Axle improvements for 40-tonne locomotives

Wilmar Sugar has a population of 24 40-tonne locomotives, each having four axles. The combined axle population for the 40-t locomotives has various designs, material properties and age in service. Superimposed on these variables are manufacturing deficiencies, assembly and maintenance deficiencies and service conditions. These axles are currently at varying risk of developing crack-like defects. Over time, the axle design and material properties have improved, but the older design axles and axles that have been involved in a derailment, or other operational high loading case, are at a higher risk of failure. Railway axles are ranked as an important part of rolling stock, because their failure will result in certain derailment. Following the failure of axle No. 3 on locomotive Hodel on 5 August 2018 at Invicta Mill, TICS were commissioned to crack-test all Wilmar Group 40-t locomotive axles during 2018. The procedures for crack testing of locomotives axles at that time had a low probability of detection for cracks. In collaboration with Rockfield and Wilmar, TICS have since developed a specific procedure for near-end low angle ultrasonic scanning, to improve the probability of detection from around 20% to around 90% for 40-t locomotive axle defects. To date, 100 axles have undergone ultrasonic testing with the improved scanning procedure developed. The findings from the testing prompted the replacement of 27 axles during the 2019 maintenance program, with a further 33 axles identified for replacement during the 2020 maintenance program. Fracture mechanics studies were carried out to determine the critical crack size in axles and an attempt was made to quantify crack size with near-end, low angle ultrasonic scanning. Due to the number of variables that influence the ultrasonic signal transmission in different axles, it was difficult to determine the crack length, but a generalisation regarding the severity of the crack was possible. The Hodel locomotive incident also triggered a re-evaluation of all processes associated with the axle life cycle, from initial procurement to final disposal. Improvements identified and developed through the 40-t locomotive axle replacement program, will significantly reduce the risk of a locomotive derailment caused by axle failure and will reduce locomotive wheel set whole of life cost. Key words Locomotive, axle, ultrasonic testing, fracture mechanics, forging