Tightening and loosening of bolted joint

Abstract

A bolted joint is a common joint encountered in mechanical structures or machine design. Its tightness is important in order to secure the structure or machine it is attached to. A proper tightness is installed at a bolted joint to resist static and dynamic loading it is subjected to. When tightness is not sufficient, it can cause failure such as loosening and in extreme cases, joint separation. In this project, a bolted joint and its tightness are objects being studied. The purpose of this project is to understand the factors affecting tightness of a bolted joint and causes of its loosening under dynamic loading (vibration). A DC motor with known transduction matrix was used as a device to measure mechanical condition of a bolted joint. By obtaining the 2x2 transduction matrix, mechanical output of a DC motor can be related with its electrical input. With this relation, a DC motor was used to tighten a bolted joint and measure its mechanical condition by controlling and measuring its electrical input. Based on this mechanical condition, tightness and dynamic condition of a bolted joint were obtained. The project is divided into 2 parts. In the first part, the tightening process of a bolted joint is studied. The experiment results show that torque is not sufficient in indicating tightness of a bolted joint. Under different friction conditions, tightness that is achieved by the same value of torque differs significantly. Thus, energy is used as a tightening indicator. With simultaneously measurement of torque and rotation speed, energy applied to a bolted joint can be measured. The experiment results show that energy is a better tightening indicator as compared to torque. In the second part, dynamic behavior of a bolted joint under dynamic loading is studied. First, mechanical impedance of a bolted joint is measured with usage of Hilbert Transform. The Hilbert Transform helps to obtain mechanical impedance in time domain. With this, change of mechanical impedance over a period of time can be monitored. By obtaining mechanical impedance, system properties can be Abstract ii derived from their relation. The results show that system properties of a bolted joint are affected by tightness of a bolted joint (preload), its surface condition and frequency of applied excitation. After obtaining system properties of a bolted joint, response of a bolted joint to a vibration, especially transverse vibration, can be calculated. The results show that a transverse vibration given to a bolted joint can cause loosening due to thread geometry and system properties of a bolted joint. Acknowledgment iii ACKNOWLEDGMENT I would like to praise my thanks to the God for the completion of this thesis report. I also would also like to thank my supervisor, Professor Ling Shih Fu, for his ideas, guidance and support in this research. Even in his busiest schedule, he still spends his time in discussing this research. Secondly, I would like to thanks my fellow friends, Lian Kar Foong, Wong Yoke Rong, Felicia Rezanda, Hossein Mousavi and Peng Xi...