The Distribution of Load in Screw Threads

Abstract

The distribution of load along the length of a nut is not uniform owing to the strains set up in the bolt and nut under load. These strains are analysed and the load distribution along the thread helix deduced. It is shown that the load may be taken as concentrated at mid-depth of the threads. In the normal bolt and nut case the maximum intensity of loading occurs at the bearing face of the nut, and may be from two to four or more times the mean, depending on the thread form, the proportions of the members, and the degree of lubrication; the maximum intensity is almost independent of the length of nut (unless yielding occurs). Possible methods of improving the load distribution are discussed—differential pitch, tapered threads, and the use of a softer material for the nut. The load distribution in the turnbuckle case is more favourable than in the bolt and nut case, and can theoretically be made uniform by suitably boring out the inner and shaping the outer member; this condition can be approached in practice by the use of “tension nuts”. The effect of yielding on the load distribution is discussed.Experimental confirmation of some of the results is included.