Do you know the most basic method of fixing a nut?

Bolt loosening is a common occurrence, but if not paid attention to, it can often cause equipment vibration, component damage, and even casualties. How to tighten a small nut has always been an enduring topic in mechanical design. Everyone is relatively familiar with things like Japan's eccentric nut, Tang's nut, and China's self-tightening king nut, but today we are not going to talk about these stars of the fastener world. Let's talk about the most basic methods of fixing nuts in our work.

01 Why do bolts get tighter and tighter the more you tighten them?

Generally speaking, we analyze bolt fracture 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 because of loosening—they are damaged as a result of loosening. This is because the situation of a bolt being broken due to loosening is largely the same as that of fatigue fracture, and in the end, we can always find the cause in terms of fatigue strength. In reality, the fatigue strength is so great that it is beyond our imagination, and the fatigue strength is simply not reached during the use of the bolt.

01 Bolt fracture is not due to the tensile strength of the bolt

Take an M20×80 grade 8.8 high-strength bolt as an example. It weighs only 0.2 kilograms, but its minimum tensile load is 20 tons, which is up to one hundred thousand times its own weight. Under normal circumstances, we only use it to fasten components weighing 20 kilograms, and we only use one thousandth of its maximum capacity. Even with the action of other forces in the equipment, it is impossible to exceed one thousand times the weight of the component. Therefore, the tensile strength of threaded fasteners is sufficient, and it is impossible for them to be damaged because the bolt strength is insufficient.

02 Bolt fracture is not due to the fatigue strength of the bolt

Threaded fasteners only need one hundred cycles to loosen in a transverse vibration loosening test, while they need one million repeated vibrations in a fatigue strength test. In other words, threaded fasteners loosen when only one ten-thousandth of their fatigue strength is used. We only use one ten-thousandth of their maximum capacity, so the loosening of threaded fasteners is also not due to the fatigue strength of the bolt.

03 The real cause of threaded fastener damage is loosening

After a threaded fastener loosens, enormous kinetic energy mv2 is generated. This enormous kinetic energy acts directly on the fastener and the equipment, causing damage to the fastener. Once the fastener is damaged, the equipment cannot work under normal conditions, further leading to equipment damage.

For fasteners subjected to axial force, the threads are damaged and the bolt is pulled apart.

For fasteners subjected to radial force, the bolt is sheared off and the bolt hole is beaten into an oval shape.

04 Selecting a threaded anti-loosening method with excellent anti-loosening effect is the fundamental solution to the problem

Take a hydraulic hammer as an example. The GT80 hydraulic hammer weighs 1.663 tons. Its side plate bolts are 7 sets of grade 10.9 M42 bolts. The tensile resistance of each bolt is 110 tons. If the pre-tightening force is calculated as half of the tensile resistance, the pre-tightening force is as high as three to four hundred tons. However, the bolts will still break. Now there are plans to change them to M48 bolts. The fundamental reason is that bolt anti-loosening cannot be solved.

When a bolt breaks, the conclusion people most easily draw is that the strength is insufficient, so most people adopt the method of increasing the bolt diameter and strength grade. This method can increase the pre-tightening force of the bolt, and its friction force is also increased. Of course, the anti-loosening effect can also be improved, but this method is actually an unprofessional method. Its investment is too large and its benefit is too small.

In short, bolts are: "If they don't loosen, they won't break; once they loosen, they break."

02 Analysis of the causes of bolt loosening

Threaded connections are designed according to the self-locking condition: ψ ≤ ρv. The friction pair generated in the thread pair enables the bolt to self-lock and thereby tighten the bolt, so under static load the connection will not loosen by itself. However, under impact, vibration, variable load, or large temperature changes, the friction force F of the screw pair will decrease or momentarily disappear. If this phenomenon occurs repeatedly, the connecting bolt will gradually loosen. After a threaded fastener loosens, the kinetic energy mv2 is generated. For fasteners subjected to axial force, the threads are damaged and the bolt is pulled apart. For fasteners subjected to radial force, the bolt is sheared off and the bolt hole is damaged.