Optimization of Crankshaft Torsional Rigidity for Fatigue Strength Improvement Using CAE 2012-01-0404

Automotive industries are using forged crankshafts for higher performance applications due to its high strength. Torsional rigidity plays an important role in the performance of crankshaft. The improved torsional rigidity of a forged crankshaft provides better torsion strength and improves engine performance. Present competitive market demands fast product development with lower cost and light weight design while maintaining fatigue strength and other functional requirements.

The study aims: a) evaluation of crankshaft torsion rigidity by analytical method and its Finite Element Analysis (FEA) correlation, b) evaluation of twist angle using virtual testing and its correlation with test bench data c) evaluate most critical geometric parameters of crankshaft (Crankpin diameter, Pin width, Throw, Web thickness and Web width) and their quantitative contribution in torsional rigidity.

An extensive analytical study for torsional rigidity is carried out for various crankshafts by varying the said geometric parameters. These analytical results are compared with FEA (Finite Element Analysis) results. Torsional rigidity evaluated by analytical method and FEA correlates with 85-95 % accuracy. Twist angle data also matches well with actual test bench data. From this study major critical geometric parameters and their quantitative contribution in torsion rigidity is evaluated.

This study shows that, Torsional Rigidity is directly proportional to pin diameter, web width, web thickness and inversely proportional to throw and pin width. Among all the geometric parameters pin diameter and web width are the major contributors to the torsional rigidity. An increase of 10% in pin diameter produces an increase about 15 to 17 % in torsional rigidity; while similar increase of pin width and throw decreases torsional rigidity about 3 to 3.8% and 4 to 4.6% respectively. An increase of 10% in web width produces an increase about 10 to 14% in torsion rigidity; while similar increase of web thickness increases torsional rigidity about 4.6 to 4.8 %. An increase of 10% in each pin width, journal width and web thickness maintains same torsional rigidity.

This study will help to predict and improve torsional rigidity at the design stage. It is also useful to suggest design modifications for existing design and develop new crankshafts for higher performance/weight ratios to achieve expected torsional rigidity and fatigue strength by combination of CAE and analytical techniques.