Thread load distribution

The development of finite element methods, since the late 1960s, has made possible the exploration of a wide range of variables relevant to the design of screwed plug closures. Work (126,128—132) on the stress at the toot of the first loaded thread, where most failures occur, and the load distribution along the thread length has led to the conclusions that the load carried by the first three threads decreases considerably as the number of active threads increases to 20, and the load carried by the second thread, is approximately 75% of the load on the first thread, f, and that on the third thread, fj, about 60% of f, that on the first thread, regardless of the number of threads. 

Empirical equations have been proposed (133) which enable a combination of thread and vessel parameters to be chosen to minimise the stresses at the toot of the first three active threads. The load distribution along the thread is sensitive to machining tolerances and temperature differentials (134) and... 

Barake N, Chaaban A. Effect of cracks on load distribution in threaded end closures and fracture detection. High Pressure Technology, PVP-Vol. 281, ASME 1994. 

Grewal AS, Sabbaghian M. Load distribution between threads in threaded connections. Journal of Pressure Vessel Technology Febmary, 1997 Vol. 119. 

Torsional deflection of lead screw. For a long and/or slender lead screw, the frequency of the first few torsional modes of vibration may be low enough to influence the system dynamics. Moreover, the winding/unwinding action of torsional deflections may affect the threads clearance and the overall load distribution causing further deviation from the models considered here. In this monograph, the lead screws are considered to be sufficiently stiff and modeled as rigid bodies. 

Misalignment. Design and/or assembly problems may lead to axial offset of the centerlines of lead screw and nut. The misalignment may also occur in the form of a skewed nut. In both of these cases, thread contact and load distribution may be affected severely. 

Wear Throughout the operating life of a lead screw drive, wear causes changes to the contacting surfaces, thereby affecting the load distribution across the threads. 

Remark 9.1. In the actual lead screw system, lubrication, surface conditions, and load distribution on the lead screw and nut threads change as the nut translates along the lead screw. However, in the mafiiematical modeling and the subsequent identification approach, all parameters are taken as their averaged values over a small travel distance of the nut along the lead screw. ... 

This scheme is usually implemented with a separate counter and some modulo function of that counter on each node (or in each execution thread). " Figure 2 shows the distribution of work for an arbitrary number of tasks on an arbitrary number of nodes for this kind of modulo load balance mechanism. It is evident from this diagram that the numbered tasks 1—16 are not ordered to produce a load-balanced computation. 

Crum ASD. The distribution of load over threads of a threaded closure, including temperature effects and local imperfections. High Pressure Technology, Material Design Stress Analysis and Applications, ASME PVP-Vol. 148 1988. 

It is a well-known fact that it is the first pitches of a screw that take mainly part in the transfer of forces. Over these pitches the outside diameter of the column thread is therefore provided with a slight taper so as to distribute the load as far as possible over the other pitches, too. 

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