76 Fastening technology EJOT SpringHead® High clamp load retention for demanding applications By Prof. Dr. Ralph Hellmig, Director Technology Management Fasteners, EJOT SE & Co. KG The clamp load is an important design parameter for planning the layout of a screw joint. It ensures that the intended joint properties are maintained even when the joint is subjected to mechanical or thermal stress. Mechanical stresses on the joint lead to additional forces that must be considered in the design. Thermal loads exhibit a direct effect on the clamp load in the joint and can alter it in various ways. Especially in case of thread-forming screws, which are typically made of carbon steel, the material for screw-in and clamping parts differs significantly. Frequently used materials for structural and clamping parts are aluminium and magnesium alloys, copper materials or engineering plastics. All these materials exhibit an increased coefficient of thermal expansion when compared to the screw material. Therefore, when heating a screw joint having such a material combination, the clamp load will initially rise. Additionally, elevated temperatures lead to relaxation effects, which in total leads to an accelerated clamp load loss. When a screw joint is cooled down, the clamp load is reduced due to different thermal expansion coefficients involved. In contrast, re-heating increases the clamp load. To ensure an appropriate clamp load during application, it may be necessary to incorporate a spring element into the joint to minimise clamp load variations. Ideally, such an element would be suitable for automatic feeding. At the same time, it should prevent high local surface pressures as these can cause creep in sensitive materials. The EJOT SpringHead® screw is a fastener that combines these features having an unlosable rolled-on flat washer. This innovative fastener comprises an integrated spring element that acts similarly to a conventional disc spring. Due to its lower stiffness, it reduces variations in clamp load in case of thermal or mechanical loads. The additional unlosable flat washer distributes screw forces more evenly into the clamping part, thereby preventing high localised surface pressures. The screw is installed in such a way that the integrated n Clamp load change due to thermal loading: EJOT SpringHead® with unlosable flat washer vs. recessed round washer head Graphics: © EJOT
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