The loosening mechanism of tin-bronze bushing assembled by interference at connecting rod small end of heavy-duty diesel engines

The commonly used spinning tin-bronze bushings assembled by interference sometimes fail causing the bushings to rotate or even come out from connecting rod small end. Previous studies have shown this failure to be related to the abnormal temperature of small end. Test samples with same material and process as the connecting rod small end were designed. The residual stress on inner surface and pressing force of bushings were tested before and after local heating. The results showed that the residual stresses on inner surface and maximum pressing force decrease with the increase of maximum temperature and thermal cycles when the temperature of bushing bottom reaches 160 degrees C. A quarter model of connecting rod was applied to reveal the loosening mechanism. It is found that plastic deformation occurs around oil holes, resulting in variations in the stress field of bushing. It lessens the bonding force between bushing and connecting rod small end. Meanwhile, the residual stresses on inner surface decrease and then surface hardness of this area reduces, which makes it easier to adhere with piston pin and generate large friction. These results are crucial for the material and structure design of connecting rod small end bushings.