ESS-38-70

Lubrication Systems for Aero Gas Turbines

Ashutosh Panda
Cranfield University, Cranfield, Bedfordshire MK43 0AL, United Kingdom, a.panda@cranfield.ac.uk

Abstract

The lubrication system is an effective means to serve the power transmitting elements of any engine and is
designed to keep the components like bearings, gears, splines, seals etc. lubricated and cooled in gas turbine engines. In
addition,it serves to remove impurities, support loads, prevent corrosion and acts as a diagnostic medium for condition
monitoring of engine parts. The lubrication system for a gas turbine engine includes a pressure system for supplying oil to
various components, scavenge system to collect the used oil, breathing unit for air evacuation and pressurization unit for
stopping the ingress of oil into gas flow path. With few exceptions, the lubricating system can be of the dry sump design
where bulk of the oil is stored in an engine-mounted separate tank. All gas turbine engine lubrication systems normally use
synthetic oil.Various properties of lubricants affect the system design and behaviour.The proper selection and use of
lubricants, as well as the care and operation of lubricating system is an essential part of any gas turbine maintenance program.
Correct maintenance procedure must be executed to ensure proper amount and type of lubricant getting used, and lubrication
system runs free of contaminants with the aid of protective devices.

Keywords: friction, dry sump design, synthetic oil, properties of lubricants, operation of lubricating system.

1. Introduction The actual area of contact between two surfaces is much

less than the apparent area and occurs only where the as-
1.1 Friction and Surface Roughness: perities of the two surfaces come in to contact. (Fig-1)
Metallic friction and subsequent wear occurs mainly as
some of the asperities break off or wear away.
Whenever engine components are subjected to motion, due The main objective of lubrication is to form a barrier
to metallic contact friction is generated and heat is between these contacting surfaces to prevent damage either
produced .Due to this,avialble shaft power is getting lost by the wearing away of the surfaces or in the more severe
through production of frictional heat.To avoid this caeses ,welding of the asperities.
problem, revolving parts needs to be lubricated while in
motion.
To counter friction and to keep it to a minimum ,
lubricants are being used which can absorb the frictional
heat and carry the heat away from the working
envirenment. Lubrication is very essential in the machines
and engines with rotary comonents..
Generally all engineering surfaces are not truley
smooth and flat.Any machined surafce will have an
undulating shape formed by peaks and valleys .

Fig- 2. Microscopic view of metallic surfaces with lubricant flow.

(Source: Author)

The lubricant film which forms between the two mating

surfaces,keep them apart through out the period of motion.
Thus shifing the metallic friction to liquid friction which is
Fig- 1. Microscopic view of metallic surfaces without lubricant. thousand times better than the mettalic friction. (Fig-2.)
(Source: Author)

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1.2 Necessity of lubrication for gas turbine engines. measure of internal fluid friction by virtue of which fluid
offers a resistance to shear displacement. Higher the
In a gas tubine engine bearings are used to support the viscosity, greater the protection against various failures.But
turbomachineries and gears, which are subjected to both too much viscosity causes excessive heat generation.
axial load and radial load.Lubrication system is essential to Generally viscosity of oil varies with time as shown in
keep the bearing and gears lubricated and also to remove Fig-3.
the heat away from the system.Prolonged safe and reliable
operation of a gas turbine engine can be possible with a
suitably designed and fully operational lubrication system.

1.3 Functions of a lubrication system.

The basic function/requirement of a lubrication system in a

gas turbine engine are as follows:

a) To reduce friction and hence wear of rubbing parts.

b) To protect them from corrosion & pitting.
c) To remove heat generated by friction & convection/
radiation in turbine.
d) To flush the worn out metal particles.
e) To dampen the vibration.
f) Also serves as diagnostic medium for condition moni- Fig- 3. Variation of oil viscosity with time.
toring. (Source: Author)

