Bolt loosening at rotating joint interface and its influence on rotor dynamics

• Time varying stiffness due to rotating bolt loosening is studied by 3D FE analysis.
• Time varying stiffness matrix is incorporated into the rotor dynamic model.
• HBM is used to calculate the rotor response with bolt loosening.
• Stiffness variation trend of bolt loosening resembles breathing transverse crack.
• Peaks at forward and backward whirling critical and subcritical speeds are excited.

The rotors of large rotating machinery involve multiple stages of disks supported by drums and shafts, where bolted joints are commonly employed to connect the adjacent disks and drums. Those connecting bolts, subjected to numerous combinations of loads during normal operation, tend to get loose, which will affect the rotor dynamics and even results in structural failures. However, little research has been done on the bolt loosening at the rotating joint interfaces of rotor systems. Thus, the influence of the bolt loosening on the rotor dynamics is studied in this paper. First, the time-varying stiffness at the joint interface with bolt loosening is investigated by means of three-dimensional (3D) nonlinear finite element (FE) models. Then, the motion equations for the rotor with bolt loosening are deduced accounting for the local stiffness variation caused by the bolt loosening. By taking a simple drum rotor with bolted joints as an example, the time-varying joint stiffness resulting from the bolt loosening and its influence on steady-state response of the rotor are calculated. The studies in this paper provide the fundamental understanding about the influence of the bolt loosening at the rotating joint interface on the rotor dynamics, and are helpful for the bolt loosening detection of rotating components in heavy-duty rotating machinery.