15.8 Bearings and Seals Flashcards
What is the main purpose of bearings in engine construction?
To support the main engine rotor.
What determines the number of bearings required in the engine rotor?
- Length of the engine rotor.
- Weight of the engine rotor.
What forces act on the weight rotor? (5)
- Weight.
- Gas.
- Shock.
- Gyroscopic.
- Imbalances.
What are the 2 forces that act on a bearing?
- Axial.
- Radial.
In what direction can radial forces be transmitted in, on a bearing?
In any direction.
In what direction can axial forces be transmitted in, on a bearing?
Only in the forward and aft direction.
What main type of bearings do aircraft use?
Anti-friction bearings.
What type of bearings do newer engines use?
Hydraulic bearings.
The outer race is surrounded by a thin layer of oil to reduce vibrations.
What are the advantages of Anti-friction bearings? (8)
- 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.
What’s the two disadvantages of Anti-friction bearings?
- They fail without warning.
- Vulnerable to FOD damage.
What type of load is the main example of an axial load?
Thrust load.
What forces do fixed bearings transmit?
Both axial and radial forces.
How are fixed bearings installed, in terms of the rotor?
They fix the rotor in axial direction, and support the rotor in the radial direction
What force do floating bearings transmit?
Radial forces only.
How do floating bearings ‘float’?
The shaft and inner bearing race can move axially in the outer race.
What type of bearing system do aircraft use?
A Triple-Bearing System.
What does a Triple-Bearing System consist of?
A fixed bearing in the middle of the rotor, with a floating bearing at each end of the rotor.
What’s the advantage of the Triple-Bearing System?
Minimises thermal expansion of the shaft end.
What’s the main type of fixed bearing used?
Large ball-bearings.
How are radial forces transmitted through a ball-bearing?
Through the bearing components to the bearing support.
How are axial forces transmitted through a ball-bearing?
Applied through the shaft fitting to the inner race.
What’s the purpose of the balls, in the ball-bearings?
Transmits the forces between the outer and inner races.
What’s the main type of floating bearing that’s used?
Roller bearing.
How do roller bearings transmit the radial forces?
The rollers are guided by the inner race, free to move in the axial direction in the outer race. (which transmits a radial force).
What’s the third type bearing used in aircraft?
Oil-dampened Bearings.
What bearings can be ‘oil-dampened’?
Both ball-bearings and roller bearings.
How do oil-dampened bearings transmit forces?
They have a film of oil between the outer race and the bearing support, which aborbs and dampens radial shock forces.
What temperature can bearings reach?
200-260 degrees C.
What two methods are used to lubricate bearings?
- Direct method.
- Under-race method (indirect).
How does the direct method lubricate a bearing?
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.
What bearings would usually use the direct lubrication method?
Bearings that take lower loads.
What is the under-race (indirect) method of bearing lubrication?
The oil is sprayed onto the inner wall of the hollow rotor shaft, which moves to the inner race of the bearing.
Which bearing lubrication method is better at removing heat, and why?
The under-race method, as the oil is in contact with the bearing materials for longer.
What are the two types of seals?
- Labyrinth.
- Thread (helicoil).
What is the main sealing feature of a Labyrinth seal?
It’s a pressurised seal.
What is the main sealing feature of a thread-type (helicoil) seal?
It soley depends on the reverse-thread to produce its seal.
What are ‘lands’ on the rotor shaft surface?
Machined, matching surfaces for the oil seals in the bearing housing.
What’s a more modern-type of seal?
Carbon seals, which are spring-loaded.
