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Why do bolts loosen and how to prevent them from loosening
Bolt loosening is one of the most common failure forms in mechanical equipment. According to industry statistics, about 70% of mechanical failures are directly related to loose fasteners. This phenomenon is particularly prominent in conditions with frequent vibration. For instance, the bolts of wind turbine blades may experience a preload reduction of over 50% within several months due to freaking wear under continuous wind vibration. What is more serious is that loosening can trigger a chain reaction: A case of a certain hydraulic hammer shows that although the M42 high-strength bolt can withstand a preload of 400 tons, it broke due to the failure of anti-loosening measures, ultimately causing deformation of the equipment's main shaft. This failure mode is covert - in the early stage of loosening, it may only manifest as abnormal equipment noise, but if it continues to develop, it will lead to the separation of the connection surface, stress redistribution, and eventually cause bolt overload fracture or thread shear failure. In the aerospace field, fastener loosening has been listed as the second most serious safety hazard after fatigue fracture, which shows its harmfulness. The root cause of bolt loosening can be attributed to the combined effect of dynamic loads and material failure. Vibration and shock are the primary triggers - when the equipment is in operation, the lateral force repeatedly breaks through the frictional resistance of the threaded pair, causing a micron-level relative displacement between the bolt and the nut. This rotational self-loosening phenomenon is similar to the sliding of a slanted wooden block after being struck by an external force. Take the car chassis bolts as an example. Road vibrations of dozens of times per second can cause the preload to be completely lost after thousands of cycles. Secondly, material deformation leads to the decline of preload force: The metal protrusions on the contact surface between the bolt head and the connected part undergo plastic deformation (settlement phenomenon) under high pressure, while the gasket material creep under long-term stress, just like a spring gradually losing its elasticity. More concealed is the temperature effect. When there is a difference in the coefficient of thermal expansion between the fastener and the connected body, a temperature difference of 200℃ can generate a thermal stress equivalent to 15% of the preload. Furthermore, the 15% metal contact rate of the thread pair means that 85% of the contact surfaces have air gaps. This structural defect enables the vibration energy to easily break through the frictional resistance. It is worth noting that false tightening (such as not reaching the specified torque) will accelerate this process. An analysis of a wind turbine tower bolt accident shows that 30% of the loosening cases are due to the failure to use a torque wrench during installation. In response to the complex causes of bolt loosening, modern engineering practice has developed a multi-level anti-loosening technology system, the core of which lies in breaking through the limitations of traditional friction anti-loosening. The following are the eight verified strategies:
I. Mechanical locking Technology
1.Double-nut anti-loosening system
The double friction surface is formed through the head-up effect of the main and auxiliary nuts, and it can be automatically compensated when the preload force decreases. A test of a heavy-duty truck wheel hub shows that this scheme enables the bolts to maintain 92% preload after 50,000 vibration cycles. It should be noted that the secondary nut should be designed to be thin to avoid stress concentration.
2.All-metal lock nut
For instance, the 30° wedge-shaped thread design of the Schbitt nut converts lateral vibration into axial pressure by changing the force application Angle. After adopting this technology, the spacecraft door connection parts can achieve zero loosening in an alternating environment ranging from -50℃ to 120℃.
Ii. Structural Enhancement Scheme
3.Pre-coated adhesive thread technology
After the anaerobic adhesive fills the thread gap and cures, a rigid connection layer is formed. In the pipeline flange renovation of a certain chemical plant, the use of Loctite 243 glue reduced the annual loosening rate of bolts from 17% to 0.3%. Disassembly requires a special de-gumming agent.
4.Elastic washer combination
The combination of the wave spring washer and the sawtooth lock washer features both elastic compensation and tooth surface embedding mechanisms. After the main shaft bolts of the wind turbine adopted this scheme, the maintenance cycle was extended from three months to two years.
Iii. Intelligent Monitoring Methods
5.Electronic torque marking bolt
The intelligent bolt with integrated strain gauge can monitor the preload force changes in real time. When the attenuation exceeds 15%, a warning will be triggered. The application of a certain high-speed railway bogie shows that this technology has reduced the failure rate of bolts by 80%.
6.Magnetorheological anti-loosening device
The viscosity of the gasket is adjusted by electromagnetic field to automatically enhance damping when vibration intensifies. Tests on helicopter rotors show that their anti-vibration performance is six times better than that of traditional solutions.
Iv. Technological Innovation
7.Hydraulic tension fastening method
The direct axial tension bolts eliminate the torque friction error. After the main pipeline connection of a certain nuclear power plant adopted this process, the preload force dispersion was reduced from ±30% to ±5%.
8.Low-temperature cryogenic treatment
The liquid nitrogen treatment at -196℃ creates a compressive stress layer on the surface of the bolts. After this treatment, the anti-loosening performance of a certain racing car connecting rod bolt has been improved by three times.
These solutions need to be combined and used according to the working conditions: for instance, for high-speed rail bogies, intelligent monitoring and hydraulic fastening should be adopted simultaneously, while for offshore platforms, magnetorheological anti-loosening and double-nut systems are preferred. It is worth noting that any anti-loosening measures must be combined with standardized tightening processes. An accident analysis of a certain wind farm shows that substandard torque control can reduce the anti-loosening effect by 40%.