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Journal (sleeve) bearings use a thin oil film between shaft and bearing surface. They are used in large generators (>1 MW) and have higher temperature tolerances. Rolling element bearings use balls or rollers and are common in smaller generators. They have lower temperature limits due to grease degradation.

ΔT (bearing temp minus ambient) is more diagnostic than absolute temperature because it eliminates the effect of varying ambient conditions. A bearing running at 80°C in 25°C ambient (ΔT=55°C) is more concerning than 85°C in 40°C ambient (ΔT=45°C).

IEEE C50.13 covers requirements for cylindrical-rotor synchronous generators. It includes guidelines for temperature limits of bearings, windings, and cooling systems. The bearing temperature limits in this tool are based on IEEE C50.13 and common manufacturer practices.

Synthetic oils (PAO, ester) offer better thermal stability and allow slightly higher operating temperatures (5–10°C more). They are recommended for high-speed applications, extreme temperatures, or extended drain intervals. Mineral oils are standard for most applications and are more cost-effective.

Normal temperature changes are gradual (1–2°C per hour during load changes). A rate exceeding 5°C per hour without corresponding load change warrants investigation. A sudden jump of 10°C or more indicates a developing problem requiring immediate attention.