Broken Rim of a Rolling Mill Transmission

The rim of a gear wheel of 420 mm width and 3100 mm in diam broke after four years of operation time in a sheet bar three-high rolling mill. The rim was forged from steel with about 0.4C, 0.8Si and 1.1Mn. The rim started to break in the tooth bottom from a fatigue fracture which extended from the gear side to more than half the rim width. A second incipient failure commenced from the opposite tooth bottom. Both fractures joined below the tooth of the rim. Both incipient cracks were fatigue fractures with several starting points, all located in the transition between tooth flank and tooth bottom. The remaining failure was a fine-grained ductile fracture. It was found that the teeth were not supported uniformly over the entire width and were thus overloaded on one side. The transition from the tooth flanks to the tooth bottom was sharp-edged, causing a tension peak there. The tooth bottom was machined only roughly. Also, the yield point was a little bit too low.