Preload Stability of Modern Bolted Joints

Abstract

The design of screw joints is very important for fastened components to ensure reliable transmission of mechanical or thermal loads between components. Suitable fastener selection and proper design are required for increasing product performance while reducing size, weight and cost ratios. A fastening system is basically characterized by component geometry, materials and tightening level. Robust clamping of components depends on time dependent clamp force, which can be represented by Bolt’s preload change. In any fastening system preload loss takes place. The contribution shows, why (seating, load plastification, creeping). Furthermore, the loss may not be too large to provide suitable behaviour over time. This means, a higher tightening preload cannot be utilized, if most of the advantage is compensated by preload loss. Important influences are clamped materials, tightening level (from assembly method) and type of thread engagement. For various combinations long time measurements of preload loss over time are shown. The paper shows also how the measurement is done. The outcome of all the combinations is that preload loss significantly exceeds the estimations of existing guidelines, e.g., VDI 2230. The reason is, that in time of development of the guidelines mainly steel components have been used and preloading was moderate. So the conclusion is to extend the calculation with a better and more flexible approach for today´s requirements. Overall, the contribution combines explaining mechanisms for preload loss, results from measurements for analysis as well as calculation for prediction in advance to extend guidelines.