2. Properties of Lubricants: Viscosity tends to keep the lubricant film from being
squeezed by the pressure of the bearing surfaces. Viscosity
The ability of a lubricant to satisfy above requirements, is usually expressed by kinematic viscosity.The ideal lubri-
together with any other requirement, depends on its funda- cant is one whose viscosity is least affected by the variation
mental characteristics, some of which may be inherent of engine operating temperatures.
properties of the lubricants while others originate from the An indication of rate of change of viscosity of oil with
use of suitable additives. temperature is expressed by its Viscosity Index (VI), higher
The important properties of lubricating oil are value of which indicates less variation with temperature.
viscosity, density, volatility, thermal stability, flash point, Synthetic oils have a higher value of Viscosity Index in
pour point, freeze point, fire point, specific gravity, comparison to mineral oils.
chemical stability, oiliness, volatility etc. Some of the
important properties of aero engine lubricants and their 2.2. Pour point
applicability are listed in Table.1.
Oil gets thicker when the temperature falls. At a particular
temperature the oil stops flowing.[1] This temperature
below which the oil stops to flow is called Pour Point. The
lower the pour point, the better the oil performance at
lower temperatures.[2] The pour point depends on, e.g., the
viscosity and chemical structure of the oil. Mineral oil has
a higher pour point than synthetic oil because of presence
of paraffin or wax in the base oil. (Fig-4)

Table- 1. Properties of lubricants with application environment.

(Source: Author)

2.1 Viscosity Fig- 4. Pour point of different oils. [2]

Undoubtedly, the most important property of any Pour point depressants are added to lubrication oil to
gas turbine lubricant is its viscosity and to be more accu- improve their pour point.
rate, its viscosity/pressure/temperature relationship. It is the

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2.3. Flash-Point surface & maintain a continuous film is known as lubricant

having high degree of oiliness. [5]
The flash point indicates the volatility of a liquid. Flash
point is the temperature at which oil begins to burn, when 2.8. Volatility
ignited with a naked flame, but the burning does not
sustain.[1] Volatility of a lubricant is its tendency to vaporise with the
In gas turbine applications often oil comes in direct increase of temperature. If the lubricant is highly volatile, it
contact with flame or enough heat generated from the will vaporise readily even at low temperature. A good
friction of moving parts will cause the oil to burn. It is lubricant should have low volatility.[5] Generally synthetic
important to use oil with a high flash point as it is more oils contains ester group due to which they are less volatile
stable and will withstand higher temperatures, burning will in comparison to mineral oils.
be less. [2] The flash point is useful in deciding the
transportation and storage temperature requiremnts of 2.9. Thermal oxidation stability
oils.[3](Fig-5)
It is the measure of how well oil can resist the formation of
hard carbon and sludge formation at high temperatures.
Thermal oxidation stability of the oil is determined af-
ter its oxidation at rated values of temperature and time in
tests, by checking the weight of emerged plates in the oil
using different metals and alloys, their condition, acid
number, and viscosity.

Fig- 5. Flash point of different oils. [1]

Flash point is used to isolate chances of any product

contamination.Unusual reduction in flash point indicates
mixing of high volatile substances in oil. [3]

2.4. Fire-Point

Fire point is the temperature at which the oil vapors catch

Fig- 6. Effect of oxidation on lubricated components.
fire with an aid of a external fire source. Fire point is (Source: Author)
usually 10% higher than the flash point.[3]This is an im-
portant property of any gas turbine lubricant, because it is Lubricated surfaces of gas turbine engines are seem to be
very useful in determining fire resistance and volatility of cleaner with the use of synthetic oil than mineral oil as
lubricants at high operating temperatures. shown in Fig-6.This is because synthetic oils have very
good thermal–oxidation stability.
2.5. Self-Ignition Point
2.10. Demulsibilty
It is a temperature at which oil ignites and sustains com-
bustion without application of any external fire source. When oil is mixed with water or water is mixed with oil,
emulsion is formed. A good lubricant is one which does not
2.6. Cloud Point form emulsion and even if it forms, the emulsion should
break quickly. [5] Demulsibility measures oils ability to get
separated from water. It is related to corrosion tendency of
It is the temperature at which dissolved solids in the oil,
lubricating oils.
such as paraffin wax, begin to form and separate from the
oil. As the temperature drops, wax crystallizes and
3. Types of lubrication
becomes visible. Certain oils must be maintained at tem-
perature above the cloud point to prevent clogging of oil
filters. [4] Considering the nature of motion between moving or
sliding surfaces, there are different types of mechanisms by
2.7. Oiliness which the lubrication is done. [6] They are:

