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10 LUBRICATION SYSTEMS
10.1 INTRODUCTION
There is always friction when two surfaces are in contact and moving, for even
apparently smooth surfaces have small undulations, minute projections and
depressions and actually touch at only a comparatively few points. Motion makes the
small projections catch on each other and, even at low speeds when the surface as a
whole is cool, intense local heat may develop leading to localised welding and
subsequent damage as the two surfaces are torn apart. At higher speeds and over
longer periods, intense heat may develop and cause expansion and subsequent
deformation of the entire surface; in extreme cases large areas may be melted by the
heat, causing the metal surfaces to weld together.
The gas turbine engine is designed to function over a wider environment and under
different operating conditions from its piston engine equivalent and therefore special
lubricants have been developed to cope with the following main problems:
a. High rpm compared with piston engines.
b. Cold starting in winter can mean initial bearing temperatures of -54°C which
rapidly increases after starting to 232°C. Therefore a good viscosity index and
adequate cooling are required.
On the other hand, the following advantages can be claimed for the gas turbine:
a. There are fewer bearings and gear trains.
b. Oil does not lubricate any parts directly heated by combustion and therefore oil
consumption is low.
c. There are no reciprocating loads.
d. Bearings are generally of the rolling contact type and therefore only low oil
pressures are needed (40 psi is normal).
Turbo-prop engine lubrication requirements are more severe than those of a turbo-jet
engine because of the heavily loaded reduction gears and the need for a high-
pressure oil supply to operate the propeller pitch control mechanisms. (For example,
a twin relief valve in the Dart provides 35 psi for engine lubrication and 70 psi, which
is fed to the propeller controller and boosted by a further pump to a pressure of 600
psi).
10.2 BEARINGS
The early gas turbines employed pressure lubricated plain bearings but it was soon
realised that friction losses were too high and that the provision of adequate
lubrication of these bearings over the wide range of temperatures and loads
encountered was more difficult than for piston engine bearings.
As a result, plain bearings were abandoned in favour of the rolling contact type as the
latter offered the following advantages:
a Lower friction at starting and low rpm.
b Less susceptibility to momentary cessation of oil flow.

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c The cooling problem is eased because less heat is generated at high rpm.
d The rotor can be easily aligned.
e The bearings can be made fairly small and compact.
f The bearings are relatively lightly loaded because of the absence of power
impulses.
g Oil of low viscosity may be used to maintain flow under a wide range of conditions
and no oil dilution or pre-heating is necessary.
The main bearings are those which support the turbine and compressor assemblies.
In the simplest case (a single spool engine), these usually consist of a roller bearing
at the front of the compressor and another in front of the turbine assembly, with a ball
bearing behind the compressor to take the axial thrust on the main shaft. “Squeeze
film” main bearings have been introduced to reduce transfer of rotor vibration to the
aircraft. In this type of bearing pressure oil is fed to a small annular space between
the bearing outer track and the housing. Figure 10.1. shows that the bearing will
therefore “float” in pressure oil, which will damp out much of the vibration. Squeeze
film bearings are fitted to the Spey and all subsequent aero engines produced by
Rolls-Royce (1971) Ltd. They have also been fitted retrospectively to existing
engines. In addition to the main bearings, lubrication will also be required for the
wheelcase, tacho-generator, CSDU, alternator, starter and fuel pump drives.

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Squeeze Film Bearing.

Figure 10.1.

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Single Spool

Twin Spool Turboprop Engine.

Bearing Location Comparison.
Figure 10.2.

