Introduction of the Equipment

In the cement manufacturing industry, the cement mill grinds the ingredients to produce the final product—cement. Cement mills are operated with the help of a powerful gearbox, which is often referred to as the most critical and essential part of the cement grinding section.

Mills operate with single-drive or double-drive gearboxes, depending on the design and manufacturer of the equipment. In our case, the cement mill is designed to operate with two gearboxes.

Due to high rates of oil contamination, we began facing an elevated risk of early gearbox failure. Through intentional efforts to discover the root cause and quick action to resolve the issue, our corrective maintenance actions saved the facility more than $1.3 million in maintenance costs and $2.8 million in production loss.

Preventive Maintenance for Main Drive Gearboxes

In the cement manufacturing industry, equipment is typically exposed to highly dusty environments. Because of these often extreme conditions, facilities must take special care of their main drive gearboxes using various techniques.

For instance, proper lubrication is vital in contamination control to prevent equipment failure and increase uptime. There are several important preventive and predictive maintenance actions cement facilities must take to properly care for gearboxes, including:

• Regularly inspecting and maintaining dust protection seals to prevent contaminants from entering the oil.
• Conducting online vibration and temperature monitoring.
• Maintaining the lubrication system through oil filters.
• Ensuring the required oil flow and pressure by utilizing flow meters and switches.
• Observing the lubrication oil condition through regular oil analysis.
• Maintaining the required temperature of lubrication oil for proper oil flow with the help of heat exchangers.
• Performing visual stopped inspections of the gearbox and oil circulation system every two weeks.
• Setting the oil sampling frequency to 90 days.
• Replacing the oil after 8,000 hours of operation or based on the contamination levels found in the oil through sampling.

Cement Mill Case Study

We were tasked with overhauling the main drive gearbox at this particular cement mill. One month after these actions were taken, we began observing an abnormally increased vibration tendency. To discover the root cause, we started by reviewing gearbox specifications and history.

Technical Gearbox Specs

The gearbox is a Combiflex Double Drive Unit DMG2 22 that operates within a cement mill with a 5,280-ton-a-day throughput. Figure 1 provides additional information about the main drive gearbox operating specifications.

Figure 1: Combiflex Double Drive Unit DMG2 22 Specifications

Typical Gearbox Inspection, Maintenance, and Overhauling Frequency

Due to the excessive dust in the operating environment, the gearbox is strictly monitored and maintained to prevent contamination. As a result, this specific gearbox must be completely overhauled after every 48,000 operating hours.

If dust penetrates the lubricating oil, it can cause wear and tear on the bearings and gear surfaces. Vibration data analysis monitors these bearing defects and gear mesh frequencies. Based on these overall vibration values, overhauls may be required more frequently.

This facility has the additional task of monitoring not one but two main drive gearboxes. When an overhaul is required, the process is typically performed on both gearboxes simultaneously.

Gearbox Maintenance and Vibration History

Both gearboxes were first started up in December of 2016. Initial vibration values were recorded and are as follows:

• Gearbox 1: 1.5 mm/sec
• Gearbox 2: 2.2 mm/sec

At the end of August 2024, when the gearboxes had only achieved 37,700 hours of operation, vibration values were trending high:

• Gearbox 1: 9.9 mm/sec
• Gearbox 2: 19.6 mm/sec

This raised an alert at the facility, and an immediate overhaul was performed on both gearboxes. Vibration values were retaken after the overhaul was completed:

• Gearbox 1: 1.8 mm/sec
• Gearbox 2: 2.1 mm/sec

Current Issue on The Gearboxes & Actions Taken

About a month after the overhaul, we once again began seeing an abnormal increase in vibration tendencies. We decided to stop mill operations to allow gearbox inspections, where we observed dust particles in the oil.

Obvious Oil Contamination Prior to Oil Change

While oil sampling was performed every 90 days, we took two additional samples over the course of a week to verify the oil condition. The lab results confirmed what we had observed—the oil was contaminated, and the contamination was increasing steadily.

An oil replacement was immediately conducted on the 5,000-liter tank. During this process, we cleaned the gearbox sumps, oil tanks, and duplex oil filter housing and replaced the oil filters. Additionally, a detailed inspection was performed on the lubrication circuit’s sealing system, which revealed that a small part of the dust seal was damaged, allowing contaminants to enter the lubrication system. This dust seal was immediately repaired.

Cement Removed from Gearbox Oil

As an improved preventive measure, the facility changed its sampling policy; now, the oil sampling frequency for all critical equipment would be every 45 days instead of 90. This has created widespread benefits, including avoiding breakdowns, decreasing unplanned maintenance costs, and reducing production losses.

Cost-Savings Breakdown

When overhauling gearboxes, they are typically broken down into two scenarios.

In the first scenario, first-time overhauling is done by only replacing the set of bearings and reversing the same gear shafts to utilize the unused face of the gears.

In the second scenario, a second overhaul is done after the first overhaul, necessitating replacing all the sets of gears and bearings because both sides of the gears have already been utilized. This comes with increased costs compared to step one.

In our case, we had already utilized both sides of the gear’s faces, and if we were to lose the gearboxes, the overhauling cost would be closer to replacing the old gearboxes with new ones, as per the second scenario. However, due to our oil analysis procedure, we saved the gearboxes from early failure while avoiding a breakdown, unplanned maintenance costs, and production losses.

Factoring in all costs, bringing down the gearboxes for rebuild would have resulted in $3.08MM in lost production and hard costs, while the loss due to proactively changing the oil was just $224K, a significant savings.

Conclusion

This case study demonstrates that a robust preventive maintenance program is essential for protecting critical assets in demanding industrial environments like cement manufacturing. The facility avoided a potentially catastrophic failure by swiftly identifying and addressing the root cause of oil contamination through vigilant monitoring and oil analysis. This resulted in substantial cost savings, exceeding $1.3 million in maintenance expenses and $2.8 million in potential production losses.

Through proactive maintenance, continuous monitoring, and oil analysis, facilities in various industries can protect critical equipment and ensure operational reliability for long-term success.