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Finally understand - thread locking is actually preventing bolt fracture

2020-09-11

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 prosperous topic in mechanical design. We know it better, such as eccentric nuts in Japan, down nuts and self tightening King nuts in China. But we won't talk about these stars in the fastener industry today. Let's talk about the most basic method of fixing nuts in our work.

Why are the bolts tighter and tighter?

Generally, we analyze bolt fracture from the following four aspects:

I. bolt quality

Second, pre tightening torque of bolts

Third, the strength of bolts

Fourth, the fatigue strength of bolts

In fact, most cases of bolt fracture are broken due to looseness, specifically broken due to looseness. Because the situation of bolt loosening and breaking is roughly 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 large that we can't imagine that the fatigue strength of bolts can't be used at all in the process of use.

1、 The bolt fracture is not due to the tensile strength of the bolt:

With an M20 × 80 grade 8.8 high-strength bolt as an example, its weight is only 0.2kg, and its minimum tensile load is 20t, up to 100000 times its own weight. Generally, 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 break through thousands of times the weight of components. Therefore, the tensile strength of threaded fasteners is sufficient and cannot be damaged due to insufficient strength of bolts.

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

The threaded fastener can be loosened only 100 times in the transverse vibration loosening test, while it needs to be repeatedly vibrated one million times in the fatigue strength test. In other words, the threaded fastener loosens when using one ten thousandth of its fatigue strength, and we only use one ten thousandth of its large capacity. Therefore, 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 looseness:

After the threaded fastener is loosened, it will produce huge kinetic energy MV2, which will directly act on the fastener and equipment, resulting in the damage of the fastener. After the fastener is damaged, the equipment cannot work under normal conditions, further leading to the damage of the equipment.

For the fastener under axial force, the thread is damaged and the bolt is broken.

For the fastener under radial force, the bolt is cut and the bolt hole is made into an ellipse.

4、 The selection of thread locking method with excellent locking effect is the fundamental to solve the problem:

Take the hydraulic hammer as an example. The weight of gt80 hydraulic hammer is 1.663t. Its side plate bolts are 7 sets of grade 10.9 M42 bolts. The tensile resistance of each bolt is 110 tons. The preload is calculated by taking half of the tensile resistance, and the preload is as high as 300-400 tons. However, the bolt will break. Now it is ready to change to M48 bolt. The fundamental reason is that the bolt locking can not be solved

When the bolt is broken, the most easy conclusion is that the strength is not enough, so the method of increasing the bolt diameter and strength grade is mostly adopted. This method can increase the pretightening force of the bolt and its friction force. Of course, the anti loosening effect can also be improved, but this method is actually a non professional method, with too much investment and too little income.

In short, the bolt is: "if it is not loose, it will break as soon as it is loose."

Cause analysis of bolt looseness

The threaded connection is designed according to self-locking conditions: ψ ≤ ρ v. The friction pair generated in the thread pair makes the bolt self-locking to fasten the bolt, so the connection will not loosen under static load. However, under impact, vibration, variable load and large temperature change, the friction 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 is generated. For the fastener under axial force, the thread is damaged and the bolt is broken. For fasteners subjected to radial force, bolts are cut and bolt holes are broken.

Bolt locking principle: limit the relative movement between thread pairs or increase the difficulty of relative movement.

Introduction to common locking methods

There are three common locking methods for bolts: friction locking, mechanical locking and permanent locking. Mechanical locking and friction locking are detachable locking, while permanent locking is called non detachable locking. There are three common nut locking methods: friction locking, mechanical locking and permanent locking.

There are three common locking methods for 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.

No.1 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 produce a continuous elastic force, so that the threaded connection pair of the nut and the bolt can maintain a friction force and produce a resistance torque, so as to prevent 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 piece, so as to prevent the bolt from rotating relative to the connected piece.
2. Counter nut (double nut) locking

3. Self locking nut

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

4. Elastic ring nut locking

No.2 mechanical locking

1. Groove nut and cotter pin locking

2. Stop gasket

3. Series steel wire locking

No. 3 permanent locking
Common permanent locking devices include spot welding, riveting, bonding, etc. This method mostly destroys threaded fasteners during disassembly and cannot be reused.

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

1. Edge punching method

After the nut is tightened, punch the end of the thread to destroy the thread.

2. Adhesive locking - nut locking fluid

Apply the nut locking fluid to the tightening part of the bolt, and then screw on the nut. After self curing, the locking effect is good.