Thread connection is widely used in all kinds of mechanical structures. It is one of the most commonly used fastening methods because of the advantages of reliable connection, simple structure and convenient assembly and disassembly. The quality of fasteners has an important influence on the level and quality of mechanical equipment.
Threaded fasteners are clamped by internal and external threads to realize fast connection of parts, and can be disassembled. Threaded fasteners also have good interchangeability and low cost. However, they are also a significant source of mechanical and other failure problems. Part of the reason for these problems is that they loose themselves in use.
There are many mechanisms that can lead to loosening of threaded fasteners. These mechanisms can be divided into rotational and non-rotational loosening.
In the vast majority of applications, threaded fasteners are tightened to apply preload in the joint sub joint. Loosening can be defined as the loss of pretightening force after tightening is complete, and it can occur by either of two methods.
Rotary loosening, usually called self-loosening, refers to the relative rotation of fasteners under external loads. Non-rotational loosening is when there is no relative rotation between the inner and outer threads, but preloading loss occurs.
The actual working conditions show that the general thread can meet the self-locking condition and the thread will not loose under static load. In practice, alternating load, vibration and impact are one of the main causes of loosening of screw connection pair.
General anti-loosening method for threaded fasteners
The essence of thread connection is to prevent the relative rotation of bolts and nuts at work. There are many conventional anti-loosening methods and anti-loosening measures.
For threaded fasteners of mechanical connection, the anti-loosening performance of threaded connection pair is also inconsistent due to different installation conditions. Considering reliability, economy, maintainability and other factors, various anti-loosening measures are adopted for threaded fasteners of mechanical connection in practice.
For decades, engineers have taken various measures to prevent the loosening of threaded fasteners. For example, check back gaskets, spring washers, split pins, glue, double nuts, nylon nuts, all-metal torque nuts, etc. However, these measures can not completely solve the problem of loosening.
Below, we discuss and compare the anti-loosening firmware from the aspects of anti-loosening principle, fastening performance and assembly convenience, anti-corrosion performance and manufacturing reliability. At present, there are four types of commonly used anti-loosening forms:
First, the friction is loose. Such as the use of elastic washers, double nuts, self-locking nuts and nylon insert lock nuts and other anti-loosening methods, to produce a can prevent the relative rotation of the joint friction. The positive pressure, which does not vary with external forces, can be tightened in axial or simultaneous two directions.
The second is mechanical anti-loosening. The use of stop cotter pin, wire and stop washer and other anti-loosening methods, directly limit the relative rotation of the connecting pair, because the stop does not have pre-tightening force, when the nut loose back to the stop position anti-loosening stop can work, this is actually not loose but to prevent falling off the way.
Third, riveting and anti-loose. When the connection pair is tightened, welding, punching and bonding methods are adopted to make the thread lose motion characteristics and become undetachable connection. The obvious disadvantage of this method is that the bolt can only be used once, and it is very difficult to disassemble. It can not be reused unless the connecting pair is destroyed.
Fourth, the structure is loose. It is the use of thread connection pair of its own structure, loose reliable, reusable, convenient disassembly.
The first three anti-loosening technologies mainly rely on third-party forces to prevent loosening, mainly using friction, and the fourth is a new anti-loosening technology, only relying on its own structure.
Post time: Nov-11-2021