B. Kenny

Load and stress distribution in screw threads

The load distribution and the normalized stress distribution in the threads of an ISO metric nut and bolt were found by frozen-stress photoelastic analysis employing a fringe-multiplying polariscope in conjunction with a recording microdensitometer. Previous investigators assumed that these distributions were identical, whereas the present study shows that this is not the case. It has been further shown that deducing the load distribution from measurements of the nut deformation is unsatisfactory. The results of the reported method correlate closely with Sopwiths theoretical load distribution. Three recent finite-element analyses of nut-bolt systems have been compared indirectly to the experimental load distribution. That of Bretl and Cook was found to give the best comparison.

A modification to the theory for the load distribution in conventional nuts and bolts

Abstract A discrepancy between experimental results and the generally accepted theory for the load distribution occurring in the threads of a nut-bolt connection is highlighted. This occurs at the loaded end of the bolt and is caused by the changing geometry of the thread form of the nut as the thread emerges from the loaded face of the nut. The result is a thread form which is not complete and which is progressively more flexible as the loaded face of the nut is approached. This results in a reduction of the load carried by these incomplete threads. Sopwiths analytical theory has been modified to allow for this effect by using finite element analysis to determine the varying stiffness of the threads. The results of this modified theory compare well with those from three-dimensional photoelastic analyses of the loads in the threads of two bolts fitted with conventional nuts.

Stress analysis of some nut-bolt connections with modifications to the external shape of the nut:

Abstract The effect on the stress levels in an axially loaded bolt has been investigated for the case where a nut which incorporated a circumferential groove in its outer surface was used. It was found from a three-dimensional photoelastic frozen stress study that the modified nut reduced the maximum stress in the bolt by 5 per cent. The addition of a bevel to the load bearing face of this nut further reduced the maximum stress to 74 per cent of its value in a standard connection. It has been established that these modifications reduce the maximum shear stress in the roots of the nut threads, and that the stress concentration associated with the groove was smaller than the maximum stress concentration in both the nut and bolt thread roots. The increase indicated by photoelastic analyses in the strength of the connection produced by these modifications, has also been substantiated by fatigue tests of steel connections, but these results are not reported in this paper.

The optimisation of the design of nuts with partly tapered threads

Abstract The angle of taper and the number of tapered threads in a nut necessary to give the lowest maximum stress concentration in the bolt were determined for an ISO nut—bolt connection subject to axial loading. Thirty-eight frozen stress models were analysed and it was found that a two degree taper for 5.5 threads from the loaded face of the nut reduced the maximum normalised tensile stress in the bolt by 41 per cent from 5.17 to 3.07. It was observed that if the angle of taper was too large the stress concentration produced at the junction of the parallel and tapered threads could negate the beneficial effects of the taper geometry.

Stress analysis of some nut-bolt connections with modifications to the nut thread form

Abstract Six photoelastic frozen stress models of ISO nut-bolt connections loaded in pure tension were studied. Four of these models had modifications to the threads at the load bearing end of the nut. These modifications included tapered truncation of the thread crests and taper of the whole thread form for a portion of the nut length. The maximum stress in the bolt always occurred within one pitch of the load bearing face of the nut. Truncating threads increased the maximum bolt stresses. Tapering the whole thread form reduced the maximum stress and produced a more uniform load distribution in the bolt. The conditions at the load bearing face of the nut were found to influence the load distribution in the bolt threads more than the stress concentrations in the bolt.

PHOTOELASTIC ANALYSIS OF DOVETAIL JOINTS FOR TURBINE BLADES

Both room temperature and frozen stress techniques of photoelasticity were used to study the stress distributions in two-dimensional models of an axially loaded dovetail type of joint. The geometry of the joint and the location of the loading were varied. Stress separations were carried out in the interior of the models as close to the loaded boundaries as was found to be feasible, using the Shear Difference method.

The elastic behaviour of tube drawing dies subjected to a sinking process

Abstract A numerical stress analysis study of a tube drawing die of conical profile is used in conjunction with a theoretical expression predicting die pressures in a sinking process, to obtain hoop strain information for the outer surface of a tube drawing die. The analysis indicates that material strain hardening rate and coefficient of friction both affect the hoop strain distributions, but their separate effects may be measured individually by appropriate positioning of strain gauges on the outer surface of the die.