INCREASING THE RELIABILITY OF TOP ROLL SHAFTS

摘要

Several top roll shafts failed in service in different sugar mills. After analysis, it was determined that the failures happened due to a fatigue process, where the cracks started at surface-located microdefects, caused by abrasive wear, machining marks or defects in the repair welds when applied. A research project to extend the life of top roll shafts with excessive wear and developing cracks was structured with the Mechanical Engineering School of Universidad del Valle (Cali - Colombia), CENICANA and Manuelita Mill. Failure locations and frequencies were documented, together with materials, torques and crushing loads. A shaft journal on the drive side was chosen for detailed analysis because of the higher frequency of failures found there. Two options were considered to obtain a shaft free of cracks: the reduction of the journal diameter and the recovery of the original dimensions using arc welding procedures and consumables. A fracture mechanics approach was used to predict the maximum defect size tolerated in a repaired shaft and the time between non-destructive testing inspections. It was found that the reliability of a welded shaft can be impaired if a stress relieving process is not used, and service life can be reduced by an estimated 63% because of residual stresses. A minimum diameter for a reduced size shaft was calculated; a 5% reduction reduces the required inspection intervals by 38%.