talk about bearing

2- BEARING material  ,Bearing damage cause

Top end bearing - Crosshead engine – transmit load from cross head pin to connecting rod , allow relative movement of con rod and cross head pin, Trunk pistion engine- transmit load from gudgeon pin to con rod , allows relative movement of con rod and gudgeon pin

Bearing lubrication - Gudgeon pin and Crosshead bearing

Fluid -film lubrication depends on the fluid's viscosity, that is, its resistance to internal motion. This resistance is due to cohesion of the molecules within the fluid; it occurs whenever there is internal motion, irrespective of whether or not this motion is caused by a change in the fluid's shape. A liquid adheres to the surface of a solid in contact with it. The motion of the solid at the interface causes internal motion in the liquid, which then resists the force acting on the solid. When a shaft rotates in a journal bearing with suitable running clearances, resistance to motion (or 'viscous drag') carries the oil with the journal round the bearing. The hydrodynamic conditions, that then come into operation, cause the journal to adopt a position, relative to the bearing, in which the running clearance on the loaded side diminishes and a high fluid pressure is built up. Provided that the oil is delivered to the bearing at a rate sufficient to compensate for end or side leakage, a fluid film established in this manner will withstand severe loading. The rate at which oil escape's from a bearing depends mainly on its viscosity, the bearing clearances and the arrangement of the oil distribution grooves. Leakage is also influenced by the condition of the bearing and the load applied to it. Thus, by suitable bearing design and by supplying oil of the correct viscosity at a rate above the minimum required, it is possible to maintain a lubricant film that will not rupture under the designed running conditions. Lubrication of the gudgeon-pin bearing of trunk-piston engines is far more difficult than that of the main, big end or thrust bearings and lubrication failure is a common cause of trouble. Seizure of the pin can lead to bending of the connecting rod or scuffing of the piston. The gudgeon-pin subjected to heavy loads and because of the heat conducted from the piston, operates at relatively high temperatures. Moreover, the small oscillatory angular motion restricts the build-up of an oil film. In 4 stroke engines, satisfactory lubrication is promoted by cyclic lifting of the gudgeon pin within the bearing that takes place towards the end of the exhaust stroke. This movement, caused by forces of inertia, accompanies reversal of the direction of load and enables the bearing to 'breathe'. So the clearances are refilled with oil. In the two stroke engine, there is no 'breathing',. since the bearing load, though fluctuating is continuously downwards as in the main bearings. Various special arrangements are therefore used to ensure satisfactory lubrication of gudgeon pins. One method is to increase the bearing area to such an extent, that the hydraulic load due to oil pressure is able to force the pin and journal out of contact with each during the minimum loading period. These conditions of gudgeon-pin lubrication also apply in general to the crosshead bearings of large, low-speed, two-stroke crosshead engines.

Owing to the oscillating movement on the bearing, it is very difficult to establish full hydrodynamic lubrication, and conditions of boundary lubrication usually prevail. Crosshead engines are therefore subject to local overloading and failure of the lubricant film, leading in turn to fatigue cracking and possible break-up of the surface. Various means have been adopted to improve lubrication of crosshead bearings. Manufacturers attempt to keep the specific loading as low as possible, for example by a large crosshead pin diameter, and by allowing self-adjustment of the bearing housing, to compensate for deformation of the crosshead pin and the head of the connecting rod. This flexibility maintains a uniform load distribution. In addtion to reduced loading and improved lubrication, other important factor in the maintenance of bearings are the alignment between them and the surface finish of the crosshead pin. By providing of hydrostatic lubrication to X head bearing , such as Fitting of reciprocating positive displacement L.O pump , utilized by movement of con. rod ,  Arrange booster pump to supply X head bearing up to 16 bar.

To combat the difficulty of Fluid -film lubrication - Sulzer engine- conjugate deflection(crosshead pin bored at center), continuous full length bottom half of crosshead bearing(MAN B&W), eccentric bored bearing & machining shell (Fiat engine), large diameter stiff crosshead pin L/D ratio less( con rod and crank throw ratio keep as low as to increase sliding velocity & improved lub oil film), make cross head pin high degree of surface finish <0.1 micron, thin shell bearing & improved material.

Man b&w crosshead bearing - The lower half of the bearing housing is formed by the top end of the connecting rod. It supports the crosshead pin over its entire length, the piston rod being bolted to the top half of the crosshead pin through a cut out in the bearing top half. Oil supply to the crosshead is via a telescopic pipe from the main LO supply at a pressure of about 2.5 bar.

 Bearing Type -The type of bearing used in the crosshead assembly is a thin shell (insert) bearing . The lower shell is a trimetal shell, i.e. the shell is composed of a steel back with cast-on white metal and an overlayer coating. The upper shell is a bimetal shell, as it does not have the overlayer coating; both the upper and lower shells are protected against corrosion with tin flash

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 Bearing Function and Configuration- Because of the oscillating movement and low sliding speed of the crosshead bearing, the hydrodynamic oil film is generated through special oil wedges on

either side of the axial oil supply grooves situated in the loaded area of the bearing.

