M15.10 Lubrication systems

Question 1

What is another name for a bearing compartment?

Answer 1

Bearing sump

Question 2

What types of seal are used in a bearing sump and shaft contact areas?

Answer 2

Labyrinth, carbon and air seals

Question 3

What are the main components of carbon seals?

Answer 3

The main components of a carbon seal are:
* The carbon ring
* The seal seat
* The seal support
* Springs
* The seal ring

Question 4

Why and how carbon seals lubricated?

Answer 4

The contact surface of the seal seat is lubricated when the engine is running.
The lubrication of the carbon seals has a number of benefits. It reduces seal
wear leading to a longer service life, the oil removes the heat caused by
friction, and the sealing is improved

Question 5

What is the difference between the pressure and scavenge system delivery and removal rates?

Answer 5

The scavenge pumps remove the lubrication oil at a faster rate than it is
delivered via the oil nozzles. This prevents the accumulation of oil in the
bearing compartments and the gearbox sumps.
This is where the term dry sump comes from.

Question 6

Because of the high removal rate, what happens to the oil as it is returned to the tank?

Answer 6

Because of their high capacity the scavenge pumps remove air as well as oil
and therefore send an air/oil foam back to the oil tank

Question 7

How can air be removed via the vent system?

Answer 7

Either through vent lines in the shaft, or by external vent lines.

Question 8

How and why is air from the tank removed?

Answer 8

Air is removed from the tank as it has been mixed with air forming a foam.
The oil tank is connected to the forward bearing sump

Question 9

How are the gearboxes replenished with air removed by the vent system?

Answer 9

Air is supplied from the forward bearing sump to the gearboxes via the radial drive shaft housing

Question 10

Explain the de/oiler on a vent system that uses it

Answer 10

Like the other system the oil tank is connected to this vent
system and air is supplied from the forward bearing compartment to the
gearboxes.
A de-oiler is connected to the end of the vent line. The de-oiler is a centrifugal
air/oil separator driven by the gearbox.
The vent air goes through the de-oiler, before it is released to the atmosphere.
The oil, which is taken out of the vented air by the de-oiler, leaves via the
de-oiler scavenge line. As you can see the de-oiler scavenge line has its own
scavenge pump.

Question 11

Explain a full flow oil system?

Answer 11

This system cannot regulate pressure. The pressure within the system is in direct response to N2 rotor speed because its this shaft the oil pump is fixed too.
This can mean that at different operating speeds and temperatures the oil pressure will either be on the upper or lower limits.

Question 12

On a full flow oil system what is the purpose of the pressure relief valve?

Answer 12

A pressure relief valve is installed between the delivery side and the suction
side of the supply pump to prevent oil pressure becoming too high

Question 13

Explain the constant pressure oil system?

Answer 13

The pressure regulating valve holds the oil pressure to a given design value at
all N2 rotor speeds from idle to maximum.
Changes in oil temperature also have no effect on oil pressure.
The control parameters are the oil pressure in the supply line and the vent
pressure. The difference between the supply pressure and the vent pressure
controls the position of the valve piston against the valve spring.
The position of the piston controls the spill flow of the supply pump from the
delivery side to the suction side. A change in the flow area has the same effect
on pressure as a leak in the full flow system.
The pressure regulating system holds the pressure difference between the oil
pressure and the vent pressure constant. This ensures that we have the same
flow at all engine speeds

Question 14

What are the disadvantages and advantages of the full flow oil system?

Answer 14

The full flow system is the most commonly used system because of its
simplicity.
Another advantage of this system is that no adjustments are necessary during
maintenance of the engine.
There are two main disadvantages of the full flow system:
* This system needs a large supply pump or a relatively high oil pressure when
the engines are at idle. This leads to very high oil pressures at higher speeds.
* We have a significant rise in oil temperature after a power reduction. The oil
removes much of the heat from the bearings. When the power is reduced,
the lower flow rate of the oil causes the oil temperatures to increase

Question 15

What are the disadvantages and advantages of the constant pressure system?

Answer 15

In contrast to the full flow system, the constant pressure system maintains
relatively low oil pressure and has no increase in oil temperature after a
power reduction. This is because the flow rate is kept constant.
However, the constant pressure system is a more complex system and it
requires adjustments during maintenance. In addition the pressure regulating
valve is often a source of trouble.

Question 16

How many mounts does the oil tank have?

Answer 16

The tank has three mounts - a front mount, an aft mount, and a top mount

Question 17

How many supply lines does the oil tank have?

