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Quantification of Clamp Loss and Subsequent Loosening of Automotive Hub-Knuckle Joints under Time-Varying Proving Ground Loading
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
Threaded fasteners or bolted joints are used extensively in automotive components. There are standard procedures to evaluate joint performance under block cycles or road loads. The deciding load case for such joint design is the slippage analysis of the joint. There are studies done to evaluate the theoretical and experimental behavior of these joints. There are different ways of understanding the interaction between the bolt and the nut under different loading scenarios. However, none has provided a satisfactory method of quantifying the bolt loosening or loss of clamp under cyclic loading where initially no slippage is observed. Under varying loads, initial relaxation of the joint is followed by a loss of clamping load to below the critical value. Below a critical value, complete loss of clamping load happens very rapidly and results in a loose joint. The loss of clamping load below the critical value happens at a transition stage when both elastic deformations of the bolt as well as external moments contribute to rapid loosening. This study provides an overall understanding of geometric parameters that contribute to sudden loss of clamping load and increased propensity of loosening, and is applied to a well-known example in automotive suspension. The experiment and subsequent analysis aims to identify factors contributing to loosening of a circular joint connected with 4 bolts along a designed bolt circle. This joint is subjected to a combinatory load which induces both translational as well as rotational load at the center of the pitch circle. This causes the loosening of the joint. The study developed empirical formulae for assessment with different input parameters and is successfully applied to a number of joints under proving ground or rig loads. The study also proposed minimum bolt pre-loads of the joint to eliminate the propensity of bolt loosening in the joint under a given set of time-varying loads. Keywords: Hub; knuckle; clamp loss; bolt loosening; bolted joints under external loading.