Current Status of Anti-loosening Fasteners

The national standard nuts we currently use only serve a connecting purpose and cannot prevent loosening, which is a well-known fact. The weakening and disappearance of many machine parts’ functions are directly related to the failure of fasteners. We often see small bolts and nuts on the road, especially those below M16, and these nuts should be said to have fallen off from various vehicles. Over a long period, people have thought of many ways to prevent nuts from loosening: 1. Adding spring washers, 2. Using double nuts, 3. Using clamping nuts, 4. Riveting and stamping threads, 5. Adding open-end pins, 6. Adding steel, nylon, and steel wire inside the nuts, etc. However, none of these methods work.

Current Status of Anti-loosening Fasteners

There are many types of anti-loosening nuts on the market: national standard non-metallic nested nuts, nuts with embedded steel plates, nuts with embedded spring steel wires, flange indented lock nuts, crushed nuts, and the iron standard variable tooth anti-loosening nuts.

Non-metallic nested nuts do not have anti-loosening properties but have some anti-detachment performance. 2. Nuts with embedded steel plates. These two types of nuts are divided into two parts internally: one is the threaded part, and the other is a non-threaded groove. The groove is fitted with a nylon ring and steel plate. During assembly, under external force, the male thread extrudes a thread on the nylon; or the male thread passes through the barrier of the steel plate and screws out, making assembly difficult and disposable. These nuts have a short thread body, many processing steps, an overall height greater than that of national standard nuts, and a cost several times higher than that of national standard nuts.

Spring steel wire nuts have two parts internally: one is the threaded part, and the other is two grooves. The grooves contain steel wires that extend out of the body of the nut. The installation and removal of these nuts must be done using special tools; once they become loose, they cannot be removed without the special tools. These nuts have many processing steps, difficult installation and removal, an overall height greater than that of national standard nuts, and a cost several times higher.

Flange indented lock nuts: These nuts basically have no anti-loosening effect. Their indented surface has a slightly greater frictional force than that of smooth nuts, but this has nothing to do with anti-loosening properties because once it is loose, it will rotate, and without positive pressure, the friction coefficient is of no use.

Variable tooth anti-loosening nuts: On May 28, 2001, the iron standard released the “Variable Tooth Anti-loosening Nut.” It has a 30-degree angle slope at the top of the 60-degree tooth profile of a standard thread. This slope serves two purposes: a. It decomposes the tightening force into an axial force parallel to the axis and a radial force perpendicular to the axis. The axial force parallel to the axis tightens the connection, while the radial force locks it. b. It is a well-known fact that 80%-90% of the axial force of standard threads is concentrated on the first two threads. After tightening, the first two threads have already undergone significant deformation, which cannot be restored but continues to increase under vibration in various machines and vehicles, leading to gaps and loosening. The slope of the variable tooth anti-loosening nut evenly distributes the axial force of the bolt over each thread of the external thread (this fact is evidenced by photoelastic tests). Thus, the force on each thread of the external thread should not exceed the elastic range. At this point, the external thread is subjected to a cantilever force. Steel has a high elastic modulus, and its ability to recover from deformation is strong. Even if the force on the external thread exceeds the elastic range, causing permanent deformation, there is a tendency for relative rotation under shear force in the vibration of the threaded connection pair, but the radial force on the nut’s slope constantly presses the external thread, preventing relative rotation, thus achieving anti-loosening and anti-detachment functions. This is its intrinsic anti-loosening principle. Its anti-loosening performance has passed the tests by the “General Mechanical Components Product Quality Supervision and Testing Center,” demonstrating excellent anti-loosening properties. Currently, this type of nut is being widely used in the accessories and lines of passenger and freight trains on the railway. Variable tooth anti-loosening nuts belong to the category of structural anti-loosening.