Summary of Causes of Loose and Fracture of Threads and Anti-loose Strategy

When two or more members are bolted, since the angle of the thread is less than the equivalent friction angle, the thread can be self-locked for the purpose of reliable connection. However, in actual service, many components are subjected to alternating load, vibration load, impact load and temperature load. The friction between the thread pairs will decrease or even disappear, so the thread will be loose, so the thread connection must be loosened. deal with.

The essence of thread joint inspection is to prevent relative movement between the thread pairs. When the thread pairs rotate relative to each other, there are two types of frictional moments between the threads to balance the rotational moment of the thread pair. The frictional moment generated by the load and the preload on the thread pair and the frictional moment between the coupling nut and the bearing surface.

Bolt breaks usually come from four sources: 1) the quality of the bolt; 2) the pre-tightening torque of the bolt is insufficient; 3) the strength of the bolt is insufficient; 4) the fatigue strength of the bolt is low. In fact, most of the bolt breaks are broken due to looseness and are broken due to looseness. Because the loosening of the bolts is almost the same as the fatigue fracture, in the end, we can always find the reason from the fatigue strength. In fact, the bolts do not use the fatigue strength at all during the use.

The looseness of the threaded fastener is not due to the fatigue strength of the bolt: the threaded fastener can be loosened only one hundred times in the transverse vibration test, and it needs to be repeatedly vibrated one million times in the fatigue strength test. In other words, the threaded fastener is loose when it is used in one ten thousandth of its fatigue strength. We only use one ten thousandth of its large capacity, so the looseness of the threaded fastener is usually not due to the fatigue strength of the bolt. insufficient.

The real cause of damage to threaded fasteners is looseness. When the threaded fastener is loose, it generates a huge amount of kinetic energy, which directly acts on the fasteners and equipment, causing the fastener to be damaged. Fasteners that are subjected to axial forces, the threads are broken and the bolts are broken. For fasteners that are subjected to radial forces, the bolts are sheared and the bolt holes are made elliptical.

There are four ways to lock the threaded fasteners: frictional anti-loose, mechanical anti-loose, riveting and anti-loose.

Frictional locking is one of the most widely used methods of securing looseness, which produces a positive pressure between the pairs of threads that does not change with external force to create a frictional force that prevents relative rotation of the pairs of threads. This positive pressure can be achieved by axially or simultaneously pressing the thread pair in both directions. Such as elastic washers, double nuts, self-locking nuts and nylon insert lock nuts. This kind of anti-loose method is convenient for the disassembly of the nut, but in the case of impact, vibration and variable load, the pre-tightening force of the bolt will decrease due to the looseness at first, and the pre-tightening force of the loss increases slowly as the number of vibrations increases. In the end, the nut will be loose and the threaded connection will be ineffective.

Mechanical anti-loose is the use of a stop to directly limit the relative rotation of the thread pair. Such as the use of cotter pins, tandem wire and stop washers. Since the stopper has no pre-tightening force, the anti-loose stopper can function when the nut is retracted to the stop position, and therefore, this method does not actually prevent loosening but prevents falling off.

The riveting and anti-loose is made by punching, welding, bonding, etc. after tightening, so that the thread pair loses the motion pair characteristics and is connected to be undetachable. The disadvantage of this method is that the bolt can only be used once, and the disassembly is very difficult, and the bolt must be broken to be detachable.

Structural anti-loose is the use of the thread's own structure, that is, the Tang's thread anti-loose. The first three types of anti-loose methods mainly rely on the third party to force the anti-loose, mainly refers to the friction, and the anti-loose effect depends on the size of the third party. The structural anti-loose does not rely on third-party forces, but only on its own structure. The structure anti-loose method is the Tang's thread anti-loose method, and it is also the most advanced and best anti-loose method.

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