Past Turbine Tips have covered the main reasons for bearings to wipe: 1) Insufficient lube oil supply, 2) Low lube oil pressure, and 3) Water in the lube oil. Every once in a while a fourth cause appears: High bearing loading.
Proper bearing loading is calculated by the elevations of the bearings, component weights and shaft alignments (bending moments, lateral, torsional). The OEM calculates the elevations and coupling alignments during the design process, based on the catenary curve (or sag chart). Calculations ofbearing loadings and alignment are usually accurate based on the design engineers’ mathematical calculations and computer model for the rotor’s geometry, speed, weight, and bearing design.
The Catenary Curve
Most of the time, high bearing loading is caused by misalignment of the turbine power train from the original design. That is, some force has moved the components from their original alignments. The source of the bearing failure can be eliminated by carefully measuring and re-aligning to the original specifications. But we have seen examples where the original calculations either were not accurate orover years of operation the bearing pedestals had moved.
Recalculating bearing loading is an arduous and potentially expensive process, so all other contributing factors should be eliminated before attempting this course. If necessary, TGM can perform the recalculation and re-alignment without the participation of the OEM. On three bearing units, it is not uncommon to utilize a dynamometer to check bearing loading during alignment and their adjustments.