December 2001

Engineering Case Histories

Case history 6: Keep those bolts tight

It’s important to keep operating loads less than the bolt preload

Sofronas, A., Consulting Engineer

If there is one rule-of-thumb that should be applied to minimize bolting failures it's "Keep the bolts tight." Loose bolts, or bolts where the operating applied load is greater than the assembly preload, can see the full effect of any externally applied cyclic load. Excessive cyclic loading can lead to bolt fatigue failure. In a later case history we will examine static loads on gaskets and how the bolts can loosen because of thermal process upsets and cause leakage. In this case history the bolts that failed in fatigue were used to secure pillow block bearings (Fig. 1). These bearings were supporting an eccentric shaft weight that drove a vibrating screen conveyor. The

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