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10.3 ENGINE LUBRICATION SYSTEMS

There are basically two types of lubrication system at present in use in gas turbine
engines:-
a) Recirculatory. In this system, oil is distributed and returned to the oil tank by
pumps. There are two types of recirculatory system:-
(i) Pressure relief valve system.
(ii) Full flow system.
b) Expendable. The expendable or total loss system is used on some small
turbo-jet engines, eg. RB 162 in which the oil is spilled overboard after
lubricating the engine.
10.3.1 PRESSURE RELIEF VALVE RE-CIRCULATORY SYSTEM
In the pressure relief valve type of recirculatory lubrication system the flow of oil to
the various bearings is controlled by a relief valve which limits the maximum pressure
in the feed line. As the oil pump is directly driven by the engine (by the HP spool in
the case of a multi-spool engine), the pressure will rise with spool speed. Above a
pre-determined speed the feed oil pressure opens the system relief valve allowing
excess oil to spill back to the tank, thus ensuring a constant oil pressure at the higher
engine speeds.
A typical relief valve type of recirculatory lubrication is shown in the figure 10.3.
The oil system for a typical turbo-prop engine is similar but, as it supplies the
propeller control system, it is more complicated. The oil supply is usually contained in
a combined tank and sump formed as part of the external wheelcase. Oil passes via
the suction filter to the pressure pump, which pumps it through the air-cooled oil
cooler to the pressure filter. A pressure regulating valve upstream of the filter
controls the oil pressure. Both oil pressure and temperature indications are
transmitted to the cockpit. The oil flows through pipes and passages to lubricate the
main shaft bearings and wheelcases. The main shaft bearings are normally
lubricated by oil jets and some of the heavier loaded gears in the wheelcases are
also provided with oil jets, while the remaining gears and bearings receive splash
lubrication.
An additional relief valve is fitted across the pump in the lubrication system of some
engines to return oil to pump inlet if the system becomes blocked.

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Cold Tank Variation

Hot Tank Variation

A Pressure Relief Valve Lubrication System for a Two Shaft Turbojet.

Figure 10.3.

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A Turboprop Full Flow Oil System.

Figure 10.4.

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10.3.2 RECIRCULATORY OIL SYSTEM – FULL FLOW TYPE

The full flow lubrication system is an alternative to the pressure relief valve oil system
and full flow systems are in use as a means of lubricating many modern high power
gas turbine engines.
The full flow system is similar in many ways to the pressure relief system just
discussed – i.e. oil is drawn from a tank by a pump and delivered, via a pressure
filter, to various parts of the engine; the oil is then returned by scavenge pumps, via
the oil cooler to the tank; also, air is separated from the oil by a de-aerator and
centrifugal breather.
The major differences from the pressure relief type of recirculatory system are as
follows:-
• The flow of oil to the bearings is determined by the speed of the pressure pump,
the size of the oil jets and the pressure in each of the bearing housings.
• A metered spill of feed oils is fed back to the tank. This spill is calibrated to match
the pump output to ensure that the oil flow to the bearings, via the oil jets, is the
same at all engine speeds.
• The relief valve in this system is set to prevent excessive oil pressure in the feed
side of the system.
10.3.3 ADVANTAGES OF FULL FLOW LUBRICATION
The advantages of full flow lubrication are those of suitable oil flow and oil pressure
at all engine speeds. A study of the graph will reveal a difference in oil pressure
between the pressure relief system and the full flow system and, it will also show that
the pressure difference continues throughout the speed range of the engines, with a
crossover point at cruising speed. The relief valve system provides too much oil
pressure at idle rev/min, but because of the relief valve, the oil pressure is below
optimum at maximum engine speed. In contrast the pressure provided by the oil
pump of a full flow system rises progressively with increased engine speed and is
nearer to the optimum value throughout the rev/min range of the engine.

Comparison of Full Flow and Relief Valve Systems.

Figure 10.5.
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Full Flow Oil System ( RR Gem).

Figure 10.6.

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10.3.4 EXPENDABLE SYSTEM

An expendable system is generally used on small engines running for periods of
short duration. The advantage of this system is that it is simple, cheap and offers an
appreciable saving in weight as it requires no oil cooler, scavenge pumps or filters.
Oil can be fed to the bearing either by a pump or tank pressurisation. After
lubrication the oil can either be vented overboard through dump pipes or leaked from
the centre bearing to the rear bearing after which it is flung onto the turbine and
burnt.

An Expendable Oil System.

