Analysis of the causes of bolt fracture

Thread Content

Generally, 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, in the end we can always find the cause in terms of fatigue strength. In fact, 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 failure is not due to 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 these 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 a lateral vibration test, whereas they require one million cycles of vibration in a fatigue strength test. 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 a tremendous amount of kinetic energy, mv². This immense kinetic energy acts directly on the fasteners and the equipment, causing damage to the fasteners. Once damaged, the fasteners can no longer keep the equipment in proper working condition, ultimately leading to further damage to the equipment. Under axial force, the threads of the fastener were damaged and the bolt broke. 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 opt to increase the bolt’s diameter or its strength grade. This method can increase the pre-tension of the bolts, as well as their friction; consequently, the anti-loosening effect is also improved. However, it is actually an unprofessional approach, as the costs involved are high while the benefits are low. In short, a bolt is: “It doesn’t loosen or break when tight, but breaks as soon as it loosens.” ”