Data from periodic oil samples were gathered, environmental and operational conditions were analyzed, magnetic plug debris, and sampling principles as well as maintenance procedur es were taken into account to add depth to the RCA thinking process. The root cause was identified, a solution implemented and best practices involved were identified and the people educated to implement. That unique failure mechanism has not occurred since. Savings in identifying/preventing similar axle problems ran into millions of dollars. This opencast mine has a fleet of 12 Cat 789s, 3 Cat 777s and 2 Cat 773s moving coal from the opencast pit to the tip which totals about four kilometers and includes a seven degree incline. Because of the 24 hour production principle the availability of these vehicles is of the highest importance. Oil samples were taken at 500 hr intervals and have always been sufficient to pick up any abnormalities which would include oil condition, contaminants , abnormal wear modes and transfer of fluid from the wet brake systems. The Failure Process The first catastrophic axle failure took place over a weekend and the main query was why the sample results didn’t show much. Oil analysis trends were looked at with the main focus on ICP and PQ (ferrous content) results which barely moved off the normal trend. No real explanation could be given. Another axle failed that week and the maintenance personnel started asking questions regarding the effectiveness of the oil analysis program altogether becau se again, the oil showed very little in terms of abnormal wear taking place. A site meeting took place and the failed components were stripped down to shed more light on the failure mode. Massive damage to the planetary gears, thrust surfaces, sheared bolts and bearing damage were observed (Figure 1). Sampling technique, bearing pre-load procedures and general maintenance principles were scrutinized but nothing was identified as the cause or having played a part in the wear process. The visual indicators all pointed in the same direction – only foreign objects in the oil can cause wear on gears as catastrophic as on these final drives. Handfuls of metal flakes, chunks and gear pieces were lying in the bottom of the failed final drive and concentrated against the lip where the final drive and differential meet (Figure 2). Thinking Process How many of these failures have occurred recently? At what time in the vehicles life did it occur? What common factor exists on the failed vehicles? These are some of the questions asked to try and identify the root cause. The only fact relevant to both vehicles was that they were in the workshop for bearing pre-loads very recently. Why Didn’t We See it in the Oil? Because of the catastrophic nature of the failure it seemed as if the natural wear generation path did not exist. The failure particles went from normal benign rubbing wear to chunks and platelets. These particles are big and heavy and tend to settle to the bottom very quickly. As samples were taken every 500 hours, the oil wasn’t drained and magnetic plugs were not checked. The oil being common throughout also had a dilution effect on the particles from the failed component being the right final drive. Procedures were put in place to make sure samples were taken correctly as soon as the vehicle reached the workshop to have as little settling effect as possible. The magnetic plugs were also removed to analyze the debris that accumulated over the period which meant draining the oil every 500 hours. The plug debris was sent to the lab where RPD ferrography was done to try to identify the wear mode. The results proved that the right final drive was the main failing component causing an increase in metal throughout the axle. Ferrography on the oil showed a decrease of large metal particles on the right final drive verifying existence of large metal particles settling in a very short space of time (Figures 3,4,5 and 6). INTRODUCTION One of our mining customers was up in arms because they were having catastrophic axle failures on CAT 789 dump trucks and the oil analysis results did not give them enough warning to prevent the axle from destroying itself. Three trucks were stranded within a three week period with a repair bill of $200,000 per failure. This case study proved that root cause analysis has to be done to prevent similar failures from occurring again. Identifying Root Cause Failures with Oil Analysis: A Mining Case Study BY JAN BACKER, WEARCHECK SOUTH AFRICA Figure 1. Visible catastrophic wear. Figure 2. Trapped debris. 73 LUBRICATION EXCELLENCE 2005 CONFERENCE PROCEEDINGS