The wetting effect of oil which reduces friction is called a. Hydrodynamic lubrication or thick film lubrication
oiliness of oil. It is the power of oil to maintain a continu- b. Hydrostatic lubrication
ous film under pressure while it is used as lubricants. A c. Boundary lubrication or thin film lubrication
lubricant which does not squeeze out from the sliding d. Extreme pressure lubrication

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3.1 Hydrodynamic lubrication or thick film lubrication 3.3 Boundary lubrication or thin film lubrication

Hydrodynamic lubrication is said to exist when the moving Boundary lubrication may happen when two surfaces slide
surfaces are separated by the pressure of a continuous or rotate against one another under heavy load, and in such
unbroken film or layer of lubrication. In this type of lubri- conditions there may be contact between the moving sur-
cation, the load is taken completely by the oil film. [6] faces and film breaks down, an oil is used which interact
The basis of hydrodynamic lubrication is the formation with the solid surface of the metal.(Fig-9)This can be pos-
of an oil wedge. When the journal rotates, it creates an oil sible by adsorption of lubricant molecules on the surfaces
taper or wedge between the two surfaces, and the pressure or by chemical reaction of the lubrication substances with
build up with the oil film supports the load. [6](Fig-7) metal surfaces.[5]

Fig-7. Mechanism of Hydrodynamic lubrication [6]

Hydrodynamic lubrication depends on the relative

speed between the surfaces, oil viscosity, load, and clear-
ance between the moving or sliding surfaces. [6]
In hydrodynamic lubrication the lube oil film thickness
is greater than outlet, pressure at the inlet increases quickly,
remains fairly steady having a maximum value a little to
the outside of the bearing centre line, and then decreases
quickly to zero at the outlet.[6]
Fig- 9. Mechanism of boundary lubrication [5]
3.2 Hydrostatic lubrication

Hydrostatic lubrication is essentially a form of 4. Synthetic Lubricants

hydrodynamic lubrication in which the metal surfaces are
separated by a complete film of oil, but instead of being Synthetic turbo oils are formulated to obtain the following
characteristics:
self-generated, the separating pressure is supplied by an
a. High thermal stability at elevated temperatures
external oil pump.(Fig-8) Hydrostatic lubrication depends b. High resistance to oxidation and corrosion
on the inlet pressure of lube oil and clearance between the c. Low deposit formation tendencies
metal surfaces, whereas hydrodynamic lubrication depends d. High viscosity index
on the relative speed between the surfaces, oil viscosity, e. High load carrying capability (to minimize wear)
load on the surfaces, and clearance between the moving f. Low volatility (minimizes lubricant evaporation loss-
surfaces.[6] es) at high temperatures and low pressures
g. High flash point and auto-ignition temperatures for
safety
h. High resistance to foaming
i. Satisfactory lubricant-elastomer compatibility.

5. Types of lubrication system

a. Closed system-Oil is re-circulated.

b. Open system - Oil is not re-circulated.
c. Wet sump - No oil tank.
d. Dry sump - Use of Oil tank.
e. Cold Tank- Oil is cooled in Scavenge line.
f. Hot Tank - Oil is cooled in Supply line.

Dry sump lubrication is used in all gas turbine engines.

Any Lubrication system is designed to ensure oil supply, to
all the supports, gears & drives of an engine in correct
conditions with regard to cleanliness, pressure, tempt and
quantity throughout all operating conditions and at all
Fig-8. Mechanism of Hydrostatic lubrication [6]
power ratings.

