Transient elastohydrodynamic lubrication film thickness in sliding and rolling line contacts

摘要

The contact behavior between cam and follower is greatly influenced by the kinematics and dynamics of the whole valve train system. This is the reason that both shape and thickness of the fluid film in the contact gap are mainly determined by applied loads and relative contact speeds as well as the curvatures of contacting elements. Most of the studies about lubricant film behavior between cam and follower have been performed without a consideration of transient effects in the contact gap. For the computational difficulties of transient effects, most contact conditions such as relative contacting speeds have been regarded as quasi-steady state during the whole operating cycle. In this work, in order to obtain stable convergence, a multigrid multi-level method is used for the computation of load capacity in the lubricant film. Nonlinear valve spring dynamics are also considered in the same way as Hanachi's. From the computational results, transient EHL film thicknesses under the conditions of different contact geometries are computed for a pushrod type valve train system during an engine cycle. Several results show the squeeze film effect, which is generally not found with conventional EHL computations of the cam and follower contact. The results are also compared with those by the Dowson-Hamrock (D-H) formula, which does not consider the dynamic film effect. Without the dynamic film effect as in D-H's formula, the minimum film thickness is highly dependent on the entraining lubricant velocity, whereas the minimum film thickness including the squeeze film effect is dependent on the applied load.