Answer 17

The three lines are:
* The pump supply line at the bottom of the tank
* The scavenge oil line at the top of the tank
* The vent line also at the top of the tank

Question 18

What is the purpose of the anti syphon valve in the oil tank?

Answer 18

An anti-siphon device is located in the pump supply line. It interrupts the oil
flow through the supply line after the engine stops. It prevents the oil from
flowing through the supply line to the lowest point in the system.
If we had no anti-siphon device, all the oil could be transferred to the
accessory gearbox.

Question 19

What is present within the oil tank?

Answer 19

Anti slosh bulkheads

Question 20

What is the purpose of the de-aerator on the oil tank?

Answer 20

A de-aerator is installed at the inlet for the scavenge oil. This de-aerator
separates the air from the incoming scavenge oil.
As the scavenge oil comes in through a spiral shaped flow path the centrifugal
force holds the oil flow at the outer wall of the tube. The air is separated from
the scavenge oil and moves up through the centre of the de-aerator and then
leaves the oil tank via the vent line.

Question 21

Why do some engine feature a pressurisation valve in the oil tank vent line?

Answer 21

To pressurise the tank somewhat, prevents oil foaming

Question 22

What materials are oil tanks made from?

Answer 22

The tank body is usually made from aluminium alloy. It is coated with a layer
of silicone to give it sufficient fire resistance.
To maintain this fire resistance, it is important that the silicone coating is not
damaged.
You can also find non-coated oil tanks. Oil tanks, which are not coated, are
made from materials which are strong enough to maintain this fire resistance

Question 23

What is fitted below the fill port?

Answer 23

A strainer and a check valve are installed below the filler cap in the filler.
The check valve prevents the discharge of air and oil through the filler when
the cap is lost during engine operation.
The strainer catches any large foreign particles during servicing

Question 24

How can oil samples be taken from the oil tank?

Answer 24

Oil samples can be taken from the tank via a sampling tube in the strainer

Question 25

What types of oil pumps are use?

Answer 25

All oil pumps are positive displacement pumps. The main types used are: * Gear pumps * Gerotor pumps * Vane pumps

Question 26

Explain a scavenge pump unit?

Answer 26

a scavenge pump unit, which houses all the scavenge pumps of the lubrication system. The unit housing has connection flanges for the scavenge oil lines from the bearing compartments, the gearbox sumps, and the de-oiler. All the scavenge oil flows through a common scavenge line to the scavenge filter.

Question 27

If gerotor pumps are used How is the pump unit typically assembled?

Answer 27

The gerotor type pumps are installed in a row on one shaft. Screens are installed in the suction line of every oil pump. They remove any large particles in the oil flowing from the individual scavenge lines to the pump. The oil usually passes a magnetic chip detector before it reaches the screen

Question 28

Explain last chance filters

Answer 28

Filters and screens are used in the lubrication system to clean the oil and to protect the system. Screens in the suction lines are used to protect the oil pumps. In some lubrication systems screens are also used to protect the oil nozzles if the supply filter is bypassed. These screens are installed in the supply lines upstream of the oil nozzles.

Question 29

Which type of filter is typically used in the pressure system?

Answer 29

A filter element with a pleated woven wire mesh is normally used in supply filters

Question 30

Which type of filter is typically used in the return system?

Answer 30

A filter element with paper as the filtering medium is normally used in scavenge filters. Paper filter elements are always disposable cartridges

Question 31

What is a hot tank oil system?

Answer 31

If it is installed in the supply line the system is called the hot tank system. The oil from the bearing compartments is not cooled before it returns to the oil tank.

Question 32

What is a cold tank oil system?

Answer 32

If the oil cooler is installed in the scavenge line the system is called the cold tank system. The oil from the bearing compartments is cooled before it returns to the oil tank

Question 33

Why might an engine have a separate air cooled oil cooler?

Answer 33

To act a secondary oil cooler for additional heat removal

Question 34

Which engines use air cooled oil coolers a the primary cooling method and why?

Answer 34

Air cooled oil coolers are usually used on turboprop engines and APUs. Large air coolers are necessary on these engines, because the fuel flow is low in relation to the amount of heat to be removed from the oil and the quantity of oil in these engines is relatively high.

Question 35

Explain a typical fuel oil heat exchanger?

