Anti-loosening Measures for Hex Bolts

Hex bolts are widely used in fields such as machinery, steel structures, rail transit, and construction machinery, relying on thread preload to achieve secure fastening. However, during equipment operation, vibration, impact, temperature fluctuations, and wear can easily cause bolts to loosen and preload to be lost, leading to equipment failures and safety hazards. Therefore, a reliable anti-loosening mechanism is essential for ensuring the stability of bolted connections and extending the service life of equipment.

I. Friction-Based Anti-Loosening

Friction-based anti-loosening is the most common passive anti-loosening method in industrial applications. Its core principle involves continuously increasing the forward friction between the threaded pair to counteract the loosening torque caused by vibration and displacement, thereby preventing relative rotation between the bolt and nut and maintaining a stable preload. This method features a simple structure, low cost, and easy installation and removal, making it a fundamental protective measure.

Mainstream applications include spring washer locking, double-nut locking, and flat washer + anti-slip washer combinations. Spring washers rely on their own elastic deformation to continuously apply compressive force to the threaded pair, maintaining friction resistance at the thread contact surface; double nuts eliminate thread clearance through the squeezing force of the upper and lower nuts, locking the relative positions of the threads.

Applications: Primarily suitable for static loads, low-frequency vibrations, and standard operating conditions at ambient temperatures, such as frames for general mechanical equipment, civil hardware components, and light steel structure connections. It is the preferred basic anti-loosening solution for standard operating conditions.

II. Mechanical Anti-Loosening

Mechanical anti-loosening is a forced locking method. Unlike the passive resistance protection of friction-based anti-loosening, its core principle involves using specialized mechanical locking components to rigidly secure bolts, nuts, and connected parts, directly restricting the relative rotation of the threaded pair. This structurally eliminates the risk of loosening, offering significantly higher reliability than friction-based anti-loosening.

Classic engineering applications include cotter pin + slotted nut anti-loosening, lock washers, and wire-linked anti-loosening. A cotter pin passes through pre-drilled holes in the slotted nut and bolt shank, using the pin’s rigidity to lock the nut in place; a lock washer forms a mechanical stop by bending one side to fit against the nut and workpiece; and wire-linked systems connect multiple bolts in series to restrict their relative rotation.

Applications: Widely used in dynamic heavy-load, high-frequency vibration, and high-safety-level operating conditions, such as critical load-bearing connections in rail transit vehicles, construction machinery chassis, aerospace components, hydraulic equipment, and lifting equipment.

III. Permanent Anti-Loosening

Permanent anti-loosening, also known as adhesive anti-loosening or curing anti-loosening, is a technology that achieves integrated locking of threaded pairs through chemical curing. The core principle involves applying a specialized thread-locking adhesive to the thread engagement surfaces. After the bolt is tightened, the adhesive fills the thread gaps and undergoes a curing reaction within the airtight thread spaces, bonding the thread engagement surfaces of the bolt and nut into a single unit. This completely eliminates the space for relative thread movement while simultaneously providing sealing, rust prevention, and corrosion protection.

Based on strength, these adhesives can be classified into low-strength removable threadlockers, medium-strength general-purpose threadlockers, and high-strength permanent threadlockers. Low-strength adhesives allow for disassembly using standard tools, while high-strength adhesives completely lock the threads after curing, making disassembly impossible and providing a permanent fastening solution.

Applications: Suitable for threaded connections in precision machinery, hydraulic piping, automotive components, and long-term maintenance-free equipment. It is particularly well-suited for anti-loosening protection of small bolts and precision fasteners.