M15.08 Bearings and seals

Question 1

What forces act on the rotor?

Answer 1

Weight forces caused by the rotor
Gas forces from the gas flow
Shock forces caused by turbulence and impact of landing
Gyroscopic forces caused by aircraft manoeuvres
Imbalance forces of the rotor components

Question 2

What are the two types of forces on a rotor?

Answer 2

Axial
Radial

Question 3

Fixed bearings to what?

Answer 3

Support the rotor in a radial direction and fixes it in the axial direction, transmits axial and radial force

Question 4

Floating bearings do what?

Answer 4

Can only transmit radial forces as it is not fixed in axial direction

Question 5

What is the benefit of having fixed and floating bearings?

Answer 5

The rotor can expand and contract with use without any stress

Question 6

Explain a triple bearing rotor

Answer 6

The middle bearing is a fixed bearing, and the bearings at each end are floating bearings. this leads to smaller expansion of the shaft ends

Question 7

What type of bearing is used in fixed bearings?

Answer 7

Ball bearings, because it can transmit radial and axial loads. large bearings are required for large engines

Question 8

What type of bearing is used in a floating bearing?

Answer 8

Roller bearings, mostly transmit radial forces

Question 9

Explain oil dampened bearings

Answer 9

Oil dampened bearing have a film of oil in the outer race between the bearing support structure, when a shock is felt the bearing then has to push the oil of the way, proving some dampening

Question 10

What is the purpose of pressure balancing?

Answer 10

Axial gas loads are applied in a forward direction on the compressor rotor and
in a rearward direction on the turbine rotor.
Pressure balancing reduces the axial loads on the fixed bearings and
therefore increases the service life of each bearing.
Pressure balancing causes a force in the opposite direction to the axial load
on the bearing.
This force is caused by air pressure in an internal cavity of the rotor.
The air pressure used for pressure balancing is usually taken from the high
pressure compressor.
The rearward force is made by the pressure that acts on the conical part of
the low pressure turbine rotor. This force pushes the shaft rearwards and
decreases the load on the fixed bearing.
Pressure balancing can be used for both the low pressure rotor and the high
pressure rotor.
Its disadvantage is that it needs complicated internal air pressure passages
and efficient air seals.

Question 10

What is the purpose of pressure balancing?

Answer 10

Axial gas loads are applied in a forward direction on the compressor rotor and
in a rearward direction on the turbine rotor.
Pressure balancing reduces the axial loads on the fixed bearings and
therefore increases the service life of each bearing.
Pressure balancing causes a force in the opposite direction to the axial load
on the bearing.
This force is caused by air pressure in an internal cavity of the rotor.
The air pressure used for pressure balancing is usually taken from the high
pressure compressor.
The rearward force is made by the pressure that acts on the conical part of
the low pressure turbine rotor. This force pushes the shaft rearwards and
decreases the load on the fixed bearing.
Pressure balancing can be used for both the low pressure rotor and the high
pressure rotor.
Its disadvantage is that it needs complicated internal air pressure passages
and efficient air seals.

Question 11

What are the two methods of bearing cooling and lubrication?

Answer 11

Direct lubrication
Under race lubrication

Question 12

Explain the direct lubrication method?

Answer 12

In the direct lubrication method, oil is sprayed directly onto the bearings with
one or two oil nozzles.
The oil droplets then fly off the bearing because of the rotation and are
collected in the sump housing. This method is normally used for bearings with
lower loads

Question 13

Explain the under race lubrication method

Answer 13

In the under-race lubrication method, oil is sprayed on the inner wall of the
hollow rotor shaft. The oil stays on the wall because of centrifugal forces.
The oil then moves outwards through the holes in the shaft and the bearing
inner race and leaves the bearing at the cage.
The under-race lubrication method removes more heat than the direct
lubrication method.
This is because the oil can reach the inner surface of the inner race and it is in
contact with the bearing materials for longer.
This permits more intense lubrication of the bearing

Question 14

How much power does the accessory drive take from the engine rotor?

Answer 14

400kw

Question 15

What is the purpose of the internal gearbox?

Answer 15

The internal gearbox, which transmits the rotation from the engine rotor to
the radial drive shaft.

Question 16

What is the purpose of the radial drive shaft?

Answer 16

A radial drive shaft, which is routed through the fan frame to the outer wall
of the fan frame case.

Question 17

What is the purpose of the transfer gearbox?

Answer 17

A transfer gearbox, also called the angle gearbox, which changes the
direction of rotation.

Question 18

What is the purpose of the horizontal drive shaft?

Answer 18

A horizontal drive shaft, which connects the transfer gearbox with the main
component of this system, the accessory gearbox also known as the external
gearbox. The pumps and control units are located on this gearbox.

Question 19

What is the disadvantage of fan section mounted accessory gearboxes?

Answer 19

Fan case mounted gearboxes increase the frontal area of an engine because
the accessories need space

Question 20

What is disadvantage of hot section mounted accessory gearboxes?

Answer 20

The accessories must now operate in very hot conditions

Question 21

What is the purpose of having two accessory gearboxes?

Answer 21

Two gearboxes are used on an engine to improve the starting capabilities of
the engine. It puts less resistance on the high pressure rotor during engine
start because the aircraft accessories are on the low pressure rotor.

Question 22

What type of gearing does the internal gearbox use?

Answer 22

Bevel gears

Question 23

What can be found on large engines on the radial drive shaft?

Answer 23

A centring bearing to ensure the shaft runs true and prevent vibrations

Question 24

What can the construction of the radial drive shaft be like on very large engines?

Answer 24

Can be made in two or three segments, splined together

Question 25

What is the purpose of the radial drive shaft?

Answer 25

These splines, located at each end, also transmit the torque between the bevel
gears and the drive shafts.
A shear neck at one end close to a spline is a defined area of the drive shaft,
which can break if there is a mechanical failure without causing severe
damage to the accessory drive system

Question 26

How are gearbox accessories normally sealed?

Answer 26

A seal between the gearbox shaft and the gearbox case prevents leakage of
engine oil.
Carbon seals are normally used. This type of seal has a seal assembly with a
carbon ring installed in the gearbox case and a mating ring, which rotates with
the shaft. This mating ring has a polished surface, which is in close contact
with the seal assembly.
When the carbon ring of the seal assembly is in contact with the mating ring, it
seals off the lubrication area inside the gearbox.