What methods are available for calculating the appropriate tightening torque for a bolt?
A high bolt preload ensures that the joint is resistant to vibration loosening and to fatigue. In most applications, the higher the preload – the better (assuming that the surface pressure under the nut face is not exceeded that is).The preload is related to the applied torque by friction that is present under the nut face and in the threads. The torque value depends primarily on the values of the underhead and thread friction values and so a single figure cannot be quoted for a given thread size.The stress that is often quoted is often taken as the direct stress in the bolt as a result of the preload. It is normally calculated as preload divided by the stress area of the thread. Typical values vary between 50% to 80% of the yield strength of the bolt material, in many applications a figure of 75% of yield is used.
A high bolt preload ensures that the joint is resistant to vibration loosening and to fatigue. In most applications, the higher the preload – the better (assuming that the surface pressure under the nut face is not exceeded that is). The preload is related to the applied torque by friction that is present under the nut face and in the threads. The torque value depends primarily on the values of the underhead and thread friction values and so a single figure cannot be quoted for a given thread size. The stress that is often quoted is often taken as the direct stress in the bolt as a result of the preload. It is normally calculated as preload divided by the stress area of the thread. Typical values vary between 50% to 80% of the yield strength of the bolt material, in many applications a figure of 75% of yield is used. Our TORKSense program uses this approach and further details on this is presented in the help file that comes with the demo program that is available for download from our
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