Reasons For Loosening And Breaking Of Bolts And Methods to Prevent Loosening
(Last Updated On: December 1, 2021)
Loosening of bolts occurs from time to time. If you do not pay attention, it will often cause equipment vibration, component damage, and even personal injury. How to tighten a small nut has always been an everlasting topic in mechanical design. For example, Japanese eccentric nuts, Down nuts and Chinese self-tightening nuts appeared to prevent the nuts from loosening. Today we will not discuss these star products in the fastener industry, but will talk about the most basic method of fixing nuts.
Why does the bolt break?
Under normal circumstances, we analyze the 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 the bolt
In fact, most cases of bolt breakage are caused by loosening of bolts. Looseness causes bolts to be broken. The situation of a bolt being broken due to looseness is roughly the same as the situation of fatigue fracture, so in the end we can always find the reason from the fatigue strength, but in fact the fatigue strength of the bolt is very large. The bolt will not break due to fatigue at all during actual use.
01. Bolt fracture is not caused by insufficient tensile strength of the bolt
Take an M20×80 8.8 grade 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. Under normal circumstances, we only use it to fasten 20 kg parts, and only use one thousandth of its maximum capacity. Even with the action of other forces in the equipment, it is impossible to break through a thousand times the weight of the component. Therefore, the tensile strength of the threaded fastener is sufficient, and it is impossible to damage the bolt due to the insufficient strength.
02. 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 experiment, and one million times of repeated vibration in the fatigue strength experiment. In other words, threaded fasteners become loose when using one ten-thousandth of their fatigue strength. We only use one ten-thousandth of its maximum capacity. Therefore, the loosening of threaded fasteners is not due to bolt fatigue strength. .
03. The real cause of damage to 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, causing the fastener to be damaged. After the fastener is damaged, the equipment cannot work under normal conditions, which further leads to Equipment damage. For fasteners that are subjected to axial force, the threads are broken and the bolts are broken. For fasteners that are subjected to radial force, the bolts are cut, and the bolt holes are punched into an ellipse.
04. Choosing the thread anti-loosening method with excellent anti-loosening effect is the fundamental solution to the problem
Take hydraulic hammer as an example
The weight of the GT80 hydraulic hammer is 1.663 tons. The side plate bolts are 7 sets of 10.9 grade M42 bolts. The tensile force of each bolt is 110 tons. The pre-tightening force is calculated as half of the tensile force. The pre-tightening force is as high as 300 to 400 tons. . But the bolts will break. Now I am going to change to M48 bolts. The root cause is that the bolts cannot be prevented from loosening.
When a bolt is broken, the easiest conclusion that people can draw is that the strength is not enough, so the method of increasing the strength grade of the bolt diameter is mostly adopted. This method can increase the pre-tightening force of the bolt and its frictional force. Of course, the anti-loosening effect can also be improved, but this method is actually a non-professional method. It requires too much investment and too little profit.
Analysis of the causes of bolt loosening
The threaded 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 by itself under static load. However, under shock, vibration, variable load, and large temperature changes, the frictional force F of the screw pair will decrease or disappear instantaneously. If this phenomenon occurs repeatedly, the connecting bolts will gradually loosen. When threaded fasteners are loosened, kinetic energy mv2 is generated. For fasteners under axial force, the threads are damaged and the bolts are broken. For fasteners under radial force, the bolts are sheared and the bolt holes are broken.
Bolt anti-loosening principle: Restrict the relative movement between thread pairs, or increase the difficulty of relative movement.
Introduction of common bolt anti-loosing methods (a brief introduction to commonly used bolt anti-loosing methods, and later this article will focus on some new and unique bolt anti-loosing methods and their bolt anti-loosing principles)
There are three commonly used anti-loose methods for bolts: friction anti-loose, mechanical anti-loose and permanent anti-loose. Mechanical anti-loose and friction anti-loose are detachable anti-loose, and permanent anti-loose is called non-detachable anti-loose.
There are three commonly used anti-loosening methods for nuts: friction anti-loosening, mechanical anti-loosening and permanent anti-loosening.
No.1 Anti-loose friction
1. Spring washer 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 of 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 into the surface of the bolt and the connected part, thereby preventing the bolt from rotating relative to the connected part.
2. Anti-loosening of the top nut (double nut)
This method is to tighten the bolt by rotating the tightening nut and the lock nut in opposite directions.
3. Self-locking nut to prevent loosening
One end of the nut is made into a non-circular narrowing or a radial narrowing after slitting. When the nut is tightened, the closing is expanded, and the elastic force of the closing is used to compress the screw threads.
4. Anti-loosening of elastic ring nut
Fiber or nylon is embedded in the screw thread to increase friction. The elastic ring also plays a role in preventing liquid leakage.
No.2 Mechanical anti-loosening
1. Groove nut and split pin anti-loosening
Punch holes on the bolts, tighten the nuts, and insert pins in the holes for tightening to prevent the bolts from loosening.
2. Stop gasket
After the nut is tightened, the single-ear or double-ear stop washer is respectively bent and tightly attached to the nut and the side of the connected part to prevent loosening.
3. Tandem steel wire anti-loose
Use low-carbon steel wire to penetrate the hole in the head of each screw, and connect the screws in series to brake each other.
No.3 Permanent anti-loose
Commonly used permanent anti-loose are: spot welding, riveting, bonding and so on. This method mostly destroys the threaded fasteners during disassembly and cannot be reused. In addition, there are other ways to prevent loosening, such as: applying liquid adhesive between the screw threads, inserting a nylon ring at the end of the nut, riveting to prevent loosening, etc. Mechanical anti-loose and friction anti-loose are called detachable anti-loose, and permanent anti-loose is called non-detachable anti-loose.
1. Flushing method to prevent loosening
After the nut is tightened, the thread is broken by punching at the end of the thread.
2. Adhesive anti-loosening-nut anti-loosing fluid
Apply the nut anti-loosing fluid to the tightening part of the bolts, and then screw on the nut. After self-curing, the anti-loosening effect is good.
The above are several methods that TorcStark summarized for bolt loosening prevention. TorcStark is a professional manufacturer of hydraulic torque wrenches and hydraulic bolt tensioners, focusing on solving the problems of tightening and dismantling bolts in the industry. Regarding bolt anti-loosening, TorcStark believes that the best solution is to tighten the bolts to the most suitable state, which requires very precise torque application tools to control. Hydraulic wrenches and bolt tensioners can solve this problem well.
If you have any questions about bolt loosening, please leave a comment!