Answer 35

The main components of a typical fuel cooled oil cooler are: * The housing * The cover * The core The core contains a series of tubes. The fuel flows through the tubes and the oil circulates around them. If the passage of the oil through the core is clogged, the oil flows through the oil bypass valve to the exit and if the passage of the fuel is clogged, the fuel flows through the fuel bypass valve to the exit.

Question 36

Explain the bearing mounted de-oiler

Answer 36

The de-oiler is installed in the bearing compartments. This de-oiler has radial ducts mounted on the engine shaft. The air/oil mist flows in a radial direction through the ducts into the vent line in the shaft.

Question 37

Explain the gearbox mounted de-oiler

Answer 37

On some engines the de-oiler is mounted on the gearbox and driven by the gearbox shaft. The air/oil mist enters the de-oiler housing and flows through the rotating radial ducts to the centre of the rotor. The oil droplets are thrown out of the rotor and against the walls of the housing by the centrifugal force. This oil is collected at the bottom of the housing and then returned to the oil tank via the scavenge pump. The air from the centre of the rotor is vented to the atmosphere via the vent air outlet. The small amount of oil released with the air represents the normal oil consumption of the engine. The de-oiler operates efficiently when the vent air flow is normal, and is less efficient when the vent air flow is higher than normal. If we have a higher vent air flow, then a lower percentage of oil is recovered from the vent air and there is a risk that the oil consumption can increase above the maximum limit.

Question 38

Explain the capacitance type oil level transmitter

Answer 38

Here you see the capacitance type transmitter. The upper part has the electronic components for the capacitance measurement and an electrical connector. The lower part, which is immersed in the oil, has two concentric tubes. These are the two plates of the capacitor

Question 39

Explain The reed type transmitter

Answer 39

The reed type transmitter has a metal tube with a float inside and a multi switch assembly. The metal tube has holes to let the oil in from the tank so that the float can move up and down with the oil level in the tank. The float assembly has permanent magnets, which activate an internal switch assembly. The multi switch assembly has a ladder of reed switches connected by resistors. The magnet in the float always closes the switch nearest to it. When the oil tank is full, then the float is at the upper limit of its travel. The magnet in the float assembly causes the top switch in the ladder to close. In this situation the resistance in the electrical circuit is at its minimum, and this gives maximum output voltage from the transmitter. You can see that when the oil level falls, the float also falls. The switch nearest to the float closes and all other switches open. The electrical resistance in the circuit changes with the switch that is closed and this gives a corresponding output voltage from the transmitter.

Question 40

How is the oil pressure measurements derived?

Answer 40

The oil pressure transmitter is connected to the oil supply line and to the oil tank vent line. The transmitter senses the pressure difference between the total oil pressure in the oil supply line and the vent pressure in the oil tank. Oil pressure information is sent from the oil pressure transmitter to the engine interface unit, which performs the measurement and then to the display unit in the cockpit.

Question 41

What are the two types of pressure transmitter?

Answer 41

There are two main types of oil pressure transmitter: * Bourdon tube type * Strain gauge type

Question 42

Explain the low pressure switch in the oil system?

Answer 42

An additional pressure switch is used in the engine oil system to initiate a low oil pressure warning. The pressure switch is also connected to the oil supply line and the oil tank vent line. If the oil pressure decreases below the limit, the low oil pressure switch closes, a signal is sent to the flight warning computer and a warning message appears on the display unit in the cockpit. Note: The engine low oil pressure warning is always accompanied by an acoustic warning in the cockpit.

Question 43

Where can the temperature be taken from on an engine?

Answer 43

The sensor can be found in the scavenge system, where it senses the hot oil temperature upstream of the oil cooler or it can be found in the pressure system, where it senses the temperature of the cooled oil.

Question 44

What are the two types of temperature sensors?

Answer 44

* The thermocouple * The thermistor

Question 45

What is the size of particles suspended in the oil will not be caught by the filters?

Answer 45

These particles range in size from 0.001 mm to 0.02 mm.

Question 46

What is the size of particle that the scavenge filter will pick up?

Answer 46

The scavenge oil filter element catches larger particles which are of a size of more than 0.015 mm.

Question 47

How do electrically monitored mag chips work?

Answer 47

This chip detector has two magnets at its tip. The resistance between the two chip detector magnets is monitored by the electronic control unit. The resistance decreases when particles connect with the magnets. When the resistance between the magnets gets below the limit, the electronic control unit sends a maintenance message for the post flight report

Question 48

What is the risk of over filling an engine with oil?

Answer 48

Excess oil will flow into the vent line and then into the FWD bearing sump