30 Questions
What type of bearings are not used as main bearings in turbine engines?
Plain bearings
What determines the number of main bearings required in a gas turbine engine?
Length and weight of the rotor shaft
What type of loading do ball bearings support in a gas turbine engine?
Both axial and radial loads
What is the advantage of using roller bearings in a gas turbine engine?
They can absorb the bulk of radial loading
What is the minimum number of bearings required to support one shaft in a gas turbine engine?
One deep-groove ball bearing and one straight roller bearing
What is the reason for using anti-friction type bearings in a gas turbine engine?
To support high-speed operations
What is the purpose of the fins on a thread type labyrinth seal?
To thread the oil back into the bearing chamber
What is the main difference between hydraulic seals and labyrinth or ring seals?
The ability to allow a controlled flow of air to traverse across the seal
What compensates for any difference in air pressure inside and outside of the bearing chamber in a hydraulic seal?
A difference in oil level on either side of the fin
What is the main characteristic of the running clearance in a ring seal?
It is smaller than that of a labyrinth seal
What is the purpose of the annulus of oil in a hydraulic seal?
To create a seal fin immersed in oil
What happens when the shaft comes into contact with the ring in a ring seal?
The ring is allowed to move in its housing
What is the primary function of the ball bearing in the engine configuration?
To handle the thrust load
What is a common disadvantage of both ball and roller bearings?
Their vulnerability to damage caused by foreign matter
What is the primary purpose of the bearing seals in the main engine bearings?
To keep the lubricating oil and oil mist from entering the engine core
What type of seal is commonly used for main bearing chamber sealing?
Labyrinth seal
Why is air cooling of the engine bearing chambers not normally necessary?
Because the lubrication system is adequate for cooling purposes
Where are the bearing chambers typically located in the engine?
In the coolest regions of the engine
What is the purpose of a double-skinned bearing housing with cooling air fed into the intermediate space?
To provide additional cooling when required
What is the purpose of the finned rotating member in a labyrinth seal?
To cut into the abradable material and create a minimum clearance
What is the effect of the pressure drop across each seal fin in a labyrinth seal?
It restricts the flow of sealing air from one side of the seal to the other
What is the purpose of the oil in a labyrinth air/oil seal?
To prevent excessive heat build-up due to seal rub caused by shaft flexing
What is the difference between a labyrinth air seal and a labyrinth air/oil seal?
The presence of an oil annulus in the air/oil seal
Why do labyrinth air/oil seals have greater fin clearance than air seals?
To allow for the entry of oil into the seal
What is the reason for not using ring seals in hot areas?
Due to oil degradation caused by heat
What is the purpose of the springs in a carbon seal?
To maintain contact between the carbon and the collar
What is the characteristic of carbon seals that requires special care during handling?
They are brittle and prone to damage
What is the material used to manufacture the bristles of a brush seal?
High-temperature nickel-cobalt alloy
Where are brush seals typically used in a gas turbine engine?
In the secondary and tertiary flow paths
What is the angle of the brush seal bristles relative to the tangent of the adjacent shaft?
45°
Study Notes
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.
Learn about the critical role of engine main bearings in supporting the rotor shaft in gas turbine engines, including factors that determine their number and type.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