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5.1 Dry sump lubrication system The return oil from the bearing supports and gear box
passes through oil cooler and flows back into the oil tank.
In dry sump lubrication system oil is stored in a separate (Fig-10)
tank outside the engine. There are at least two oil pumps in
a dry sump -one pulls oil from the sump and sends it to the 5.2.2 Hot tank lubrication system
tank, and the other one sucks oil from the tank and sends it
to lubricate the engine parts..[7] In this lubrication system the oil cooler is usually placed in
Dry sump systems gives certain advantages over wet pressure lines. Hot scavenged oil returns directly from the
sumps:[7] engine supports into the oil tank without cooling.(Fig-11)
a. Engine’s centre of gravity can be lowered which im-
proves aerodynamics. The oil capacity of a dry sump is
much more than that of wet sump system.
b. Less chances of oil leakage in to the engine.
c. Less oil consumption.[7]

5.1.1 Subsystems of a dry sump lubrication system

A dry sump lubrication system used in gas turbine engines

is basically consists of the following four sub systems.

a. Oil feed system, lubrication and cooling (Pressure

System) consists of pressure pump, pressure relief valves,
high pressure filter, pipe lines and nozzles.
b. Return oil system (Scavenge system) consists of no of
scavenge pumps, filters, check valves, chip detectors, air
separator, pipe lines, oil coolers etc.
c. Breathing system (Venting) consists of centrifugal
breather, safety valves, check valves and pipe lines.
d. Oil space pressurization system consists of cavity
structure, seals, labyrinths, selector valve, breather valves
and pipe lines.
There are two types of dry sump lubrication system.

5.2.1 Cold tank lubrication system

In a Cold Tank Dry Sump lubrication system oil is getting

cooled in the scavenge line before entering in to the oil
tank. Fig - 11. Schematic layout of Dry sump Hot tank lubrication.
(Source: Author)

6. Lubrication system components

The dry-sump system usually contains several system

components for effective opeartion of the engine
lubrication.

6.1 Oil tank.

Usually constructed of welded sheet aluminium or steel,

it provides a storage place for the oil. In most engines the
tank is pressurized to ensure a constant supply of oil to
the pressure pump. The tank can contain—

 Venting system.
 De-aerator to separate entrained air from the oil.
 Oil level transmitter or dipstick.
 Rigid or flexible oil pickup.
 Coarse mesh screens.
 Various oil and air inlets and outlets.

To allow servicing, devices for filling and draining the oil

tank are provided. Typical locations for the oil tanks on the
Fig -10. Schematic layout of Dry sump Cold tank lubrication.
engine are either the fan case or the accessory gearbox.
(Source: Author)

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The oil tank is usually placed close to the engine and high
enough above the oil pump inlet to ensure gravity feed.
Oil tank capacity and size varies with the different
types of engine and aircraft, but generally it is sufficient to
ensure an adequate supply of oil to for the rotating compo-
nents. Oil tank vent line is provided to ensure proper tank
ventilation with variation of flight altitude. [9]

Fig -13. Schematic layout of oil pressure pump. [12]

(Source: Author & internet)

6.3 Oil filters

To prevent foreign matter from reaching internal parts of

the engine, filters and screens or stainers are provided in
the engine lubricating system.They are usually placed-
Fig -12. Schematic layout of oil tank of a gas turbine engine.  After Pressure Pump
(Source: Author)  Before scavenge pump
 Near supply jets
The typical oil tank has three connections to the lubrica-
 Oil delivery line in to the oil tank, and
tion system. These are the oil supply line to the pressure oil
pump, the oil return line from the scavenge pumps and the  Oil intake in to the pump in the oil tank.
vent line. Scavenge pumps deliver a scavenge oil/air mix-
ture into the tank. (Fig-12) This air is vented through a Various types of oil filters are used in gas turbine lu-
static de-aerator within the tank. [8] brications system and need to be cleaned at regular inter-
In oil tank, pressurization valve is installed in the vent vals to ensure blockages free operation in engines. Filters
line which keeps the air pressure slightly above the ambient are generally cleaned by ultrasonic method.
pressure. This facilitates the oil supply to the pressure Coarse type filters are fitted in each oil return line to
pump during engine start. [8] collect any debris from the lubricated components before
entry in to the scavenge pumps. An example of a pressure
6.2 Pressure pump and scavenge filter is shown in fig.14. [10]

