(i) Dissimilarity of Materials in Contact Surfaces
Bearing design normally uses a hard/soft material combination, such as a steel journal running in white metal, bronze, or tin-based Babbitt bearing material. This difference in material properties is intentional.
The softer bearing material:
- Can embed foreign particles such as dirt and wear debris, preventing scoring of the harder journal.
- Can deform slightly to accommodate minor misalignment.
- Wears preferentially, thereby protecting the more expensive shaft or journal.
If the materials are too similar in hardness, both surfaces may wear together and adhesive wear, galling, or pick-up can increase because there is no sacrificial surface.
If the bearing material is too soft, it may suffer rapid wear, extrusion, and fatigue cracking under load.
Dissimilar metals can also produce electrochemical or galvanic corrosion when contaminated lubricant or moisture is present, resulting in corrosive pitting.
(ii) Relative Speed of Sliding Between the Surfaces
Bearing wear is closely related to the type of lubrication achieved at different speeds.
- At low speeds, particularly during starting and stopping, the oil film may not be fully established. Boundary or mixed lubrication occurs, resulting in some metal-to-metal contact and increased wear.
- At the correct operating speed, a full hydrodynamic oil wedge separates the surfaces. Direct metal-to-metal contact is then greatly reduced and wear becomes very small.
- At excessively high speed, frictional heat increases, causing the oil temperature to rise and its viscosity to decrease. The oil film may become thinner, increasing the risk of overheating, wiping, and bearing damage.
Therefore, a large proportion of bearing wear can occur during starting, stopping, or prolonged low-speed operation when the oil film is insufficient.
(iii) Roughness of the Surfaces
Surface roughness consists of small high points or asperities on the bearing and journal surfaces.
- If the surfaces are too rough, the asperities may penetrate the oil film and come into contact with the opposite surface.
- This causes increased friction, local heating, scoring, and wear.
- During initial running-in, some high spots are normally removed. However, excessive initial roughness causes accelerated wear and produces wear particles.
- These particles may then cause three-body abrasive wear.
- Rough surfaces also reduce the effective contact area, concentrating the load over fewer points and increasing local pressure.
Thus, excessively rough surfaces require a greater oil-film thickness to prevent metal-to-metal contact.
(iv) Incompatibility of Lubricant and Bearing Material
The lubricant must be suitable for both the operating conditions and the bearing material.
- Incorrect oil viscosity may result in insufficient oil-film thickness at the operating temperature and load, causing boundary lubrication and increased wear.
- Some lubricant additives, particularly certain sulphur- or chlorine-containing EP additives, may chemically attack bearing materials such as white metal, copper-lead, or silver, causing corrosive wear and pitting.
- Water contamination or acidic degradation products in the oil can corrode the bearing surface.
- An incompatible lubricant may also cause excessive foaming, rapid oxidation, or poor removal of heat and contaminants, indirectly increasing bearing wear.
Cause | Signs During Inspection | Operational Corrective Action | Maintenance Corrective Action |
Dissimilar materials | Embedded debris in the soft bearing metal; scoring of the journal; galvanic pitting; uneven wear pattern. | Maintain correct lubricant condition, avoid operation with contaminated oil, and monitor bearing temperature trends. | Re-metal/re-babbitt bearing shells to the correct specification; use the correct replacement material grade; check bearing clearances during renewal. |
Relative sliding speed | Wear concentrated around starting/low-speed areas; wiped or smeared metal, particularly on the bottom half; heavy wear associated with turning-gear operation. | Ensure adequate pre-lubrication using priming pumps before starting; avoid prolonged slow-speed operation where possible; ensure lubricating oil is supplied when using turning gear. | Check and restore correct running clearances; verify lubricating-oil pressure and priming-pump operation; inspect bearing crush and fit. |
Surface roughness | Scratched, dull, or matte bearing surface instead of a smooth running-in finish; increased wear debris in oil filters or oil analysis. | Maintain effective oil filtration; prevent entry of abrasive contaminants such as dust, sand, and metal particles; follow the correct running-in procedure after overhaul. | Re-machine, lap, or scrape the bearing surface to obtain the correct finish; polish the journal; renew the journal/bearing if scoring exceeds permissible limits; improve filtration where necessary. |
Lubricant incompatibility | Discoloration; corrosion or pitting; sludge or varnish deposits; unusual oil odour; oil analysis indicating incorrect additives or oil degradation. | Use only the manufacturer-approved lubricant grade; avoid mixing different oil types; regularly monitor oil condition through sampling and analysis. | Drain and flush the lubrication system; refill with the specified lubricant; renew corroded bearing components; review and improve filtration and purifier settings where necessary. |
General Inspection Methods
The following methods can be used to identify bearing wear and its causes:
- Visual inspection of the bearing shell for colour changes, pitting, wiping, scoring, embedded particles, and abnormal wear.
- Clearance measurement using a feeler gauge, Plastigauge, or micrometer, comparing the results with the manufacturer's specified tolerances.
- Lubricating-oil analysis for wear-metal content, viscosity, contamination, TAN/TBN, and other relevant parameters.
- Vibration monitoring and trending of bearing temperatures.
- Crankshaft deflection measurements for main and crankpin bearings to identify possible misalignment-related wear.
General Corrective Principle
The root cause must be identified and corrected before simply renewing the bearing. Installing a new bearing without correcting the underlying problemāsuch as incorrect lubricant, contamination, poor alignment, or incorrect clearanceācan result in repeated bearing failure.