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 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 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.
Video auf YouTube (Englisch)
https://www.youtube.com/watch?v=7Ld–v9pLZ4&list=PLTD_RZurdUv6t2rXQ1Zxs6j65fn5Bl1E2&index=3
Caption I:
Clamp load change due to thermal loading:
EJOT SpringHead® with unlosable flat washer vs. recessed round washer head
Caption II:
FEA simulation of stress distribution in the clamping part:
EJOT SpringHead® with unlosable flat washer vs. pan head
Caption III:
Schematic torque-angle curve and possible fastening strategy
for an EJOT SpringHead® screw
Caption IV:
Combined system (left) vs. EJOT SpringHead® system (right)
Aufmacherbild (Deutsch / Englisch):
