Engine Bearings: Purpose and Function

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Questions and Answers

What type of bearings are not used as main bearings in turbine engines?

Plain bearings (correct)

Roller bearings

Deep-groove bearings

Ball bearings

What determines the number of main bearings required in a gas turbine engine?

Type of bearing used

Length and weight of the rotor shaft (correct)

Type of compressor

Speed of operation

What type of loading do ball bearings support in a gas turbine engine?

Neither axial nor radial loads

Only axial loads

Both axial and radial loads (correct)

Only radial loads

What is the advantage of using roller bearings in a gas turbine engine?

They can absorb the bulk of radial loading Signup and view all the answers

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 Signup and view all the answers

What is the reason for using anti-friction type bearings in a gas turbine engine?

To support high-speed operations Signup and view all the answers

What is the purpose of the fins on a thread type labyrinth seal?

To thread the oil back into the bearing chamber Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

What is the main characteristic of the running clearance in a ring seal?

It is smaller than that of a labyrinth seal Signup and view all the answers

What is the purpose of the annulus of oil in a hydraulic seal?

To create a seal fin immersed in oil Signup and view all the answers

What happens when the shaft comes into contact with the ring in a ring seal?

The ring is allowed to move in its housing Signup and view all the answers

What is the primary function of the ball bearing in the engine configuration?

To handle the thrust load Signup and view all the answers

What is a common disadvantage of both ball and roller bearings?

Their vulnerability to damage caused by foreign matter Signup and view all the answers

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 Signup and view all the answers

What type of seal is commonly used for main bearing chamber sealing?

Labyrinth seal Signup and view all the answers

Why is air cooling of the engine bearing chambers not normally necessary?

Because the lubrication system is adequate for cooling purposes Signup and view all the answers

Where are the bearing chambers typically located in the engine?

In the coolest regions of the engine Signup and view all the answers

What is the purpose of a double-skinned bearing housing with cooling air fed into the intermediate space?

To provide additional cooling when required Signup and view all the answers

What is the purpose of the finned rotating member in a labyrinth seal?

To cut into the abradable material and create a minimum clearance Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

Why do labyrinth air/oil seals have greater fin clearance than air seals?

To allow for the entry of oil into the seal Signup and view all the answers

What is the reason for not using ring seals in hot areas?

Due to oil degradation caused by heat Signup and view all the answers

What is the purpose of the springs in a carbon seal?

To maintain contact between the carbon and the collar Signup and view all the answers

What is the characteristic of carbon seals that requires special care during handling?

They are brittle and prone to damage Signup and view all the answers

What is the material used to manufacture the bristles of a brush seal?

High-temperature nickel-cobalt alloy Signup and view all the answers

Where are brush seals typically used in a gas turbine engine?

In the secondary and tertiary flow paths Signup and view all the answers

What is the angle of the brush seal bristles relative to the tangent of the adjacent shaft?

45° Signup and view all the answers

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.

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Description

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.