The oil film generated in this manner can be rather thin. This makes the demands for pin surface roughness and oil wedge geometry very important parameters for the assembly

to function. A further requirement is effective cooling which is ensured by the transverse

oil grooves. The pin surface is superfinished.  The lower shell is executed with a special surface geometry (embedded arc) which extends over a 120 degree arc, and ensures a uniform load distribution on the bearing surface in contact with the pin. The lower shell is coated with an

overlayer , which enables the pin sliding geometry to conform with the bearing surface.

In MAN B&W engines, a set of channels have been machined in the lower crosshead bearing, in

which the cooling oil can pass. The geometry is designed in such a way that all the loaded square centimetres of the pin are flushed with cooling oil twice every engine cycle. In contrast, the Sulzer crosshead has a plain lower bearing without channels. In order to inject oil between pin and bearing, they have to supply oil at a much higher pressure. The injection will take place at around 20 degrees crank angle before TDC, where the cylinder pressure is still low and upward inertia forces on piston is still high. There is a short interval, in which the bearing pressure is lower than the oil pressure.

cosmetic wiping - this wiping is of a cosmetic nature; but it can sometimes cause blockage of the oil-wedges that normally build up the oil film to the "pads" inside the bearing,  Disturbance of the oil film build-up inside the bearing could result in slight fatigue damage just behind the blocked area of the oil-wedge. If the phenomenon is observed at an early stage during inspections. The problem is solved by removing the wiped lead from the oil-wedge.

Sulzer RTA 96

Main bearing oil system

Main bearing oil is also used to cool the working piston  via a toggle lever at an operating pressure of 4.0 BAR TO  4.8 bar. The main bearing and crosshead oil systems are interconnected through a non-return valve . The integrated axial detuner, and if there is a vibration damper as well as an electrical balancer built-on at the free end (by 6 Cyl. engines), are supplied and cooled with bearing oil.

Should the crosshead lubricating oil pump  fail, then the crosshead bearing oil system is supplied with main bearing oil pressure. Under such conditions the engine can only be operated at reduced load (Load Indicator Position, LIP max. 4.5).

Crosshead bearing oil system

The operating pressure of the high pressure oil is 10 to 12 bar. The lubrication of crosshead pins as well as of connecting rod bottom end bearings is effected through toggle lever.

The oil supply for the exhaust valve actuator pumps to the exhaust valve control  passes from the crosshead bearing oil system through the air separator . For actuating the exhaust valves, the oil pressure is raised to about 160 bar by the actuator pumps .The reversing servomotors on the camshaft are actuated by crosshead bearing oil.

Main bearing – support crankshaft and keep it aligned. To remove heat produced by friction.

Bottom end / crankpin bearing – transmit load from con rod to crankshaft. Allow relative movement of con rod & journal.

The MC engines were originally designed with white metal bearings for the crosshead, crankpin and main bearings. The main bearings were of the so-called thick shell design,whereas the crosshead and crankpin bearings were of the thin shell design.

Thin shell main bearings

The development towards higher specific engine outputs has resulted in the gradual introduction of the thin shell design for the main bearings. Sn40Al (tin-aluminium), which has been applied with great success on the main bearings for the smaller two-stroke engines, has been introduced on the S46-70MC-C engines, on which good service has been experienced .In addition to the above, an advantage of the thin shell design is the reduced risk of fretting-corrosion between the main bearing saddle and shell when the bearing housing is well designed. Running-in on testbed and during sea trials has in a number of cases caused light seizure of Sn40Al main bearing shells on the S-MC-C engines. This seizure can be avoided either by prelubrication with grease or high-viscosity oil, or by PTFE-coating of the running surface of the shells. The Sn40Al bearings have been introduced on the engine with the same well-proven specific load level as that for white metal bearings. (A thin shell bearing design was introduced to more recent engine types. The main bearings of the small and medium bore models (S46MC-C to S70MC-C)are lined with AlSn40 and provided with a PTFE running-in coating as standard.

The bearings of the large bore models are lined with white metal.)

Nip – the external circumference of a pair of bearing shell is slightly larger than the bore of housing. This difference is called nip.

Locating tangs - allow correct axial positioning of bearing shells thereby ensuring alignment of oil transfer passages and clearance between the ends of the bearing and crankshaft fillets. The tang is located in a corresponding recess in the housing and must be a clearance fit, otherwise the bearing surface may distort. It is also advantageous to relieve the tang below the level of the bearing joint face, thus reducing localized pressure on the tang during assembly. Tangs are not intended to resist rotation of the bearing shell; that is achieved by the contact pressure. If, under seizure conditions, this contact pressure cannot prevent rotation, the tangs are either sheared or flattened.

