Dynamic analysis of a rotating stepped shaft with and without defects

Abstract

Rotating machines have many applications, in several mechanical systems. They typically contain a rotary shaft as an influence conversion unit, that is subject to multiple loads in operation. One of the most widely-used rotary machines is the pump, and pump shafts are exposed to many forces due to fluids, unbalancing due to slight bending in the shaft or errors of design or bearings, bearings-induced forces, etc. These forces can reach an unacceptable level once the rotor is functioning close to its natural frequencies. Excessive vibration levels within the device cause fatigue and thus tiny cracks may grow to a serious extent and ultimately cause failure. In this study, dynamic analysis of the pump shaft was made, with and without cracking. Fundamental bending natural frequencies and torsional natural frequencies, response shaft, the equivalent stresses and total deformations in each dynamic and static cases were evaluated. The dynamic load factor (DLF) was calculated in the presence of cracks of various depths (4mm, 6mm, 8mm, 10mm, 12mm) set at diverse locations (x=80mm, x=166mm, x=210mm) measured from the point of overhanging. The finite element method by ANSYS package was used to conduct the numerical analysis for this study, and a specific experimental test rig was built to verify the experimental results. Results showed that increasing depth of the cracks will lead to reducing in the natural frequency and, as a result, increase instability in the shaft. When the location of the crack is close to the highest bend point in the shaft, the natural frequencies will increase. In addition, the equivalent stress depends on cracking location and it is increased with increased depth of cracking.