10.4 MAIN COMPONENTS Figure 10.7.
In any aircraft oil system, we have a number of components that may be thought of
as the main components and we have some that are incorporated to safeguard the

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system (ie. to act as safety devices). The main components, on which the operation
of the system depends, include the oil tank, the oil pump and the oil cooler; these are
considered in the paragraphs immediately following. The safety devices, which
include the various valves and filters, are considered later.
10.4.1 OIL TANK
The oil tank is usually mounted on the engine; it may be a separate unit or part of an
external gearbox called the sump. It has provision to allow the system to be filled
and drained and a sightglass or dipstick to allow the oil contents to be checked.
Usually, the oil level sightglass on the side of the tank is graduated in half-pint or in
litre increments, between LOW and FULL marks. The tank is replenished either by
pressure or by gravity feed. The pressure filler connection contains a non-return
valve and a bayonet adapter to which the oil replenishment trolley pipe is connected.
A de-aerator tray is mounted in the top half of the tank and receives the return oil
from the scavenge pumps. The oil in its passage through the system will become
aerated and steps must be taken to remove the air. As the oil/air mixture flows over
the tray, the
oil separates
and drains
down into the
sump, whilst
the air is
vented to
atmosphere.

Typical Oil Tank

Figure 10.8

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10.4.1.1 Oil Pumps

The oil pumps fitted in a recirculatory system are normally gear-type or Gerotor type
pumps. The pumps are usually mounted in a pack containing one pressure pump
and several scavenge pumps. They are driven by a common shaft through the
engine gear train.
Gear type pumps (Fig. 10.10. ) require suitable machining of the gear teeth, or the
provision of a milled slot in the casting (adjacent to the delivery side of each pump),
to prevent pressure locking of the gears.
Gerotor type pumps (Fig. 10.11.) use an inner and outer rotor, where the inner rotor
is driven by the engine, and the outer rotor which has an extra gear tooth rotates with
it. The inner rotor is eccentric to the outer and it is the stepping of the teeth that
pumps the oil. The pump also requires kidney shaped slots as inlet and outlet ports.
The scavenge pumps have a greater capacity than the pressure pump to ensure
complete scavenging of the bearings in a dry sump system. Furthermore, air tends
to leak into the bearing housings from the air pressurised seals and this aeration of
the oil means that the scavenge pumps have to pump an increased oil/air volume.
As we saw in the previous paragraph the air is subsequently removed by the de-
aerator.

Typical Gear Type Oil Pump.

Figure 10.9.

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Gear Type Pump.

Figure 10.10.

Gerotor Type Pump.

Figure 10.11.

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10.4.2 OIL COOLING

All engines transfer heat to the oil by friction, churning and windage within a bearing
chamber or gearbox. It is therefore common practice to fit an oil cooler in
recirculatory oil systems. The cooling medium may be fuel or air and, in some
instances, both fuel-cooled and air-cooled coolers are used.
Some engines which utilise both types of cooler may incorporate an electronic
monitoring system which switches in the air-cooled oil cooler (ACOC) only when it is
necessary. This maintains the ideal oil temperature and improves the overall thermal
efficiency.
The fuel-cooled oil cooler (FCOC) has a matrix which is divided into sections by
baffle plates. A large number of tubes convey the fuel through the matrix, the oil
being directed by the baffle plates in a series of passes across the tubes. Heat is
transferred from the oil to the fuel, thus lowering the oil temperature.
The fuel-cooled oil cooler incorporates a bypass valve fitted across the oil inlet and
outlet. The valve operates at a pre-set pressure difference across the cooler and
thus prevents engine oil starvation in the event of a blockage. A pressure
maintaining valve is usually located in the feed line of the cooler which ensures that
the oil pressure is always higher than the fuel pressure. In the event of a cooler
internal fault developing, the oil will leak into the fuel system rather than the
potentially dangerous leakage of fuel into the oil system.

Typical Fuel Cooled Oil Cooler.

Figure 10.12.

