When the equally distributed load is applied to the expected helix diameter between bolt and nut, the fundamental engagement length and the longitudinal bolt force is formed. When the nonlinear distribution load is applied to the expected helix diameter in the fundamental engagement length based on the equally distributed load, the existing engagement length meets to the height of nut, the existing load changes to the equally distributed load making the superposed stress at root diameter of bolt to minimum. Since the deformation of bolt due to bolt tightening can precisely be estimated, correct bolt tightening could easily be achieved.

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Majority of refinery, petrochemical and chemical plant has been operated more than 40 years in Japan. The bolt and nut especially under insulation corroded severely and the height of nut reduced due to corrosion. Therefore, establishment of maintenance guideline required to make appropriate maintenance. This paper will theoretically make it clear the permissible thinning ratio of nut

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