The thread profile design of anchor bolts is one of the core factors affecting their load-bearing capacity. Its geometry, thread angle, pitch, and other parameters directly determine the mechanical properties of anchor bolts under complex working conditions. Common thread profiles include triangular, trapezoidal, and rectangular profiles. Different profiles, by altering stress distribution patterns, contact areas, and frictional characteristics, have varying effects on the tensile, shear, and fatigue load-bearing capacities of anchor bolts.
Triangular thread profiles (such as ordinary threads), due to their 60° thread angle design, are widely used in anchor bolts. This profile, with its large side slope, generates a significant radial force component in the thread under stress, thereby enhancing the frictional self-locking capability between threaded pairs. Under axial tensile force, the stress concentration phenomenon at the thread root of triangular thread profiles is relatively mild, and the stress distribution gradually decreases along the thread profile, helping to delay the initiation and propagation of fatigue cracks. However, the load-bearing efficiency of triangular thread profiles is limited by their smaller effective contact area. When subjected to lateral loads or alternating stresses, their insufficient shear resistance must be compensated for by increasing the number of thread turns or optimizing the preload.
Trapezoidal thread profiles (such as trapezoidal threads) are characterized by a 30° thread angle and an isosceles trapezoidal cross-section. They are commonly used in anchor bolts for mechanical connections to withstand heavy or dynamic loads. Their larger thread height and steeper flank slope result in a more uniform stress distribution under load, making them particularly suitable for impact loads or vibration conditions. The advantage of trapezoidal thread profiles lies in their significantly higher shear strength compared to triangular thread profiles. By increasing the effective contact area of the thread teeth, shear stress can be effectively dispersed, reducing local stress concentration. Furthermore, trapezoidal thread profiles have better centering than triangular thread profiles, automatically correcting eccentricity errors during installation and reducing the impact of additional bending stress on load-bearing capacity.
While rectangular thread profiles are widely used in transmission applications, they are rarely used alone in anchor bolts for mechanical connections. The 90° tooth angle design of rectangular thread profiles results in almost no radial force component generated by the thread teeth under stress, leading to weak frictional self-locking capability between thread pairs. However, rectangular thread profiles have extremely high load-bearing efficiency, and their full-tooth contact characteristics allow them to transmit larger loads per unit length. To compensate for the lack of self-locking, rectangular thread profile anchor bolts are usually used with anti-loosening devices (such as double nuts or spring washers) or adhesives are used to enhance connection reliability, making them more suitable for applications requiring high space compactness and stable loads.
The pitch parameter of the thread profile also has a significant impact on load-bearing capacity. Increasing the pitch increases the load-bearing area per thread turn, but reducing the number of thread turns may lead to a decrease in the total contact area. In the design of anchor bolts for mechanical fasteners, the relationship between pitch and load-bearing efficiency must be balanced based on material properties (such as the elastic modulus of steel and the creep properties of aluminum alloys). For example, high-strength steel anchor bolts can use a larger pitch to reduce the number of machining turns, while aluminum alloy anchor bolts need to increase the contact area by reducing the pitch to compensate for insufficient material strength. Furthermore, the surface roughness and machining accuracy of the thread profile indirectly affect load-bearing capacity. Precision-machined threads reduce fretting wear during assembly, lowering the risk of stress corrosion cracking. In corrosive environments, surface treatments (such as galvanizing and Dacromet coating) can further enhance thread durability, ensuring it maintains its designed load-bearing capacity during long-term service.
The thread profile design of anchor bolts requires comprehensive consideration of load type, material properties, installation conditions, and environmental factors. Triangular profiles are suitable for general applications, trapezoidal profiles are geared towards heavy and dynamic loads, while rectangular profiles emphasize space efficiency and high load density. By optimizing profile parameters and related processes, the connection reliability of anchor bolts can be significantly improved, meeting the stringent safety and durability requirements of engineering structures.
