Vibration Analysis of Connecting Rod Big End Bearings

Abstract

The aim of the work described in this paper was to compare 'finite bearing' and 'short bearing' models of connecting rod big end bearings with measured practical performance. The effects of bearing shell distortion were also to be investigated, and an assessment made of a novel approach to collecting experimental data from a running internal combustion engine. Theoretical results were obtained for models based both on a 'finite bearing' solution and on the 'short bearing' approximation. Results were calculated for both a circular bearing profile and also for a distorted bearing profile, such as might be caused by the interference 'nip' between the bearing shell and the connecting rod. Experimental data was collected from a single cylinder diesel engine. The results of the work show that a 'full' bearing analysis predicts smaller values of minimum oil film thickness than those obtained when the short bearing approximation is used, and that bearing shell distortion leads to smaller values of minimum oil film thickness. Values of minimum oil film thickness inferred from practical measurements were generally found to be larger than those predicted by theory however, probably because of further bearing shell deflection caused by the oil film forces. The experimental procedure investigated proved useful for measuring changes in oil film thickness, but less useful for measuring absolute oil film thickness.