Generator bearing failures are rarely sudden events. In most cases, the bearing gives warning signs well before it reaches the point of catastrophic failure — vibration that trends upward, temperatures that rise without an obvious cause, oil that darkens or contains metal particles. Recognising these signals early gives you the window to plan a controlled inspection instead of responding to an emergency shutdown.
Why Bearings Fail — and Why It's Rarely Sudden
Generator journal bearings support the rotor through a pressurised oil film. When this film is intact and the oil is clean and at the right temperature and viscosity, bearing wear is minimal. Failure occurs when the film breaks down — through contamination, inadequate oil pressure, wrong viscosity, thermal effects, or mechanical damage from a preceding event.
The progression from early damage to failure typically takes hours, days or even weeks. The challenge is that the early stages produce signals that are easy to dismiss as normal variation. Trending is essential: a single measurement tells you the current state, but a trend tells you whether the state is stable or deteriorating.
Vibration Signals
Vibration is often the first measurable signal of bearing deterioration. What matters is not just the absolute level but the change relative to the established baseline for that machine.
- Gradual increase in 1× (synchronous) vibration — can indicate bearing damage, changing alignment due to thermal effects, or rotor imbalance developing over time. A steady upward trend without any changes to operating conditions is a significant warning.
- Sub-synchronous vibration at low amplitude — in generators, sub-synchronous components can indicate oil film instability. Even small amplitudes warrant investigation if the frequency is unusual for the machine.
- Increased broadband noise floor — random high-frequency content in the vibration spectrum can indicate bearing surface damage, particularly in the early stages of Babbitt wear.
- Change in vibration character after a load change or temperature shift — if vibration behaviour changes noticeably with operating conditions in a way it didn't previously, something has changed in the bearing.
Compare current readings to the baseline established after the last bearing maintenance, not to generic alarm limits. A machine that historically runs at 2.5 mm/s reaching 3.8 mm/s is a significant change, even if 3.8 mm/s is below the alarm level.
Temperature Signals
Bearing metal temperature is one of the most direct indicators of bearing health. Most generators have embedded thermocouples or RTDs in the bearing Babbitt layer that give continuous temperature readings.
Normal bearing temperature depends heavily on machine design, oil temperature, speed, load and ambient conditions. It is not meaningful to compare to a fixed absolute limit — the comparison must be to the historical operating temperature for those specific conditions.
- Slow upward drift — temperature that climbs 2–5°C over weeks or months without a change in operating conditions. Often dismissed as seasonal variation but worth logging carefully.
- Temperature difference between the two generator bearings — if one bearing consistently runs hotter than the other, and this was not previously the case, investigate. Possible causes include different oil flow rates, bearing condition differences, or rotor loading imbalance.
- Rapid temperature rise — a fast rise above normal operating temperature is an urgent warning. If temperature rises more than 10°C above the established operating temperature without explanation, investigate immediately before continuing to operate.
- Temperature that does not stabilise after a load change — temperature should return to a predictable steady state after load changes. If it continues to drift upward after the load is stable, something is consuming energy in the bearing that wasn't before.
Most bearing temperature trips are set at 100–120°C depending on machine design. This is a safety trip, not an operational target. If bearing temperature is within 15°C of the trip setpoint, this is already a serious situation requiring investigation — do not wait for the trip.
Oil Condition Signals
Oil condition is a highly informative diagnostic parameter that is often underused. The oil that lubricates the generator bearings passes through them continuously and carries information about what is happening inside.
- Increased metal particle count — oil particle analysis (ferrography or spectrometry) can detect Babbitt particles, steel particles from journal wear, or copper particles from bearing cage material. A step change in particle count is a significant finding.
- Oil darkening or discolouration — clean turbine oil is light amber. Darkening indicates thermal degradation, contamination or metal oxide formation. This is worth investigating rather than accepting.
- Foam in the oil system — air entrainment reduces oil film load capacity. Investigate air ingress paths if foam is visible in the oil reservoir or oil sight glass.
- Water contamination — water in the oil (detected by the Karl Fischer method or by the "crackle test") dramatically reduces film strength and accelerates Babbitt fatigue. Any detectable free water is a problem that needs to be found and eliminated.
Seal Oil Indicators
Hydrogen-cooled generators use a seal oil system to prevent hydrogen leakage along the shaft. This system interfaces directly with the generator bearings. Changes in seal oil system behaviour can be early indicators of bearing-related issues.
- Increased seal oil consumption without visible external leakage
- Hydrogen purity degradation (hydrogen contaminated by oil vapour from a deteriorating seal)
- Abnormal differential pressure across the seal ring
How to Assess Severity
A single warning sign is worth noting. Multiple simultaneous warning signs indicate that an inspection should be planned. The following combinations represent escalating severity:
| Signals present | Assessment | Recommended action |
|---|---|---|
| Gradual vibration increase, no temperature change | Monitor closely | Increase monitoring frequency; set tighter alert levels |
| Vibration increase + temperature increase | Investigate | Plan inspection at next available opportunity; investigate root cause |
| Vibration + temperature + abnormal oil condition | Urgent | Plan controlled shutdown for inspection; consult technical support |
| Rapid temperature rise + high vibration + any other signal | Critical | Controlled shutdown; do not continue operating |
What to Have Ready for a Technical Assessment
If you contact Axerion or any technical support for an assessment, have the following available:
- Current vibration readings at each bearing (amplitude and, if available, frequency spectrum)
- Trend of vibration over the past 3–6 months
- Current bearing temperatures and temperature trend
- Date and scope of last bearing inspection or replacement
- Oil analysis results if available
- Any recent operating events (load changes, trips, unusual conditions)
With this information, it is possible to give a meaningful assessment of severity and recommended next steps without needing to be on site.