It doesn’t matter if the bolt is loose, it will break once it is loose. What is the reason?
Bolt loosening is common, but if you do not pay attention, it will often cause equipment vibration, damage to components, and even casualties. How to tighten a small nut has always been a prosperous topic in mechanical design. Everyone knows better, such as Japanese eccentric nuts, tang’s nuts and Chinese self-tightening nuts, but we don’t talk about the stars in these fastener circles today. Let’s talk about the most basic method of fixing nuts in our work.
Why are bolts screwed more and more tightly?
Under normal circumstances, we analyze bolt fracture from the following four aspects:
- The quality of the bolt;
- Pretightening moment of Bolt;
- Strength of Bolt;
- Fatigue Strength of bolts.
In fact, most cases of bolt fracture are broken due to loosening. Because the situation of Bolt loosening and breakage is basically the same as that of fatigue fracture, finally, we can always find the reason from fatigue strength. In fact, the fatigue strength is so large that we cannot imagine that the fatigue strength cannot be used at all during the use of the bolt.
1.1 Bolt fracture is not due to the tensile strength of the bolt.
Take a m20×80 8.8grade high-strength Bolt as an example, its weight is only 0.2kg, and its minimum tensile load is 20 tons, up to 100,000 times of its own weight, under normal circumstances, we will only use it to fasten 20kg of components, and only use one thousandth of its maximum capacity. Even for the function of other forces in the equipment, it is impossible to exceed 1000 times of the weight of the component. Therefore, the tensile strength of the threaded fastener is sufficient, and it is impossible to be damaged because of the strength of the bolt.
1.2 The fracture of the bolt is not due to the fatigue strength of the bolt.
Threaded fasteners can loosen only one hundred times in the horizontal vibration experiment, while in the fatigue strength experiment, they need to vibrate for one hundred thousand times repeatedly. In other words, the threaded fastener is loose when using one in ten thousand of its fatigue strength, and we only use one in ten thousand of its large capacity, so it is said that the loosening of the threaded fastener is not due to the fatigue strength of the bolt.
1.3 The real cause of thread fastener damage is loose.
After the screw thread fastener looses, it will generate huge kinetic energy mv2, the huge kinetic energy of which acts directly on the fastener and equipment, leading to the damage of the fastener. After the fastener is damaged, the equipment can not work under normal condition, further lead to damage to the equipment.
The fastener thread acted by axial force is damaged and the bolt is pulled off.
The fastener bolt affected by radial force is cut off, and the bolt hole is oval.
1.4 Choosing a thread anti-loose method with excellent anti-loose effect is the fundamental to solve the problem.
Take the hydraulic hammer as an example. The weight of GT80 hydraulic hammer is 1.663 tons, and its side plate bolts are 7 sets of 10.9 grade M42 bolts. The tensile strength of each bolt is 110 tons, four hundred tons. However, the bolt will also break, and now I am going to change it to M48 bolt. The root cause is that the Bolt cannot be solved.
When the bolt is broken, the most easy conclusion is that the strength is not enough, so most of them adopt the method of increasing the strength grade of bolt diameter. This method can increase the pretightening force of the bolt, and its friction force is also increased. Of course, the anti-loosening effect can be improved, but this method is actually a non-professional method, which is too much investment, the gains are too small.
In short, the bolt is: “not loose, break once loose.”
Cause Analysis of Bolt loosening
Thread connection is designed according to self-locking condition: friction generated in the thread pair of ψ ≤pv can make the bolt self-lock and then tighten the bolt, so the connection will not loosen automatically under static load. However, under the impact, vibration and variable load, when the temperature changes greatly, the spiral pair friction F will decrease or disappear instantly. If this phenomenon occurs repeatedly, the connecting Bolt will gradually loosen. After the threaded fasteners loosen, the kinetic energy mv2 is generated. Due to the action of axial force, the fasteners are damaged and the bolts are pulled off. The fastener bolt affected by radial force is cut off and the bolt hole is broken.
Bolt locking principle: Limit the relative motion between screw thread pairs, or increase the difficulty of relative motion.
Introduction to common anti-loosening methods (briefly introduce the common anti-loosening methods, and will focus on some new and unique anti-loosening methods and their anti-loosening principles).
There are three kinds of locking methods commonly used for bolts: friction locking, mechanical locking and permanent locking. Mechanical anti-loose and friction anti-loose are detachable anti-loose, while permanent anti-loose is called non-detachable anti-loose.
There are three kinds of widely used anti-loose nut methods: friction anti-loose, mechanical anti-loose and permanent anti-loose.
No.1 Friction anti-loose
1. Spring gasket anti-loose
The anti-loose principle of the Spring washer is that after flattening the Spring washer, the Spring washer will generate a continuous elastic force to keep the thread connection between the NUT and the bolt continuous maintaining a friction force and generating resistance torque, so as to prevent nuts from loosening. At the same time, the sharp corners of the opening of the Spring washer are embedded respectively in the surface of the bolt and the connecting piece, so as to prevent the rotation of the bolt relative to the connecting piece.
2. Top nut (Double Nut) anti-loose
The use of nut from the top action to make the bolt type by additional tension and additional friction. Due to the multi-use of a nut, and the work is not very reliable, has been rarely used.
3. Self-locking nut
One end of the nut is made into a non-circular closed mouth or a radial closed mouth after opening. When the nut is tightened, the mouth is swelled, and the screw thread is pressed tightly by the elastic force of the mouth.
4. Elastic ring nut anti-loose
Fiber or nylon is embedded in the thread spinning to increase friction. The elastic ring also plays a role in preventing liquid leakage.
No.2 Mechanical lock
1. Groove nut and split pin anti-loose
After the groove type nut is tightened, use the opening pin to pass through the small hole at the tail of the bolt and the groove of the nut.
2. Stop gasket
After the nut is screwed, make the single-ear or double-ear stop washer bending to the side of the nut and connected parts respectively to realize the anti-loosening.
3. Series steel wire anti-loose
Use low-carbon steel wire to penetrate into the holes of the heads of the screws, and connect the screws in series to make them brake each other.
NO.3 Permanent lock
The commonly used permanent anti-loose are: spot welding, riveting, bonding, etc. This method mostly destroys threaded fasteners during disassembly and cannot be reused.
In addition, there are other anti-loose ways, such as: coating liquid adhesive between spinning and closing threads, inserting nylon rings at the ends of nuts, and riveting and anti-loose. Mechanical anti-loose and friction anti-loose are called detachable anti-loose, while permanent anti-loose is called non-detachable anti-loose.
Damage the thread at the punch point at the end of the thread after tightening the nut.
2. Bonding and loosening-nut anti-loosening fluid
Apply the anti-loosening liquid of the nut to the place where the bolt is screwed, and then screw in the nut. After self-curing, the anti-loosening effect is good.
Finally, let’s look at the bolt manufacturing process:
Source: China Bolts Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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