Bearing Failure Analysis: ISO 15243 Failure Modes, Root Causes and How to Prevent Repeat Failures

A failed bearing is evidence, not scrap. When a bearing in a critical pump or fan fails early and is thrown away, the plant usually buys another one, mounts it the same way and gets the same failure a few months later. Structured bearing failure analysis breaks that cycle. This guide uses the classification in ISO 15243, the international standard for rolling bearing damage terms, characteristics and causes, to explain how to read the damage, connect it to a root cause and decide what to change so it does not happen again.

Why ISO 15243 Matters

ISO 15243 gives maintenance teams, bearing manufacturers and insurers a common language. It classifies damage that occurs while the bearing is installed and operating, based on what can be seen on the rings, rolling elements and cage, and it describes the typical causes of each appearance. The current edition is ISO 15243:2017, which replaced the 2004 edition. The standard does not cover manufacturing defects. It also recognises that several mechanisms often act together, so the visible damage may be a mix, and the analyst must find the primary mode that started the chain.

The Six Failure Modes at a Glance

Rolling Contact Fatigue

Fatigue is the failure mode that L10 life predicts. Repeated stress cycles create microcracks that grow until material flakes from the surface. Subsurface-initiated fatigue starts below the surface in the zone of highest shear stress and is the classic end-of-life failure in a clean, well-lubricated bearing. If it appears early, look for higher load than designed: belt over-tension, unbalance, misalignment, or an interference fit that removed internal clearance and preloaded the bearing.

Surface-initiated fatigue begins at the surface where the lubricant film is too thin to separate the contacts. The early sign is a frosted or glazed band and fine micro-pitting; later it develops into flaking. Root causes include wrong viscosity for the operating temperature, degraded or insufficient lubricant, and dents from contaminants that act as stress raisers. In hot climates, a grease selected for a lower operating temperature is a frequent contributor.

Wear: Abrasive and Adhesive

Abrasive wear removes material evenly and leaves dull, matt raceways and greyish lubricant loaded with fine metal. It is the typical result of sand, dust or process particles entering through worn seals or being pumped in during relubrication with dirty grease guns. Increased internal clearance follows, which raises vibration and noise.

Adhesive wear, often called smearing, happens when rolling elements slide instead of roll. Material transfers between surfaces and they look torn and discoloured. It is common on large roller bearings that run with very light load at high speed, during rapid acceleration, or where the lubricant is too stiff for start-up. Solutions include ensuring a minimum load, choosing a suitable lubricant and, in some designs, changing cage or internal geometry.

Corrosion: Moisture and Frictional

Moisture corrosion appears as dark stains or rust, often at the spacing of the rolling elements, because water collects at the contacts during standstill. Sources include washdown water, rain on outdoor equipment, condensation in humid coastal air and water in the lubricant. Pitting from corrosion later becomes a starting point for fatigue.

Frictional corrosion has two forms. Fretting corrosion occurs between a ring and its seat when the fit is too loose or the seat is not round, leaving reddish-brown oxide on the bore or outside diameter. False brinelling occurs on the raceways of stationary bearings subjected to vibration, for example a standby pump next to a running unit or equipment transported without securing the shaft. The marks sit at rolling element pitch, but unlike true brinelling the original grinding marks are worn away.

Electrical Erosion

When current passes through the rolling contacts, it melts tiny areas of the surface. Excessive current, such as welding current earthed through a machine, causes craters and burn marks. Current leakage, a lower current over longer periods, typically produces fluting: a regular washboard pattern of grey lines across the raceway, with darkened, burnt grease. Fluting is strongly associated with motors fed by variable frequency drives, where common-mode voltage discharges through the bearings. Prevention combines shaft grounding devices, current-insulated or hybrid ceramic bearings, and correct drive cabling and earthing. Never connect a welding earth clamp so that the current path crosses a bearing.

Plastic Deformation

Overload deformation, or true brinelling, leaves shallow dents at rolling element pitch, often from hammering a bearing during mounting or from an impact load. Indentation from debris produces many small dents with raised edges when hard particles are over-rolled; each dent is a future fatigue starting point. Indentation from handling includes nicks and gouges from dropping bearings or using the wrong tools. Using induction heaters and correct mounting tools, and keeping bearings in their packaging until fitting, removes most of these causes.

Fracture and Cracking

Forced fracture comes from stress beyond the material strength, for example an inner ring driven too far up a tapered seat or an impact. Fatigue fracture develops from repeated bending, such as an outer ring poorly supported in a housing. Thermal cracking forms across the ring when sliding generates high frictional heat, for example a ring creeping on its seat or a seal rubbing hard. Cracks are serious because the bearing can seize suddenly; finding the source of the excessive stress or heat is essential before restarting.

