Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: Lubrication

LUBRICATION

The necessity for lubrication

28. The primary object of lubrication is to reduce the friction which occurs when one surface rubs against another. Without effective lubrication, not only would a great deal of power be wasted in overcoming friction but the heat produced would be very difficult to dispose of and would eventually cause damage or melting of the metal of which the bearing surfaces are made. The bearing surfaces of an engine are lubricated in two ways ; by direct supply of fresh oil under pressure, (for main and big end bearings, etc.), and by splash, that is, by oil spray thrown out by the moving parts of the engine (for pistons, gears etc.). In a properly lubricated bearing there is no actual contact between the metal surfaces, they are separated by an oil film. It is not, however, the pressure of the oil supply which keeps them apart, but the rotation of the shaft and the viscosity (resistance to flow) of the oil, and a bearing may even be made to operate satisfactorily with no external oil pressure at all. High oil pressure is, however, necessary mainly to ensure a rapid flow of oil through the bearings and so to carry away the heat which, even when the friction is reduced to a minimum, is generated within them. This rate of oil flow may amount to several gallons a minute. Ball and roller bearings, in which there is theoretically no sliding friction, need hardly any lubrication, but they must be kept oily to avoid corrosion.

The dry sump system

29. In all aero engines, the dry sump system is used. The oil is not retained in the engine but after passing through the bearings it runs down the crankcase and collects in a small reservoir or sump from which it is pumped by a " scavenge " pump to a separate oil tank. On the way to the tank it is passed through a cooler which is usually provided with a by-pass valve to cut it out of the circuit for warming up purposes. From the tank, the cooled oil, after passing through a filter, is forced into the engine by a pressure pump and begins its circuit once more. It is usually forced into the hollow crank-shaft through one or more of the main bearings, and then passes into the crankpins from which it lubricates the big ends (and wrist pins in radial engines). From the big ends it escapes from there to lubricate the pistons and cylinders. In " in-line " engines there is usually a separate oil supply from the pump for the camshafts etc., on radials the cam gear usually receives oil from the front of the crankshaft. An important point to notice is that as in every engine the cylinders are lubricated by splash, the engine must not be allowed to run too slowly, especially when cold : otherwise there may not be enough oil thrown off by the big ends to lubricate the cylinders and pistons properly. Figs. 105 and 106 shew the details of two typical oiling systems, of " in-line " and radial engines respectively, and Fig. 107 gives the outlines of the external circulation system. The type of oil pump used in aero engines is known as a gear pump, it consists of two gear wheels meshing with one another and revolving inside a very close-fitting housing. It will be seen from Fig. 108 that the oil is carried round between the teeth of the gears and the housing, but cannot pass backwards between the wheels themselves owing to the close meshing of the teeth. The scavenge pump is always larger than the pressure pump. This ensures that any oil which may accumulate in the crankcase when the engine is not running, is quickly removed when the engine is started.

(A larger version of this chart can be viewed here. )

(Figure 106 missing in the original)

30. The oil pressure is kept approximately constant by a spring loaded relief valve on the delivery side of the pressure pump, it is indicated to the pilot by a gauge in the cockpit. The gauge is of the " transmitting type ", which is arranged so that the gauge itself is not exposed to the oil in the system. It is very important that the pressure should be maintained, for without it the oil flow may be insufficient to keep the bearings cool and the engine may be seriously damaged. It is also important to keep the temperature within limits, because, as has been said, the correct lubrication of the bearings depends on the viscosity of the oil and this in turn depends on the temperature. If the oil gets too hot, the viscosity will get less and may not be enough to maintain a good film in the bearings. If, on the other hand, it is too cold, the oil may be so " thick " that it will not flow properly through the passages and some parts may not get enough oil. In extreme conditions it may be necessary to heat the oil before starting the engine. The usual limits of oil temperature are from about 15°C. to 90° C., but the actual figures are laid down for each engine in the handbooks. After it has been in use for some time the oil gets contaminated in various ways. Fuel sometimes leaks past the piston rings ; particles of metal from the bearings and carbon from the pistons find their way into the oil system and the oil becomes black in appearance, it is therefore, customary to change it after about 40 hours flying. The impurities can be removed by filtering, and the cleaned oil can be used again.