Unfortunately, most designers today do not know the importance of preload on a fastener. Preload is necessary to assure a tight joint and maximize fatigue performance. Bolts should be tightened to a sufficient preload to ensure proper fatigueperformance. The tighter the joint, the less alternating loads the bolt will see. The relationship between alternating loads (fatigue) and fastener preload is illustrated in Figure 1. An axially loaded bolted joint in which there is no bolt preload isrepresented by line OAB. When the joint varies between P{sub}a and P{sub}b, the bolt load is equal to P{sub}(Ba) and P{sub}(Bb). When a preload of P{sub}(B1) is added to the bolt, the joint is compressed and bolt load changes more slowly than the jointload indicated by the line P{sub}(B1)A. The slope of P(B1)A is less than OAB because some of the load is absorbed as a reduction of compression in the joint. Consequently, when the joint varies between P{sub}a and P{sub}b, the bolt load is equal toP{sub}(Ba') and P{sub}(Bb'). This condition results in a considerable reduction in cyclic bolt-load variation and thereby increases the fatigue life of the fastener and dynamically loaded joints have less tendency to slip and loosen. This is why it isoften desirable to preload a bolt into its plastic region. This article will discuss preloads in the plastic region. Figure 2 defines what the elastic and plastic regions are.
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