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rotating shaft. The oil is then collected and returned
to the gearbox.
Materials
27. To reduce weight, the lightest materials possible
are used. The internal gearbox casing is cast from
aluminium but the low environmental temperatures
that an external gearbox is subjected to allows the
use of magnesium castings which are lighter still.
The gears are manufactured from non-corrosion
resistant steels for strength and toughness. They are
case hardened to give a very hard wear resistant
skin and feature accurately ground teeth for smooth
gear meshing.
Accessory drives
71
Fig. 7-5 An external gearbox with auxiliary gearbox drive.
Rolls-Royce Tay
Bristol Theseus
This engine was conceived in 1940 as a 4000
hp turbo-prop but was later scaled down to
2000 hp. Named the Theseus the engine was
type tested in December 1946. the world’s first
turbo-prop to reach this stage of development.
After extensive flight testing in an Avro
Lincoln, four Theseus engines were installed
in a Handley-Page Hermes 5.
INTRODUCTION
1. The lubrication system is required to provide
lubrication and cooling for all gears, bearings and
splines. It must also be capable of collecting foreign
matter which, if left in a bearing housing or gearbox,
can cause rapid failure. Additionally, the oil must
protect the lubricated components which are manufactured
from non-corrosion resistant materials. The
oil must accomplish these tasks without significant
deterioration.
2. The requirements of a turbo-propeller engine are
somewhat different to any other types of aero gas
turbine. This is due to the additional lubrication of the
heavily loaded propeller reduction gears and the
need for a high pressure oil supply to operate the
propeller pitch control mechanism.
3. Most gas turbine engines use a self-contained
recirculatory lubrication system in which the oil is
distributed around the engine and returned to the oil
tank by pumps. However, some engines use a
system known as the total loss or expendable system
in which the oil is spilled overboard after the engine
has been lubricated.
LUBRICATING SYSTEMS
4. There are two basic recirculatory systems,
known as the ’pressure relief valve1 system and the
’full flow’ system. The major difference between them
is in the control of the oil flow to the bearings. In both
systems the temperature and pressure of the oil are
critical to the correct and safe running of the engine.
Provision is therefore made for these parameters to
be indicated in the cockpit.
8: Lubrication
Contents Page
Introduction 73
Lubricating systems 73
Pressure relief valve system
Full flow system
Total loss (expendable) system
Oil system components 77
Lubricating oils 83
73
Pressure relief valve system
5. In the pressure relief valve system the oil flow to
the bearing chambers is controlled by limiting the
pressure in the feed line to a given design value. This
is accomplished by the use of a spring loaded valve
which allows oil to be directly returned from the
pressure pump outlet to the oil tank, or pressure
pump inlet, when the design value is exceeded. The
valve opens at a pressure which corresponds to the
idling speed of the engine, thus giving a constant
feed pressure over normal engine operating speeds.
However, increasing engine speed causes the
bearing chamber pressure to rise sharply. This
reduces the pressure difference between the bearing
chamber and feed jet, thus decreasing the oil flow
rate to the bearings as engine speed increases. To
alleviate this problem, some pressure relief valve
systems use the increasing bearing chamber
pressure to augment the relief valve spring load, This
maintains a constant flow rate at the higher engine
speeds by increasing the pressure in the feed line as
the bearing chamber pressure increases.
6 Fig. 8-1 shows the pressure relief valve system
for a turbo-propeller engine and indicates the basic
components that comprise an engine lubrication
system. The oil pressure pump draws oil from the
tank through a strainer which protects the pump
gears from debris which may have entered the tank,
Oil is then delivered through a pressure filter to the
pressure relief valve which maintains a constant oil
delivery pressure to the feed jets in the bearing
chambers. Some engines may have an additional
relief valve (pressure limiting valve) which is fitted at
the oil pressure pump outlet. This valve is set to open
　
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