Bolted Joints - Still a Key Part of Efficient Powertrains and a Challenge for Simulation

Abstract

The bolt is one of the most standardized and most commonly used machine elements. On the other hand, since the mechanics of highly stressed bolted joints and the thread fatigue are complex issues, the design and evaluation of such joints is frequently carried out with major simplifications and assumptions, leading to either over-engineered solutions or to premature failures of the prototypes.The simulation techniques and the computing power that are now available theoretically allow a precise evaluation of the fatigue safety in the most heavily loaded areas through the application of very fine FE models of the thread regions. However, due to the modeling and calculation effort, this is still only acceptable in practice for structures with a limited number of bolts. Furthermore, uncertainties related to material specification including the behavior in the plastic range as well as difficulties in considering thread manufacturing tolerances or residual torsional stresses from the assembly state, makes this approach difficult to apply widely in the engineering practice.An interesting alternative method is a combination of the FE analysis of highly stressed bolted joints with historically well-proven analytical considerations, as proposed in the initially released second part of the VDI 2230 standard. In this way, most of the aspects that are difficult to be effectively simulated by FEA are considered in the VDI 2230 limits defined by hardware tests, while in parallel the unique behavior of the joint including the tension, bending and shearing of the bolt can be precisely simulated.The effective application of the above approach is shown in this paper using the example of a straight-split, inclined-split and a marine type connecting rod. The differences in joint behavior and thus the design requirements as well as the limitations and benefits of the applied evaluation technique are discussed in detail.