Comparative analysis of contact and non-contact method for determining the natural frequencies of a bolted joint

ABSTRACT:  To ensure high assembly quality, a proper clamping force in bolted joints is essential. The existing tightening technology rely on torque and angle; however, this thesis proposes a method for obtaining the clamping force of a bolted joint based on measuring the natural frequencies. This process is comparable to the process of tuning a guitar string. By measuring the natural frequencies, the guitar string can be tuned to the desired sound. By using this principle, a bolt in a bolted joint could be tightened to a desired clamping force by measuring the natural frequencies.

This thesis investigates whether the vibrations of a bolted joint could be related to the desired clamping force. A contact method and a non-contact method are suggested for measuring vibrations of a bolted joint and acquiring the natural frequencies. The contact method involves usage of an acceleration sensor that has direct contact with the bolted joint and the non-contact method involves usage of a microphone that requires no contact with the bolted joint.

By simulating a bolt and a bolted joint and conducting a modal analysis, the theoretical natural frequencies were obtained as an additional measure to validate the contact and non-contact methods. The models indicated high accuracy as the theoretical natural frequencies compared well to the natural frequencies obtained by implementing the two methods. By conducting experiments, the feasibility of the contact and non-contact methods was evaluated and compared in terms of precision.

The results showed high precision for both methods, and indicated that the methods perform equally well for the purpose of determining the natural frequencies of a bolted joint. This methodology may be promising for further development of existing tightening technology and future tightening tools.