Cause of thin shell bearing shifting – 1-defective tag or improper fitting precedure, 2-insufficient nip clearance, 3-suddenly applied extreme load(pounding), 4-incorrect size of bearing use, 5-due to overtightening bolts(elongation of bolts), 7- friction force from back of the shell and keep. The overall thickness/diameter ratio is not critical,but typically varies from 0.05 at 40 mm diameter to 0.02 at 400 mm

Interference fit or bearing crush - On assembly the excess peripheral length creates a circumferential stress around the bearing, and a radial contact pressure between the bearing

back and the housing bore. This contact pressure resists relative movement, thus preventing fretting. Unfortunately, there is no theoretically correct level; housings with high flexibility require more contact pressure than stiffer ones. On early engines, having thin shell bearings, a contact pressure as low as 2MPa was usually sufficient to resist fretting, but as engine ratings increased, and housing stress analysis became more sophisticated, higher pressures became necessary, often reaching 8-10 MPa today. In these very high interference fit assemblies, particular care has to be taken to ensure that the joint face clamping bolts have sufficient capacity to assemble the bearing, and to resist dynamic separating forces generated by engine operation.( need to follow the instruction to tighting correct hydraulic pressue at con rod bolts)

Thick shell main bearings (Optimum Lemon Shape (OLS) bearing with flexible edges)

The design of the thick shell bearings has been updated in order to ensure reliable performance. The major updates are summarised in the following. Optimum Lemon Shape (OLS) shells,have been introduced to increase the minimum oil film thickness. optimum lemon shape (OLS)-type main bearing as an evolution of the Mark 5 bearing type, featuring reduced top and side clearances.

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Flex –Edge thick-shell main bearing design. SL03-414/AAB-Engine types: S/L50-80MC

Flex-Edge. Design  is fully interchangeable with previous types and provides an increased margin against fatigue damage occurring in the edge area. There have been a few reports of local bearing fatigue damage still occuring near the bearing shell edges, in a few highly edge-loaded main bearing positions. New, advanced calculations, including journal tilt and housing deformation, have provided detailed information on the load pattern mechanisms of the main bearings. The calculations indicate that a certain radial flexibility of the bearing edge will

significantly increase the minimum oil film thickness near the edges. Likewise, the maximum oil film pressure near the edges will be reduced. The Flex-Edge design feature has been tested with success, for more than a year, in positions which initially suffered repeated problems.

The flexibility has been achieved by machining a 3 mm deep round-edged groove on

the back of the shell, with a width equal to the thickness of the bearing shell, thus

avoiding contact with the bearing support in this area. The unsupported part of the shell will flex slightly when load is applied, thereby increasing the effective bearing area in situations where the centrelines of the shaft and the bearing are not in alignment. A further feature is static deformation of the unsupported part, which occurs when tensioning the bearing housing..

Apart from the flexible edges, the properties of the bearing are similar to those of the Optimum Lemon Shape type. This design provides a larger safety margin in the event of geometrical

non-conformities. The deformation ensures a controlled and smooth convex shape, 10-20 μm larger bore toward the edges. Calculations confirm that such a shape further increases the safety margin against high edge-loading.

Bearing Metals

Tin based White Metal (There is no interlayer in white-metal based bearings)- Tin-based white metal is an alloy with minimum 88% tin (Sn), the rest of the alloy composition is antimony (Sb), copper (Cu), cadmium (Cd) and small amounts of other elements that are added to improve the fineness of the grain structure and homogeneity during the solidification process. This is important for the load carrying and sliding properties of the alloy. Lead (Pb) content in this alloy composition is an impurity, as the fatigue strength deteriorates with increasing lead content, which should not exceed 0.2 % of the cast alloy composition. The fatigue strength of the basic white metal composition  can be increased by adding cadmium, which goes into solution

in the matrix, and by adding trace quantities (0.1 per cent) of chromium, which refines the distribution of the copper-tin compound. Tin based white metal is used in the main bearings, crankpin bearings, crosshead guide shoes, camshaft bearings and thrust bearings because of its excellent load carrying and sliding properties.

Tin Aluminium (AlSn40)- In AlSn40 bearings, the overlayer is always bonded to the bearing metal by a very thin interlayer of Ni (nickel), or Ag (silver). Tin aluminium is a composition of aluminium (Al) and tin (Sn) where the tin is trapped in a 3-dimensional mesh of aluminium. AlSn40 is a composition with 40% tin. The sliding properties of this composition are very similar to those of tin based white metal but the loading capacity of this material is higher than tin based white metals for the same  working temperature; this is due to the ideal combination of tin and aluminium, where tin gives the good embedability and sliding properties, while the aluminium mesh functions as an effective load absorber.Tin aluminium is used in main bearings and crosshead bearing lower shells.

