Reducing bearing failure in wood pellet mills

Wood pellet mills are tough environments for industrial equipment. Operating conditions include extreme temperatures and contaminants inherent to the pelletizing process.

As a result, bearing failures are common, causing unscheduled downtime and increased maintenance costs. For an industry where 24/7 production is critical to success, any unscheduled downtime is a serious concern.

To help keep equipment running despite these conditions, operators need to use the right grease.

The role of grease in maintaining uptime in wood pellet mills

Roller bearing temperatures in wood pellet mills often run continuously at between 250 degrees and 300 degrees Fahrenheit (or between 120 and 150 degrees Celsius), depending on ambient temperature, wood type and machine design. These temperatures put significant stress on the lubricating grease. Many greases fail, which can result in premature bearing failure.

Additionally, the viscosity thinning effects of high temperatures combine with the contaminants inherent in the pelleting process to create more challenges. Grease is used as a seal to keep out contaminants, but if the lubricant film ruptures or is bridged by contaminants, metal-to-metal contact can occur, causing bearing wear. To maintain the seal, OEMs set high feed rates. However, continually replenishing grease can be expensive.

For these reasons, wood pellet operations should be using advanced greases, such as Mobil Centaur calcium sulfonate greases, to help protect bearings. These greases can provide excellent high temperature bleed control, high inherent extreme pressure (EP) protection, and stay-in-place performance to reduce re-lubrication requirements.

High-temperature protection

Continuous high equipment temperatures can cause grease to release oil excessively. This oil escapes from the bearings, leaving them unprotected. It also leaves behind concentrated thickener in the bearings and feed lines, which can block new grease from reaching the bearings. Greases formulated to withstand these high temperatures can maintain consistency by resisting high-temperature bleeding and caking.

Figure 1: Despite operating in extreme temperatures, greases such as Mobil Centaur™XHP 462, pictured here, maintain consistency and resist hardening as well as excessive oil bleed.

Extreme pressure

To prevent bearing wear from metal-to-metal contact, operators should use greases formulated to provide exceptional EP protection. In Mobil Centaur greases, for example, this EP protection is inherent in the calcium sulfonate thickener. In the 4-Ball EP Test, Mobil Centaur greases typically handle weld loads of 400 kilograms or higher.

Reduced grease consumption

Compared to standard lithium complex lubricants, advanced greases can allow for longer re-lubrication intervals – up to 50 percent higher in some cases – which can help lower maintenance and product replacement costs.

Additional properties

Advanced greases can also provide outstanding water resistance, wet mechanical stability and corrosion protection. In the wet environments common in pellet mills, grease is used as a sealant. However, most greases soften when they absorb water, which can cause them to run out and leave bearings unprotected. Some greases designed to resist water absorption will maintain stability, but the water they leave in the bearing can lead to rust and corrosion.

Advanced greases are able to absorb water while maintaining consistency and adhesion.

Figure 2: Mobil Centaur™ grease retains consistency and remains in place on the bearings despite having absorbed water.

 

Hopefully this quick primer has conveyed the importance of using the right grease to help keep wood pellet mills operating at full speed. If you have any specific questions about grease performance in wood pellet operations not answered in this post, please reach out in the comments section below.

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