A Theoretical and Experimental Study of the Tribiology of a Cam and Follower

The development of more fundamental knowledge of the tribology of the cam and
follower mechanism calls for a more comprehensive theoretical analysis and experimental
investigation than has been previously reported. A mixed lubrication analysis has been
applied to the problem to give an estimation of the nominal minimum film thickness and
friction force associated with the contact in such mechanisms. The analysis showed that
the roughness height and the distribution of the roughness between the two contacting
surfaces had important effects on the lubrication performance of the contact. A full
numerical transient EHL analysis was carried out allowing the normal velocity to vary
along the conjunction. This revealed that local squeeze film velocity provided an
increased damping effect which contributed to the persistence of the minimum film
thicknesses in the two zero entraining velocity regions. An approximate technique for
determining the minimum film thickness of a transient EHD line contact associated with
rough surfaces was developed and applied to the mixed lubrication analysis of a
four-power polynomial cam and non-rotating flat faced follower arrangement. The results
demonstrated that under certain circumstances mixed lubrication predominated in the
conjunction of the cam and follower with the surfaces being separated by an EHL film on
the cam flanks.
Existing experimental apparatus was improved to test the effects of altering the bulk
temperature and camshaft rotational speed by measuring the friction torque and
electrical resistivity across the contact. By adopting advanced techniques for data
sampling and processing the instantaneous friction torque was successfully obtained with
the camshaft rotational speed exceeding (2000 rpm). The wear characteristics were also
examined. The bulk temperature showed a mild effect on the wear characteristics of the
cam and follower as it was increased from (75° C) to (105° C), whilst, a substantial
influence was found as the temperature was further increased to (120°C). Increasing the
bulk temperature caused an increase in both the friction torque and power loss o f the
valve train, but this increase was not considerable.
Based upon the theoretical analyses and experimental observations, a theoretical
model for evaluating the tribological performance of the valve train was developed. A
multi-aspect comparison between theoretical and experimental results was made. The
excellent agreement between theoretical and experimental results showed that the model
provided a reliable prediction o f the tribological characteristics of the cam/flat faced
follower. Three critical portions of the cycle could be identified — one over the cam nose
and two in the vicinity of the zero entraining velocity regions. The minimum separation
between the cam and follower occurred near the falling flank of the cam.