How To Diagnose Plain Bearing Failures
Diagnosing plain bearing failures is all about recognising the tell-tale signs of different wear out and other failure modes. Careful inspection of the failed bearing will usually reveal vital visual clues that can lead to a potential cause for the failure. However, do not stop at this stage. More than one mechanism of failure can lead to superficially similar bearing damage. Moreover, once failure has been initiated, the original mechanism may lead to other failure mechanisms and it is essential to identify the original cause.
A Picture is Worth A 1000 Words…
This page lists many typical plain bearing failure modes with images showing the associated failure pattern. The failure causes listed point towards a solution or areas for further investigation so that this type of failure can be prevented in the future. The information on this page is taken from our Tribology Handbook – a PDF version is available from our site here.Need some more advice? – Get in touch and we’d be happy to help
Foreign Matter - With Fine Scoring
Foreign Matter - With Severe Scoring
Causes: Contamination of lubricant by excessive amounts of dirt particularly non-metallic particles which can roll between the surfaces
Wiping Wear Marks
Characteristics: Surface melting and flow of the bearing material, especially when of low-melting point, e.g. whitemetals, overlays.
Causes: Inadequate bearing clearance, overheating, insufficient oil supply, excessive load, or operation with a non-cylindrical journal
Fatigue - Mosaic Cracking
Characteristics: cracking, often in a mosaic pattern, and loss of areas of lining.
Causes: Excessive dynamic loading or overheating causing reduction of fatigue strength; over-speeding causing imposition of excessive centrifugal loading.
Fatigue Cracking - Resulting in Lining Loss
Characteristics: Loss of areas of lining by propagation of cracks initially at right angles to the bearing surface, and then progressing parallel to the surface, leading to isolation of pieces of the bearing materials
Causes: Excessive dynamic loading which exceeds the fatigue strength at the operating temperature.
Characteristics: Welding, or pick-up of metal from the housing on the back of bearing. Can also occur on the joint faces. Production and oxidation of fine wear debris, which in severe cases can give red staining.
Causes: Inadequate interference fit; flimsy housing design; permitting small sliding movements between surfaces under operating loads.
Characteristics: Uneven wear of bearing surface, or fatigue in diagonally opposed areas in top and bottom halves.
Causes: Misalignment of bearing housings on assembly, or journal deflection under load.
Dirty Assembly - Example 1
Characteristics: Localised overheating of the bearing surface and fatigue in extreme cases, sometimes in nominally loaded areas.
Causes: Entrapment of large particles of dirt (e.g. swarf), between bearing and housing, causing distortion of the shell, impairment of heat transfer and reduction of clearance (see next column).
Dirty assembly - Example 2
Characteristics: Local areas of poor bedding on the back of the bearing shell, often around a ‘hard’ spot.
Causes: Entrapment of dirt particles between bearing and housing. Bore of bearing is shown in previous column illustrating local overheating due to distortion of shell, causing reduction of clearance and impaired heat transfer.
Characteristics: Removal of bearing material, especially soft overlays or whitemetal in regions near joint faces or grooves, leaving a roughened bright surface.
Causes: Changes of pressure in oil film associated with interrupted flow.
Discharge Cavitation Erosion
Characteristics: Formation of pitting or grooving of the bearing material in a V-formation pointing in the direction of rotation.
Causes: Rapid advance and retreat of journal in clearance during cycle. It is usually associated with the operation of a centrally grooved bearing at an excessive operating clearance.
Cavitation erosion - Example 2
Characteristics: Attack of bearing material in isolated areas, in random pattern, sometimes associated with grooves.
Causes: Impact fatigue caused by collapse of vapour bubbles in oil film due to rapid pressure changes. Softer overlay (Nos. 1, 2 and 3 bearings) attacked. Harder aluminium – 20% tin (Nos 4 and 5 bearings) are usually not attacked under these particular conditions.
Characteristics: Removal of lead phase from unplated copper-lead or lead-bronze, usually leading on to fatigue of the weakened material.
Causes: Formation of organic acids by oxidation of lubricating oil in service. Consult oil suppliers; investigate possible coolant leakage into oil.
Tin Dioxide Corrosion
Characteristics: Formation of hard black deposit on surface of white-metal lining, especially in marine turbine bearings. Tin attacked, no tin-antimony and copper-tin constituents.
Causes: Electrolyte (sea water) in oil.
Characteristics: Deep pitting and attack or copper-base alloys, especially phosphor-bronze, in high temperature zones such as small-end bushes. Black coloration due to the formation of copper sulphide.
Causes: Attack by Sulphur compounds from oil additives or fuel combustion products.
Wire Wool Damage - Example 1
Characteristics: Formation of hard black scab on whitemetal bearing surface, and severe machining away of journal in way of scab, as shown on the right.
Causes: It is usually initiated by a large dirt particle embedded in the whitemetal, in contact with journal, especially chromium steel.
Wire Wool Damage - Example 2
Characteristics: Severe catastrophic machining of journal by ‘black scab’ formed in whitemetal lining of bearing. The machining ‘debris’ looks like wire wool.
Causes: Self-propagation of scab, especially with ‘susceptible’ journals steels, e.g. some chromium steels.
Characteristics: Pitting of bearing surface and of journal; may cause rapid failure in extreme cases.
Causes: Electrical currents from rotor to stator through oil film. Often caused by faulty earthing.
Fretting Due To External Vibration
Characteristics: Pitting and pick-up on bearing surface.
Causes: Vibration transmitted from external sources, causing damage while journal is stationary
Characteristics: Extrusion and cracking, especially of whitemetal-lined bearings.
Causes: Operation at excessive temperatures.
Thermal Cycling Damage
Characteristics: Surface rumpling and grain-boundary cracking of tin-base whitemetal bearings.
Causes: Thermal cycling in service, causing plastic deformation, associated with the non-uniform thermal expansion of tin crystals.
Faulty Assembly - Example 1
Characteristics: Localised fatigue or wiping in nominally lightly loaded areas.
Causes: Stagger at joint faces during assembly, due to excessive bolt clearances, or incorrect bolt disposition (bolts too far out).
Faulty assembly - Example 2
Characteristics: Overheating and pick-up at the sides of the bearings.
Causes: Incorrect grinding of journal radii, causing fouling at fillets.
Incorrect Journal Grinding
Characteristics: Severe wiping and tearing-up of bearing surface.
Causes: Too coarse a surface finish, or in the case of SG iron shafts, the final grinding of journal in wrong direction relative to rotation in bearing
Inadequate Oil Film Thickness
Characteristics: Fatigue cracking in proximity of a groove.
Causes: Incorrect groove design, e.g. positioning a groove in the loaded area of the bearing.
Characteristics: Seizure of bearing.
Causes: Inadequate pump capacity or oil gallery or oilway dimensions. Blockage or cessation of oil supply.
Bad Bonding of Lining to Shells
Characteristics: Loss of lining, sometimes in large areas, even in lightly loaded regions, and showing full exposure of the backing material.
Causes: Poor tinning of shells; incorrect metallurgical control of lining technique