The oil to be supplied is first sucked and pressurized by a

gear type pump. This pump is a positive displacement
pump which generally consists of two pairs of meshed spur
gears that revolves inside a pump casing.
The pump sucks oil form the oil tank via a suction pipe
and delivers to the main pressure line of the oil system.
The pump is generally mounted on the engine gear box
and gets the drive through a splined shaft on to one of the
gears known as the driver gear. Seals are used to prevent
leakage around the drive shaft. (Fig-13)
Generally pressure pumps are provided in association
with a scavenge pump and are housed in a single pump
block.[12]

Fig- 14 A typical pressure and scavenge filter in oil system [10]

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The high pressure filters are generally accompanied Oil pressure at the outlet from delivery pump is adjusted by
by a bypass valve which by passes the unfiltered oil to the pressure relief valve.
engine when the filter blocks, thereby saving the engine
components from any damage during flight. 6.5 Scavenge pumps

Thread-type filters (fig.15) are often fitted as a last chance- Scavenge pumps are also of gear type positive displace-
filter immediately upstream of the oil jets. ment pumps .Separate scavenge pumps are used to collect
return oil from each support bearings and gear boxes. Some
of the scavenge pumps are fitted in a common housing
known as scavenge pump block and gets drive from the
engine gear box. (fig-17)

Fig-15 A typical thread type filter in oil system [10]

6.4 Pressure relief valve

Pressure pumps are usually fitted with a spring operated

pressure relief valve which by passes excess oil in to the
suction inlet of the pump in case of excess pressure
generation by the pump.(Fig-16)

Fig-17 Scavenge pump block of an aero engine.

(Source: Author)

6.6 Oil Cooler

Heat exchanger (Fig.18) cools the oil with fuel supplied to

the main combustion chamber. Cooling of oil takes place
on all operating modes of the engine.
The heat exchanger cools the oil supplied to engine
support, during their intensive heating-up during flights at
high speeds. The fuel supplied to the afterburner chamber
does cooling of oil in a separate heat exchanger.
Switching on the heat exchanger is carried out on the
command from the after burner fuel pump while starting
Fig-16 Oil pressure pump and associated units. [9]
the afterburning mode.[11]

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Dynamic air separators are also used in which oil-air emul-

sion is separated by centrifugal action of baffles mounted
on a driving shaft inside a separate housing .They are
generally mounted on the engine gear box.

6.7 Centrifugal Breather

It communicates the engine transmission / oil cavities with

atmosphere and maintains the cavity pressure within limit
throughout the flying altitude. It also separates oil from air
–oil mixture produced from the bearing cavities. It pre-
vents the oil being discharged to atmosphere together with
air under all operating conditions of the engine.

COMPONENTS

1.CASING. INLET FOR

Inlet of air-oil
AIRemulsion
+ OIL
MIXTURE
2.IMPELLER.
3. GEAR.
4. CHECK VALVE.
5. CASING OF VALVES.
6. BAROSTATIC VALVES
7. ANEROID CAPSULE
1 - Cowling; 6.Tube matrix Air outlet

2,4,7,10 - Union; 8.Valve

3 - Valve; 9.Bracket
5 - Cover; 11.Cover
12.Tube plate 13.Partition Through aneroid valve

