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.