Page 88 - The Jet Engine

Lubrication

system contents to be checked. The filler can be
either the gravity or pressure filling type; on some
engines both types are fitted. Provision is also made
for a continuous supply of oil to be made available in
aircraft which are designed to operate during
inverted flight conditions. Since air is mixed with the
oil in the bearing chambers, a de-aerating device is
incorporated within the oil tank which removes the air
from the returning oil.

13. The oil pumps are vital to the efficient operation
of the engine. Failure of the pumps will necessitate a
rapid shutdown of the engine. For this reason, the oil
pump driveshafts do not incorporate a weak shear-
neck (Part 7) because they must continue to supply
oil for as long as possible, regardless of damage.
14. As the feed oil is distributed to all the lubricated
parts of the engine a substantial amount of sealing
air (Part 9) mixes with it and increases its volume.
Additionally the bearing chambers operate under
differing pressures. Therefore, to prevent flooding it
is usually necessary to have $. scavenge pump for
each chamber.

15. Gear type pumps are normally used in recircu-
latory oil systems but vane and gerotor pumps are
employed in some engines. The simplicity of single-
shot pumps (para. 19) make them ideal for engines
which run for a short duration and use the total loss
type of oil system. Fig. 8-5 Principle of a gear pump.
16. Gear pumps (fig. 8-5) consist of a pair of inter-
meshing steel gears which are housed in a close cylinder is achieved by a spring forcing the piston to
fitting aluminium casing. When the gears are rotated, its original position. This reduces the pressure
oil is drawn into the pump, carried round between the between the cylinder and the oil tank which allows
teeth and casing and delivered at the outlet.
the oil replenshing valves to open until the cylinder is
17. Since a small quantity of incompressible oil recharged.
becomes trapped in the gear mesh, which can cause
a hydraulic lock and possible pump damage, a relief 20. The most common type of oil distribution device
is a simple orifice which directs a metered amount of
slot is machined into the end faces of the casing to oil onto its target. These jet orifices are positioned as
provide an escape route for the oil.
close to the target area as possible to overcome the
18. Gear pumps are used both as pressure (feed) possibility of the local turbulent environment
pumps and scavenge (return) pumps and are incor- deflecting the jet of oil. The smallest diameter of a jet
porated within a common casing. The oil pumps pack orifice is 0.04 inch which allows a flow of 12 gallons
is driven by the accessory drive system (Part 7). per hour when operating at a pressure of 40 lb. per
sq. in. The use of restrictors upstream can reduce the
19. Single-shot pumps (fig. 8-6) have a quantity of flow rate if required.
oil contained within a cylinder. When the piston is
forced up the cylinder bore, under the control of the 21. All engines transfer heat to the oil by friction,
throttle unit, the oil forces the outlet valves to open churning and windage within a bearing chamber or
allowing a flow of oil to the parts required to be gearbox. It is therefore common practice to fit an oil
lubricated. When the piston reaches the top of the cooler in recirculatory oil systems. The cooling
cylinder bore the outlet valves close due to the medium may be fuel or air and, in some instances,
reduced oil pressure. Recharging of the oil pump both fuel-cooled and air-cooled coolers are used.

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