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Clamp force relaxation of self tapping screw joints in plasic components

Compared to other dismantable joining processes, self-tapping screw joints in plastic are very economical. However, because of the polymers visco-elastic deformation, the prestressing force decreases as a function of time and temperature. The great decrease in prestressing force occurs in the first hours. The relaxation rate then decreases with time. However, the loss of prestressing force may continue for a very long time.

Superimposed on time-dependent material-specific effects are differences in thermal expansion of plastic component and metal screw/metal attachment as well as volume reducing effects (cooling, drying out of the polyamide, post-crystallisation/ shrinkage). Under cyclic temperatures these effects lead to a characteristic progress of the prestress force. However, mainly relaxation effects occur during the first heating up of the joint. In spite of irreversible deformation in the region of the flank engagement the residual prestress force is proportional to the initial prestress force.

Essential for residual prestress forces on a high level are the joint design (boss upper surface area and boss height) and the material stiffness in dependence of temperature (fiber reinforcement, conditioning of polyamide). In joints with metallic inserts prestress force relaxation also occurs for load transfer via the boss cross-section (tensile load).

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Author
 Axel Tome

Axel Tome
Lehrstuhl für Kunststofftechnik (LKT)
Universität Erlangen-Nürnberg

Information

Free keywords: Clamp Force reduction, relaxation, prestress force, temperatur cycle test, self tapping screws, inserts in brass, residual clamp force, design of joints criterias, viso elastic deformation, relaxation rate
Institute / chair: Technische Fakultät der Universität Erlangen-Nürnberg
Language: German
Technical consultant for expert services: Prof. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein (Betreuer), Prof. Dr.-Ing. Klaus Feldmann
Publication year: 2000
Provider: Technische Fakultät der Universität Erlangen-Nürnberg

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