The main reasons for bearings to wipe are:
- Insufficient lube oil supply
- Low lube oil pressure
- 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). These forces have a direct impact on the stress of the equipment and thus, need to be a part of the equation.
The OEM calculates the elevations and coupling alignments during the design process, based on the catenary curve (or sag chart). Calculations of bearing 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 following Catenary Curve graphic depicts bearing loading:
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. This may include temperature, rotor vibration and other forces associated with the wear-and-tear of the operation.
The source of the bearing failure can be eliminated by carefully measuring and re-aligning to the original specifications. We have also seen examples where the original calculations were not accurate or after 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.
It is important to maintain proper bearing loading to avoid the worst-case-scenario of any impact damage, equipment fatigue, or even catastrophic malfunction. Preventative maintenance can help you be proactive and reduce any bearing challenges that can cause a more severe event in your operation. On three bearing units, it is not uncommon to utilize a dynamometer to check bearing loading during alignment and their adjustments.
PSG can perform the recalculation and re-alignment without the participation of the OEM. If you believe your turbine may be affected by any of the above mentioned reasons for bearing wipe, please contact us today for a free assessment of your current situation.