AKARSHAN SOOD GRADUATE MARINE ENGINEER
[email protected] MAIN ENGINE BEARING Bearings are used in the marine engines along with lubrication to reduce frictional forces between components have relative motion. Learn about main engine bearings in this article
Introduction 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.
MAIN BEARINGS These are inser inserting ting type remova removable ble shell bearing. The main bearing shells are made of steel and lined with white metal at the working surface. The back of shell is machined such that when fitted in housing it is pressed with an interference fit. The bearing surface is lubricated from the forced feed system. system. Lubricated oil is supplied at the centre and then to bearing side pockets. The rotating shaft draws the oil in the clearance space and then to the bottom of the bearing. The thrust bolts are tensioned simultaneously by hydraulic pressure.
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. The important physical properties are crushing strength i.e. i.e. yield yield point point under under compre compress ssion ion,, tensil tensilee strengt strength, h, enduran endurance ce to fatigu fatigue, e, therma thermall expansion , conductivity and melting point etc.
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. White metal alloys or sliding bearing antifriction metals usually known as Babbitt. Babbits are divided into tin based or led based alloys. Tin based tin, antimony-copper white metals are widely used in modern diesel d iesel engine bearings. Typical Typical composition is mentioned below: SN ------- 87.65% Sb -------- 7.5 % Cu --------4.5% Pb ---------.35 %
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 h and 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 in which which the white metal metal is bonded bonded by combin combinati ation on of mechani mechanical cal anchorage anchorage and
chemical bond. The mechanical bonding of white metal lining prevents free escape of fragments in event of cracking. It is constructed in two parts for ease in fitting and operational convenience and the bearing is force lubricated.
DEFECTS
Bearings are very important to reduce friction, yet the same bearings can even lead to engine seizure if allowed to run in the damaged state so you should know how to check them for defects. Bearings tend to undergo wear and tear during their normal operation and can develop several types of defects during their lifetime. It is important for a marine engineer to know and identify these defects so that faulty bearings can be isolated and changed.
Types of Defects Wear (Adhesive wear) – Galling, Galling, scuffing scuffing and scoring are the phenomenon which comes under adhesive wear. The relatively softer surface of the bearings rubs against the harder surface to which they are providing the bearing surface. Hence when two metals of different hardness rub together, the one with the lesser hardness with wear more and this wear is accelerated by the fact that impurities and abrasive particles can enter between these surfaces thus speeding up the wear by plough it. The loss of metal thus caused is sometimes termed as wiping off .
Corrosion – bearings are lubricated with lubricating oil from the engine systems and this lubricating oil can turn acidic in due course of time with effect of combustion products, temperature and so on. Oil additives may be decomposed to form acids. This acidic lubricating oil can cause corrosion of the bearings especially the white metal parts of the bearings. White metal containing copper-lead is more susceptible to oil acid corrosion.
Overloading/Overheating – a bearing can get damaged due to local concentration of stress or temperature and certain regions which in turn can be due to several factors. This can lead to various types of defects like –
If load distribution is unequal. The alignment of engine is upset. The bearing runs with too high or too low oil clearances. Oil is contaminated or supply of oil is inadequate. The rubbing surfaces are uneven.
A bearing may be run at a temperature higher than the normal but below its melting point. Yielding Yielding may results locally at hard spots. Such bearing leaves a black hard incrustation of tin oxide at the surface. This hard surface may be removed as it is likely to cause local wear of the journal.
Surface cracks- cracks may appear as small hair line cracks, as cracked areas or network of cracks. Cracks results as a result of failure of the metal to fatigue. Such condition as a defective bond, excessive peak load, concentration of loads on small areas etc. exceeds the endurance e ndurance strength of the material to fatigue failure. Small areas of white metal may be dislodged from the surface.
PICTURES
Pictures which show different types of defects which have occurred due to wear, loss of lubrication etc. This should give you an idea about the manner in which a defective bearing appears from outside without using an y measuring tool for checking for Defects.
INSPECTION AND PRECAUTIONS To prevent the bearing failures following features are worthy to note:
Presence of ovality, trilobity, trilobity, taper and roughness at the surface are undesirable.
Continued Continued check on alignment, clearances clearances etc. are to be maintained.Since maintained. Since larger
clearance may result in cracking, seizure or lubricating oil pressure drop, periodic overha overhaul ul is requir required. ed. Otherw Otherwise ise,, bearing bearing metal metal wear wear causes causes increa increased sed crank crank deflection, bringing about a dangerous crankshaft condition due to the application of repetitious stress by internal moment. If the bearing is found to be too much worn or having other defects and needs replacement, the same should be done by removing the bottom shell, and fitting a new shell of similar dimensions after scraping it with a mandrel cutter or such tool to ensure proper alignment of the crankshaft crank shaft journal. Carefully check back side of bearing metal and mating surface for fretting, traces of seizure of buildup of foreign matter; measure deg ree of wear.
Steel back is favored as against cast iron or alloy steel. Presence of free graphite in cast iron machined face and chromium or nickel in alloy steel will interfere with continuity of the bond. A thinner layer enhances the property of white metal against fatigue failure. Generous feed of uncontaminated lubricating oil will increase the life of bearing. Uniform loading should be aimed for power and peak pressure.
MEASURING CLEARANCES 1. Metal thickness measurement Extract bearing metal and measure thickness. 2. Measurement by means of lead wire Extract main bearing cap and put a soft lead wire (0.5mm) on upper part of crankshaft. Then replace cap and tighten to prescribed torque. Remove cap and measure with a micrometer the thickness of the lea d wire thus pressed.
3. Measurement with bridge gauge Apply bridge gauge across crank journal and measure clearance between gauge and circumference of journal. An optional bridge b ridge gauge is available. A bridge gauge is also used to take the reading while the bottom shell is in its genuine position plus taking another reading after removing the shell and seeing the difference between them.
4. Measurement of outside diameter of crankshaft and inside diameter of metal Remove crankshaft from sole plate and measure outside and inside diameters. This is the most reliable method for obtaining correct value s. 5) When replacing main bearing metal, replace upper and lower shells as a pair.
