Introduction
We studied about the main engine exhaust valve in our previous article. We will now continue with our study of marine diesel engine components and learn about bearings. Bearings are an important part of any engine and the marine diesel engines used on ships are no exception to this rule. Bearings help to reduce friction mainly by converting sliding friction into rolling friction as the value of the latter is much less than the former as we know from our study of physics and the phenomenon of friction and motion.
Forces on Bearings
The bearings in diesel engines are subject to various kinds of forces which include forces due to gas pressure plus inertial and centrifugal forces due to different reciprocating and rotating motion of the engine parts. Hence the bearings should be designed keeping these forces in mind. Moreover the bearing material should be tolerant towards minor surface irregularities.
In case of marine diesel engines the lubricating oil is used along with bearings at several places and there is a possibility that the oil has small abrasive particles which can lodge themselves within these bearings, hence they should also be capable to withstand such particles without getting seized.
Materials Used for Construction
The above forces and circumstances are taken care of by using special white metal alloys which are normally tin or lead based alloys. The only disadvantage of these alloys is their weak mechanical strength which is also necessary apart from the good anti-friction properties; hence steel is used to provide the required backup of strength. The bonding between the alloy and the back up material is either through mechanical anchorage or chemical bonding. Steel is preferred over cast iron since it provides better bonding between the two materials.
Main Engine Bearing Arrangement
It is time to take a look at the main engine bearings and their arrangement in a typical main propulsion engine now. I would suggest you take a close look at the diagram below to understand how the arrangement is made. The sketch is fairly self descriptive and all the parts are labeled clearly on the picture.
The left hand side shows the front view while the sketch on the right hand side shows the side view of the arrangement, along with various parts such as the thrust bolts, tie rod, crankshaft and so forth. As you must have noticed the bearings used here are of shell type which is constructed in two parts for ease in fitting and operational convenience and the bearing is force lubricated.
In the next article we will learn about the wear that occurs in marine diesel engine bearings, and their maintenance such as taking clearances and the procedure to change the main engine bearings.
Image Credits
Image of Main Engine Bearing: Principles and Practice of Marine Diesel Engines by D.K. Sanyal