|Fig. 8-11 A thread-type oil filter.|
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)
|Fig. 8-10 A centrifugal breather.|
|Fig. 8-12 A typical pressure and scavenge filter.|
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 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.
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 tempera-
tures 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 tempera-
tures, 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
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
can be made in temperatures as low as -40 deg. C. without having to pre-heat the oil.
34. Turbo-propeller engines use a slightly higher viscosity oil due to the additional requirements of the
reduction gear and propeller pitch change mechanism.