Key Findings
Threaded‑fastener loosening divides into rotational (self‑loosening) and non‑rotational pre‑load‑loss types; alternating vibration and shock are the primary triggers even though static‑load threads satisfy theoretical self‑locking conditions. Four mainstream anti‑loosening approaches have distinct pros‑cons; no single conventional method can fully eliminate loosening. Structural anti‑loosening relying on thread‑pair inherent geometry represents a reusable new technical direction.
| Anti‑Loosening Category | Working Principle | Representative Products | Main Shortcoming |
|---|---|---|---|
| Frictional anti‑loosening | Sustain friction‑pressure between thread‑pair | Spring washers, double nuts, nylon‑insert / all‑metal prevailing‑torque nuts | Performance degrades under heavy‑shock conditions |
| Mechanical anti‑loosening | Physically block nut rotation | Cotter pins, lock washers, wire lacing | Only prevents detachment after loosening starts, cannot stop initial rotation |
| Rivet‑punch anti‑loosening | Destroy thread movability for permanent joint | Welding, staking / punching | Non‑reusable; disassembly requires component destruction |
| Structural anti‑loosening | Leverage intrinsic thread‑pair structural geometry | Special‑profile threaded pairs | Emerging solution, reusable and easy to disassemble |
Loosening is defined as pre‑load loss after tightening. Rotational loosening means relative turning between bolt and nut; non‑rotational loosening means pre‑load drop without thread‑pair rotation. Static‑load threads satisfy self‑locking criteria, yet alternating vibration and shock induce pre‑load decay in real‑world machinery.
Frictional, mechanical and rivet‑punch methods rely on external restraining forces. Mechanical locking parts only function once loosening has already begun, so they serve as anti‑drop rather than true anti‑loosening measures. Rivet‑punch‑type locking makes connections permanent and prohibits repeated disassembly. The newer structural anti‑loosening mode depends purely on fastener intrinsic geometry, delivering both reliable locking and reusability. Select solutions according to reliability, cost and maintainability requirements for each application scenario.
Anti-Loosening Solution for Threaded Fasteners
Threaded connections are widely used in various mechanical structures. Due to their reliable connection, simple structure and convenient assembly and disassembly, they are one of the most commonly used fastening methods. The quality of fasteners has an important impact on the level and quality of mechanical equipment.
Threaded fasteners are tightened by the engagement of internal and external threads, enabling rapid connection of components and allowing for disassembly. They 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 issues is that they tend to loosen on their own during use.
1. The main reason for the loosening of the fasteners
There are many mechanisms that can cause threaded fasteners to loosen, which can be classified into: rotational and non-rotational loosening.
In the vast majority of applications, threaded fasteners are tightened to apply preload in the connection joint. Loosening can be defined as the loss of preload after tightening is completed, and it can occur through either of two methods.
Rotational loosening, often referred to as self-loosening, refers to the relative rotation of fasteners under the action of external loads. Non-rotational loosening means that there is no relative rotation between the internal and external threads, but preload loss may occur.
Actual working conditions show that common threads can meet the self-locking condition and will not loosen under static load. However, alternating loads, vibration and shock in practical use are among the main reasons for the loosening of threaded connections.
2. Conventional anti-loosening methods for threaded fasteners
The essence of preventing loosening in threaded connections lies in preventing the relative rotation of bolts and nuts during operation. However, there are many conventional methods and measures for preventing loosening.
Due to the different working conditions of the installation sites for mechanical connection threaded fasteners, the anti-loosening performance requirements for the threaded connection pairs also vary. Considering factors such as reliability, economy, and maintainability, various anti-loosening measures for threaded fasteners are adopted in actual mechanical connections.
For decades, engineers have adopted various measures to prevent the loosening of threaded fasteners. For instance, they have used lock washers, spring washers, cotter pins, adhesive coatings, double nuts, nylon nuts, and all-metal effective torque nuts, among others. However, these measures still cannot completely solve the loosening problem.
Currently, the commonly used anti-loosening forms are as follows:
- Frictional anti-loosening. For instance, anti-loosening methods such as elastic washers, double nuts, self-locking nuts, and nylon insert lock nuts can be adopted to generate a frictional force that can prevent the relative rotation of the connection pair. This constant normal pressure can be achieved by axially or simultaneously pressing the connection pair in two directions to prevent loosening.
- Mechanical anti-loosening. Anti-loosening methods such as using cotter pins, wire lacing, and lock washers are employed to directly restrict the relative rotation of the connection pair. Since these anti-loosening parts do not have a pre-tightening force, the anti-loosening and locking effect only takes place when the nut has loosened to the locking position. In fact, this is not anti-loosening but rather a method to prevent detachment.
- Riveting, punching and anti-loosening. After the connection pair is tightened, welding, punching and bonding methods are adopted to make the thread lose its motion characteristics and become an irreversible connection. The obvious disadvantage of this method is that the bolt can only be used once, and it is very difficult to disassemble. The connection pair must be destroyed to disassemble it and it cannot be reused.
- Structural anti-loosening. It utilizes the inherent structure of the threaded connection pair, ensuring reliable anti-loosening, being reusable and convenient for disassembly and assembly.
The first three anti-loosening techniques mainly rely on external forces for anti-loosening, mainly by utilizing frictional force, while the fourth one is a new type of anti-loosening technique that solely depends on its own structure.
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