Tension bolts are widely used in engines, in machinery of various kinds, and in structures. They have unique advantages in their contribution to ease of assembly and dismantling. They also give a comparatively rigid and compact joint, and from the standpoint of static strength are extremely efficient. Unfortunately they have not got a good reputation from the standpoint of fatigue. Some designers go to considerable trouble to avoid them. Others accept their limitations and ensure safety by care in detail design and by adopting conservative figures for design stresses. Where necessary, a limitation is placed on working life, at the expiration of which the nut and bolt are replaced by new ones.