Through plate valve

AIR OUTLET OIL OUTLET
Fig-18 Layout of a Fuel-oil heat exchanger.
(Source: Author)
Fig-20 Centrifugal breather
(Source: Author)
In case of increase of resistance in the inter-pipe cavity of
the heat exchanger, the valve 3 opens, and a part of oil goes
The breather is mounted on engine accessory gearbox, gets
into the engine, by-passing the heat exchanger. Fuel-oil
its rotation from the high-pressure rotor through gears of
heat exchanger is mounted in the upper part of the external
contour casing. gear box. On the shaft of the pinion is mounted an impeller,
separating oil from air. The separated oil is supplied into
6.6 Air separator the cavity of same gear box, and the air is exhausted into
the atmosphere.(Fig-20)
Air separator is meant for separation of oil coming from
mainlines of scavenging system, from air. The air separator Breathing up to a height of 5 - 8 kms in flight is carried out
(Fig- 19) is located inside the oil tank. Oil-air emulsion is through the barostatic valve controlled by a set of sensitive
supplied to the pipeline in a tangential direction to the air elements. After closing the barostatic valve, the non-return
separator in which oil, being separated from air, flows valve of the system maintains the pressure, above the
down the oil tank, and air through holes of the air separator atmospheric (0.10-0.15 kgf/cm2), necessary for operating
leaves in the upper part of the oil tank. the oil-scavenging pump in high-altitude conditions.

6.8 Metal chip detector

Chip detectors are usually employed to detect an excess

amount of metal chips produced in the engine oil sys-
tem.(fig-21)

4. Current conductors. 5. Insualtors.

Fig-19 A typical static air separator. Fig-21 A typical metal chip detector.
(Source: Internet) (Source: Author)

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As soon as the metal particles enter the clearances between ized oil, the same is supplied to different rotating compo-
the current conducting washers, the resistance of the sensi- nents like gears, bearings and seals through supply pipe-
tive element is sharply decreased and a signal is fed. They lines and flow nozzles.
are generally fitted in the scavenge line of the oil system . The used oil gets collected by separate scavenge pumps
fitted with different bearing supports and gear boxes. The
6.9 Oil space pressurization unit scavenge pumps collect return oil through scavenge filters
and pressurize it and delivers the same to oil cooler through
This unit is meant for protection of air gas path of engine air separator and chip detectors. The cooled oil form the
against the falling of oil from rotor supports. The oil in the cooler gets back to the oil tank, thus completing one cycle.
bearing cavities is subjected to differential air pressure Oil tank, gear boxes and all the bearing supports are
across the sealing rings in the bearing housing. For doing connected to atmosphere through separate vent lines for
this compressed air is used which is tapped from fan or LP breathing.Pressure controlled in all the three cavities of the
compressor end during initial periods and then from the pressurization unit ensures constant pressure drop in all the
higher stage HP compressor stages during higher modes and flight of the aircraft (from the pressurization
modes.(Fig-22) cavity to the oil cavity), on the oil contact seals, thus
excluding the possibility of entering of oil and vapors in
the air-gas path of the engine.The filtration capacity of the
oil filter should not be coarser than 40 to 60 microns.
Each rotating engine component needs lubrication and
has its own specific amount of oil requirement for proper
lubrication and cooling. The oil flow to the lubricated areas
as per the need is provided by the cross section of the
supply lines and the oil nozzles.

8. Oil System Indications and condition Monitoring

There are certain performance parameters of oil system

which are measured and/or monitored for effective engine
lubrication throughout flight. The following data must be
indicated to the pilot for effective condition monitoring.

 Oil Quantity
 Oil Pressure
 Oil Temperature
 Low or high Oil Pressure Warning
Fig-22 Bearing cavity pressurization unit.  High oil temperature warning.
(Source: Author)  Oil Filter Clogging.
The sensors are located at predefined locations within the
7. Functioning of a typical aero-engine lubrication lubrication system. The data provide by the different sen-
system . sors are used to monitor the system functioning. In addition
to this, different types chip detectors are used for wear
A schematic description of an aero engine lubrication debris monitoring and are usually placed in the oil scav-
system is shown below enge lines.

