Analysis of the Causes of Recent Roll Shaft Failures in Natal Sugar Mills

Details of 93 roll shaft breakages which have occurred at eight selected sugar mills in Natal since 1979 have been collated and analysed to determine the most likely causes of failure. Theoretical analyses of shaft stresses and fatigue stress concentration factors have been carried out to determine whether present shaft design, machining practices, material specifications and shell assembly techniques are satisfactory and whether they can be improved. The feasibility of using adhesive to fix the shell to the shaft is discussed and some recommendations to users and manufacturers on roll shaft and shell specification, design, assembly and operation are given. (a) Breakages of shafts in service occur more frequently when the roll is being used as a top roll. The percentage of top roll failures is 66% of the total. (b) The most likely position for a break to occur is at the inner fillet radius on the drive side of the roll. The frequency of this occurrence was 42%. (c) The next most likely position for a break to occur is at or near the drive side end of the shell. The frequency of this occurrence was 29%. (d) The average age of a shaft which fails in service is 5,6 seasons. A diagram of the frequency of failure at different points along the shaft is given in Figure 1.