Abstract The purpose of this investigation is to verify the mechanism of spontaneous loosening of bolted joints under continuous transverse vibration conditions by evaluating friction coefficient changes on tightening and untightening processes. The first test method is a tightening test by measuring bolt preloads by applying certain constant tightening torque under various transverse vibration conditions using Junker vibration machine to calculate each friction coefficient on tightening process. As a result of the first test, calculated friction coefficient of bolted joints by applying tightening torque has decreased significantly and measuring generated preload have increased along with the increase of the acceleration and velocity of clamped parts slippage, and this tendency was applicable for another type of bolted joints. The conclusion of this test on tightening process was that the friction coefficient decrease during transverse vibration conditions could be estimated by the transverse acceleration and velocity between clamped parts. The second test method is an untightening test by measuring spontaneous loosening torque under various vibration conditions to verify the tendency when bolts and nuts are loosening spontaneously. In the result of the second test, values of measured spontaneous loosening torque were significantly smaller than the value of untightening torque under non-vibration conditions. And friction ring lock nuts which have typical anti-loosening function by additional friction force as prevailing torque also behaved same tendency in certain transverse vibration conditions.
Takenaka et al. (Sun,) studied this question.