In the industrial realm, the combination of bolts and nuts stands as the "unsung heroes of the mechanical world." From the high-speed rails slicing through the landscape to the steel girders of cross-sea bridges, from spacecraft cabins to the colossal blades of wind turbines, this dynamic duo silently shoulders the critical tasks of connection, load-bearing, and safety. Their reliability hinges not only on the mechanical ingenuity of thread design but also on the breakthroughs in anti-loosening technologies. This article delves into the mechanical principles of thread engagement and explores the pivotal applications of anti-loosening techniques (such as lock washers and thread-locking adhesives) in high-speed rail and bridge engineering.

The Mechanical Code of Thread Design: From Inclined Planes to Precision Engagement

The core connection mechanism of bolts and nuts originates from the principle of inclined planes. As a bolt is screwed into a nut, the inclined planes of the threads convert rotational motion into axial tension, generating a preload force in the bolt shank that clamps two or more components together through friction. This process involves three key mechanical parameters:

Thread Lead Angle and Self-Locking Conditions

The thread lead angle (the angle between the thread's inclined plane and the bolt axis) determines connection reliability. When the lead angle is smaller than the friction angle between materials, the thread system achieves self-locking, preventing loosening without additional measures. For example, the lead angle of common coarse threads is approximately 2°–3°, sufficient for most static loads but requiring auxiliary anti-loosening under vibration or cyclic loading.

Preload and Friction Coefficient

Preload is the cornerstone of bolt connections, directly influencing clamping force and fatigue resistance. According to VDI 2230 standards, preload typically ranges from 70% to 90% of the bolt's yield strength. The friction coefficient is a critical variable: discrepancies between the thread friction coefficient and the bearing surface friction coefficient can cause actual preload to deviate from theoretical values by ±30%. High-precision applications thus require lubricants or surface treatments (e.g., zinc plating, phosphating) to optimize friction performance.

Stress Distribution and Fatigue Life

Under cyclic loading, bolts experience periodic stress variations. Stress concentration at thread roots is the primary cause of fatigue fractures. Techniques such as thread rolling to harden surfaces and adopting large-radius transitions significantly enhance fatigue life. For instance, MJ threads (U.S. military standard threads) used in aerospace improve fatigue strength by over 40% through optimized profiles.

Anti-Loosening Technologies: Defying Vibration and Time

Despite thread design's self-locking capability, scenarios like high-speed rail and bridges, which endure prolonged vibration or impact, demand supplementary anti-loosening measures. The core objective of these technologies is to prevent relative rotation of thread pairs, achieved through three primary approaches: friction-based, mechanical, and permanent locking.

1. Friction-Based Anti-Loosening: Increasing Rotational Resistance of Thread Pairs

The Mechanics of Lock Washers

Spring washers provide axial pressure through elastic deformation, increasing thread friction. Serrated lock washers (e.g., Nord-Lock) employ wedge-shaped teeth that bite into the bearing surface, converting axial forces into radial locking forces. In high-speed rail bogie axle box connections, serrated lock washers resist vibrational angles up to ±15°, ensuring bolts remain tight at speeds of 350 km/h.

The Chemical Bonding of Thread-Locking Adhesives

Anaerobic thread-locking adhesives (e.g., Loctite 242) cure when isolated from air, filling thread gaps and forming a adhesive layer. Their mechanism combines friction enhancement with chemical bonding, offering superior disassembly ease compared to welding. In bridge steel structure assembly, thread-locking adhesives compensate for manual torque variations, reducing preload fluctuations to ±10%.

2. Mechanical Anti-Loosening: Physically Restricting Thread Pair Movement

Rigid Constraints of Cotter Pins and Slotted Nuts

In heavy machinery drive shaft connections, slotted nuts paired with cotter pins provide irreversible locking by inserting pins into nut slots. Although requiring pin destruction for disassembly, their reliability is unmatched.

Deformation Locking of Tab Washers

Single- or double-ear tab washers lock rotation by bending tabs into bolt heads or component slots, converting rotational motion into plastic deformation of the washer. In nuclear power plant pressure vessel flange connections, tab washers endure over 10⁶ fatigue cycles without failure.

3. Permanent Anti-Loosening: Irreversible Connection Solutions

Fusion Applications of Spot Welding and Riveting

In aerospace, critical bolt connections may use spot-welded nuts or high-lock bolts (Hi-Lok) instead of traditional nuts. Hi-Lok bolts achieve permanent connections through preset interference and locking collars, offering threefold vibration resistance over conventional bolts.

High-Speed Rail and Bridges: The Ultimate Testbeds for Anti-Loosening Technologies

The operating conditions of high-speed rail and bridges impose stringent demands on bolt connections: high-speed rail must withstand high-frequency vibration, thermal cycling, and extreme wind loads, while bridges must resist long-term creep, seismic impacts, and marine corrosion.

High-Speed Rail Bogies: Zero Loosening Under Dynamic Loads

The CRH380A high-speed train bogie's axle box positioning node employs a triple anti-loosening scheme: double nuts + serrated lock washers + thread-locking adhesive. Double nuts create additional friction through preload differentials, serrated washers resist lateral vibration, and thread-locking adhesive eliminates fretting wear. Field tests show this approach reduces bolt loosening torque decay from 30% (traditional methods) to below 5%.

Cross-Sea Bridges: Long-Term Fastening in Corrosive Environments

The Hong Kong-Zhuhai-Macao Bridge's immersed tube tunnel joints use A325 high-strength bolts with Dacromet coatings and silicone sealants. Dacromet provides over 1,000 hours of salt spray resistance, while silicone sealants block seawater ingress. To mitigate long-term creep risks, designers increased initial preload by 20% and deployed online monitoring systems to track bolt stress in real time.

Future Challenges: Balancing Lightweighting and Intelligence

As Industry 4.0 advances, bolt design faces new contradictions:

Lightweighting Demands: Aerospace and electric vehicles require 30%+ bolt weight reduction, necessitating a shift from high-strength steel to titanium or aluminum alloys. However, lightweight materials are prone to thread stripping and adhesive wear.

Intelligent Monitoring: Bolts embedded with fiber-optic sensors enable real-time preload monitoring, but compatibility between sensors and threads, along with data transmission stability, remain technical bottlenecks.

For example, the Airbus A350's composite fuselage partially uses titanium blind-hole bolts with laser-clad thread profiles, paired with wireless strain gauges for health monitoring. This solution reduces bolt weight by 25% while achieving 1.5× the fatigue life of conventional designs.

Conclusion: The Invisible Guardians of the Mechanical World

The combination of bolts and nuts epitomizes humanity's mastery of mechanical principles and materials science. From thread engagement to anti-loosening innovations, this golden duo has long served as the "invisible guardian" of industry. Looking ahead, as new materials, sensors, and artificial intelligence converge, bolt connections will evolve toward lighter, stronger, and smarter forms—yet their core mission remains unchanged: safeguarding every sprint, every crossing, and every soar, silently and reliably.