High bearing temperature in a generator is a warning signal, not a normal operating state. The causes range from simple (low oil flow) to complex (electrical bearing currents), and they each call for different responses. Treating them all the same — reducing load and hoping the temperature drops — often delays the diagnosis without addressing the underlying cause. This article covers the most common causes, how to distinguish between them and what to do.
What "Normal" Looks Like
Generator bearing temperatures vary significantly between machine designs. A large turbogenerator bearing might normally operate at 70–85°C, while a smaller industrial generator might be designed for 55–70°C. Neither figure is universal.
What matters for diagnostics is the deviation from the established baseline for that specific machine under the same operating conditions. A bearing that normally runs at 72°C reaching 85°C is more significant than a bearing that normally runs at 82°C reaching 85°C — even though the absolute temperature is the same in both cases.
The Most Common Causes
Insufficient oil supply — pressure or flow
The oil film that supports the shaft requires adequate flow and pressure to maintain its load-carrying capacity. Insufficient supply — from a failing lube oil pump, a blocked oil filter, a restriction in the bearing supply line, or a faulty pressure regulating valve — reduces the film thickness and increases metal-to-metal proximity, generating heat.
Signature: Temperature increases at one or both bearings simultaneously. Oil supply pressure at the bearing is below the normal operating range. Both turbine and generator bearings may show elevated temperature if the issue is in the shared oil supply system.
Immediate check: Verify oil supply pressure at the bearing header. If pressure is low, identify the cause (pump, filter, valve) before the temperature rises further.
Wrong oil viscosity or degraded oil
Oil viscosity determines the thickness and strength of the hydrodynamic film at operating temperature. Oil that is too thin (wrong grade, or oil that has thermally degraded) supports less load at the same film thickness. Oil that is too thick increases viscous drag and generates more heat.
Signature: Temperature drift that tracks ambient temperature changes more closely than usual. Oil that appears dark, cloudy or has unusual odour when sampled. Temperature difference may exist between winter and summer operation beyond normal seasonal variation.
Bearing damage — Babbitt wiping or scoring
A bearing that has already experienced partial Babbitt damage — from a previous low-oil event, a foreign particle, or a transient overload — has reduced load-carrying surface area. The remaining Babbitt runs at higher temperature to carry the same load. This is a progressive situation: the damaged bearing generates more heat, which accelerates further degradation.
Signature: Temperature elevated at one specific bearing, not matching changes to oil supply pressure or operating conditions. May be associated with vibration changes. Oil analysis may show elevated metal particle content from that bearing location.
If Babbitt damage is suspected, the bearing must be inspected at the next available opportunity. Operating with a damaged bearing is not a stable situation — the degradation tends to accelerate, and the failure mode if allowed to continue is a catastrophic bearing wipeout.
Shaft misalignment
Misalignment between the generator and the turbine (or between the generator and the exciter, in machines with separate exciters) creates additional loading on the bearings nearest the coupling. This additional radial load increases oil film pressure and generates heat.
Signature: Temperature elevated primarily at the bearing nearest the coupling. May be associated with 2× vibration at the same bearing. Temperature may change significantly with load (since misalignment forces change with torque in some coupling types).
Electrical bearing currents
In large generators, differences in magnetic flux can induce shaft currents that discharge through the bearings — essentially turning the bearing into a spark erosion device. The damage is distinctive: pitting on the bearing surface and journal, progressive temperature increase as the damage accumulates, and ultimately a bearing with significant surface erosion.
Signature: Temperature increase that is gradual and progressive over months rather than sudden. Oil analysis shows fine metallic particles from erosion. Bearing inspection shows characteristic "frosted" pitting on the Babbitt surface and shaft journal. Insulation of the non-drive-end bearing is a common engineering solution to limit shaft current path.
Cooling circuit issues
Many large generator bearings are cooled not only by oil flow but by dedicated bearing cooling water circuits. If cooling water flow is reduced (due to fouled heat exchanger, reduced flow, or elevated cooling water inlet temperature), bearing temperature rises even if oil supply is normal.
Signature: Temperature increase at the bearing that tracks cooling water temperature. Temperature rises during periods of high ambient temperature or when cooling water system is under load. Check cooling water flow rate and inlet temperature.
How to Distinguish Between Causes
| Observation | Most likely cause | First check |
|---|---|---|
| Both bearings high, oil pressure low | Oil supply issue | Oil pump, filter, header pressure |
| One bearing high, other normal | Bearing damage or alignment | Inspect bearing; check coupling alignment |
| Temperature tracks ambient/cooling water | Cooling system | Cooling water flow and temperature |
| Gradual rise over months, vibration normal | Electrical bearing currents, oil degradation | Oil analysis; bearing inspection at next outage |
| High temp + high 2× vibration at same bearing | Misalignment | Alignment measurement at next available opportunity |
| Rapid rise + high vibration | Bearing damage, acute event | Controlled shutdown and inspection |
Immediate Response Steps
When bearing temperature exceeds the normal operating range by more than 10°C, or is trending upward without explanation:
- Verify oil supply pressure and temperature at the bearing header
- Check cooling water flow rate and inlet temperature if applicable
- Review vibration trend at the affected bearing
- Check oil sample from the reservoir — visual check for colour and condition
- Increase monitoring frequency — check every 15–30 minutes rather than shift-based
- Notify engineering and prepare for a possible controlled shutdown
Stop the machine immediately if: temperature reaches within 15°C of the trip setpoint without an identified and corrected cause; temperature rises faster than 2°C per minute; temperature is rising in combination with a simultaneous vibration increase; oil supply pressure falls below the minimum operating limit.