It's okay if the bolt is not loose, but it will break if it is loose? what is the reason?

Loosening of bolts is a common occurrence, but if you do not pay attention, it will 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 knows it well, such as Japanese eccentric nuts, Tang nuts and Chinese self-tightening king nuts, but we will not talk about these fasteners today. The star of the world, let's talk about the most basic method of fixing nuts at work.

In general, we analyze bolt fracture from the following four aspects:

First, the quality of bolts

Second, the pre-tightening torque of the bolt

Third, the strength of the bolt

Fourth, the fatigue strength of bolts

In fact, most of the bolts are broken because of looseness, and they are broken due to looseness. Because the situation of bolt loosening and breaking is basically the same as that of fatigue fracture, in the end, we can always find the reason from the fatigue strength. In fact, the fatigue strength is so great that we cannot imagine that the bolt does not use fatigue strength at all during use.

1. The 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, its weight is only 0.2 kg, and its minimum tensile load is 20 tons, which is as high as 100,000 times its own weight. Generally, we will only use it to tighten Even with a 20kg component, only one-thousandth of its maximum capacity is used. Even if it is the action of other forces in the equipment, it is impossible to break through a thousand times the weight of the component, so the tensile strength of the threaded fastener is sufficient, and it is impossible to damage it due to insufficient strength of the bolt.

2. The fracture of the bolt is not due to the fatigue strength of the bolt:


Threaded fasteners can be loosened only one hundred times in the transverse vibration loosening test, and one million times in the fatigue strength test. In other words, threaded fasteners loosen when using one ten-thousandth of its fatigue strength, and we only use one ten-thousandth of its maximum capacity, so the loosening of threaded fasteners is not due to the fatigue strength of bolts .

3. The real reason for the damage of threaded fasteners is looseness:

After the threaded fastener is loosened, a huge kinetic energy mv2 is generated. This huge kinetic energy directly acts on the fastener and the equipment, resulting in damage to the fastener. After the fastener is damaged, the equipment cannot work in a normal state, which further leads to Equipment damage.

For fasteners subjected to axial force, the threads are destroyed and the bolts are pulled off.

For fasteners subjected to radial force, the bolts are sheared and the bolt holes are punched into ellipses.

4. The root of the problem is to choose the thread anti-loosening method with excellent anti-loosening effect:

Take the hydraulic hammer as an example. The weight of the GT80 hydraulic hammer is 1.663 tons, and its side plate bolts are 7 sets of 10.9 grade M42 bolts. The tensile force of each bolt is 110 tons. Ton. But the bolts will also break, and now I am going to change them to M48 bolts. The root cause is that the bolts cannot be loosened.

When the bolt breaks, the easiest conclusion people can draw is that the strength is not enough, so most of them adopt the method of increasing the strength grade of the bolt diameter. This method can increase the pre-tightening force of the bolt, and its friction force has also been increased. Of course, the anti-loosening effect can also be improved, but this method is actually a non-professional method. Its investment is too large and its benefit is too small.

In short, the bolt is: "If it is not loose, it will break when it is loose."

Cause Analysis of Bolt Loosening

The thread connection is designed according to the self-locking condition: ψ ≤ ρv, the friction pair generated in the thread pair makes the bolt self-lock and tighten the bolt, so the connection will not loosen automatically under static load. However, under shocks, vibrations, variable loads, and large temperature changes, the friction force F of the screw pair will decrease or disappear instantaneously. If this phenomenon occurs repeatedly, the connecting bolts will gradually loosen. After the threaded fastener is loosened, the kinetic energy mv2 generated, the thread of the fastener subjected to the axial force is destroyed, and the bolt is pulled off. For fasteners subjected to radial force, the bolts are sheared and the bolt holes are broken.



Bolt anti-loosening principle: limit the relative movement between thread pairs, or increase the difficulty of relative movement.

Introduction to common anti-loosening methods (briefly introduce the commonly used anti-loosening methods, and then focus on explaining some new and unique anti-loosening methods and their anti-loosening principles)

There are three commonly used anti-loosening methods for bolts: frictional anti-loosening, mechanical anti-loosening and permanent anti-loosening. Mechanical anti-loosening and friction anti-loosening are detachable anti-loosening, while permanent anti-loosening is called non-removable anti-loosening.

There are three commonly used anti-loosening methods for nuts: frictional anti-loosening, mechanical anti-loosening and permanent anti-loosening.

There are three commonly used anti-loosening methods for bolts: frictional anti-loosening, mechanical anti-loosening and permanent anti-loosening. Among them, mechanical anti-loosening and friction anti-loosening are called detachable anti-loosening, and permanent anti-loosening is called non-detachable anti-loosening.
No.1 friction lock

1. Spring gasket anti-loosening



The anti-loosening principle of the spring washer is that after the spring washer is flattened, the spring washer will generate a continuous elastic force, so that the threaded connection between the nut and the bolt will continue to maintain a frictional force and generate a resistance torque, 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, thereby preventing the bolt from rotating relative to the connected part.

2. Anti-loosening of top nuts (double nuts)



3. Self-locking nut anti-loosening



One end of the nut is made into a non-circular closing or a radial closing after slits. When the nut is tightened, the closure expands, and the elastic force of the closure is used to compress the screw threads.

4. Elastic ring nut anti-loosening



Fiber or nylon is embedded in the threaded entry to increase friction. The elastic ring also acts to prevent liquid leakage.

No.2 Mechanical anti-loosening

1. Slot nut and cotter pin anti-loosening





2. Stop washer


After the nut is tightened, the single-ear or double-ear stop washer is respectively bent and attached to the nut and the side of the connected part to prevent loosening.


3. Series steel wire anti-loosening



Use low-carbon steel wires to penetrate the holes in the head of each screw, connect the screws in series, and make them brake each other.


No.3 Permanent anti-loosening

Commonly used permanent anti-loosening methods include: spot welding, riveting, bonding, etc. This method mostly destroys the threaded fasteners during disassembly and cannot be reused.

Spot welding, riveting, bonding, etc.

This method mostly destroys the threaded fasteners during disassembly and cannot be reused.

In addition, there are other anti-loosening methods, such as: applying liquid adhesive between screw threads, inlaying nylon ring at the end of the nut, riveting anti-loosening, etc. Mechanical anti-loosening and friction anti-loosening are called detachable anti-loosening, while permanent anti-loosening is called non-removable anti-loosening.


1. Punching method to prevent loosening

After the nut is tightened, the punching point at the end of the thread destroys the thread.


2. Bonding and anti-loosening - nut anti-loosening liquid


Apply the nut anti-loosening liquid to the tightened part of the bolt, and then screw on the nut. After self-curing, the anti-loosening effect is good.