15.8 Bearings and Seals

Question 1

What is the main purpose of bearings in engine construction?

Answer 1

To support the main engine rotor.

Question 2

What determines the number of bearings required in the engine rotor?

Answer 2

• Length of the engine rotor.
• Weight of the engine rotor.

Question 3

What forces act on the weight rotor? (5)

Answer 3

• Weight.
• Gas.
• Shock.
• Gyroscopic.
• Imbalances.

Question 4

What are the 2 forces that act on a bearing?

Answer 4

• Axial.
• Radial.

Question 5

In what direction can radial forces be transmitted in, on a bearing?

Answer 5

In any direction.

Question 6

In what direction can axial forces be transmitted in, on a bearing?

Answer 6

Only in the forward and aft direction.

Question 7

What main type of bearings do aircraft use?

Answer 7

Anti-friction bearings.

Question 8

What type of bearings do newer engines use?

Answer 8

Hydraulic bearings.

The outer race is surrounded by a thin layer of oil to reduce vibrations.

Question 9

What are the advantages of Anti-friction bearings? (8)

Answer 9

• Little rotational resistence.
• Precision alignment.
• Inexpensive.
• Easily replaced.
• Withstand high momentary loads.
• Accomodate both axial and radial loads.
• Easy cooling, lubrication and maintanence.
• Resistent to elevated temps.

Question 10

What’s the two disadvantages of Anti-friction bearings?

Answer 10

• They fail without warning.
• Vulnerable to FOD damage.

Question 11

What type of load is the main example of an axial load?

Answer 11

Thrust load.

Question 12

What forces do fixed bearings transmit?

Answer 12

Both axial and radial forces.

Question 13

How are fixed bearings installed, in terms of the rotor?

Answer 13

They fix the rotor in axial direction, and support the rotor in the radial direction

Question 14

What force do floating bearings transmit?

Answer 14

Radial forces only.

Question 15

How do floating bearings ‘float’?

Answer 15

The shaft and inner bearing race can move axially in the outer race.

Question 16

What type of bearing system do aircraft use?

Answer 16

A Triple-Bearing System.

Question 17

What does a Triple-Bearing System consist of?

Answer 17

A fixed bearing in the middle of the rotor, with a floating bearing at each end of the rotor.

Question 18

What’s the advantage of the Triple-Bearing System?

Answer 18

Minimises thermal expansion of the shaft end.

Question 19

What’s the main type of fixed bearing used?

Answer 19

Large ball-bearings.

Question 20

How are radial forces transmitted through a ball-bearing?

Answer 20

Through the bearing components to the bearing support.

Question 21

How are axial forces transmitted through a ball-bearing?

Answer 21

Applied through the shaft fitting to the inner race.

Question 22

What’s the purpose of the balls, in the ball-bearings?

Answer 22

Transmits the forces between the outer and inner races.

Question 23

What’s the main type of floating bearing that’s used?

Answer 23

Roller bearing.

Question 24

How do roller bearings transmit the radial forces?

Answer 24

The rollers are guided by the inner race, free to move in the axial direction in the outer race. (which transmits a radial force).

Question 25

What’s the third type bearing used in aircraft?

Answer 25

Oil-dampened Bearings.

Question 26

What bearings can be ‘oil-dampened’?

Answer 26

Both ball-bearings and roller bearings.

Question 27

How do oil-dampened bearings transmit forces?

Answer 27

They have a film of oil between the outer race and the bearing support, which aborbs and dampens radial shock forces.

Question 28

What temperature can bearings reach?

Answer 28

200-260 degrees C.

Question 29

What two methods are used to lubricate bearings?

Answer 29

• Direct method.
• Under-race method (indirect).

Question 30

How does the direct method lubricate a bearing?

Answer 30

Oil is sprayed directly onto the bearings via one/two oil nozzles.

The excess oil is spun-off the bearing and collected by a sump.

Question 31

What bearings would usually use the direct lubrication method?

Answer 31

Bearings that take lower loads.

Question 32

What is the under-race (indirect) method of bearing lubrication?

Answer 32

The oil is sprayed onto the inner wall of the hollow rotor shaft, which moves to the inner race of the bearing.

Question 33

Which bearing lubrication method is better at removing heat, and why?

Answer 33

The under-race method, as the oil is in contact with the bearing materials for longer.

Question 34

What are the two types of seals?

Answer 34

• Labyrinth.
• Thread (helicoil).

Question 35

What is the main sealing feature of a Labyrinth seal?

Answer 35

It’s a pressurised seal.

Question 36

What is the main sealing feature of a thread-type (helicoil) seal?

Answer 36

It soley depends on the reverse-thread to produce its seal.

Question 37

What are ‘lands’ on the rotor shaft surface?

Answer 37

Machined, matching surfaces for the oil seals in the bearing housing.

Question 38

What’s a more modern-type of seal?

Answer 38

Carbon seals, which are spring-loaded.