How to Improve the Strength of Bolted Connections?

The strength of bolts is influenced by many factors, including material, structure, size parameters, manufacturing, and assembly processes. Here are some measures to improve the strength of bolts:

Improve the load distribution between threads: With a standard nut, the axial load distribution among the engaged threads is uneven, with the first thread bearing the most load. Using a suspended (tension) nut or a ring groove nut can improve the distribution by reducing the difference in thread pitch variation, resulting in a more even load distribution.

Avoid or reduce additional stresses: Due to design, manufacturing, or installation oversights, bolts may be subjected to additional bending stresses, which greatly affect the fatigue strength of the bolt. This should be avoided by using structures like boss or countersunk seats to ensure a flat support surface when installing bolts on unprocessed surfaces of castings or forgings.

Reduce stress concentration: The root of the thread, and the junction between the bolt head and the shank, are areas of stress concentration that are prone to fractures. Increasing the fillet radius at the thread root or at the transition part of the bolt head, or cutting relief grooves, can reduce stress concentration and improve fatigue strength.

Reduce stress amplitude: When the maximum stress on the bolt is constant, a smaller stress amplitude results in higher fatigue strength. Reducing the stiffness of the bolt or increasing the stiffness of the connected components can achieve this, but the preload force should be increased accordingly. Measures to reduce bolt stiffness include increasing the bolt length, partially reducing the shank diameter to create a hollow structure (flexible bolt), or installing an elastic element under the nut.

Improve manufacturing processes: The manufacturing process has a significant impact on the fatigue strength of bolts, especially for high-strength steel bolts. Cold working hardening in the case of rolled threads, as well as processes like carbonitriding, nitriding, and shot peening, can improve the fatigue strength of bolts.

These measures, when implemented together, can significantly enhance the connection strength and durability of bolts.