This research is purpose to knowing the reason of the fracture that happen in drive pinion gear of the swing motor unit Excavator Caterpillar 349 d. In this research is doing  by several stage it is visual observation and macro observation, testing of metal structure composition, and load and latch modeling usingstudent version of autodesk inventor 2014 before getting the final result. The results obtained from this study are that there is a lack of precision between drive pinion gear and planetary gear due to planetary pins that cannot function as planetary gear holders so that planetary rotations are prone to hilarious triggering collisions resulting in defects. In the process of stress analysis using the student version of autodesk inventor 2014 software, the results of the analysis of the value of von misses is 9.798 Mpa are much smaller than the limit of material elasticity of 790 MPa so that the loading factor does not cause a fault