Analysis of Preload Loss With Corrosion Protection Layers in the Flow of Force

Abstract

This contribution investigates the tightening behavior and preload loss of bolted joints with painted coating systems for corrosion protection of components. Friction coefficients are measured with test stand, preload loss is measured by strain gauges. Further evaluation is done with optical roughness measurement to see damage of the painted surfaces in contact areas. Resulting from this hints for applications get obvious. Research progress focuses the quantification of preload loss in different applications. Mechanical loads are neglected at that stage because of minor influence as the temperature. To sum up, it can be formulated that the tested, robust coating systems can endure yield-point-tightening of high-strength screws and also exist as a complete and coherent layer after the loosening. Therefore corrosion protection could be guaranteed also after a yield-point-tightening, especially underneath the screw head. This is in particular relevant for applications which require repeated tightening of the same screw. Corrosion protection layers within the flow of force usually change the friction coefficients compared with bare material. This effect influences the assembly of bolted connections dominantly. Controlling the assembly process is getting more difficult because of the increasing scattering from steering parameters, like torque or angle. There might be huge deviations, up to a factor of 6.2, between torque values of a similar assembly process. Therefore process safety cannot be guaranteed, especially if their values miss originally defined tolerances. To overcome these difficulties it is necessary to use the right assembly method as well as the right coating system. After our opinion the yield-point controlled tightening is the only method which leads to a safe assembly preload, even with coated parts. If the preload loss will be acceptable depends on the coating system and the external conditions.