In an ideal scenario, shaft ‘C’ rotates around its central axis point ‘A’ without any noticeable movement of this axis. This occurs when the rotor is well balanced and the bearings are in excellent condition. However, when a defect arises, as depicted in the figure on the right, the shaft starts to vibrate. Consequently, point ‘A’ traces a path denoted as ‘B’ during each rotation. This indicates that the displacement, and subsequently the velocity and acceleration of point ‘A’ (and thus the entire shaft), undergo variations from segments ‘A-B,’ ‘B-C,’ and so forth. The change in velocity between these segments represents the alteration in the shaft’s instantaneous angular speed.
Just as accelerometers analyze velocity, acceleration, and displacement using Fast Fourier Transform (FFT) analysis, the study of torsional vibrations involves subjecting angular displacement, velocity, and acceleration to FFT examination.