Shaft crack monitoring via torsional vibration analysis; Part 1 - Laboratory tests

Torsional vibration signature analysis has shown the potential to detect shaft cracks during normal machinery operations of rotating equipment. The method tracks characteristic changes in the natural torsional vibration frequencies that are associated with shaft crack propagation. The method is generally applicable to many types of rotating equipment. A laboratory scale rotor test bed was developed to investigate shaft cracking detection techniques under controlled conditions. A sample shaft was seeded with a semi-elliptical surface crack, which was propagated in three point bending. The fatigue crack was incrementally grown in nine steps, with depths ranging from approximately 0 - 60% of the shaft diameter. After the crack was grown to each pre-defined depth, the shaft was installed in the rotor test bed and the changes in shaft torsional vibration features observed. The first torsional natural frequency is shown to be sensitive to the shaft crack depth, which for the crack depths tested produced a 2 Hz frequency drop. The relationship between crack depth and torsional natural frequency is nonlinear. The test data show that changes in the torsional shaft frequency in the range of 0.1 to 0.2 Hz. can be detected by a visual inspection. This study points to the potential of using online torsional signature analysis as a diagnostic for shaft crack monitoring in rotating equipment.