An FEM Based Approach to Crankshaft Dynamics and Life Estimation 980565
Crankshafts developed wide applications even before methods to design them got evolved. Initially, they were assumed to be made up of beam segments. Since, the length to width ratios of the segments are of the order of one, these methods were not effective. Therefore, till recently they were designed based mostly on in-house experience and by using empirical formulae. Of late, with the advent of powerful computers, analysis based on finite element method has come into use for crankshaft design. In this work, we generate, from fundamentals, a systematic procedure to design crankshafts for finite life.
Our method uses the advantages of both the classical method and the finite element technique. For the initial and the approximate results, we use the classical method. A software developed, TVAL, based on this method, quickly gives the natural frequencies, critical modes, displacements and stresses. For more accurate results, we model the crankshaft using solid finite elements. The simulated boundary conditions represent both radial and bending stiffnesses of the bearings, which have a large effect on crankshaft dynamics. Time varying radial and tangential forces acting on the crankpin are derived from the cylinder pressures. The model considers the effect of dampers. We find out displacements and stresses at all locations of the crankshaft using the mode superposition method. Stress concentration factors (SCF) are found out by a method based on zooming. We use an engine duty cycle for generating stress histories for accelerated fatigue testing. We conduct rainflow counting on thus obtained variable stress-time history. Life is estimated by assessing the damage by the Miner's rule. We have also discussed variation of lives of crankshafts due to changes in their material properties. Results of analysis of crankshafts belonging to three different classes of engines are presented here.