Bolt

Why does a bolted joint fail as a whole when the surface is only crushed a tiny bit? This article uses the model of a 'stretched rubber band clamping a sponge' to see through the mechanics of the bearing-pressure check: from microscopic peak crushing and macroscopic embedding (Setzen), to the elastic recoil that causes an irreversible loss of preload (clamping force), to the chain failure of loosening and fatigue fracture — and gives engineering measures such as controlling the limiting surface pressure and enlarging the bearing area.

Where does the reduction factor κ in bolt pre-selection come from? Starting from the two-way stress state where axial tensile stress and torsional shear stress combine during tightening, this article uses the von Mises yield criterion to derive the definition, formula and engineering values of κ, explaining how thread friction acts like an 'invisible tax' that cuts the bolt's axial load capacity, and why it appears in the denominator of the Kübler equation.

For a given working load, how large should the bolt be? This article covers the preliminary design and pre-selection of bolts: first read the nominal diameter and property class from a quick pre-selection table by load range, then use the Kübler equation to back out the thread stress cross-section from the axial working load and the required clamping force, combined with the tightening factor, reduction factor and simplified fatigue and bearing-pressure checks, to help engineers quickly size a bolt in the early design stage.

The bolted joint is one of the most widely used connection methods in mechanical engineering. Starting from the inclined-plane nature of a thread, this article sets out thread geometry and classification: the lead angle, the pitch, the pitch diameter d2, the minor diameter d3 and other key geometry parameters; common thread profiles and technical standards such as metric ISO coarse/fine, pipe threads and trapezoidal threads; and the functional distinction between fastening bolts, motion screws and sealing/adjusting bolts — laying the geometric groundwork for the later bolt strength calculation and pre-selection.