A Step-by-Step Bearing Failure Analysis Procedure

1. Collect operating history: run hours, trend data, temperature, alarms and the last maintenance actions.
2. Before removal, note the bearing position, load direction and any visible leakage, and take a lubricant sample.
3. Mark the load zone on the outer ring and the orientation in the housing before dismantling.
4. Photograph each component: rings, rolling elements, cage, seals, shaft seat and housing bore.
5. Examine the running track pattern. Its position and width show load direction, misalignment and fit problems.
6. Classify the damage using the six ISO 15243 modes and identify the primary mode.
7. Check the shaft and housing dimensions, roundness and fits against the drawing.
8. Identify the root cause and decide the corrective action, then record it in the maintenance system.

What the Lubricant and Seals Tell You

The lubricant is often the fastest route to the root cause. Grease that is dark brown or black and hard has usually oxidised from high temperature or an extended relubrication interval. Grey or silvery grease points to wear particles, while a milky or emulsified appearance shows water ingress. Gritty grease that feels sandy between the fingers confirms external contamination. Where possible, send a sample for laboratory analysis of water content, particle count and wear metals, and compare it with a sample of the fresh grease from stores.

Seals and shields deserve the same attention. Worn or hardened lip seals, a damaged labyrinth, a missing V-ring or a blocked grease relief port all change the operating conditions of the bearing. A seal running on a scored shaft surface will leak whatever grease is chosen, and a housing with no free path for old grease to escape will overheat after every relubrication. Record seal condition with the same care as the bearing itself.

Reading Running Track Patterns

The path where the rolling elements have run is often more informative than the damage itself. A normal pattern under a stationary radial load is centred on the outer ring raceway and covers less than half its circumference, while the rotating inner ring shows a band all around. A track running around the entire outer ring indicates preload, typically from an excessive interference fit or a non-locating bearing that cannot float. A pattern offset to one side shows axial load, and a diagonal track on the stationary ring shows misalignment between shaft and housing. Comparing these patterns with the expected loads tells you quickly whether the problem lies in the bearing or in the machine.

GCC Field Factors That Shape Bearing Damage

Plants in Oman and the wider GCC see some failure modes more often than others. Fine sand drives abrasive wear on outdoor fans, conveyors and cooling tower drives when seals are not maintained. High ambient temperature thins lubricants and accelerates oxidation, leading to surface-initiated fatigue. Humid coastal air at desalination and port sites encourages moisture corrosion, particularly on standby equipment, and vibration from neighbouring machines creates false brinelling on idle units. Widespread use of variable frequency drives in water and HVAC services makes electrical erosion a common finding. A practical response is to rotate standby equipment on a schedule, check seal condition at every relubrication and include drive grounding in motor inspections.

Prevention and Monitoring

• Use correct mounting tools and heaters; never transmit mounting force through the rolling elements.
• Keep lubricants clean and use dedicated, labelled grease guns for each grease type.
• Align couplings and check soft foot after every motor change.
• Verify shaft and housing fits during overhauls.
• Monitor vibration and temperature to detect damage early; our overview of vibration monitoring system types explains the options, and the vibration monitoring category lists hardware.
• Use proper tooling and lubricants from the lubrication and bearing maintenance range.

Bearing Failure Analysis Checklist

• Operating history and condition monitoring trends gathered.
• Lubricant sample taken before cleaning.
• Load zone, orientation and running track recorded.
• All components photographed in good light.
• Damage classified into one primary ISO 15243 mode.
• Shaft and housing fits measured.
• Root cause identified and corrective action recorded.
• Replacement bearing designation confirmed, including clearance and seal type.

Bearing Maintenance Products We Supply

Many of the failure modes above start with poor mounting or lubrication. These products address both, plus detection.

Send the bearing designation, machine type and operating conditions through our quote request form.

Sourcing Replacement Bearings Through Seven Star LLC

Seven Star LLC supplies replacement bearings, mounting tools and lubrication products from established manufacturers on request. If the failure analysis points to a different clearance class, seal type or an insulated design, we help cross-check part numbers and confirm equivalents between brands. Availability and lead time are confirmed at quotation. See our roller bearings range for typical types.

Send us the failed bearing’s designation, photos of the damage and the machine datasheet or nameplate, and we will reply with suitable replacement options in our quotation.