Jo May 19, 2024

Rotor system, which plays an important role in modern industries, is used in many machines such as internal combustion and jet engines, various types of motors, generator turbines and so on. High speed and high power are the development direction of modern rotating machinery, and fatigue crack is apt to happen as one of the most serious damages in the heavy loading environment. Therefore, detecting the crack in a timely way is very important for safe operation.

Kim Yu Jong, a researcher at the Faculty of Mechanical Science and Technology, has proposed a finite element model of a hollow shaft with open cracks. She derived the time-varying stiffness matrix of finite element based on the transformation matrix by which node displacements are converted from the rotating coordinates into the fixed ones. She employed the harmonic balance method (HBM) to find the responses of a cracked rotor system.

She analyzed the nonlinear vibration of rotor system and compared the results with some published results, considering the eccentricity of disk, the dimensionless crack depth, the ratio between the inner radius and outer radius of the rotor and the angle between the crack and imbalance directions.

As a result, she drew the following conclusions.

The subcritical rotational speeds corresponding to the first critical forward whirling speeds are not excited in the open cracked hollow rotor system. For the deep crack, the first forward whirling rotational speeds decrease more rapidly than the first backward ones as the dimensionless crack depth increases.

The whirl orbits near the subcritical backward whirl speeds have the same shape and behavior for both breathing and open crack models. However, the variations of the whirl orbits shape with the change in the rotational speeds are slower at the deep crack than at the shallow crack.