8/3/2019 Identifying Root Cause Failures With Oil Analysis, A Mining Case Study

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Data from periodic oil samples were gathered, environmental and

operational conditions were analyzed, magnetic plug debris, and samplingprinciples as well as maintenance procedures were taken into account toadd depth to the RCA thinking process. The root cause was identified, asolution implemented and best practices involved were identified and thepeople educated to implement. That unique failure mechanism has notoccurred since. Savings in identifying/preventing similar axle problems raninto millions of dollars.

This opencast mine has a fleet of 12 Cat 789s, 3 Cat 777s and 2 Cat773s moving coal from the opencast pit to the tip which totals about fourkilometers and includes a seven degree incline. Because of the 24 hourproduction principle the availability of these vehicles is of the highestimportance.

Oil samples were taken at 500 hr intervals and have always beensufficient to pick up any abnormalities which would include oil condition,contaminants, abnormal wear modes and transfer of fluid from the wetbrake systems.

The Failure ProcessThe first catastrophic axle failure took place over a weekend and the

main query was why the sample results didn’t show much. Oil analysistrends were looked at with the main focus on ICP and PQ (ferrous content)results which barely moved off the normal trend. No real explanation couldbe given. Another axle failed that week and the maintenance personnelstarted asking questions regarding the effectiveness of the oil analysisprogram altogether because again, the oil showed very little in terms ofabnormal wear taking place.

A site meeting took place and the failed components were stripped downto shed more light on the failure mode. Massive damage to the planetarygears, thrust surfaces, sheared bolts and bearing damage were observed(Figure 1).

Sampling technique, bearing pre-load procedures and generalmaintenance principles were scrutinized but nothing was identified as thecause 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 thebottom of the failed final drive and concentrated against the lip where thefinal drive and differential meet (Figure 2).

Thinking ProcessHow 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 workshopfor 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 fromnormal benign rubbing wear to chunks and platelets. These particles are bigand heavy and tend to settle to the bottom very quickly. As samples weretaken every 500 hours, the oil wasn’t drained and magnetic plugs were notchecked. The oil being common throughout also had a dilution effect on theparticles from the failed component being the right final drive.

Procedures were put in place to make sure samples were taken correctlyas soon as the vehicle reached the workshop to have as little settling effectas possible. The magnetic plugs were also removed to analyze the debristhat accumulated over the period which meant draining the oil every 500hours. 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 drivewas the main failing component causing an increase in metal throughoutthe axle.

Ferrography on the oil showed a decrease of large metal particles on theright final drive verifying existence of large metal particles settling in a veryshort space of time (Figures 3,4,5 and 6).

INTRODUCTIONOne 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 $200,000per 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 StudyBY JAN BACKER, WEARCHECK SOUTH AFRICA

Figure 1. Visible catastrophic wear.

Figure 2. Trapped debris.

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8/3/2019 Identifying Root Cause Failures With Oil Analysis, A Mining Case Study

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Although we were now able to identify the component that failed or wasbusy failing, the root cause had not been identified. We had to change ourthinking and go back to basics.

Setting bearing preloads and assembly and disassembly of componentsinvolves procedures and tools. The procedures are cast in stone but thetools! – It hit me like a 3 ton truck – the torque wrench! We booked out thewrench from the mechanical store and had a calibration test done by acertified company. The result, a torque wrench under calibrated by nearly100%. What does this mean? The retainer is supposed to be torqued to e.g.,

100Nm but the actual figure is 200Nm. This puts massive forces on thebolts and because of the load, incline from the pit and the direction oftravel caused the retainer bolt heads to pop off ending up in the gears(Figure 7).

A new torque wrench was bought and a calibration station installed toverify accuracy before and after use. This failure mode has not occurredsince at this mine.

Summary

Always remember that an oil analysis report is only relevant to oil takenfrom the sample itself. It is up to the customer to make sure the sample isrepresentitive of the component. If that is not possible then filter analysis,magnetic plug analysis and BS&W samples need to be incorporated to seethe full picture.

Figure 3. Left final drive (December).

Figure 4. Left final drive (February).

Figure 5. Right final drive (December).

Figure 6. Right final drive (February).

Figure 7. Sheared retainer bolts.

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