In the event of an lubrication failure to the turbocharger bearing system and shaft, I often get asked why the main bearing of the engine survive a lubrication failure.
There are numerous reasons for this.
- The turbo is generally the furthest away from the oil pump and is the first to suffer when oil pressure drops or fails completely
- The crank turns at an average of 5000 rpm in general operation conditions and the turbo at 160,000 rpm.
- The main and big end bearings are made from a much more complex material and operating design and have completely different load functions to contend with. Where main bearing are designed to absorbs impurities, the turbo bearings are not.
- Turbo bearings are more susceptible to two types of wear. Abrasive wear. Abrasive wear is the characteristic for a third material to get in between the bearing and shaft. Carbon deposits and impurities from machining and cam grinding.
- The heat and speed generated inside the turbo shaft system makes it vulnerable to adhesive wear. This is when the two surfaces begin to fuse together or bond and pieces are torn off of one of the surfaces and bonded to the other
- The oil supply pipe from the block to the turbo is generally pretty close to the exhaust and heat source.In engine shut down periods this oil can boil in the pipe and form deposits which are then forced through the bearings. These deposits cannot be removed by general cleaning.
Its a known fact that any small impurities like carbon get embedded into the main bearing shell and do not cause wear. Bearing material in the crankshaft bearing is designed to do this function to prolong bearing life in an engine. Turbocharger bearing are generally made from a brass alloy as a bush and have very small clearances and impurities will cause wear to both shaft and bearing in a very short amount of time. The limited friction properties of brass materials cause the galling pattern of wear, caused by insufficient lubrication.
Adhesion wear is a result of micro-junctions caused by welding between the opposing rough surfaces rubbing on the counter-bodies. The load applied to the contacting surfaces is so high that they deform and adhere to each other forming micro-joints. The motion of the rubbing counter bodies result in rupture of the micro-joints where some of the material is transferred by its counter-body. This effect is called scuffing or galling. Eventually this will cause the seizure of one of the bodies by the counter-body. This is a common sight found when stripping a turbocharger and inspecting the bearing and shaft journals. The blue to purple discoloring is caused by excessive heat and the brown colouring on the shaft is the brass adhesion onto the shaft.
With this in mind it is imperative that the engine builder and, or turbo fitter has made sure that the inside of the motor and oil galleries have been cleaned properly and using the correct cleaning material. Unfortunately paraffin is one of the worst cleaning materials when used on its own. After cleaning it is essential that it gets washed again with HOT WATER and soap at high pressure. Oil supply lines need to be replaced as deposits CANNOT be removed by cleaning.
Most “warranty” failures are cased by this.