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The air-cooled oil cooler is similar to the fuel-cooled type both in construction and in
operation – except, of course, that air replaces the fuel as the cooling agent. On
some engines, the airflow through the matrix is controlled by a flap valve, which is
automatically operated when the temperature of the return oil rises to a pre-
determined value. A turbo-propeller engine may be fitted with an oil cooler that
utilises the external airflow as a cooling medium. This type of cooler incurs a large
drag factor and, as kinetic heating of the air occurs at high forward speeds, it is
unsuitable for turbo-jet engines.
10.4.2.1 Pressure Filter
The pressure oil filter housing contains a wire-wound or mesh, Paper or felt
elements and incorporates a by-pass valve. The filter housing can be drained
independently of the main oil system. This is done through a drain valve in the
housing base. When drained, the filter can be removed for examination, servicing, or
replacement, as necessary, without disturbing the rest of the system. Typical
pressure filters are illustrated in figure 10.13.

Wire Wound and Paper Type Oil Filters.

Figure 10.13.

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Filters are usually fitted with an impending by-pass indicator. This is usually a red
pop out indicator which will pop out and stay out it the differential pressure across the
filter element exceeds a predetermined value. This value will be less than the by-
pass valve value, to allow the filter to be replaced before the filter goes into by-pass.
The pop out usually has a thermal lock on it, which prevents the pop out extending
when the oil is cold and thick.

Filter Bowl with Pop Out Indicator.

Figure 10.14.

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10.4.3 LAST CHANCE FILTER

Some of the gears in the gearboxes and also the main bearings of the engine are
lubricated through oil jets. These jets are usually protected by thread-type or small
fine mesh filters. These are often referred to as last chance filters.

Thread Type Last Chance Filter

Figure 10.15.

10.4.4 SCAVENGE OIL STRAINERS

When the oil has been distributed to all parts of the engine and has done its job, it is
returned to the oil tank by either gravity or pressure from the scavenge pumps. Each
pump returns the oil from a particular part of the engine and is protected by a coarse
filter (or strainer) in the return line. This arrangement protects the pump gears. It also
gives an indication of impending component failure if the strainers are examined for
metal particles during periodical inspection.

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10.4.5 MAGNETIC CHIP DETECTOR

Magnetic detectors may be fitted into the oil system at various points to collect and
hold ferrous debris. They are normally fitted in gearboxes and in the scavenge pump
return lines to the tank. The collection of ferrous particles on the chip detector
provides a warning of impending (or incipient) failure of a component. Some
detectors are designed so that they can be removed for periodical examination
without having to drain the oil system; others may be checked externally by
connecting a suitable test circuit to the plug; finally, some are connected to a cockpit
warning system to give an in-flight indication of failure. The chip detector (see figure
10.16.) fits into a self-sealing housing and has a bayonet-type fitting for easy
removal.

Magnetic Chip Detector.

Figure 10.16.

10.4.6 DE-AERATOR
We have already noted that air from the bearing sealing system mixes with the oil
and causes frothing. If the air is allowed to remain in the oil it may cause a
lubrication failure. To prevent this, a de-aerating device may be installed; this
removes air from the oil before the oil is re-circulated round the engine by the
pressure pump; the air can be vented to atmosphere via the centrifugal breather.
De-aerators are usually tray types fitted in the oil tank or centrifugal type as a
separate item.

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10.4.7 CENTRIFUGAL BREATHER

When the oil/air mixture returns
to the tank the air is separated by
the de-aerator tray and passes
through to the gearbox via a vent
line. It carries some of the oil with
it in the form of a fine mist. The
oil/air mist in the gearbox can
then pass to the centrifugal
breather (see figure 10.17). As
the vanes of the centrifugal
breather rotate, the oil in the
mixture is caught in the vanes
and thrown back into the
gearbox; the air being vented to
atmosphere.

Centrifugal Breather.
Figure 10.17.

10.4.8 PRESSURE RELIEF VALVE

The pressure relief valve shown in the
figure 10.18. controls the oil pressure
at the pre-set value demanded by the
system. The valve is normally
integral with the pump assembly and
protects the system from excessive
pressure. It is usually a spring-loaded
plate-type valve, and can on some
engines provide adjustment of
pressure setting.

