Main engine bearings lubrication is being discussed in this article and information about the lubrication of all the major bearings and type of lubrication happening between the surfaces.
Here in this article we will discuss lubrication in following bearings:
- Main engine shaft Bearings or Journal Bearings or crank pin bearing.
- Crosshead Bearing Lubrication
- Crosshead guide surface lubrication
- Gear Wheel of the main engine
- Piston Ring Lubrication
The lubrication in all the above surfaces is different as the relative motions are either rotary, sliding and point contacts. So let’s discuss one by one starting from main shaft bearing.
Main Engine Bearing Lubrication
First let’s pick the main shaft bearings. Shaft has a rotary motion as compared to the bearing material. And hence the type of lubrication is hydrodynamic and builds when the shaft actually starts rotating.
When engine is not running, the shaft actually touches and sit on the bearing only. But gradually when it picks up speed, the oil around starts to accumulate in between bearing and the shaft. But occasionally it touches bearing when running at very low speed.
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As engine picks up speed, the oil is drawn into the inside of the shaft, and thus it forms a wedge. And bearing is floating on the layer of oil.
Note: As we can see the oil wedge which floats the shaft away from the bearing, forms when only there is a considerable speed and relative motion between the two. Thus at the time of starting or change of speed of the engine, the wear on the bearing will be too much. Because, hydrodynamic lubrication is disturbed.
Cross Head Bearing Lubrication.
The crosshead bearing is more or less similar to the one which we use for the main bearing. But there is a problem with the lubrication here. If you can remember, crosshead does not have a rotary motion, and have movement of some degrees (small end of crankpin bearing).
Thus hydrodynamic lubrication is not possible here. Combustion forces are passed on from piston to the crosshead. And since there is no hydrodynamic lubrication here. Chances of bearing getting damaged are quite high as there is no proper lubrication and load acting on to it is too much.
Solution to this problem is to feed high pressure lubrication oil in to the bearing like in Sulzer or have drilled passage from the main lubrication system as in MAN Engines.
Also the surface finish for such bearing is kept better during manufacturing and the shell material is more smooth as compared to other shell. Crosshead pin is made more bigger in diameter so that it can take higher load.
Crankpin (Big End Bearing Lubrication)
Lubrication in this is same as that happens in the main bearing of the engine. Internal drilled passages supply the oil for the bearing
Crosshead Guide Lubrication
As we know that guide have a sliding surface and friction is too much. One way to reduce this friction is to apply oil film called as boundary layer. Oil is sprayed over the sliding surfaces, and because of the boundary layer, lubrication takes place.
Piston Ring And Liner
Lubrication between the piston rings and the liner is a very challenging issue considering the extreme temperature and combustion products present in this area. And the kind of lubrication changes across the liner. At the very top most part of the liner, lubrication film does not last much longer as it is burned off very easily. Also, the lubricant which is used for the lubrication should have a good viscosity that it can last in such high temperature.
Most of the cylinder oil which is used for lubrication is burned off. Around a viscosity of 120-150cst at the temperature of 50 degree celsius is used. Oil is usually fed through various holes around the liner called as quills and usually injected at the time when piston is going upwards.
Note: The feed rate for the cylinder lubrication oil should be so that not much and not less oil is present for the lubrication. Excess oil will lead to unburnt carbon carry overs in the exhaust leading to the chances of the soot fire and also scavenge space fires, if the oil leaks by the piston. Also less oil will mean localized contact of piston rings with the liner surface without any film of oil and hence it will damage the liner.
In gear wheel boundary lubrication works as the oil is sprayed all over the contact surface. The oil film made, makes sure that when two teeth touch, there is a film of oil to prevent the metal to metal contact in a dry state.
It also helps in cooling the contacts once the gear teeth messes together.
This was all from the article on the Main engine bearing lubrication. Hope you enjoyed reading it.