In tension-type bolted connections, the overall strength of the joint is mainly determined by the load-bearing capacity of the bolt itself. Bolt strength is affected by multiple factors, including material properties, structural form, dimensional parameters, manufacturing processes and assembly techniques. To effectively improve the bearing capacity, fatigue resistance and overall reliability of bolted connections, strength enhancement can be achieved through structural optimization, process improvement and assembly standardization. This article systematically introduces the key technical measures for improving the strength of bolted connections.
01 Improving Load Distribution among Thread Teeth
When standard ordinary nuts are used, the axial load is unevenly distributed among each engaged thread turn. Starting from the nut bearing surface, the first thread carries the largest portion of the load, and the load gradually decreases on subsequent threads. Theoretical analysis and experimental verification show that a greater number of engaged thread turns leads to more severe load unevenness. Threads beyond the 8th to 10th turns barely bear any load. Therefore, simply increasing nut thickness and thread engagement turns cannot improve the strength of bolted connections.
Suspended tension nuts can significantly improve thread load distribution. The tapered suspended section of the nut and the bolt shank undergo synchronous tensile deformation, reducing the pitch deformation difference between the nut and bolt, thereby achieving uniform load distribution on all engaged threads. Ring-grooved nuts operate on a similar principle, effectively equalizing thread stress, improving thread engagement efficiency, and enhancing the overall connection strength.
02 Avoiding and Reducing Additional Bending Stress
In the processes of design, machining and assembly, bolts are prone to additional bending stress caused by structural errors, machining deviations and non-standard installation. Additional bending stress greatly deteriorates bolt fatigue strength and serves as a major cause of bolt fracture, which must be eliminated in design and assembly practices.
When bolts are installed on unmachined rough surfaces of castings or forgings, uneven contact surfaces easily cause eccentric loading and bolt bending. In engineering applications, bosses and counterbore seats are commonly adopted with precision cutting to form flat bearing surfaces. This ensures full surface fitting, eliminates assembly eccentricity, and prevents the generation of additional bending stress.
03 Reducing Structural Stress Concentration
Thread roots and the transition zone between bolt head and shank are typical stress concentration regions and critical failure-prone positions. Among these areas, stress concentration at thread roots has the most significant impact on bolt fatigue strength. Optimizing structural geometry to relieve stress concentration is essential for extending bolt fatigue life.
Common optimization measures include increasing the fillet radius of thread roots to mitigate local stress peaks, enlarging the transition fillet between the bolt head and shank, and designing stress-relief grooves and relief notches at high-stress positions. These methods disperse concentrated stress, reduce peak stress values, and improve the bolt's fracture resistance.
04 Reducing Stress Amplitude to Improve Fatigue Performance
Under a constant maximum working stress, a smaller stress amplitude results in better fatigue performance and longer fatigue life. With steady working load and residual preload, reducing bolt stiffness or increasing the stiffness of connected components can effectively lower the stress amplitude and enhance fatigue strength. In such cases, the preload should be appropriately increased to ensure connection reliability.
Common methods for reducing bolt stiffness include increasing the effective bolt length, moderately reducing the diameter of the smooth shank, adopting hollow flexible bolt structures, and installing elastic elements under the nut to achieve a flexible bolt effect. Flexible bolts feature large deformation capacity and excellent energy absorption, making them suitable for working conditions with impact and vibration loads.
To improve the stiffness of connected components, low-stiffness deformable gaskets should be avoided, as gasket compression increases stiffness disparity and raises bolt stress amplitude. For sealing connections, high-stability sealing rings are preferred to maintain overall connection stiffness and optimize stress distribution.
05 Optimizing Bolt Manufacturing Processes
Manufacturing processes directly determine the surface microstructure, residual stress state and fatigue performance of bolts, especially for high-strength steel bolts. Thread rolling introduces surface work hardening, forms continuous and intact metal flow lines, and generates beneficial compressive residual stress on the thread surface. Rolled threads exhibit substantially higher fatigue strength compared with conventionally turned threads.
Furthermore, surface strengthening treatments such as carbonitriding, nitriding and shot peening can refine surface microstructures, eliminate surface micro defects, and introduce uniform residual compressive stress. These treatments inhibit the initiation and propagation of fatigue cracks and significantly improve the fatigue strength and overall load-bearing performance of bolts.
