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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 shearneck
(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 recirculatory
oil systems but vane and gerotor pumps are
employed in some engines. The simplicity of singleshot
pumps (para. 19) make them ideal for engines
which run for a short duration and use the total loss
type of oil system.
16. Gear pumps (fig. 8-5) consist of a pair of intermeshing
steel gears which are housed in a close
fitting aluminium casing. When the gears are rotated,
oil is drawn into the pump, carried round between the
teeth and casing and delivered at the outlet.
17. Since a small quantity of incompressible oil
becomes trapped in the gear mesh, which can cause
a hydraulic lock and possible pump damage, a relief
slot is machined into the end faces of the casing to
provide an escape route for the oil.
18. Gear pumps are used both as pressure (feed)
pumps and scavenge (return) pumps and are incorporated
within a common casing. The oil pumps pack
is driven by the accessory drive system (Part 7).
19. Single-shot pumps (fig. 8-6) have a quantity of
oil contained within a cylinder. When the piston is
forced up the cylinder bore, under the control of the
throttle unit, the oil forces the outlet valves to open
allowing a flow of oil to the parts required to be
lubricated. When the piston reaches the top of the
cylinder bore the outlet valves close due to the
reduced oil pressure. Recharging of the oil pump
cylinder is achieved by a spring forcing the piston to
its original position. This reduces the pressure
between the cylinder and the oil tank which allows
the oil replenshing valves to open until the cylinder is
recharged.
20. The most common type of oil distribution device
is a simple orifice which directs a metered amount of
oil onto its target. These jet orifices are positioned as
close to the target area as possible to overcome the
possibility of the local turbulent environment
deflecting the jet of oil. The smallest diameter of a jet
orifice is 0.04 inch which allows a flow of 12 gallons
per hour when operating at a pressure of 40 lb. per
sq. in. The use of restrictors upstream can reduce the
flow rate if required.
21. All engines transfer heat to the oil by friction,
churning and windage within a bearing chamber or
gearbox. It is therefore common practice to fit an oil
cooler in recirculatory oil systems. The cooling
medium may be fuel or air and, in some instances,
both fuel-cooled and air-cooled coolers are used.
Lubrication
78
Fig. 8-5 Principle of a gear pump.
22. Some engines which utilize both types of cooler
may incorporate an electronic monitoring system
which switches in the air-cooled cooler only when it
is necessary. This maintains the ideal oil temperature
and improves the overall thermal efficiency.
23. The fuel-cooled oil cooler (fig. 8-7) has a matrix
which is divided into sections by baffle plates. A large
number of tubes convey the fuel through the matrix,
the oil being directed by the baffle plates in a series
of passes across the tubes. Heat is transferred from
the oil to the fuel, thus lowering the oil temperature.
24. The fuel-cooled oil cooler incorporates a bypass
valve fitted across the oil inlet and outlet. The valve
operates at a pre-set pressure difference across the
cooler and thus prevents engine oil starvation in the
event of a blockage. A pressure maintaining valve is
usually located in the feed line of the cooler which
ensures that the oil pressure is always higher than
the fuel pressure. In the event of a cooler internal
fault developing, the oil will leak into the fuel system
rather than the potentially dangerous leakage of fuel
into the oil system.
25. The air-cooled oil cooler is similar to the fuelcooled
type in both construction and operation; the
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