Why does a bolt get tighter when you keep tightening it? Generally, we analyze bolts breakage from the following four aspects: First, the quality of the bolt. Second, the preload torque of the bolt. Third, the strength of the bolt. Fourth, the fatigue strength of the bolt.
In fact, the vast majority of bolt fractures are caused by loosening and breakage.
Specifically, it is due to the damage caused by loosening. Because the situation of bolt breakage due to loosening is largely the same as that of fatigue fracture, in the end, we can always find the cause from the fatigue strength. In reality, the fatigue strength is so high that it is beyond our imagination, and the bolt never reaches the fatigue strength during its use.
1. The fracture of the bolt is not due to its tensile strength.
Take an M20×80 8.8 grade high-strength bolt as an example. It weighs only 0.2 kilograms, but its minimum tensile load is 20 tons, which is a hundred thousand times its own weight. Generally, we only use it to fasten components weighing 20 kilograms, which is only one-thousandth of its maximum capacity. Even if other forces act on the equipment, it is impossible to exceed a thousand times the weight of the component. Therefore, the tensile strength of the threaded fastener is sufficient, and it is impossible for the bolt to break due to insufficient strength.
2. The fracture of the bolt is not due to the fatigue strength of the bolt.
In the transverse vibration loosening test, the threaded fastener loosens after only a hundred times, while in the fatigue strength test, it needs to be vibrated repeatedly for a million times. In other words, the threaded fastener loosens when only one ten-thousandth of its fatigue strength is used. We have only utilized one ten-thousandth of its maximum capacity. Therefore, it can be concluded that the loosening of the threaded fastener is not due to the fatigue strength of the bolt.
3. The real cause of damage to threaded fasteners is loosening.
After loosening, a huge kinetic energy mv² is generated, which directly acts on the fasteners and equipment, causing damage to the fasteners. Once the fasteners are damaged, the equipment cannot operate in a normal state, further leading to equipment damage. For fasteners subjected to axial force, the threads are damaged and the bolts are pulled apart. For fasteners subjected to radial force, the bolts are sheared off and the bolt holes are deformed into ellipses.
4. Selecting a superior anti-loosening method for threads is the fundamental solution to the problem.
Take the hydraulic hammer as an example. The GT80 hydraulic hammer weighs 1.663 tons. Its side plate bolts are 7 sets of 10.9 grade M42 bolts, each bolt having a tensile strength of 110 tons. The preload is calculated as half of the tensile strength, which is as high as three or four hundred tons. However, the bolts still break. Now it is planned to change to M48 bolts.
The fundamental reason is that the anti-loosening of the bolts cannot be solved. When bolts break, the most common conclusion people draw is that the strength is insufficient, and thus they mostly adopt the method of increasing the diameter and strength grade of the bolts.
This method can increase the preload of the bolts, and the friction force is also increased, and of course, the anti-loosening effect can be improved. But this method is actually a non-professional one, as it requires too much investment and yields too little benefit. In short, bolts are like this: “They don’t break when they are not loose, but break as soon as they are loose.”
Analysis of the Causes of Loose Bolts
Threaded connections are designed based on the self-locking condition: ψ ≤ ρv. The friction pair generated in the threaded pair enables the bolt to self-lock and thus secure the bolt. Therefore, under static load, the connection will not loosen by itself. However, under impact, vibration, variable loads, or significant temperature changes, the friction force F between the threaded pair will decrease or disappear momentarily.
If this phenomenon occurs repeatedly, the connecting bolt will gradually loosen. After the loosening of the threaded fastener, the kinetic energy mv² is produced. For fasteners subjected to axial force, the thread is damaged and the bolt is pulled off.
For fasteners subjected to radial force, the bolt is sheared off and the bolt hole is damaged. The principle of bolt anti-loosening is to restrict the relative motion between the threaded pair or increase the difficulty of relative motion.
Introduction to Common Anti-loosening Methods
There are three common methods for preventing bolts from loosening: frictional prevention, mechanical prevention and permanent prevention. Among them, mechanical prevention and frictional prevention are called detachable prevention, while permanent prevention is called non-detachable prevention.
I. Frictional Anti-loosening
1. The anti-loosening principle of spring washers is that after the spring washer is flattened, it will generate a continuous elastic force, which keeps the threaded connection pair of the nut and the bolt in a continuous friction force, generating a resistance moment, thereby preventing the nut from loosening.
At the same time, the sharp corners at the opening of the spring washer are respectively embedded in the surface of the bolt and the connected part, thus preventing the bolt from rotating relative to the connected part.
2. Anti-loosening with opposing nuts (double nuts)
3. Self-locking nuts for anti-loosening
One end of the nut is made into a non-circular closed mouth or slit and then radially closed. When the nut is tightened, the closed mouth expands, and the elastic force of the closed mouth is used to press the engaged threads tightly.
4. Anti-loosening with elastic ring nuts
II. Mechanical Anti-loosening
1. Anti-loosening with slotted nuts and cotter pins
2. Locking washers
After the nut is tightened, the single-ear or double-ear locking washer is bent towards the side of the nut and the connected part respectively and pressed tightly to achieve anti-loosening.
3. Anti-loosening with series steel wire
Low-carbon steel wire is inserted into the holes at the heads of each screw to connect them in series, so that they can restrain each other.
III. Permanent Anti-loosening
Common permanent anti-loosening methods include spot welding, riveting, and bonding. These methods usually damage the threaded fasteners during disassembly and thus cannot be reused. Other anti-loosening methods include applying liquid adhesives between the mating threads, embedding nylon rings at the end of nuts, and riveting and punching anti-loosening.
Mechanical and friction anti-loosening are called detachable anti-loosening, while permanent anti-loosening is referred to as non-detachable anti-loosening.
1. After the anti-loosening nut is tightened by the edge-punching method, the thread end is punched to damage the thread.
2. Anti-loosening by bonding – Anti-loosening liquid for nuts
Apply the anti-loosening liquid for nuts to the tightened bolt, then screw on the nut. After it cures by itself, the anti-loosening effect is good.
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