Two analytical structural models of the connecting rod cap

The mathematical models available in the pertinent literature for designing the cap of a connecting rod are critically examined, and it is shown that some currently adopted assumptions are inaccurate. Spurred by these critical remarks, two more realistic mathematical models are developed of the contact between the crankpin and the cap. In both these models, the crankpin is modelled as rigid, and the cap is idealized as a purely flexural, curved beam of constant cross-section, in progressive contact with the crankpin. In the first model, the cap is mimicked as a twice twice-statically redundant structure solved with the Castigliano theorem. The cap structural response is mechanically examined starting from the central part of the cap-crankpin contact. In the second model, the cap is mimicked as a statically determined structure. Particular attention is paid to the cap change of curvature when moving along the cap axis. The beginning of the cap-crankpin contact is determined by requiring that the curvature of the deformed cap equal the curved border of the crankpin. A result common to the two approaches is the definition of a combined factor summarizing the mechanical effect of several variables, namely the total load, the initial clearance, Young’s modulus, and the cap geometry; the last factor is recapped by the radius defining the centre of mass, and by the moment of inertia. To evidence the merits and limits of the two approaches, the extent of the cap-crankpin angular contact, and the value of representative cap stresses, are determined and compared to Finite Element (FE) forecasts. Furthermore, the effect of the ratio between the moment of inertia of the abutment and the cap central cross-section is investigated. This ratio influences the prediction of the contact angle, when the ratio is higher than 4, the numerical FE results significantly differ from the analytical calculations.