Overlayers - An overlayer is a thin galvanic coating of mainly lead (Pb) and tin (Sn), which is applied directly (on to the white metal ) or,( via an intermediate layer, on to the tin aluminium sliding surface of the bearing). The overlayer is a soft and ductile coating, its main objective is to ensure good embedability and conformity between the bearing sliding surface and the pin surface geometry.

Flash layer, Tin (Sn) - A flash layer is a 100% tin (Sn) layer which is applied galvanically; the thickness of this layer is from 2 µm to 5 µm. The coating of tin flash is applied all over and functions primarily to prevent corrosion (oxidation) of the bearing. The tin flash also functions as an effective dry lubricant when new bearings are installed and the engine is barred.

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copper-tin-lead alloy bearing (lead – bronze alloy ). lead-bronze bearing linings were introduced when fatigue of white metal became a serious problem. They are used in very highly loaded bearings because of their high fatigue strength; their drawback is poor tribological

behaviour. That is why they require an electroplated overlay in most applications. It is always overlay-plated, primarily to protect the lead phase from corrosion but also because the alloy is relatively hard and incompatible. Standard composition for con-rod and main bearings is 78 % Cu, 20 % Pb, 2 % Sn. The alloy is used with electroplated overlay or cast babbit running layer

These bearings can be found in marine diesel medium speed engines.

Material		Maximum loading (MPa)
AlSn6Cu1Ni1	Aluminium alloys:	38
CuPb26Sn1.5	Lead-bronze alloys:	38
AlSn20Cu1	Aluminium alloys:	35
AlSn40Cu0.5	Aluminium alloys:	20
SnSb8.5Cu3.5Cd1Cr0.1	White metal alloy	14
SnSb7.5Cu3.3	White metal alloy	12
PbSb 10Sn6	White metal alloy	12

General Bearing Requirements and Design Criteria

Bearings are vital engine components; therefore, the correct bearing design and the proper choice of bearing metal is necessary for reliable engine performance. Bearing design criteria depend on the bearing type and, in general, on:a) Bearing sliding surface geometry.b) The surface roughness of the journal or pin, which determines the permissible bearing pressure and required oil film thickness. This is necessary to ensure effective and safe functioning of the bearing.c) The correct flow of cooling oil to prevent heat accumulation, which is obtained through a flow area, provided either through the clearance between the journal and the bearing bore or through axial grooves in the bearing sliding surface.

The compactness of engines and the engine ratings influence the magnitude of the specific load on the bearing and make the correct choice of bearing metals, production quality and, in certain bearings, the application of overlayer an absolute necessity.

Scraping of the bearing surfaces is strictly prohibited, except in those repair situations of oil wedges, tangential run out, bore relief, removing ridges and high spots from wiped bearing as approved by manufacturer.

Incorrect scraping has often proved to have an adverse effect on the sliding properties of the bearing, and can result in damage.

Bearing Design - Plain bearings for MC engines are manufactured as steel shells with a sliding surface of white metal or tin aluminium. Tin aluminium bearings are always of the thin shell design  while the white metal bearings can either be of the thick shell or thin shell design.The bearing surface is furnished with a centrally placed oil supply groove and other design features such as tangential run-outs, oil wedges and/or bore reliefs.

Tangential Runout - A tangential runout is transition geometry between the circumferential oil supply groove and the bearing sliding surface. This special oil groove transition geometry

prevents an oil scraping effect and reduces the resistance to the flow of oil towards the loaded area of the bearing 

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Bore Relief - The bearing sliding surface is machined at the mating faces of the upper and lower shells to create bore reliefs. Their main objective is to compensate for misalignments which could result in a protruding edge (step) of the lower shell's mating face to that of the upper shell. Such a protruding edge can act as an oil scraper and cause oil starvation. Fig A-A above.

Axial Oil Grooves and Oil Wedges - Oil grooves and wedges have the following functions: a) To enhance the oil distribution over the load carrying surfaces. (The tapered areas give improved oil inlet conditions). b) Especially in the case of crosshead bearings  to assist the formation of a  hydrodynamic oil film between the load carrying surfaces. c) To provide oil cooling (oil grooves). In order to perform these functions, the oil must flow freely from the lubricating grooves, past the oil wedges, and into the supporting areas ! where the oil film carries the load.

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B.E DESIGN BEARING

The blended edge (BE) design was introduced on thin shell white metal main bearings to better cope with crankshaft inclinations and thereby increase its resistance towards edge fatigue failures.

The blended edge design is the corresponding thin shell feature as for the thick shell flex-edge design in MC engines.

The BE design was introduced in 2004 and has since that time successfully been used for new engines. Today, the BE white metal bearing is featured in the S46, S50, S60, S65, S70, S80, K80, S90, K90 and K98 engine series.

