Finite element and durability modelling of roller shells and shafts : final report JCU6S
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The technology that has been adopted by the Australian raw sugar industry for roller shells and shafts has evolved over the past 40 years. Apart from Crawford's theoretical calculations on shell design and Cullen's factory experiments on operational stress states in roller shafts, little has been done to address the current problems associated with up to 10 mill roller failures each year. Ultrasonic testing of shafts has been used as a means of identifying problem shafts for many years. However, despite this, cracked rollers still prevail. The problem is exacerbated when one considers the operational costs associated with the removal of problem rollers during continuous crushing or an unplanned shutdown due to a roller failure.This project has utilised finite element analysis to investigate the complex stress state that occurs in mill rollers during crushing. Predictions coupled with factory observations indicate key problem areas that must be addressed to reduce the occurrence of fatigue related damage to mill rollers. The report addresses these issues specifically through:1. Detailed analysis of the complex stress state in an existing mill roller. Interference fits, nominal roll and torsional loads were applied and resulting stresses were investigated. The effect of roll lift and associated misalignment loads on roller stress was also analysed;2. Analysis of ten alternative roller designs was undertaken. Variations in geometry, shell material and attachment methods were investigated. The resulting stress states in the critical fillet and shell-end regions highlight specific problems with existing designs. An alternative design is presented which should have a substantially improved durability; and3. Durabilty modelling of the existing roller design. A simple model to evaluate the structural integrity of a cracked mill roller is presented and then verified using finite element methods. The model is user-friendly and can be used by factory engineers to assess the likelihood of catastrophic failure of a cracked roller shaft under known loading conditions. A case study predicting the critical crack depth and crack propagation rate for a candidate shaft material is presented.