Dynamic Model of a Fluctuating Rocker-Arm Ratio Cam System

Abstract

This study is focused upon one example from a class of cam systems with a fluctuating rocker-arm ratio, known as “finger follower” cam systems. This class of cam systems is typically built in practice with a hydraulic tappet as the pivot-end support for the finger-follower, and the rocker-arm ratio varies as much as 34 percent from the baseline value during the cam cycle. A six-degree-of-freedom dynamic model is formulated to predict the forces as well as the motions of the cam system components. Successful dynamic modeling of such a system requires an accurate model for the hydraulic tappet as well as for the cam system, and so, six separate dynamic phenomena are identified in the tappet and the resulting nonlinear dynamics included in the cam system model. Lateral and rotational motions, as well as the customary longitudinal displacement, are admitted for the valve, and it is found that although the extra degrees of freedom change the cam contact force but little, they strongly influence where in the cam system toss and impact occur.