Blended Edge Design

The blended edge design consists of a smooth radius that allows the main bearing shaft to incline without risking touching the bearing edge or causing high oil film pressure near the edge. The blended edge is described by two dimensions, its length and its depth. The actual values depend on engine size and configuration.

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With a good blended edge design, the high edge load can be reduced and distributed over a larger area, thus resulting in a decreased max. oil film pressure and increased safety against edge fatigue failure.

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fatigue damage on the edge in bearing position No. 5, where a straight-edge main bearing was originally installed. The static bearing imprint indicates that a concave geometry exists in this bearing assembly, and a “straight-edge” check reveals that the main journal is concave. In this case,

a BE lower bearing shell was installed, and the issue was eliminated.

Lower Oil Groove Removal(tangential runout removal)

The oil groove feeds and distributes oil into the bearing, but it also decreases the bearing area. On engines with high horizontal load, the combination of blended edge and lower oil groove causes poor fluid film conditions. The influence of the lower oil groove for the oil supply is marginal and it is therefore considered better to remove the lower oil groove to improve the horizontal bearing load capacity.

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Spare Part Bearings

The trade off with blended edge bearings is its decreased bearing load capacity when the inclinations are small and the bearing force is very high. In such positions, “straight” cylindrical geometry bearings are the better choice, and if blended edge bearings are specified elsewhere in the same engine, it must be equipped with both plain and blended edge spare parts. If an engine is delivered with a BE spare part bearing only, BE bearings can be used in all

main bearing positions.

This type of bearing has a steel back with the required stiffness a) To ensure against distortion of the sliding surface geometry, and b) To support the cast-on white metal in regions where the shell lacks support, for example in the area of the upper shell mating faces.The top clearances in this bearing design are adjusted with shims, while the side clearances are a predetermined result of the summation of the housing bore, shell wall thickness, journal tolerances, and the influence of the staybolt tensioning force which deforms the bedplate around the bearing assembly.

Thin shell bearings have a wall thickness between 2% and 2.5% of the journal diameter.

The steel back does not have the sufficient stiffness to support the cast-on white metal alone. The bearing must therefore be supported rigidly over its full length. This type of bearing is manufactured with a circumferential overlength (crush/nip) which, when the shells are mounted and tightened up, will produce the required radial pressure between the shell and the bearing housing. The maximum/minimum top clearance in this shell is predetermined and results from a summation of the housing bore, shell wall thickness, journal/pin diameter tolerances and, for main bearings, the deformation of the bedplate from the staybolt tensioning force.

Cause of thin shell bearing shifting

1-defective tag, 2-insufficient nip clearance, 3-suddenly applied extreme load (pounding), 4-improper fitting, 5-correct size of bearing use, 6-due to overtighting bolts, 07-frictional force from the back of shell and keep.

Top Clearance - Correct top clearance in main bearings, crankpin bearings, and crosshead bearings is necessary to sustain the required oil flow through the bearing, and hence stabilize the bearing temperature at a level that will ensure the fatigue strength of the bearing metal.  In the main and crankpin bearings, the clearance ensures the necessary space to accommodate the journal orbit so as to avoid mechanical overload tendencies on the bearing sliding surface (especially in the main bearing).The bearings are checked in general by measuring the top clearances.In service, clearance measurements can be regarded:1. as a check of the correct re-assembly of the bearing. For new thin shell bearings and new/ overhauled thick shell bearings the clearances must lie within the limits specified in the maintenance manual. 2. as an indicator to determine the condition of the bearing at a periodic check ‘Checks without opening-up’. In both cases, it is vital that the clearance values from the previous check are available for comparison. Therefore, it is necessary to enter clearances in the engine log book with the relevant date and engine service hours.

Wear - Under normal service conditions, bearing wear is negligible. Excessive wear is due to abrasive or corrosive contamination of the system oil which will affect the roughness of the journal/pin and increase the wear rate of the bearing.

The reduction of shell thickness in the loaded area of the main, crankpin and crosshead bearing in a given time interval represents the wear rate of the bearing. Average bearing wear rate is 0.01 mm/10,000 hrs. As long as the wear rate is in the region of this value, the bearing function can be regarded as normal.  For crosshead bearings, the wear limit is confined to about 50% reduction of the oil wedge length.

Journals/Pins Surface Roughness

Journal/pin surface roughness is important for the bearing condition. Increased surface roughness can be caused by: a) Abrasive damage due to contamination of the system oil.

b) Corrosive damage due to sea water contamination of the system oil (acidic) or oxidation of the journals due to condensate. c) Spark erosion (only in main bearings).

With increasing journal/pin roughness, a level will be reached where the oil film thickness is no longer sufficient, causing metal contact between journal/pin and the bearing sliding surface. This will cause white metal to adhere to the journal/pin, giving the surface a silvery white appearance.  When such a condition is observed, the journal/pin must be reconditioned by polishing, and the roughness of the surface made acceptable. In extreme cases, the journal/pin must be ground to an undersize.

