Investigation Into Self-loosening Phenomenon Of Bolt Joints In Tower Cranes

The threaded connection is currently one of the most widely used fastening methods in mechanical industry. It is also used as a primary joining method on standard sections of the tower crane. The self-loosening of bolt joints is a main form of bolt connection failures, which would pose a serious threat to the security of tower cranes. If the self-loosening phenomenon happens on bolt joints between standard sections of tower crane, the structure stiffness would decrease, and the standard sections might distort. This paper took a bolt joint between standard sections of tower crane QTZ5510 as the research object to learn the mechanism of its self-loosening phenomenon under specific working conditions by simulating the loosening process with a finite element model.First of all, it studied Yamamoto method, an analytic method which could learn the load distribution of the threaded portion in a bolt joint, and got analytical results of the bearing capacity of this portion under the action of static pre-tightening force. By combinational usage of finite element software Pro/E, Hypermesh and Abaqus, it established a finite element model of a bolt joint. Firstly, it set up a three dimensional finite element model with the helix Angle using Pro/E. Secondly, it processed the pre-treatment the finite element analysis in Hypermesh. Thirdly, the finite element model was imported into Abaqus for the final static analysis to get the simulated results of the distribution of bearing capacity in the bolt joint under the pre-tightening force. By comparing the analytical results and simulation results of distributions of the threaded portion in the bolt joint, the validity of this finite element model was verified.Then, a new three-dimensional finite element model was established based on bolt joints between standard sections of tower crane QTZ5510 by combinational usage of that finite element software. In this finite element model, the mesh quality was optimized, and the pre-treatment processes such as exerting pre-tightening force, setting boundary conditions and load steps were accomplished in Abaqus. The explicit dynamic finite element analysis method was used and a new method for exerting pre-tightening force was adopted to guarantee the analysis convergent. After getting the simulated results of the distribution of bearing capacity in the bolt joint under the pre-tightening force, Yamamoto method was used again to obtain corresponding analytical results. Comparing the simulated results and the analytical ones, the validity of the finite element model and the feasibility of the new method for exerting pre-tightening force were verified.Finally, combining with vibration frequencies of tower crane measured under specific working conditions, dynamic analysis was carried out for investigating the influence of vibrational frequencies, amplitudes of external displacement, friction coefficients of threads interfaces and pressured surfaces of bolt joints on the self-loosening of bolt joints. Thus it studied mechanisms of self-loosening of bolt joints in the tower crane.The conclusion reached in this paper is not only beneficial to understand the principle of self-loosening phenomenon of bolt joints, but also of great significant to prevent the occurrence of this phenomenon in standard sections of tower cranes. It is also of application values and practical significance to improve the reliability and safety of tower crane under repeated usages, ensure the safety of the structure of tower cranes, avoid the occurrence of serious accidents and reduce the damage of accidents that have happened.