A Study on Optimum Structural Design of Big End and Serration of Connecting Rod

The big end of a connecting rod is separated into two parts. On its separated surfaces, a serration is usually employed. Fretting failures sometimes occur on the serration surfaces. The reliability of the connecting rod depends upon the behavior of the serrations. So we carried out the following studies concerning the serrations; F.E.M analysis, photoelasticities, and the loading test. Through the investigations, the following were clarified. (1) The separation angle of the connecting rod is hoped to be larger than 35° to reduce the stresses of the serrations. (2) The most suitable shape of the connecting rod is introduced here by the optimum structural design of F.E.M. (3) The suitable shape of the serration is the one with the teeth of 90° in angle and with relief on the tops of the teeth. (4) The small size serration is good for the uniform contact, but is not so good for the initial loosening of the tightening bolts, although the large size serration has the contrary effects. (5) For giving the uniform contact pressure between the serrations, four bolts tightening of the connecting rod is better than two bolts tightening. (6) Getting the contact pressure between the serrations near to zero, there come out the possibility to give rise the fretting failure. (7) When the serrations separate each other, the fretting failure occurs between the separate teeth and the contact teeth.