After repeated failures of its crushers caused production losses, a large copper mine’s maintenance reliability team knew something had to change. In this case, it was the gear oil protecting its bushings.

With this one change, they increased equipment availability to production by 2.2%. They also reduced lubricant usage by 83%, resulting in cost savings and a reduction in safety risks and carbon footprint. Total annual savings exceeded $2.8 million.

The mine, in South America, was having to do full rebuilds up to five times a year on the bushings of each of its two primary crushers, resulting in excessive production losses. The mine processes more than 280,000 t/d with these two crushers. This excessive unplanned downtime was largely caused by tramp metal ingested by the crushers.

When either of these two crushers ingests a shovel tooth or other metal object greater than 9 in (229 mm), it typically causes irreparable bushing damage, which necessitates full rebuilds.

The crusher had been lubricated with a “premium” gear oil, which was filtered with a 20 µm filtration system. ISO particle counts typically ran at 24/22/18. The introduction of tramp metal initiated extreme pressures and temperatures that would last as long as needed for the metal to clear itself. Once, the crusher came to a complete stop due to the sheer size and mass of the ingested metal. Each of these tramp metal incidents started a cascade of events that literally toasted the bushing and its base plate.

When looking at these failures, it is helpful to focus on the particles generated from the tramp metal ingestions. Other sources of contamination were also present, but they were insignificant in comparison to the tramp metal.

Lubricant failure leads to primary & secondary damage

The oil’s foremost responsibility is to keep lubricated metal surfaces from touching each other. When the oil fails at this task, contamination particles are generated and certain types of wear appear on the metal surfaces. This wear can be classified as “primary damage”. Particles created from primary damage can be very abrasive depending on their source. Those formed from soft bearing material might be the least destructive. Those formed from hardened steel are especially destructive.

Particles from primary damage tend to create their own damage by lodging between metal surfaces and creating new particles. This is “secondary damage” and is a direct result of particles from primary damage creating additional particles in a cascade effect. During the life of an oil change, the cascade can go as high as 19:1 in a diesel engine. Gear trains are less, but 10:1 is not unusual.

Stopping the primary damage will have a large impact on the amount of secondary damage. In this case, the extreme pressure additive in the lubricant is imperative to the lubricant’s success. The EP additive in the gear oil being used in the crushers had failed to stop metal-to-metal contact. The resulting particle generation was immense, as was the creation of secondary damage particles that followed.

The mine’s maintenance & reliability team thought it was impossible for gear oil to do anything to stop such catastrophic failures caused by tramp metal. They examined the current physical procedures on tramp metal elimination and determined they were at a high standard, with only minor tweaking required to maximise current processes.

The only exception was the older stockpiled material they occasionally used. Although purported to be clean, its composition is largely unknown with respect to tramp metal. It had been accumulated before current tramp metal remediation practices were applied. The mine has a significant amount of this stockpiled material that requires processing. Given the sheer volume of this material, it must be processed with the two crushers. The challenge for the team was to stop the crusher failures.

The crushers are considered critical assets, for which unplanned downtime has severe production and cost consequences. Ongoing losses associated with this problem had to be resolved. It could not be considered business as usual. Given the stockpiled material could not be improved, focus was directed toward the bushing destruction and what, if any, preventative options were available.

It was determined the gear oil being used to protect the bushing could not withstand the spike in extreme pressure caused by a sudden stoppage of the mantle. It was time to look at oil again. The crushers were critical assets that required a critical lubricant. Repeated failures had demonstrated that routine lubricants were not performing to their needs.

After the team tried various suppliers’ maximum pressure gear oil products with no discernible difference, the mine’s senior reliability superintendent approached Jim Thomas of Lubrication Engineers about this issue. Thomas had supported several North and South American mine sites for over a decade with lubrication, filtration and monitoring devices and has achieved previous success with this application.

The superintendent said that the mine continued to have significant issues with the crushers’ inner bushings, where tooth ingestion created significant metal debris and high return oil temperatures (>120°F). He said it was common for them to change this bushing due to the bushing being burned.

He said this problem would likely prevent the mine from achieving their next planned maintenance interval, which was becoming the norm. It also was having a significant negative impact on its ability for continual processing downstream.

New oil puts stop to failures

As an initial measure, Thomas suggested conducting a trial with a synthetic gear oil containing a unique and proprietary extreme pressure additive – Duolec – which was developed by Lubrication Engineers and is not available from other suppliers.

The result of this trial was an immediate halt to the catastrophic failures. After the new oil was applied, the tramp metal ingestions continued, including four more from shovel teeth and a large steel plate. One of these ingestions even brought the system to a complete halt; however, the new gear oil – Duolec Syn Gear Lubricant – protected the bushing, and no rebuilds were required after clearance was achieved. The convex bearing and base plate were also unscathed.

The oil and its additive package put a stop to nearly 90% of the primary damage. Current ISO readings are 20/18/16 and relatively steady. Secondary damage remained the same because the filtration system wasn’t changed.

Critical assets require the right lubricants to minimize unplanned downtime and costs. Each bushing change costs $360,000 in lost production, parts and labour. During the past 12 months since implementing use of the advanced EP gear oil, the mine’s two crushers combined have avoided eight rebuilds.

After one year, the maintenance & reliability team has documented the following results:

• Total annual savings exceeded $2.8 million;
• Availability to production increased 2.2%;
• Payback was achieved in less than two months;
• Lubricant usage dropped from 12,000 to 2,000 gallons per year for both crushers, resulting in cost savings as well as a reduced carbon footprint; and
• Safety risks lowered due to less unplanned maintenance.

Next step: Filtration and monitoring

About that secondary damage, the expense of the lubricant can be defrayed if it is kept clean, cool and dry. Superior filtration can enable use of the same oil for up to five years. Upgrading the filtration system would further improve the mine’s results.

A new depth media filtration system – CCJensen HDU 4x27x108 – is set to be added, which will remove 99% or more of particles to 3 µm. This will greatly reduce secondary damage and effectively extend the oil life to five years.

An oil monitoring system will provide real time oil contamination levels (ISO 4406), water intrusion, oxidation, temperature and pressure. With the addition of VeriTAI vibration analysis, the program can provide up to three months of advanced notice of impending maintenance issues, allowing the maintenance & reliability team time to plan. It also answers the question will the equipment keep running until the next PM.

Lubrication is a very small part of an operating budget, typically 3-5%. However, it literally touches every rotating and moving surface. In this case, use of the LE gear oil for the critical asset added 2.2% availability to production for a relatively small increase in cost with less than two-month payback. This kind of leverage can be available for any critical asset.