A Photo-Elastic Study of Stresses in Screw Threads

The several components of the deformation of a screw subjected to tension through a nut have been examined theoretically by Sopwith (1948)‡. He showed that the distribution of load along the thread was far from uniform and that it reached its maximum intensity at the plane of the bearing face of the nut. Sopwith's conclusions have now been investigated photo-elastically with a Fosterite model of a stud and nut, using the “frozen-stress” method. An additional investigation was made of the stress distribution in a tension nut with a recessed stud, suggested by Sopwith as a means of rendering the load distribution more uniform.

The experimental results confirm the theory in both cases, showing a large rise in stress in the stud near the bearing face of the ordinary nut and a much more uniform distribution of stress when the tension nut is used. The stress concentration factors determined for the normal nut are higher than those found by Hetényi, who used Bakelite B.T.61-893 as the model material. This difference may be accounted for partly by improvements in technique and partly by the greater accuracy which becomes possible with the use of Fosterite.