Many maintenance and reliability professionals focus on particle count as a metric for understanding how their machines are affected by solid contaminants. It is true that quite a few problems can be identified quickly and easily by monitoring the number and size distribution of particles in oil samples. Still, there are more aspects to consider when it comes to these destructive passengers in the oil that lubricates our machines.
To fully understand the impact that solid particles can have on machines, we should consider the properties of these particles that can lead to problems like oil oxidation, increased machine wear, sludge, and other issues that tend to increase maintenance costs and decrease machine reliability.
How particle shape affects machine wear
Solid particle contaminants come in many shapes and sizes, and they can enter lubricants in a variety of ways. Even “new” lubricants that are fresh from the supplier may arrive in the loading dock with high levels of contamination. Leaks, environmental conditions, and oil changes are just a few other ways that contaminants are introduced into lubricants over time.
Each particle contributes to machine wear in its own way, but together they act almost like liquid sandpaper, circulating through machines and components over and over and causing more damage with each pass. As they circulate, larger particles tend to break into multiple, smaller particles. This process increases the number of particles overall, but these smaller particles are also more highly angular.
Highly angular particles are those which have more acutely-angled facets as opposed to a rounded surface. These angular particles are more likely to cause three-body abrasion, a common type of wear mode that involves more than just the two machine surfaces in relative motion.
Three-body abrasion by a particle between two machine surfaces.
Different wear patterns can occur depending on particle hardness
Three-body abrasion occurs when a hard particle is moving between two surfaces. This particle is the “third body” in the interaction. Particles that are the same size as the lubricating film will scrape through the small clearance between lubricated components. As this happens, the particle can cause damage to both surfaces. The damage to each surface may be different depending on its relative hardness.
More spherical-shaped particles act much like ball bearings, causing surface indentations as they roll. These particles are much less likely to cut or abrade machine surfaces in the way that more angular particles do, but the damage they cause is still a concern. They may also be a symptom of greater problems.
What spherical particles can tell us
Although a number of possible origins for the source of spherical particles have been advanced by researchers, there is a general consensus that the size and quantity of spherical wear debris can reveal the severity of rolling-contact fatigue wear. It is estimated that millions of spherical particles form during rolling element bearing failures.
Spherical wear particles from a ball bearing
Because large spherical particles are often the product of high metal-to-metal contact and high frictional temperature, their presence is considered a supporting symptom for assessing the wear severity levels.
For example, sliding wear with large spherical particles is considered more advanced than similar sliding wear without spherical particles because spherical particles indicate higher temperatures are being reached.
Even spherical particles are likely to break into smaller, and often more destructive, pieces over time.
Small particles are a big problem
When these particles become small enough, down to nano scale, they become nearly impossible to remove via conventional filters.
Such particles are sometimes referred to as “ghost riders” because they tend to ride along in lubricants undetected, causing ongoing wear. Without an oil regeneration system like DST, an oil change may be the best solution for dealing with these particles.
Relative hardness of particles
In addition to angularity, a particle’s hardness is a major factor in its destructive capacity. It may come as a surprise to some that dirt or dust particles are harder than most machine surfaces. This is because common dirt particles consist largely of silica and alumina, which are harder than many types of steel.
How hard a particle is relative to the surface it contacts contributes significantly to its ability to cause wear and fatigue.
This chart details the relative hardness of contaminant particles and their appearance (Source: Machinery Lubrication)
Contaminants that lead to varnish and sludge
Despite the wear that environmental dust and dirt cause, they are generally chemically inert. However, the wear particles generated by dust in lubricants are far from inert. Iron, copper, or tin particles, although less hard and abrasive, promote oil oxidation.
Oil oxidation causes many issues of its own. It contributes to the formation of corrosive acids, varnish and sludge that can greatly impact the performance and reliability of machines.
Conclusion
As lubrication knowledge and technology continue to evolve, more facilities are taking a proactive approach to addressing particle contamination. Instead of reacting to failure or attempting to prevent it by detecting the onset of failure, the proactive approach seeks to address failure’s root causes.
Though it can take on many forms, particle contamination is a root causes of many failure modes. But by setting cleanliness targets properly and matching them with the right actions, it is possible to extend the life of machines significantly. At the same time, using highly clean oil can help reduce long-term maintenance costs and improve overall machine reliability.
While it may seem intimidating to shift to a proactive maintenance strategy, solutions like Double-Separation Technology (DST) can aid in this transition. Used as part of our oil as a service offering, DST can provide a more holistic way to address particle contamination in your lubrication program, unlocking the reliability and machine life benefits that come with highly clean lubricants.