Simple Pressure Relief Valve

Figure 10.18.

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It is more usual to find a pressure relief valve that varies the pressure with engine
speed or breather pressure. These valves are usually adjustable but usually only
effect the max speed oil pressure see Figure 10.19.

Pressure Relief Valve That Uses Breather Pressure to Vary Pressure.

Figure 10.19

This type of valve uses the oil system breather pressure and an adjustable spring to
balance the oil pressure in the main oil feed line to the engine bearings.
Consider Fig. 10.19. With the engine running, the breather pressure plus the spring
push the sliding valve to the left and restrict the pump spill back to return. This is
balanced by the pressure from the main feed line trying to move the slide valve to the
right. Should the pressure in the main feed line fall, the breather pressure and spring
will move the slide valve further to the left and restrict the oil spill still further. This will
allow more oil to flow to the system, and the oil pressure in the main feed line will
increase. The slide valve will then move to the right, and the oil spill to the return will
be controlled by the main feed line pressure balancing the spring and breather
pressure.
10.4.9 BY-PASS VALVE
This is similar in construction to the normal pressure relief valve just discussed. It is
connected in the system in such a way that, should the oil cooler or the pressure filter
become blocked (so that the oil flow is restricted), the appropriate by-pass valve will
operate to re-route the oil. Although the cooling or the filtering has now been by-
passed, oil starvation of the oil bearings is prevented. Pop–out indicators are used to
warn of an impending by-pass.
The oil cooler will usually have a thermal by-pass valve which will by-pass the cooler
when the oil is cold, thus ensuring that the oil gets up to running temperature quickly.

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10.4.10 INDICATIONS AND WARNINGS

Indications and warnings vary from aircraft to aircraft, in both the warnings given and
the priority that they are given.

10.4.11 LOW PRESSURE WARNING LAMP

If the oil pressure drops below the safe operating value for the particular system, a
pressure-sensing switch will initiate a visual warning; the warning usually consists of
a red or amber lamp switching on in the cockpit accompanied by an audio warning.
The sensing switch may be a differential pressure switch that senses the pressure
difference between the feed oil pressure and the return oil pressure or a simple
pressure switch. When the pressure or difference falls below a pre-determined level,
the switch operates to activate the warning circuit. To reduce the cockpit noise during
taxiing, the audio warning may be inhibited, as engines are often shut down before
reaching the stand.
Although this system is simple, its warning factor may not be quick enough to prevent
serious damage to the engine. This is due to the fact that the warning pressure must
be below the normal oil pressure at idle RPM. This means that the engine could be
running for some time with a low oil pressure before the warning occurs. To
overcome this problem multiple pressure switches are used and activated at differing
engine RPM’s. For instance, above 85% RPM the low oil pressure warning will come
‘ON’ at 50 psi, below 85% the warning will come on at 20psi.
This is a serious warning and the engine must be shut down as soon as possible.
10.4.12 OIL FILTER CLOGGED INDICATION
Some aircraft employ a differential pressure switch to provide an indication to the
flightdeck of impending blockage of the engine main oil filter.
10.4.13 OIL PRESSURE, TEMPERATURE AND QUANTITY INDICATION
See section 14 engine indications for details of these systems.

10.5 OIL SEALS

Oil seals have been covered in section 8.
10.6 SERVICING
The engine oil level is usually checked after flight or after an engine run. It is not
checked straight after shut-down, as entrained air will give a false reading. It cannot
be checked accurately if left too long as the oil may run out of the tank into the
gearbox. So it is normally checked between 20 minutes and 2 hours or as defined in
the aircraft maintenance manual.
The oil system magnetic chip detectors will be checked at the periodicity defined in
the maintenance schedule. Spectrometric Oil Analysis Program (SOAP) samples of
the oil may be taken when required.
Filters are replaced when required by the maintenance schedule or if the pop out
indicator is out.

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Intentionally Blank

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