How to Limit the Primary Cause of Bearing Failure – Part 2

In part one of this two-part blog, we discussed conventional lubrication strategies for bearings using grease or oil. While these solutions work for a significant majority of applications, they do come with some limitations. Today, we will look at solid film lubricants (also referred to as dry film lubricants), which excel in the most demanding bearing applications and operating environments.

Grease performs very well in typical operating conditions, but there are certain ones that expose its limitations. These include:

• Extreme Temperatures - when temperatures exceed 250°F, grease tends to break down
• Vacuum Conditions - when subject to high vacuum or low-pressure applications, oil-based lubricants release trapped gases or vapors (outgas), potentially leaving the bearing surfaces unlubricated
• Caustic/Corrosive Atmospheres - these can degrade the chemistry of the grease, while volatile contaminants can compromise its structure

Traditional grease is just not practical in these extreme scenarios. Fortunately, solid or dry film lubricants can get the job done! These fundamentally work differently than grease. Materials such as molybdenum disulfide or graphite have low physical resistance properties. Rather than flow, these allow them to shear against each other. They also have unique metallurgical characteristics to them. When they are in between the ball and raceway, they will perform their critical tasks: preventing metal to metal contact, reducing friction, and supporting the load just as oil would. The most important characteristic is that solid film lubricants bond in place, thus creating a very durable low maintenance, if not a virtually maintenance-free solution.

While there are many solid film lubricants available, the two most common are Molybdenum Disulfide and Graphite. Let us take a look at each:

Molybdenum Disulfide - has a layered crystalline structure, giving it exceptional load-carrying capacity. This makes it ideal for both high-temperature and high-pressure applications.

Graphite - also has a layered crystalline structure, with hexagonal sheets of carbon atoms. These are closely bonded to each other, but the forces between layers are weak. This allows them to slide over each other with minimal resistance. Interestingly, graphite requires water vapor to work optimally. Water molecules get absorbed onto the graphite’s surface, and further weaken the bonds, resulting in greater lubricity.

There are two negative factors when it comes to using a solid film lubricant. Lifecycle and cost. They typically will offer a shorter lifecycle than grease and will have speed and load limitations that vary depending on the application. That is why high speed applications, such as machine tools, still rely on oil mist systems over solid films.

Molybdenum and other solid film lubricants are significantly more expensive. In certain cases, specialty greases can be considered. These have additives to withstand high-temperature or corrosive environments. Some even approach similar performance levels to solid film lubricants.

Ultimately, the decision between traditional grease vs solid or dry film lubrication comes down to understanding how your bearing will operate. Since lubrication problems are the leading cause of bearing failure, you will want to get this right. For most general duty applications, grease will suffice. However, for the most demanding requirements such as extreme temperatures, corrosive environments, or vacuum conditions, solid film lubricants will deliver in ways that grease simply cannot match.