When a bearing fails, the instinct is to replace it and restart. The bearing itself is rarely the root cause: it is the component that reports a problem that originated somewhere else - in the fit, the alignment, the lubrication, the load, or the environment. Replacing it without asking why it failed guarantees the same failure at a similar interval, and each occurrence consumes a production window.
The value of a failure analysis is that a bearing carries evidence of its own history. The way it failed narrows the causes so sharply that a trained inspection often identifies the mechanism from the damaged part alone, and that is what turns a repair into a correction.
| What the bearing shows | Mechanism it points to | Where to look |
|---|---|---|
| Even, bright wear around the whole raceway | Insufficient or degraded lubricant | Lubricant specification, quantity, interval and contamination |
| Flaking concentrated on one side of the race | Misalignment or a bent shaft | Shaft straightness, housing squareness and coupling alignment |
| Discrete dents spaced at rolling-element pitch | Impact or false brinelling during transport or standstill | Handling, mounting practice and vibration while idle |
| Rusted or discoloured raceways | Moisture ingress or condensation | Seal condition, wash-down practice and thermal cycling |
| Fretting and scoring on the bore or outside diameter | Incorrect fit, or a housing that has worn and lost its grip | Fit tolerances against the shaft and housing drawings |
| Discoloured, softened or cracked metal | Overheating, often from inadequate clearance or overload | Operating temperature, load estimate and internal clearance |
The pattern is read against the load direction, which is why knowing whether the bearing carries a radial load, an axial load or both is part of the diagnosis rather than a background detail. A failure concentrated opposite the loaded zone points one way; an even failure around the whole circumference points another.
Half of the bearing failures that repeat are selection failures rather than maintenance failures. A bearing chosen on bore size alone may be adequate in normal operation and inadequate under the actual load, speed and temperature of the application, particularly where the duty involves shock, high ambient temperature, or continuous running near a limiting speed.
The four numbers that decide the choice are the load, the speed, the required life and the environment. The load determines the size, the speed sets the limiting and the lubrication regime, the life requirement decides whether the chosen size is sufficient at that load, and the environment decides the sealing, the material and the lubricant. A bearing selected without the third of these is selected by habit.
Clearance is the specification most often omitted and most likely to cause trouble. A bearing with standard clearance that runs hot will lose its internal clearance entirely and then fail from preload, and the correction is not a better lubricant but the correct clearance class for the operating temperature. Where the application runs hot, clearance is a selection decision made before purchase, not a fault to be diagnosed afterwards.
A bearing is only as good as the surfaces it sits on. An incorrect shaft fit produces either creep - the inner ring rotating on the shaft and wearing it - or an installation that preloads the bearing and destroys it at commissioning. The same applies to the housing: an outside diameter that is too loose lets the outer ring creep, and one that is too tight distorts the raceway and changes the internal clearance.
Induction heating of the inner ring, rather than hammering, is the practice that prevents most installation damage, and the temperature has to be controlled because overheating changes the material. For precision or high-speed applications, the mounting method and the fits are worth stating on the drawing so they are decided by the designer rather than by whoever assembles the machine.
Most bearings in industrial machines fail from lubrication problems, and most lubrication problems are avoidable. The failures come from the wrong lubricant for the temperature and speed, the wrong quantity, an interval that does not match the duty, or contamination entering through a failed seal. A bearing that is over-greased can churn and overheat just as a starved one can score.
The workable practice is to write the lubricant, the quantity and the interval into the maintenance schedule for each bearing position rather than leaving it to a technician's judgement, and to record what was actually added. The same discipline applies to seals: a seal that is weeping is admitting contamination as well as losing lubricant, and the replacement is part of the job rather than a separate future task.
Rotating machines give notice of a developing bearing problem in vibration, temperature and sometimes noise, long before the failure. Vibration measurement is the most informative, because the frequency of the vibration relates to which part of the bearing is deteriorating, and its trend over time is what indicates how fast it is progressing.
What makes the warning useful is a baseline recorded while the machine was healthy. Without one, an absolute reading is difficult to judge; with one, the rate of change tells you how much time is available. That is also the point at which the spares position becomes the deciding factor: a warning that gives three weeks is only worth having if the bearing, the seal and the skills to fit them are available inside three weeks.
Only if the failed bearing was performing correctly before it failed, and the failure analysis shows the cause was not the selection. Where the failure mechanism points to insufficient load capacity, the wrong clearance or a sealing problem, replacing like for like simply restarts the clock. Diagnose first, then decide whether the part number is still right.
Monitor it. Rising temperature, a change in vibration signature or audible noise under the same load indicates deterioration, and a bearing already running hot or making noise is on its way out rather than in need of grease. Re-greasing a bearing that has already started to fail from contamination does not recover it.
Installation and fits, almost always. Forcing a bearing onto a shaft, hammering it through the rings, an incorrect shaft or housing fit, or a sealing arrangement that lets contamination in at the first wash-down. The failure pattern usually shows it - flaking concentrated at an edge, or fretting on the bore - and correcting the mounting practice prevents the repeat.
Bearings fail slowly and loudly, and the evidence needed to prevent the next one is present in the failed part. ZH Industrial Machinery supplies and services industrial machinery, and we are glad to look at a repeated bearing failure with the maintenance record and the failed component in front of us rather than changing another part and hoping.
The component itself and its geometry are described under bearing, and the lubrication regime that decides most of its life under lubrication. The signals that give warning of deterioration are covered under vibration and condition monitoring. Fits, clearance and material requirements are expressed in ASTM International standards, and the dimensional traceability behind them rests on the NIST framework.