Shaft cracks are critical faults in rotating machinery that can lead to catastrophic failure if undetected. Vibration analysis provides a powerful tool for early crack detection during machine operation. Characteristic vibration responses include changes in 1X and 2X amplitude and phase, reduction in critical speed, and internal loop orbit shape [1–3]. In some cases, strong 3X components are also observed as discussed in [4–6]. This article reviews the vibration mechanisms as

Fluid-induced instabilities such as whirl and whip are critical self-excited vibration phenomena that challenge the reliability of high-speed rotating machinery. These instabilities arise from the dynamic interaction between the rotor and the circumferential fluid in bearings, seals, or any configuration where two cylinders rotate with fluid in the gap. Whirl typically manifests as a rigid-body forward precession orbit at frequencies near fluid resonance (λΩ) tracking with ma