Limits to surface roughness

The surface roughness should always be within the specified limits.1. For main and crankpin journals:

a)    New journals 0.8 Ra

b)   Roughness approaching 1.6 Ra (journal to be reconditioned).

2. For crosshead pins:

a)    New or repolished 0.05 Ra

b)   Acceptable in service 0.05-0.1 Ra

c)    Repolishing if over 0.1 Ra

Determination of the pin/journal roughness

Measure the roughness with an electronic roughness tester, or Evaluate the roughness with a Ruko tester, by comparing the surface of the pin/journal with the specimens on the Ruko tester. When performing this test, the pin surface and the Ruko tester must be thoroughly clean and dry. Hold the tester close to the surface and compare the surfaces. If necessary, use your finger nail to run over the pin/journal surface and the Ruko specimens to compare and determine the roughness level.

BWM(bearing wear monitoring) Vs OMD -A main bearing damage was recently experienced on an MAN B&W S-MC-C main engine of medium bore size. The damage was caused by a sudden ingress of foreign matter into the bearing via the lubricating oil system, and it developed rapidly into a bearing seizure.

Operation of the engine continued until the oil mist detector (OMD) alarm set off and requested slow-down. At this stage, the bearing damage had evolved into a situation where comprehensive repair of the crankshaft and bearing housing (bedplate) was called for.

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Normally, main, crank pin and crosshead bearings have very low wear rates. Wear rates up to 0.01 mm/10,000 hrs.

are considered acceptable, but lower values can be expected under normal running conditions. Conversely, abnormal operating conditions, e.g. pollution of the lubricating oil, scratches on journals or spark erosion may give rise to significantly higher wear rates and ultimately to bearing failure.

Continuous operation with an undetected bearing failure may cause excessive heating, possibly damaging the shaft and bearing housing. In particular, in case of a main bearing failure, the repair work may become quite comprehensive.

To minimise the risk of bearing damage, and possible consequential damage, inspection of the bearings is therefore recommended at regular intervals.

External inspection of bearings

The following inspections are recommended to be carried out at regular intervals.

Inside engine:

 Bearing clearance measurements

 Bearing edge check

 Inspection of crankcase for bearing metal

 Crankshaft deflection measurement

Lubrication system:

 Inspection of oil filters

 Oil analysis as described in the “Operation” section of the instruction book.

Open-up inspection of bearings

It is not recommended to open up bearings for inspections unless this is found justified by the above-mentioned external inspections or by other observations. The only exception is the open-up inspection of crosshead bearings and crank pin bearings for every three piston overhauls on engines not fitted with the monitoring systems(BWM)

Systems for monitoring and protection of bearings

Recommended by MAN Diesel & Turbo (MDT) to protect against bearing failure and consequential damage.

These can be categorised in two groups:

 Bearing protection systems serving to detect particular conditions of the bearing environment that may cause

bearing damage. MDT recommends Shaftline Earthing Device Monitoring (SEDM) and Water In Oil Monitoring (WIOM).

 Engine protection systems serving to detect a bearing damage before it develops to cause damage to major parts of the engine. MDT recommends Bearing Wear Monitoring (BWM) and Bearing Temperature Monitoring (BTM)

SEDM. Shaftline Earthing Device Monitoring. A well-functioning shaft line earthing system is necessary to avoid spark erosion in main bearings. Monitoring the electrical potential between

shaft and hull by a separate slip ring connected to the alarm system ensures correct function of the system.

WIOM - Water In Oil Monitoring. In several cases, water in the lubricating oil has resulted in poor bearing performance. The lead-based overlayer used in crosshead bearings is sensitive to water (corrosion), but also main bearings of both the White-Metal and Tin-Aluminium types have been seen to suffer from water contamination.

BWM. Bearing Wear Monitoring is a system that detects wear in any of the three crank train bearings, the main, crank pin or crosshead bearings, by measuring the position of the crosshead assembly in the bottom dead centre.

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The BWM system has threshold values for release of alarm and engine slowdown, respectively.

BTM. Bearing Temperature Monitoring systems have been available for MAN B&W engines for several years. A variety in measuring principles and executions exists, but all systems serve to release an alarm/slowdown when a bearing failure has developed to an extent where heat is developed. (bearing housing design feature – bearing shells are in place by interference fit, there is no relative movement of housing & shell, effective heat transfer between shell & housing is essentail, cap holding bolts are to be close pitched to prevent distortion, housing in robust to prevent excessive strain on shell, housing is not too stiff to prevent localized load concetration on bearing)

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Types of bearing defects - 1.crack, 2. Fatigue failure of white metal, 3.squeezing of white metal, so oil grooves are partially blocked, 4.wiping, 5. Faulty casting and faulty machining, 6.tin oxide corrosion, 7.acid corrosion,8.thermal ratcheting, 9. Electrical potential, 10.fretting

Bearing damage cause –

Abrasion

This is still probably the most common form of bearing damage, even though filtration standards are generally very high. Long term operation with very fine debris in the oil, or short term operation with coarser contaminant can result in abrasion and scoring of the surface of a bearing roughening it to such an extent that overheating can occur due to the surface roughness penetrating the thin oil film.

