ball bearings and are preferred where thrust loads are not present. The ball bearing handles the thrust load, and roller bearings provide additional supports where needed. This configuration allows the engine flexibility when it expands and contracts due to temperature changes while operating. Ball and roller bearing Disadvantages of both ball and roller bearings include their vulnerability to damage caused by foreign matter and their tendency to fail without appreciable warning. Therefore, proper lubrication and sealing against entry of foreign matter are essential. Commonly used types of oil seals are labyrinth, helical thread and carbon. Main engine bearings are housed in a bearing support or housing. The support forms a chamber which separates the bearing from the engine core cavity. Bearing seals keep the lubricating oil and oil mist from entering the engine core. Oil leakage into the core cavity enters the airflow and gas stream. 2022-08-24 B1-15a Gas Turbine Engine Page 197 of 244 CASA Part 66 - Training Materials Only  Aviation Australia Bearing chamber 2022-08-24 B1-15a Gas Turbine Engine Page 198 of 244 CASA Part 66 - Training Materials Only  Bearing Chamber Sealing Labyrinth Seals Labyrinth seals are commonly used for main bearing chamber sealing. They have no contacting parts. They prevent oil leakage out by controlling opposing air leakage into the chamber. Compressor bleed air, which is at a greater pressure than bearing chamber vent pressure, causes air to flow from the outside to the inside of the chamber. Air cooling of the engine bearing chambers is not normally necessary since the lubrication system is adequate for cooling purposes. Additionally, bearing chambers are located, where possible, in the cooler regions of the engine. In instances where additional cooling is required, it is good practice to have a double-skinned bearing housing with cooling air fed into the intermediate space. There are several variations of labyrinth seal design. Aviation Australia Bearing chamber 2022-08-24 B1-15a Gas Turbine Engine Page 199 of 244 CASA Part 66 - Training Materials Only  Labyrinth Air Seal A labyrinth seal used as an air seal comprises a finned rotating member with a static bore which is lined with a soft abradable material or a high-temperature honeycomb structure. On initial running of the engine, the fins lightly rub against the lining, cutting into it to give a minimum clearance. The clearance varies throughout the flight cycle, depending on the thermal growth of the parts and the natural flexing of the rotating members. Across each seal fin is a pressure drop which results in a restricted flow of sealing air from one side of the seal to the other. Labyrinth seal 2022-08-24 B1-15a Gas Turbine Engine Page 200 of 244 CASA Part 66 - Training Materials Only  Labyrinth Air/Oil Seal To prevent excessive heat build-up due to seal rub caused by shaft flexing, a non-heat-producing seal is used where the abradable lining is replaced by a rotating annulus of oil. When the shafts deflect, the fins enter the oil and maintain the seal without generating heat. Labyrinth air/oil seals have greater fin clearance than air seals. Labyrinth air/oil seal 2022-08-24 B1-15a Gas Turbine Engine Page 201 of 244 CASA Part 66 - Training Materials Only  Thread Type Labyrinth Seal The fins on thread type labyrinth seals form a helical path similar to a screw thread. As the seal rotates, it ‘threads’ the oil back into the bearing chamber. As with labyrinth seals, fins allow for a metered amount of compressor air to flow into the bearing chamber. Pressure within the bearing chamber is maintained slightly above atmospheric. Thread type labyrinth seal 2022-08-24 B1-15a Gas Turbine Engine Page 202 of 244 CASA Part 66 - Training Materials Only  Hydraulic Seals The hydraulic method of sealing is often used between two rotating members to seal a bearing chamber. Unlike the labyrinth or ring seal, it does not allow a controlled flow of air to traverse across the seal. Hydraulic seals are formed by a seal fin immersed in an annulus of oil which has been created by centrifugal forces. Any difference in air pressure inside and outside of the bearing chamber is compensated by a difference in oil level on either side of the fin. Aviation Australia Hydraulic seal 2022-08-24 B1-15a Gas Turbine Engine Page 203 of 244 CASA Part 66 - Training Materials Only  Ring Seals A ring seal comprises a metal ring which is housed in a close-fitting groove in the static housing. The normal running clearance between the ring and rotating shaft is smaller than that which can be obtained with the labyrinth seal. This is because the ring is allowed to move in its housing whenever the shaft comes into contact with it. Ring seals are used for bearing chamber sealing, except in the hot areas where oil degradation due to heat would lead to ring seizure within its housing. Aviation Australia Ring seal 2022-08-24 B1-15a Gas Turbine Engine Page 204 of 244 CASA Part 66 - Training Materials Only  Carbon Seals Carbon seals consist of a static ring of carbon which constantly rubs against a collar on a rotating shaft. Several springs are used to maintain contact between the carbon and the collar. This type of seal relies on a high degree of contact and does not allow oil or air leakage across it. The heat caused by friction is dissipated by the oil system. Aviation Australia Carbon seals Aviation Australia Seal race mounted on rotating shaft The seal race is mounted on the rotating shaft. 2022-08-24 B1-15a Gas Turbine Engine Page 205 of 244 CASA Part 66 - Training Materials Only  Carbon seals are brittle and need to be handled carefully. Special care should be taken not to damage the polished sealing surfaces. Brush Seals Brush seals are circumferential seals which may be used in place of traditional labyrinth seals in large or small gas turbine secondary and tertiary flow gas paths. They are manufactured from materials such as high-temperature nickel-cobalt alloy bristles. Brush seals are comprised of an array of densely packed, high-temperature resistant, low wear bristles configured on an angle of approximately 45° with the tangent of their adjacent shaft. Current brush seal mating surface material can be ceramic, chromium carbide, tungsten carbide or alumina, depending on where it is located in the engine and brush material. The bristles form a flexible porous medium, reducing leakage by up to 5 times that of a typical labyrinth seal. The seal takes up slightly more radial room than a conventional straight-toothed labyrinth, but significantly less axial space, and weighs far less than machine labyrinths. Brush seals are being applied in today’s gas turbines as replacement for labyrinth, other air-to-air seals and oil seals to improve engine efficiency, reduce fuel consumption and increase engine thrust. Brush seal 2022-08-24 B1-15a Gas Turbine Engine Page 206 of 244 CASA Part 66 - Training Materials Only  Brush seal example 2022-08-24 B1-15a Gas Turbine Engine Page 207 of 244 CASA Part 66 - Training Materials Only