Bearings in hot applications run longer with dry lube

The following is an excerpt from an article written by SKF. The company featured in its Evolution business and technology magazine.

 

Many bearings operate in high temperature environments – but few lubricants can withstand the heat. SKF has developed bearings that incorporate dry lubrication for extreme temperature applications, providing benefits for customers, among others in the metal processing, building materials and food and beverage industries.

Dry lubrication is particularly suitable for applications that run at slow speeds at extremely high temperatures.

In such applications, where bearings are typically subjected to very high temperatures (in the range of 200 to 350°C), normal lubricants such as grease and oil deteriorate rapidly, leading to poorly lubricated bearings that fail prematurely. Dry lubrication does not deteriorate at these high temperatures, can work under heavy loads, has high thermal stability and can offer a virtually maintenance-free application. This is why dry lubricated bearings have considerable potential, particularly within the metals industry. In cooling beds for sheet metal, a typical installation may contain around 5,000 bearings. These bearings often run intermittently at very high temperatures, and due to the nature of the equipment they are very difficult to relubricate.

SKF has long experience in manufacturing dry lubricated deep groove ball bearings and Y-bearings with a graphite cage (VA208 and VA228 variants). These ball bearings are often used in kiln trucks, cooling beds and continuous ovens. With the introduction of SKF DryLube bearings, a broad range of dry lubricated bearings, the benefits of dry lubrication can now be applied to virtually any SKF bearing – including roller bearings and plain bearings.

Dry lubricated bearings

SKF DryLube bearings incorporate graphite and molybdenum disulphide as the dry lubricants. In these materials, the lubricating properties are the consequence of a lamella layer structure that is created under load, when the lubricant adheres to the contact surfaces in the bearing. The layered structure enables sliding movement of the parallel plates. Weak bonding between the plates provides low shear strength in the direction of the sliding movement but high compression strength in the direction perpendicular to the sliding movement. Also, a solid lubricant having high compression strength is capable of withstanding heavy loads without creating metal-to-metal contact. Finally, the dry lubricant has good adhesion to the substrate surface. This property provides a presence of solid lubricant on the bearing surfaces even at high shear stresses.

For graphite, the lubricating properties are further enhanced when vapor is present in the graphite layers, as this reduces shear forces and friction. Graphite can function as a lubricant at temperatures well above 500°C. To further enhance the performance or increase service life of the SKF DryLube bearings, there are variants where the dry lubrication is combined with additional lubricating high-temperature oil additives and lubricating nanoparticles.

To create an SKF DryLube bearing, the dry lubricant and a resin binder are injected into the free space in the bearing around the rolling elements and raceways. A curing process ensures that the lubricant solidifies. The process makes it possible to turn virtually any SKF bearing into an SKF DryLube bearing, as long as the bearing is equipped with a metal cage and has sufficient free space for the dry lubricant. This makes the SKF range extremely dynamic and able to meet virtually any high-temperature application requirement.

In operation, the dry lubricant maintains a very thin film on the raceways and rolling elements that helps avoid metal-to-metal contact.

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