Bolt looseness is common, but if you don’t pay attention, it will often cause equipment vibration, component damage, and even casualties. How to tighten a small nut has always been a long-lasting topic in mechanical design. Today, let’s talk about the most basic method of fixing nuts in work.In the past ten years, DIN933 Defeated many competitors, courageously advanced in the struggle, and polished many good products for customers. https://www.chinatianlong.com/
In general, we analyze the bolt fracture from the following four aspects:
First, the quality of bolts
Second, the pre-tightening torque of bolts
Third, the strength of bolts
Fourthly, the fatigue strength of bolts.
In fact, most cases of bolt fracture are caused by looseness, specifically, it is damaged due to looseness. Because the situation of bolt loosening and breaking is basically the same as that of fatigue fracture, finally, we can always find the reason from the fatigue strength. In fact, the fatigue strength is so great that we can’t imagine it, and the fatigue strength is not used at all in the use of bolts.
01
The bolt fracture is not due to the tensile strength of the bolt.
Take a M20〜80 8.8-grade high-strength bolt as an example. Its weight is only 0.2kg, and its minimum tensile load is 20t, which is as high as 100,000 times its own weight. Under normal circumstances, we only use it to fasten 20kg parts, and only use one thousandth of its maximum capacity. Even under the action of other forces in the equipment, it is impossible to exceed 1,000 times the weight of the parts, so the tensile strength of threaded fasteners is sufficient and it is impossible to damage them because of insufficient strength of bolts.
02
The fracture of bolts is not due to the fatigue strength of bolts.
Threaded fasteners can be loosened only 100 times in the transverse vibration loosening experiment, but they need to be vibrated repeatedly for one million times in the fatigue strength experiment. In other words, the threaded fastener is loose when using one ten thousandth of its fatigue strength, and we only use one ten thousandth of its great capacity, so the loosening of the threaded fastener is not due to the fatigue strength of the bolt.
03
The real reason for the damage of threaded fasteners is looseness.
When the threaded fastener is loosened, it generates huge kinetic energy mv2, which directly acts on the fastener and equipment, causing the fastener to be damaged. After the fastener is damaged, the equipment cannot work in a normal state, further causing the equipment to be damaged.
When the fastener is subjected to axial force, the thread is damaged and the bolt is pulled off.
When the fastener is subjected to radial force, the bolt is cut and the bolt hole is oval.
04
Choosing thread locking mode with excellent locking effect is the fundamental solution to the problem.
Take hydraulic hammer as an example. The weight of GT80 hydraulic hammer is 1.663 tons, and its side bolts are 7 sets of M42 bolts of grade 10.9. The tensile force of each bolt is 110 tons, and the pre-tightening force is half of the tensile force, and the pre-tightening force is as high as 300 to 400 tons. However, the bolt will break, and now it is ready to be changed to M48 bolt. The fundamental reason is that the bolt locking cannot be solved.
When a bolt breaks, it is easy for people to conclude 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 is also increased. Of course, the anti-loosening effect can also be improved, but this method is actually a non-professional method, and its investment is too large and its income is too small.
In short, the bolt is: “If you don’t loosen it, it will break.”
Cause analysis of bolt looseness
The threaded connection is designed according to the self-locking condition: ψ ＋ρv, and the friction pair generated in the threaded pair makes the bolt self-lock to tighten the bolt, so the connection will not loosen automatically under static load. However, under impact, vibration, variable load and great temperature change, the friction force f of the screw pair will decrease or disappear instantly. If this phenomenon occurs repeatedly, the connecting bolt will gradually loosen. After the threaded fastener is loosened, the kinetic energy mv2 is generated, and the fastener subjected to axial force, the thread is destroyed and the bolt is pulled off. Fasteners subjected to radial force, bolts are cut and bolt holes are broken.
Bolt locking principle: limit the relative motion between screw pairs or increase the difficulty of relative motion.
Introduction of common anti-loosening methods
There are three commonly used methods for locking bolts: friction locking, mechanical locking and permanent locking. Among them, mechanical locking and friction locking are called detachable locking, while permanent locking is called non-detachable locking.
01
Friction locking
1. Spring gasket locking
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 pair between the nut and the bolt will maintain a friction force continuously and generate a resistance moment, thus preventing the nut from loosening. At the same time, the sharp corner at the opening of the spring washer is embedded into the surface of the bolt and the connected piece respectively, thus preventing the bolt from rotating relative to the connected piece.
2. Lock the top nut (double nut)
3. Self-locking nut locking
One end of the nut is made into a non-circular convergent or radially convergent after being slit. When the nut is tightened, the closure expands, and the screwing threads are pressed by the elastic force of the closure.
4. Locking of elastic ring nut
Fiber or nylon is embedded in the thread to increase friction. The elastic ring also plays a role in preventing liquid leakage.
02
Mechanical locking
Locking of groove nut and cotter pin
2. Stop gasket
After the nut is tightened, the single-ear or double-ear stop washer is bent and attached to the side of the nut and the connected part respectively to realize locking.
3. Locking of series steel wire
Use low-carbon steel wire to penetrate the holes in the head of each screw, and connect the screws in series to brake each other.
03
Permanent locking
Commonly used permanent locking are: spot welding, riveting, bonding, etc. This method mostly destroys threaded fasteners during disassembly and cannot be reused.
In addition, there are other ways to prevent loosening, such as: coating liquid adhesive between screwing threads, embedding nylon rings at the end of nuts, riveting and locking. Mechanical locking and friction locking are called detachable locking, while permanent locking is called non-detachable locking.
1, punching method to prevent loose
After the nut is tightened, the thread is damaged at the end of the thread.
2. Adhesive Locking-Nut Locking Solution
Apply the nut locking liquid to the bolt tightening position, and then screw on the nut. After self-curing, the locking effect is good.