The influence of the kinematics of the thread forming process on the quality of the threaded connection

Abstract

The effectiveness of assembly operations technologies using threaded parts is determined by the productivity, economy and quality of the resulting connections. The use of thread-forming fasteners in assembly processes reduces labor intensity by eliminating the preliminary thread-cutting operation, and the resulting connections are more accurate and have locking qualities. Thread-forming parts are screwed into smooth holes and form a thread by cutting or deformation. Such connections are called smooth-threaded. In production where the formation of chips is undesirable, thread-extruding screws are used. Threaded connections operate under conditions of alternating loads and for them cyclic durability is an important factor. However, despite the fact that the thread formed by plastic deformation has high strength, the cyclic durability of such a thread is lower than that of a cut thread. Improving the quality of smooth-threaded joints by loading with optimal axial force during thread forming is the topic of this publication.
The reason for the decrease in the fatigue characteristics of rolled threads is the reverse movement of the metal, which is caused by a violation of kinematics during the formation of the thread profile. The purpose of this work is to establish, based on the theory of plasticity, the regularities of the material flow process in the zone of the greatest deformations under the action of axial and radial thread forming forces. By solving the problem of introducing a rigid wedge into the rigid-plastic region, to establish an analytical dependence for calculating the compensating force of thread forming. To experimentally confirm the influence of axial uncompensated force on the quality of smooth-threaded joints.
Deformation in the process of thread forming is characterized by the fact that movement or deformation occurs everywhere in one plane. This is a state of plane deformation. The method of slip lines allows to calculate contact stresses and deforming forces, to determine stress fields in sections of the deformed metal, to analyze local phenomena in solving technological problems. The characteristics of plane plastic deformation make it possible to solve by the graphical method. This method determines the picture of the field of slip lines, its geometric parameters, which establish the relationship between the maximum tangential stress and the deforming force.
Based on the provisions of the theory of plasticity, regularities were established that describe the processes of metal flow in the zone of the greatest deformations under the action of axial and radial thread-forming forces. Based on the solution of the problem of the theory of plasticity on the introduction of the identifier into the rigid-plastic region, analytical dependencies were obtained for calculating the technological parameters of thread-forming. Based on the analysis of the rigid-plastic zone in the region of the greatest deformations, regularities of the influence of the axial thread-forming force on the parameters of the resulting profile were established.
The results of the work will allow to improve the quality of the connection with the thread-forming fastener by introducing a compensating axial load into the thread-forming process. Ensuring the correct screw movement during thread formation prevents the reverse movement of the metal, the occurrence of surface defects and relaxation of useful compressive stresses, which increases the cyclic durability of the thread. And the formation of a symmetrical profile increases the strength of the connection and improves its locking qualities. Analyzing the regularities of the formation of the surface layer, the choice of the angle of rise of the western turn is justified.