A pressure relief valve type oil system,Pressure relief valve system,INTRODUCTION,Lubrication

Lubrication :

INTRODUCTION :

Fig. 8-1 A pressure relief valve type oil system.

1. The lubrication system is required to provide lubrication and cooling for all gears, bearings and

splines. It must also be capable of collecting foreign matter which, if left in a bearing housing or gearbox,

can cause rapid failure. Additionally, the oil must protect the lubricated components which are manu-

fractured from non-corrosion resistant materials. The oil must accomplish these tasks without significant

deterioration.

2. The requirements of a turbo-propeller engine are somewhat different to any other types of aero gas
turbine. This is due to the additional lubrication of the heavily loaded propeller reduction gears and the
need for a high pressure oil supply to operate the propeller pitch control mechanism.

3. Most gas turbine engines use a self-contained re circulatory lubrication system in which the oil is
distributed around the engine and returned to the oil tank by pumps. However, some engines use a
system known as the total loss or expendable system in which the oil is spilled overboard after the engine
has been lubricated.

LUBRICATING SYSTEMS



4. There are two basic re circulatory systems, known as the 'pressure relief valve1 system and the
'full flow' system. The major difference between them is in the control of the oil flow to the bearings. In both
systems the temperature and pressure of the oil are critical to the correct and safe running of the engine.
Provision is therefore made for these parameters to be indicated in the cockpit.



Pressure relief valve system


5. In the pressure relief valve system the oil flow to the bearing chambers is controlled by limiting the
pressure in the feed line to a given design value. This is accomplished by the use of a spring loaded valve
which allows oil to be directly returned from the pressure pump outlet to the oil tank, or pressure
pump inlet, when the design value is exceeded. The valve opens at a pressure which corresponds to the
idling speed of the engine, thus giving a constant feed pressure over normal engine operating speeds.
However, increasing engine speed causes the bearing chamber pressure to rise sharply. This
reduces the pressure difference between the bearing chamber and feed jet, thus decreasing the oil flow
rate to the bearings as engine speed increases. To alleviate this problem, some pressure relief valve
systems use the increasing bearing chamber pressure to augment the relief valve spring load, This
maintains a constant flow rate at the higher engine speeds by increasing the pressure in the feed line as
the bearing chamber pressure increases.


6 Fig. 8-1 shows the pressure relief valve system for a turbo-propeller engine and indicates the basic
components that comprise an engine lubrication system. The oil pressure pump draws oil from the
tank through a strainer which protects the pump gears from debris which may have entered the tank,
Oil is then delivered through a pressure filter to the pressure relief valve which maintains a constant oil
delivery pressure to the feed jets in the bearing chambers. Some engines may have an additional
relief valve (pressure limiting valve) which is fitted at the oil pressure pump outlet. This valve is set to open
at a much higher value than the pressure relief valve to return the oil to the inlet side of the oil pressure
pump in the event of the system becoming blocked.

A similar valve may also be fitted across the pressure filter to prevent oil starvation of the bearing chambers
should the filter become partially blocked or the oil having a high viscosity under cold starting conditions
preventing sufficient flow through the filter. Provision is also made to supply oil to the propeller pitchcontrol system, reduction gear and torque meter system. Scavenge pumps return the oil to the tank
via the oil cooler. On entering the tank, the oil is de-aerated ready for re circulation.