Identification and Solution of Pump/Gearbox/Driveshaft/Turbine System Vibration Problem

A major petrochemical company had a serious gear box failure problem in some service water pumps, driven at variable speed by a steam turbine though a right angle 1:1 gear box and hollow drive shafting. Many experts from the pump, turbine, and gear manufacturers had attempted without success to understand and cure the problem over a 3 year period. Replacement of the gear boxes with some built to more stringent standards had no effect. External consultants, using test or analysis also had failed to solve the problem.

Analysis by MSI engineers of torsional and lateral rotor and structural frequencies confirmed earlier consultant findings that the design should be sound in this regard. Torsional modal testing, using testing techniques that could be applied while the pump/gear/turbine operated, found that all rotor system natural frequencies were close to their predicted values, and with even more damping than predicted. However, impact modal testing on the hollow drive shaft showed that it had a "bell-mode" at the gear meshing frequency, where the hollow shaft ovalized with very little damping, causing the shaft length to change through the "Poisson effect". The driving force was shown by the test to be the combined torsional and axial load from the bull/pinion gear meshing. The drive shaft was filled with grease to damp out this unusual vibration. The gear box noise immediate fell a factor of five, and all gear box problems ceased.