The silent killer of mill gears - dealing with contamination

While improper lubrication could cause a sudden failure overnight, contamination is a far more discrete condition that progressively leads to mill gear failure without any obvious symptoms. Indeed, contamination is to gear failures what high blood pressure is to heart attacks - a silent killer. And that silence could be deadly for your grining mill's gears. But just like there are ways to detect and control high blood pressure, there are also many ways to minimize gear contamination to benefit from the asset’s expected 25-year lifecycle when designed to the most stringent industry standards such as AGMA 6014 – Gear Power Rating for Cylindrical Shell and Trunnion Supported Equipment.

Types of contaminants

While anyone understands the definition of contamination, its occurrence is most often overlooked due to its sneaky nature. Despite the popular expression to “throw sand in the gears”, little attention is spent on monitoring and controlling the many different types of contaminants that could be categorized as solid or liquid.

Solids

Solid contaminants, also referred to foreign materials, are finer particles that are either suspended matter such as dust infiltration or coarser particles such as dirt and debris that ingress from the surrounding harsh environment. Due to their relatively small size, these contaminants easily find their way through the mud and gear guards that do not always provide proper sealing in certain environments. Hence, the particles will tend to stick to the gear and pinion’s tacky lubricant and get caught through the gear mesh during operation, accelerating wear and reducing lifespan.

Dust infiltrations are very common in dry grinding applications such as cement plants where constant dust clouds are present within the plant. In wet grinding applications, dirt and debris accumulation occur from material transfer such as ore falling from the feed chute or from other equipment adjacent to the mill.

Liquids

Liquid contaminants, often referred to as slurry infiltration, are any unwanted fluid from the operating environment such as spills from the mill operation, leaks from pipes or hosing for maintenance. Liquids easily find their way through the gear guard, which could either wash out the gear’s lubricant or get trapped in the mesh, both resulting in severe wear and scuffing, sometimes within a short period.

Slurry infiltration is found in wet grinding applications where leaks often occur from the mill itself such as from broken liner bolts or splashing from the feed or discharge end, depending on which end the gear is installed.

How to avoid contamination

In an ideal world, one could fix problems directly at the source. However, efforts to minimize contamination, while very effective in some cases, will never completely eliminate its presence due to the harsh conditions found within a mineral processing plant. Indeed, the use of dust removal systems in dry processes or splash guards in wet ones need to be combined with adequate equipment protection and maintenance practices to establish a preventive and corrective approach in the battle against contamination. 

Preventive approach with mud guard sealing

Mud guard sealing can be an effective way to prevent contamination. Open gears are protected with both a gear guard that is wrapped around the gear and secured to the mill foundation, and a mud guard that is bolted to the gear rim face. Together, they form a chicane that provide a last line of defense against contaminants entering the gear guard. Most mill operators might only have seen one mud guard type installed on their gears and will not question its design or request improvements. As a gear OEM, several designs have been produced and manufactured, each having their own pros and cons.

In wet grinding applications, the most common are the L and T guard designs. Observations at customer sites demonstrate that the T guard is best at preventing leakage by keeping lubricant inside the gear guard. Inversely, it has been observed that L guards are better at keeping contamination out. By combining the two designs and with the addition of a deflector on the gear guard upper segments, better protection is achieved by creating a very elaborate serpentine obstacle to prevent contamination from foreign particles and slurry infiltration.

In dry grinding applications, dusty environments require additional protection for the gear and pinion through the use of contact seals. Using either felt or rubber contact seals will maintain contamination outside the gear guard. This sealing proves most effective when combined with a pressurized system to provide positive pressure inside the gear guard by continuously operating a blower and filtering system to push air into the guard and avoid having contaminants enter the seal.

It is important to mention that combinations of labyrinth and contact seals are used in wet grinding applications to provide superior sealing properties against contamination. In addition, mud guards could also be customized to suit the customer's needs.

Corrective approach – Gear wash

Despite minimizing all sources of contamination and selecting an appropriate sealing arrangement, unexpected events can still cause foreign materials to enter the gear guard and contaminate the gear lubricant that could trigger failure mechanisms immediately after the event. Therefore, it is highly recommended to perform a gear wash to thoroughly clean the gear as soon as possible to remove all contaminants stuck in the lubricant that could cause progressive damage to the gear and pinion.

While additional downtime from an emergency stop is far from an appealing option to any mill operator, progress in lubrication technology in the past decades has seen innovative gear cleaning products hit the market that can clean the gear during operation. Indeed, oil degreasers with Extreme Pressure (EP) additives can be used where the additives will ensure gear protection while washing out the lubricant through manual sprayers. Within hours, the contaminated lubricant can be removed during operation with only an hour or two of downtime required to manually relubricate the gear.  

Bottom line

Although risks of contamination can be minimized, they can never be eliminated entirely. However, progressive wear and reduced asset life can be prevented with proper monitoring and controlling of foreign particles originating from the mill and its surroundings. Metso Outotec is a provider of expert solutions to ensure the reliability, integrity and longevity of a grinding mill’s most critical components. Through close monitoring with the use of advanced instrumentation, periodic mill inspections and preventive maintenance practices, our services offering can bring significant safety and sustainability benefits to your entire grinding operation.