Reasons for the fracture of high-strength bolts

Thread Content

Generally, when analyzing bolt failure, people start by examining the strength; since the strength usually meets the requirements, they then turn to analyze the fatigue strength. In fact, the fatigue strength is so high that it’s beyond our imagination; fatigue strength is not even necessary for bolts during use. The loosening of threaded fasteners is not due to the fatigue strength of the bolts: threaded fasteners loosen after just 100 cycles in transverse vibration tests, whereas they require one million cycles of vibration in fatigue strength tests. In other words, threaded fasteners loosen when only one ten-thousandth of their fatigue strength is utilized; we are using only one ten-thousandth of their maximum capacity, so the loosening of threaded fasteners is not due to the fatigue strength of the bolts. 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 function properly, which in turn leads to further 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 performance is the key to solving this problem: currently, the most advanced and effective anti-loosening method is the Dow thread fastener anti-loosening method. Down’s thread possesses the characteristics of both left-handed and right-handed threads. It can be used with both left-hand threads and right-hand threads. Use nuts with two different thread directions when connecting. The nut on the working support surface is called a fastening nut, while the nut on the non-support surface is called a locking nut. When in use, first pre-tighten the fastening nut, and then pre-tighten the locking nut. Under vibration and shock, the fastening nut tends to loosen. However, since the direction of loosening of the fastening nut is the same as the direction in which the locking nut is tightened, the tightening of the locking nut prevents the fastening nut from loosening.