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Written by: Hiro1945 Edited by: Lamar Stonecypher

Published Sep 18, 2009

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Home > Engineering > Marine Engineering > Marine Diesel Engines

Lubrication In Diesel Engines - Fluid Film vs Boundary Lubrication

Learn about the types of lubrication mechanisms that play an important role in marine engine lubrication

Marine Diesel Engine Lubrication Technologies

We have talked quite a bit about the lubrication aspects of the main propulsion plant on ships including

properties of lub oil, cylinder lubricationand crankcase lubrication. Follow this link if you want to know about

the importance of lubricationin marine engines. Of course these concepts apply more or less equally to all

engines since the basic working principles are the same. In this article I will talk in detail about the two main

types of lubrication technologies (rather techniques) used in marine diesel engines.

Lubrication of machine parts takes place in two ways namely

fluid film lubrication1.boundary lubrication2.

In the first type, copious supply of suitable grade of lubricant is fed to the bearing in which the journal rotates

at moderate speed at a designed load. However, when the load on the journal is large and the journal rotates

at slow speed, it presses on the bottom part of the bearing and tends to squeeze the oil out of the clearance

space between the journal and the bottom of the bearing until a stage is reached when metal surfaces of the

journal and the bearing are about to touch. This is termed as boundary lubrication. During boundary

lubrication the heat generated is higher and there is increase in the rate of wear.

Theoretically, in an ideal situation, when a bearing is perfectly lubricated and operates normally, the bearing

friction is independent of load on the journal. This does not happen in practice, as, when a bearing gets

overheated, one of the obvious steps to be taken is to reduce the load. The reason for this is that an

overheated bearing is not perfectly lubricated. Therefore, in practice, there is a maximum limit for the load

that can be put on a bearing.

When this limit is exceeded, the bearing will get overheated. The reason is not the friction that increases with

the load but it is the tendency of the clearance space getting reduced due to overload squeezing out the oil

from between the rubbing surfaces and causing overheating due to reduced supply of lubricant. Further

increase in load may completely destroy the oil film between the rubbing surfaces causing quick rise in

temperature resulting in melting and wiping out of the bearing metal.

FLUID FILM LUBRICATION

In a properly lubricated bearing, a film of oil is maintained between the rubbing surfaces. This is formed as a

result of relative movement between the surfaces and depends upon the viscosity of the oil and the relative

speed between the moving parts. The oil is drawn between the most heavily loaded parts of the bearing.

More the relative speed and higher the viscosity of the oil, greater will be tendency to draw the oil between

the most heavily loaded parts of the bearings. This is the reason when the load on an engine is increased, it is

advisable to increase the pressure of lubricating oil so that any tendency to reduce the oil film thickness due

to overload will be nullified by increased pressure of the oil, which tends to increase the clearance and

maintain the oil film.

From the above it is clear that whenever two metallic surfaces slide against each other in a machine, an effort

should made so that these sliding surfaces are prevented from coming in physical contact with each other so

that. The medium that brings about this separation of the sliding surfaces should offer minimum friction and is

termed as Lubricant. Therefore, while designing a machine with sliding parts, arrangement should be made

to produce and maintain a thin film of lubricant between the sliding surfaces. The thickness of the film varies

with the operating conditions and will depend on factors like load on the bearing, the viscosity of oil and the

operating pressure of the oil.

In order to understand the mechanism of how such thin film of oil is formed between the sliding metal surfaces

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Food Grade Oil / LubricanShanghai Ping Yiao supplies food gradelubricants from Americanwww.cpi-lube.cn

Industrial Lubricant MfgManufacture of industrial greases,lubricants & antiseize compounds.www.TSMoly.com

cylindrical rollercylindrical roller bearing compactstructure,carrying capacitywww.lysnbearing.com

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ine diesel engine lubrication technologies explained http://www.brighthub.com/engineering/marine/articles/49332.aspx

3 20 08 2011 18:11