Calculation of a Bolted Connection under Variable Load

Machine parts are joined using bolted connections. Under variable load, the strength of the threaded parts is reduced due to metal fatigue. When machine parts fail due to static loads, they undergo large deformations, since the stress exceeds the limit of elasticity. Static failure can be prevented because it is noticeable. Fatigue failure occurs suddenly, without any external signs, which makes it very dangerous. The maximum variable stresses are well below the material strength limit and below the yield strength. Repeated stresses are characteristic of metal fatigue. Studies show that 15 % of fractures occur at the cross section under the bolt head, 20 % at the beginning of the cut portion of the bolt, and 65 % in the threaded part of the rod in the plane of the nut support surface. Usually, the most effective way to increase the durability of a threaded joint under variable load is the maximum permissible tightening of the nut or bolt. In most tightened joints, about 25 % of the external load acts on the bolt, while 75 % unloads the joint of the parts. Due to this, a well-tightened bolt has the greatest durability. The strength of the bolt under variable load after the optimum tightening of the joint is calculated in this paper. Depending on the amplitude limit and the mean stress according to Goodman and Gerber criteria, the fatigue strength of the bolt is determined. The optimum tightening ensures equal strength of the bolt and tightness of the joint parts.

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