Critical Fasteners, Highly Loaded Bolted Joints
Due to clamping of the parts, the compressive stress is induced at the joint surface. The compressive stress keeps the joint away from opening out. The reliability of the joint depends on what is the reliability of the fasteners geometry, correct material selection and heat treatment, reliability of load analysis and reliability in achieving expected joint clamping load during assembly of the relevant parts. In this chapter, the preload on critical fasteners is explained using the force diagram of a bolted joint and the settling force, pre-loading on the bolts and tightening by torque-controlled tightening, angle-controlled tightening and other methods is described in detail. Important practical aspects like thread engagement length, limiting surface pressure and fatigue loading are dealt with at length. The influence of temperature especially in cylinder head bolts is important to consider while designing. The high temperature fasteners form a special category in design and development. Various methods to improve the fatigue strength of the joints are given. The time-honoured design process as per VDI 2230 is explained and examples are provided in the appendix. Also, the typical failure modes for which the design work is carried out are provided.
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