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the ring is allowed to move in its housing whenever
the shaft comes into contact with it.
18. Ring seals are used for bearing chamber
sealing, except in the hot areas where oil
degradation due to heat would lead to ring seizure
within its housing.
Hydraulic seals
19. This method of sealing is often used between
two rotating members to sea a bearing chamber.
Unlike the labyrinth or ring seal, it does not allow a
controlled flow of air to traverse across the seal,
20. Hydraulic seals (fig. 9-7) are formed by a seal
fin immersed in an annulus of oil which has been
created by centrifugal forces. Any difference in air
pressure inside and outside of the bearing chamber
is compensated by a difference in oil level either side
of the fin.
Carbon seals
21. Carbon seals (fig. 9-7) consist of a static ring of
carbon which constantly rubs against a collar on a
rotating shaft. Several springs are used to maintain
contact between the carbon and the collar. This type
of seal relies upon a high degree of contact and does
not allow oil or air leakage across it. The heat caused
by friction is dissipated by the oil system.
Brush seals
22. Brush seals (fig. 9-7) comprise a static ring of
fine wire bristles. They are in continuous contact with
a rotating shaft, rubbing against a hard ceramic
coating. This type of seal has the advantage of withstanding
radial rubs without increasing leakage.
Hot gas ingestion
23. It is important to prevent the ingestion of hot
mainstream gas into the turbine disc cavities as this
would cause overheating and result in unwanted
thermal expansion and fatigue. The pressure in the
turbine annulus forces the hot gas, between the
rotating discs and the adjacent static parts, into the
turbine disc rim spaces. In addition, air near the face
of the rotating discs is accelerated by friction causing
it to be pumped outwards. This induces a complementary
inward flow of hot gas.
24. Prevention of hot gas ingestion is achieved by
continuously supplying the required quantity of
cooling and sealing air into the disc cavities to
oppose the inward flow of hot gas. The flow and
pressure of the cooling and sealing air is controlled
by interstage seals (fig. 9-5),
CONTROL OF BEARING LOADS
25. Engine shafts experience varying axial gas
loads (Part 20) which act in a forward direction on the
compressor and in a rearward direction on the
turbine. The shaft between them is therefore always
under tension and the difference between the loads
is carried by the location bearing which is fixed in a
static casing (fig. 9-8). The internal air pressure acts
Internal air system
91
Fig. 9-6 A generator cooling system.
Internal air system
92
Fig. 9-7 Typical seals.
upon a fixed diameter pressure balance seal to
ensure the location bearing is adequately loaded
throughout the engine thrust range.
AIRCRAFT SERVICES
26. To provide cabin pressurization, airframe antiicing
and cabin heat, substantial quantities of air are
bled from the compressor. It is desirable to bleed the
air as early as possible from the compressor to
minimize the effect on engine performance.
However, during some phases of the flight cycle it
may be necessary to switch the bleed source to a
later compressor stage to maintain adequate
pressure and temperature.
Internal air system
93
Fig. 9-8 Control of axial bearing load.
Rolls-Royce Gem 60
Rolls-Royce AJ65 Avon
Work commenced early in 1945 on the AJ65
axial flow turbo-jet with a design thrust of 6500
lb. This figure was reached in 1951 with the
100 series RA3. In 1953 the considerably
redesigned 200 series RA14 was type tested
at 9500 lb thrust. Development culminated in
the 300 series RB146 which produced 17.110
lb thrust with afterburning.
10: Fuel system
Contents Page
Introduction 95
Manual and automatic control 96
Fuel control systems 99
Pressure control (turbo-propeller engine)
Pressure control (turbo-jet engine)
Flow control
Combined acceleration and speed
control
Pressure ratio control
Electronic engine control 111
Speed and temperature control amplifiers
Engine supervisory control
Low pressure fuel system 112
Fuel pumps 112
Plunger-type fuel pump
Gear-type fuel pump
Fuel spray nozzles 114
Fuel heating 116
Effect of a change of fuel 116
Gas turbine fuels 117
Fuel requirements
Vapour locking and boiling
Fuel contamination control
INTRODUCTION
1. The functions of the fuel system are to provide
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