Fatigue

Typical fatigue damage of white metal is due to a combination of load, high temperature, and the inconsistency of white metal structure resulting from the direct lining of a large variable-cross-section housing.

Corrosion

The different bearing alloys suffer corrosion under different conditions. Tin-base white metals, particularly in the slow-speed diesel engine, can form a hard, dark (almost black) brittle surface

deposit when water is present in the lubricating oil.  Under these conditions, the bearing

becomes prone to overheating and seizure. Alternatively, due to the brittle nature of the layer,

it can flake off the surface, leaving a generally pitted appearance.

Copper-lead lining, if exposed to a lubricating oil which has thermally degraded to form organic acids and peroxides, or become contaminated by sulphur containing fuel oils, blow-by

of products of combustion of such fuels, or cooling water with antifreeze additives, is prone to corrosion of the lead phase of the lining, be this either of the cast or sinter type.

Aluminium-based linings are completely resistant to engine oils, and to their high-temperature degradation products. However, in direct contact with water, a film of aluminium oxide can form in the bore, together with corrosion of the steel backing and possible lifting of the lining material, although in an oil environment in an engine, this is virtually impossible.

Service Letter SL05-460/NHN / November 2005 – corrosive wear of crosshead bearing

Water in the lube oil system may lead to corrosive wear of the overlayer and, eventually, mechanical damage to the crosshead bearing, with high costs as a consequence. If the oil system becomes contaminated with an amount of water exceeding our limit of 0.2% (0.5% for short periods), acute corrosive wear of the crosshead bearing overlayer may occur. The higher the water content, the faster the wear rate. A water content higher than 1% could lead to critical damage within a few days in operation. The overlayer in crosshead bearings consists of a thin, soft lead-tin-copper galvanic alloy covering of the bearing lining metal. It is used in both white-metal and AlSn40 (tin-aluminium) lined crosshead bearings. In AlSn40 bearings, the overlayer is always bonded to the bearing metal by a very thin interlayer of Ni (nickel), or Ag (silver). There is no interlayer in white-metal based bearings, but the risk of corrosion is still present. Wear of the overlayer changes the geometry of the bearing surface and, thereby, the intended "embedded arch" geometry. A change in the bearing geo-metry will obstruct the oil film formation, which is critical for the correct functioning of the crosshead bearing.

Furthermore, excessive wear of the overlayer in AlSn40 bearings could, eventually, expose the interlayer to nickel-to-steel contact with the cross-head pin, and result in scuffing action between the pin and the bearing shell.

In addition to damaging the compon-ents, there is, in extreme cases, a risk of a crankcase explosion.  AlSn40-based crosshead bearings must urgently be replaced as soon as possible if an open-up inspection shows more than 5% of the nickel layer exposed. If one bearing unit has more than 5% of the Ni-layer exposed, it is very likely that all the other units suffer from a similar extraordinary wear, and they should therefore be replaced as well. To decide which bearings to inspect first, all top clearances should be measured and compared with the shop test/sea trial records. If large deviations are found, and/or if bearing "dust" is found in the surrounding area in the frame box, those bearings should be the first to be inspected.the lead content in the oil system can be used as a guideline: 0-4 ppm Normal, 5-10 ppm-Inspect filters and crankcase for bearing debris, and prepare inspection of crosshead bearings when convenient, >10 ppm-Inspect filters and crankcase for bearing debris, and prepare inspection of crosshead bearings as soon as possible.

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Wiping

This type of damage can occur with any lining material, and is caused by insufficient lubricating and cooling oil on the bearing surface. This results in overheating and eventually melting of the lowest melting-point phase of the lining alloy. Potential causes of insufficient oil on the bearing

surface include inadequate generated oil film thickness, insufficient clearance, housing distortion, restriction in oil supply system, excessively worn bearings in other locations and

inadequate oil pump capacity.

Cavitation

Cavitation erosion damage to bearing surfaces is a form of micro fatigue cracking, initiated by the collapse of vapour cavities.

Fretting

fretting of a bearing due to insufficient contact pressure, local welding and tearing having

taken place between the bearing back and housing bore, resulting in transfer of metal from one to the other. Where fretting of the joint faces of the bearing occurs, this usually implies inadequate joint face clamping force. Fretting of the housing joints can also be expected under these conditions.Before fitting new bearings where fretting has occurred, all trace of fretting build-up in the housing must be removed to avoid premature catastrophic damage of the new bearing.

