Machine Design and Shop Practice (MDSP) Inhouse Practice Exam

Preloading a bolted joint creates an initial clamp force that keeps the joint surfaces pressed together. In cyclic or tensile loading, if the parts can separate slightly, slip occurs at the contact surfaces and the stress reverses in the surrounding material, producing large alternating stresses. The clamp from preload keeps the surfaces in contact, so the external load is shared between the bolt and the joined parts but the joint remains tight throughout the cycle. This reduces the stress range experienced by the members and bolts, cutting the alternating (fatigue) stress and improving fatigue life.

External loading by itself is just the applied force, and bolting or pretightening without sustaining a clamp doesn’t prevent slip or large stress reversals. Precising isn’t a standard method in this context.