Two Dimensional Effects in the Lubrication of the Cam-Tappet Contact

The cam-tappet contact experiences very high loads and the lubrication conditions are Elasto-Hydrodynamic (EHD). In EHD lubrication the oil film thickness (OFT) is mainly determined by the entrainment speed of the oil and the oil viscosity. Under the high load present in the cam-tappet contact the OFT reaches very small values in the order of magnitude of a fraction of one micrometer and asperity contacts cannot be avoided. In this situation the friction force has two components, a boundary and a viscous one, and non-Newtonian behavior of the oil prevails. The friction force is determined by the rheological properties of the oil and the geometry of the contact (combined radius of curvature, cam width and combined asperity of the rubbing surfaces). For valve trains with bucket type tappets, the mechanism is design to rotate the tappet during engine operation. In this situation, both the entrainment speed and the radius of curvature are varying along the contact line determining variations in the OFT and, consequently, variations in the contribution of the boundary friction component. Based on a simulation model that considers the tappet spin under the action of the friction force between cam and tappet, and the friction force between tappet and its bore, the entrainment speeds and their directions were calculated along the contact line. It was found that the boundary friction component reaches a maximum value at the inner edge of the cam, at the beginning of the valve-closing event. Because the boundary component of the friction force is mainly responsible for wear, an increased wear of this region should occur. Visual inspection of a used camshaft seems to confirm this finding.