Table of Contents
3.5 The oil system and the bearings
We notice the oil system in our car motor usual not till there is a warning if its alredy too late. Short term breakdown of the oil supply can damage the sleeve bearings of the connecting rods and the crankshaft. At least there exists the danger that the life of the motor is considerably shortened. Also a degradation of the oil during operation , e.g. aging , overheating, fuel or condensation can cause failures which the filter can not prevent. So we look at least after specification conform oil as the car manufacturer prescribes in his operation manual. There is a risk, that dubious low cost oils save money at the wrong place. Better is ist to rely on a garage of our trust which proceeds after reliable principles.
Quite similar is the situation with our gas turbine. But if a failure occurs much higher costs are involved. Coke formation in the oil supply can lead to oil starvation. Oil coke can reduce the life time of the bearing slide faces or pitch surfaces dramatically. The experience shows that especially in high speed small gasturbines an oil shortage can trigger in very short time (seconds) catastrophic failures.
There is a call for always higher efficiency and performance concentration of gas turbines. This trend is in accordance with rising compressor end pressures and gas temperatures in the gas generator together with higher rotation speeds. The result is a correspondent historic trend of the oil temperatures with the further development of the engines. The principle of the supply of the bearing chambers through the oil system is presented in "Ill. 3.5-1" High oil temperatures demand appropriate (synthetic) oils. These oils are very thin in upper temperature regions and form correspondingly thin oil films in the rolling bearings. The bearing is, thus, more sensitive for smaller foreign particles ( "Ill. 3.5-2") that penetrate through the oil film. With higher oil and component temperatures olso the risk of oil fire ( "Ill. 3.5-6") increases.
The main bearings of the engine absorb the axial thrust loads and the unbalanced radial forces. Because the thrust loads are produced through the ram forces of the rotor components ( "Ill. 2.5-1"), they are decidedly dependent on the pressure differences and also on the locally effective pressures. These are, in their turn, determined by the labyrinth seals (Chapter 3.1.2.4). The condition of such seals, as well as their alteration with the operation condition and life play a decisive role on the bearing loads. An overload of the bearing is to be avoided as much as conditions where the bearing is unloaded. Therefore, undefined running conditions, raceways that are not optimal and the possibility of vibrations that are not allowed are formed.
