For connecting members, it's essential to consider the quality of each individual component before assembly, as well as the precision of the assembly process and future maintenance. If a bolt is excessively long—typically exceeding 15 times its diameter—the design must account for potential reductions in connection strength. The coupling hole surfaces should be parallel and smooth to prevent bending stress, which can compromise the integrity of the joint.
When dealing with studs and screw connections, the material strength of the components should also be taken into consideration. This helps determine whether high-strength fasteners are necessary to ensure proper compatibility. Using a torque wrench to tighten bolts within the specified range is crucial for maintaining the required pre-load and avoiding over-tightening or under-tightening.
Periodic inspection of high-strength bolted joints is vital because the initial pre-tightening force usually falls within the elastic deformation range of the bolt. However, if the material of the connected parts is not uniform, local deformations may occur. These deformations can lead to stress relaxation, reducing the pre-tightening force over time. Therefore, it's common practice to check and re-tighten these connections after some operational period, especially for critical applications where failure could be catastrophic.
During the fastening process, inconsistencies may result in some bolts being under-tightened while others are over-tightened. Issues such as uneven flatness of components, bent bolt heads, or damaged galvanized coatings can further reduce the preload. Thermal expansion, plastic deformation, or changes in loading direction may cause gaps, leading to impacts, chemical corrosion, or electrolytic erosion (like rust or seizing).
Improper thread alignment or poor fit can also affect the performance of the connection. Ensuring correct threading and proper fit is essential for reliable fastening.
Regarding the service life of high-strength bolts:
1) If they are used according to the manufacturer’s torque specifications, and the threads and screws remain undamaged and free from corrosion, the bolts can continue to be used.
2) For critical components, such as the bolts securing slewing bearings in tower cranes, it's recommended to replace them after seven years of normal operation.
3) Any of the following defects necessitate replacement: cracks, damaged threads, severe rust, excessive wear beyond acceptable clearance, surface decarburization, loss of galvanization, bending, tensile deformation, or insufficient pre-tightening force that doesn't meet the required elasticity range under normal conditions.
In conclusion, high-strength bolts offer good toughness and manufacturing precision. For large bolted joints, the fastening method and inspection procedures are critical. Continuous practice and experience are needed to achieve reliable high-strength bolt connections. Regular monitoring and adherence to best practices significantly enhance the safety and longevity of these critical structural elements.
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