Threaded components have found ubiquitous use in many systems and structures. Although frequently overlooked, they represent a complex and often critical design element. One can identify numerous instances where such elements are subjected to vibratory conditions, yet their behavior in such an environment is still poorly understood. In this paper, we report on a series of experiments that were run to examine the motions of threaded fasteners subjected to axial harmonic vibration. The components are loaded by gravity and excited over a broad range of conditions. It is found that significant relative twisting motion can occur both with and against the load. This indicates that threaded components may loosen or tighten in the presence of vibration. It is shown that the direction of twist depends on the frequency and amplitude of the vibratory input as well as various physical parameters. Previous theoretical analyses of threaded components under vibration are based on static equilibrium conditions, and only predict a loosening action, i.e., twist with load. In Part 2 of this paper, a kinematic model is developed which predicts twisting both with and against load as observed in the experiments.

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