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main difference is that air is used as the cooling
medium.
26. Magnetic plugs, or chip detectors (fig. 8-8), are
fitted on the scavenge (return) side to collect ferritic
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79
Fig. 8-6 A single-shot oil pump.
Lubrication
80
Fig. 8-7 A low pressure fuel-cooled oil cooler.
Fig. 8-8 A magnetic chip detector.
debris from each bearing chamber. They are
basically permanent magnets inserted in the oil flow
and are retained in self-sealing valve housings.
Safety features incorporated in the design ensure
correct retention within the housing. Upon
examination they can provide a warning of
impending failure without having to remove and
inspect the filters. They are designed to be removed
during maintenance inspection, for condition,
monitoring purposes (Part 24), without oil loss
occurring. Additionally they may be connected to a
cockpit warning system to give an in-flight indication.
27. In some engines, to minimize the effect of the
dynamic loads transmitted from the rotating
assemblies to the bearing housings, a ’squeeze film’
type of bearing is used (fig. 8-9). They have a small
clearance between the outer race of the bearing and
housing with the clearance being filled with oil. The
oil film dampens the radial motion of the rotating
assembly and the dynamic loads transmitted to the
bearing housing thus reducing the vibration level of
the engine and the possibility of damage by fatigue.
28. To prevent excessive air pressure within the oil
tank, gearboxes and bearing chambers, a vent to
atmosphere is incorporated within the lubrication
system. Any oil droplets in the air are separated out
by a centrifugal breather prior to the air being vented
overboard. Some breathers may incorporate a
porous media, forming de-aerator segments, which
improves the efficiency of the oil separation (fig, 8-
10).
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81
Fig. 8-9 A squeeze film bearing.
Fig. 8-10 A centrifugal breather.
Fig. 8-11 A thread-type oil filter.
29. To prevent foreign matter from continuously
circulating around the lubricating system, a number
of filters and strainers are positioned within the
system.
30. Coarse strainers are usually fitted at the outlet
of the oil tank or immediately prior to the inlet of the
oil pumps to prevent debris from damaging the
pumps. A fine pressure filter is fitted at the pressure
pump outlet which retains any small particles which
could block the oil feed jets. Thread-type filters (fig.
8-11) are often fitted as a ’last chance’ filter
immediately upstream of the oil jets. Sometimes
perforated plates or gauze filters are used for this
application, Scavenge filters are fitted in each oil
return line to collect any debris from the lubricated
components. An example of a pressure and
scavenge filter is shown in fig. 8-12. They are
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82
Fig. 8-12 A typical pressure and scavenge filter.
invariably of tubular construction with a pleated
woven wire cloth, or a resin impregnated with fibres,
as the filtering medium. Some filters comprise one or
more wire wound elements but these tend to be
insufficient for fine filtration. A ’pop up indicator’ may
be fitted to the filter housing to give a visual warning
of a partially blocked filter.
LUBRICATING OILS
31. Early gas turbines used thinner oils than those
used in piston engines but were produced from the
same mineral crude oil. As gas turbines were
developed to operate at higher speeds and temperatures
these mineral oils oxidized and blocked the
filters and oilways. The development of low viscosity
(thin) synthetic oils overcame the major problems
encountered with the early mineral oils.
32. The choice of a lubricating oil is initially decided
by the need to start the engine at very low temperatures,
when the viscosity of the oil is high, whilst
being able to survive in an engine environment which
exhibits very high temperatures. Having met these
fundamental requirements, the need to provide
improved lubrication characteristics using additives
must also be investigated. Special laboratory and
engine tests are done to prove the suitability of a
particular oil for a specific type of engine.
Assessments are made as the extent to which it
deteriorates and the corrosive effects it may have on
the engine.
33. Most gas turbines use a low viscosity oil due to
the absence of reciprocating parts and heavy duty
gearing. This reduces the power required for starting,
particularly at low temperatures. In fact normal starts
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