77 spring element is properly loaded, but not flattened. This ensures the effective use of the spring element’s elastic properties. The typical fastening method, which relies on the tightening torque to stop the screw-driving system, is not suitable for this purpose. The geometry of the EJOT SpringHead® fastening element creates a highly specific torque curve. As is well known from conventional screw connections, the torque increases significantly as soon as the fastening element touches the clamping part. However, due to the spring geometry, the torque reaches a relatively constant level during tightening. It then rises sharply once the spring element is flattened. To properly fasten the screw, the fastening process should be stopped when this constant torque level is reached. This can be achieved Video EJOT SpringHead® Product Animation https://t1p.de/z5njl using an automated fastening strategy involving either head contact detection or threshold torque, followed by angle-controlled final tightening. The advantage of this strategy is that the clamp load is determined by the geometry and material properties of the integrated spring element rather than by friction. Combining a standard screw with a disc spring and a flat washer is clearly inferior to the EJOT SpringHead® system. The different components must be assembled separately. This significantly complicates handling and makes automated assembly practically impossible. A combined element manufactured by captive rolling of the disc spring and flat washer, followed by heat and surface treatments, carries a high risk of hydrogen embrittlement, particularly at the disc spring, since it is typically made of steel with an increased carbon content. This type of clamp load-retaining connections is particularly useful when certain system properties need to be guaranteed by a screw. For example, this could involve holding seals together against internal pressure, which may even change, or maintaining electrical contacts within a design. The latter suggests the use of screws with integrated spring elements for electromobility applications. Schematic torque-angle curve and possible fastening strategy for an EJOT SpringHead® screw FEA simulation of stress distribution in the clamping part: EJOT SpringHead® with unlosable flat washer vs. pan head Combined system (left) vs. EJOT SpringHead® system (right)
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