8.1 Oil level sensor

Generally capacitance type level sensors are used to read

the exact oil quality remaining inside the oil tank while
flying as well as on ground.(Fig-24)

Fig.23 Typical layout of a lubrication system.

(Source: internet)

In this engine oil drawn by the gear type pressure pump

from the oil tank and after pressurization it passes through Fig- 24 Oil level sensor.
high pressure filter. After proper filtration of the pressur- (Source: Internet)

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8.2 Oil pressure sensor. 9. Oil system Maintenance Procedure

The oil pressure sensor is connected to one of the oil pipe 9.1 Oil consumption
lines in the supply system.[8]They are usually fitted in
pressure line just before delivery to the supply nozzle and Oil consumption check is carried out to collect information
pipelines. (Fig-25) about the sealing efficiency of the bearing compartment
seals and for the early detection of leaks.[8]

9.2 Oil analysis

Spectrographic oil analysis program (SOAP) to analyze

the concentration of different type’s metal particles in the
used oil. The results of this test give a fair amount of idea
about condition of different engine components. [8]

9.3 Oil replacement.

With prolonged engine operation oil used in gas turbine

lubrication system degrades over a period of time. Due to
oxidation, chemical reactions and acidic environment
viscosity of oil increases with formation of gummy depos-
its, varnishes and sledges from wear debris.
Therefore after a stipulated period of engine running
time, all the oil from the lubrication system is evacuated
and the system is charged with fresh oil.
Fig- 25 Oil pressure sensor
(Source: Author)
10. Conclusion
8.3 Oil temperature sensor.
The lubrication system plays a vital role in safe and reliable
exploitation of any gas turbine engine. It improves engines
The oil temperature sensor can be located in scavenge
life by ensuring optimum engine performance with least
system or in the supply system. The position in individual
degradation of engine components. The functionality of
system is selected by the designer to keep the maximum
any lubrication system is linked to the good quality lubri-
indicated values at approximately 200°C. [8] cants and correct maintenance habits.

8.3 Low or high Oil Pressure Warning References

Low/High pressure warning switches are provided in addi- [1] http://www.nesteoil.com .(Accessed 19th January 2013)
tion to a pressure gauge to indicate that a minimum pres- [2] http://www.ehow.com. ( Accessed 19th January 2013)
sure is available for continued safe operation of the aero [3] http://www.engineersedge.com/lubrication
engine. These switches are generally connected to a warn- (Accessed 19th January 2013)
ing lamp in the flight compartment and the lamp illumi- [4] http://www.engineersedge.com/lubrication
(Accessed 23rd January 2013)
nates if the pressure falls below an acceptable minimum.
[5] http://www.vidyabhartitrust.org
(Accessed 27th January 2013)
8.4 Oil filter clogging indicator. [6] http://www.brighthubengineering.com
(Accessed 27th January 2013)
It is used for early detection of filter blocking .Whenever [7] http://auto.howstuffworks.com
filter elements are clogged, resistance of the elements (Accessed 29th January 2013)
increased and at a pressure difference of 0.4 to 0.6 kg/cm² [8] http://www.springer.com
across the filter, the pressure sensor gives to the indication (Accessed 29th January 2013)
[9] http://www.greatwarreplicaaircraft.com
in the cockpit. With this early warning pilot can take neces-
(Accessed 30th January 2013)
sary action avoiding damage to bearings. [10] Jet engine © Rolls-Royce plc 1986, ISBN 0902121 235
Fifth edition. Reprinted 1996 with revision.
8.5 High oil temperature sensor [11] http://www.aircraftextras.com
(Accessed 30th January 2013)
In some gas turbine engines a special type of thermo chip [12] http://www.pumpfundamentals.com
detectors are used for generating high oil temperature (Accessed 30th January 2013)
signal. Generally signal is provided by melting of a fusible
insert in an electric circuit. The signal is generated as the
oil temperature reaches nears about 90% of its fire point.

10