Thread Content
Under normal circumstances, we analyze bolt failure from the following four aspects: First, the quality of the bolt; second, the pre-tightening torque of the bolt; third, the strength of the bolt; fourth, the fatigue strength of the bolt. In fact, in the vast majority of cases, bolts break due to loosening – they are damaged as a result of being loose. Since the situation of bolts coming loose and breaking is largely similar to that of fatigue fracture, we can ultimately find the cause in terms of fatigue strength. In reality, the fatigue strength is so high that it’s beyond our imagination; during normal use, bolts simply never need to rely on their fatigue strength at all. The loosening of threaded fasteners is not due to the fatigue strength of the bolts: Threaded fasteners can become loose after just 100 cycles in a lateral vibration test, whereas they require one million cycles of repeated vibration in a fatigue strength test. In other words, the threaded fastener became loose when only one ten-thousandth of its fatigue strength was utilized. We used merely one ten-thousandth of its total capacity; therefore, the loosening of the threaded fastener is not attributable to the bolt’s fatigue strength. The real cause of damage to threaded fasteners is loosening: When threaded fasteners become loose, they generate significant kinetic energy equal to mv2. This large amount of kinetic energy acts directly on the fasteners and the equipment, resulting in damage to the fasteners. Once the fasteners are damaged, the equipment cannot operate properly, which further leads to damage to the equipment. In fasteners subjected to axial forces, the threads are damaged and the bolts break. In fasteners subjected to radial forces, the bolts are sheared and the bolt holes become elliptical. Choosing a thread locking method with excellent anti-loosening properties is the key to solving this problem. At present, the most advanced and effective anti-loosening method is the Dow thread fastener anti-loosening technique. Downs bolts are used in four-roll crushers and hydraulic breakers; their strength does not increase, yet the bolts no longer break.