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This type of fretting was experience at L23/30H man b& w engine --- (L23/30H man b& w engine (aux engine) main bearing shell fretting  - fretting/corrosion between the main bearing shell and the main bearing cap on engines without main bearing temperature sensors.The fretting/corrosion is caused by relative movement between the bearing shell and cap during engine operation. The result is that water and oil can migrate between the bearing shell and cap through the sensor hole and cause the observed wear patterns on the bearing running surface.( In order to stop further development of the fretting/corrosion, need  to plug the hole in the main bearing cap and fill the bore with a sealing compound; e.g. EN14AP.)

Design faults (above example of L23/30H )

Major design faults will not generally be evident in production engines because they would have been found out very quickly during pre-production development testing

Incorrect assembly

The commonest causes of incorrect assembly are associated with locating devices. Incorrect positioning will mean that oil feed connections are misplaced and can block off the oil feed.

Incorrect location of a tang into its recess means that the bearing back will not be in contact with the housing, and the clearance between the bearing bore and the crankshaft can be lost, resulting in local overheating of the bearing surface, and possibly seizure. Having located the bearing correctly into its housing, care must be taken to ensure that the housing bolts are correctly tensioned. Insufficient bolt load due to incorrect tightening can result in the damage also excessive dynamic stressing of the bolts and ultimate fatigue fracture. The bolts should also be tensioned in the prescribed sequence given in the engine manual.

Environmental factors

Electrical discharge - with inadequate earthing, a discharge of current can occur through the oil film between the journal surface and the bearing bore, resulting in fine, cleanly defined

pitting of the bearing surface. Damage such as this has been found to occur with an electrical potential in excess of only 50 mV.

Spark Erosion - Spark erosion is caused by a voltage discharge between the main bearing and journal surface. The cause of the potential is the development of a galvanic element between the ship’s hull, sea water, and the propeller shaft/crankshaft. The oil film acts as a dielectric. The puncture voltage in the bearing depends on the thickness of the oil film. With increasing engine ratings, the specific load in the main bearing is increased. This will reduce the oil film thickness, and enable the discharge to take place at a lower voltage level. Since the hydrodynamic oil film thickness varies through a rotation cycle, the discharge will take place at roughly the same instant during each rotation cycle, i.e. when the film thickness is at its minimum. The roughening will accordingly be concentrated in certain areas on the journal surface. In the early stages, the roughened areas can resemble pitting erosion - but later, as the roughness increases, the small craters will scrape off and pick up white metal – hence the silvery white appearance. Therefore, to ensure protection against spark erosion, the potential level must be kept at maximum 80 mV, which is feasible today with a high efficiency earthing device. If an earthing device is installed, its effectiveness must be checked regularly. Spark erosion is only observed in main bearings and main bearing journals.  The condition of the bearings must be evaluated to determine whether they can be reconditioned or have to be discarded. Manufacturer is consulted.

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Journal wear ridge

Wear of the journal surface can occur due to embedded hard contaminant in the bearing surface. Less wear occurs in the region associated with grooves, gradually producing a ridge on

the journal surface, which in turn causes wear of the bearing surface of partially grooved bearings


Ridge wear will inevitably develop over time at the crank pin journal. The wear pattern is caused by abrasive impurities that remain in the lube oil. Efficient lube oil cleaning is therefore essential to keep the development of ridge wear as low as possible in trunk engines. Careful inspection and judgement of the crankpin journals must be carried out whenever a new crankpin bearing is installed.If ridge wear is observed, this must be rectified before new upper and lower bearing shells are installed.(need to ground down the crankpin to undersize to remove ridge wear)

 Misalignment

Misalignment through whatever cause is typified by a 'D' pattern of wear, with the longest side on one end of the bearing, Causes can be taper in the bearing housing or journal surface, debris trapped between shell and housing, bruising of the steel backing of the bearing shell or housing bore during assembly, taper through the bore of the bearing, or a variation in alignment of a series of main bearing bores along the engine.

Geometric factors

Severe wear of localized regions of the bearing surface are generally caused by specific geometric inaccuracies. Heavy wear in the axial centre region is usually caused by a 'barrelled'

journal, whereas heavy wear at both ends of the bearing results from an 'hour glass' profile.

Thermal ratcheting – caused by alternate cooling and heating of bearing, result in bearing deformation, indication of high bearing temperature, most occurred at thrust pad bearing surface.

Static fretting  - The damage shown is caused by vibration of a crankshaft within the bearing clearance. The term 'static' comes from the fact that such damage only occurs when the engine is not running. The damage can occur during transport of assembled engines to site. In marine installations, generating sets mounted on flexible tank tops are prone to similar damage. In such cases, the simplest palliative is to ensure that oil is pumped through the bearing clearance every few hours while the engine is inoperative(= must continuous run the pre-lube oil pump when stop generator engine) , although in very severe instances it may also be necessary to redesign the mountings (= installed on the resilience mounting).