THREAD REINFORCEMENT OF SCREW CONNECTIONS IN LIGHTWEIGHT DESIGN

摘要

Design engineers today desire to develop more lighter and compact components with higher performance capabilities against the background of saving resources. For this purpose, low strength and lightweight metals such Aluminum or Magnesium are preferred on one hand. On the other hand there is an increase in load requirements which needs higher clamp force between these components. In such applications length of thread engagement requires special focus. At the same time from industry perspective it is essential to have effective dimensioning methods to develop new products in shorter time and at a reasonable cost (economics). Besides the increase in length of thread engagement, the tribology of mechanical contacts (i.e. increased friction coefficients and limited loading capacity due to higher contact pressure) plays an important role in the tightening process. The result is that the desired preload is often not achieved with a standardized tightening specification (reasons: high friction decreases preload for same tightening torque by high torsional stresses). In this contribution experimental results of tightening screws with nut components made from aluminum alloys are analyzed for their frictional behavior by using a multi-channel screw assembly test stand. Risks and dangers associated with tightening process are highlighted and suggestions are made for improvement in thread reinforcement. An established method to realize thread reinforcement is using wired thread inserts. For these elements several standards concerning geometry and strength exist but no dimensioning method is available. These thread inserts themselves have a higher strength than the nut thread material and transmit the preload force of the screw with a greater surface area than the original nut component. This leads theoretically to an increase in the load capacity of the nut thread. The approach of existing dimensioning methods by established rules and regulations for the design of bolted connections, e.g. VDI 2230, are not able to predict the behavior of reinforced thread engagements. Hence, an extension for the dimensioning of threaded engagement is essential for modern requirements. Experiments of tightening screw joints with wired thread inserts and with normal thread engagement are comparatively analyzed along with numerical calculations using Finite Element Analysis (FEA). Conclusions are drawn from experimental and FEA results with the perspective of formulating an extended dimensioning method for thread reinforcement of screw connections in lightweight design. These points are interesting for every design engineer, especially in the field of lightweight design.