Are bolts such that: \"they don’t loosen but also don’t break, yet they break as soon as they loosen\"?

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This post was last edited by wx_Catherine_Mi on 2020-7-8 at 11:24; it was reproduced from WeChat’s Pressure Vessel Professionals (remove if unauthorized). Everyone is welcome to discuss the following statement! Under normal circumstances, we analyze bolt fractures 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 breakage due to loose bolts is roughly similar to that of fatigue fracture, we can ultimately find the cause in terms of fatigue strength. In fact, the fatigue strength is so high that it’s beyond imagination, and bolts don’t even need to rely on such high fatigue strength during use. 1. Bolt fracture is not caused by the bolt’s tensile strength: Taking an M20×80 high-strength bolt of grade 8.8 as an example, its weight is only 0.2 kilograms, while its minimum tensile load is 20 tons – that is, 100,000 times its own weight. Under normal circumstances, it is used to fasten components weighing 20 kilograms at most, which represents just one thousandth of its maximum capacity. Even due to other forces acting on the equipment, it is impossible for those forces to exceed a thousand times the weight of the components; therefore, the tensile strength of the threaded fasteners is sufficient, and damage cannot occur due to insufficient strength of the bolts. II. The fracture of the bolt was not due to its fatigue strength: 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. III. 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. IV. Choosing a thread locking method with excellent anti-loosening performance is the key to solving the problem: taking a hydraulic hammer as an example. The weight of the GT80 hydraulic hammer is 1.663 tons. Its side plate bolts are 7 sets of grade 10.9 M42 bolts, with each bolt having a tensile strength of 110 tons. The pre-tensioning force is calculated as half of this tensile strength, resulting in a pre-tensioning force of three to four hundred tons. But the bolts can still break; now we plan to switch to M48 bolts, as the fundamental issue is that the bolt locking mechanisms are not effective enough. When a bolt breaks, the most common conclusion is that its strength is insufficient; therefore, people usually increase the bolt’s diameter or its strength grade. This method can increase the preload on the bolts, as well as their friction; consequently, the anti-loosening effect is also improved. However, it is actually an unprofessional approach, as it requires excessive investment with minimal returns. In short, a bolt is: “It doesn’t loosen or break when tight, but it breaks as soon as it loosens.” ”