Shaft detection with vibration analysis

Introduction

This article describes how to detect a crack in a shaft with Vibration Analysis made with a vibration analyzer. This vein belonged to a roll of a paper machine.

How modern rotating machines are designed to operate with greater mechanical efficiency, velocity, power and load increase, While, simultaneously the weight and dimensional tolerances are reduced. The result is a significant increase in the stress level in materials and components, during its rotation. As a consequence, many practical rotor-dynamic systems contain shaft/rotor elements that are highly susceptible to transverse cracking due to fatigue.

The detected vibrations

In the course of your machine inspection work, com vibration measurement, under the program predictive maintenance from a paper machine, a rise in vibration levels was detected in a bearing.

In the frequency spectrum the rotation speed harmonics were very clear. De facto, the second harmonic had a relatively large amplitude, sense this an indication of rigidity asymmetry.

a symmetrical vein, no cracks, has constant stiffness in any position and, therefore, a fixed deflection under load, regardless of rotation angle. in a split shaft, the split portion of the cross section is not capable of withstanding a tensile stress. So the displacement, as a function of rigidity, is minimum when the crack is closed and maximum when the crack is open. This periodic variation in stiffness gives rise to a vibratory component at twice the rotation speed, in the spectrum of vibrations.

In addition, an abnormal noise was identified..

Recommendation made - shaft crack detection with vibration analysis

Upon completion of the diagnostic work, it was recommended to stop the machine and inspect the shaft, to check for cracks.

What was found to exist can be seen in the photograph below..