Is Liquid Sandpaper Lurking in Your Machine?

Jim Fitch, Noria Corporation

Is Liquid Sandpaper Lurking in Your Machine?

Certain subjects merit emphasis and should be frequently revisited. Contamination control is one such subject. Particles are like a virus, invisible to the naked eye but packing a powerful punch. The most destructive particles are typically less than ten microns in size (for comparison, a thousandth of an inch is roughly 25 microns).

The microscopic size of these destructive particles makes them hard to measure and count. Over time, they keep accumulating in the oil. Even in relatively high concentrations, their presence can remain hidden from view and undetectable to the touch. This explains why they are often referred to as “ghost riders”. 

Figure 1. Microscopic view of descriptive silt-size
particles that can contribute to the liquid sandpaper effect.
Courtesy of R&T.

Think of ultrafine sandpaper at 1000 grit. The size of abrasive particles that are bound to the paper range from 6.8 - 9.3 microns in diameter. Lubricants with high populations of suspended particles of that size range effectively form a medium with the potential to exact considerable harm to the machine.

Compared to larger particles, small silt-size particles easily evade even the finest filters, and larger particles are often quickly crushed into many smaller particles (comminution). As shafts rotate or slide against opposing surfaces (bearings, seals, gears, pistons, cams), microscopic excavations occur (dents, gouges, scratch marks), producing even more particles. Over time, the concentration of these small particles forms a medium that is functionally equivalent to liquid sandpaper.

Think of a belt sander. Even fitted with fine 1000-grit paper, it can polish away vast amounts of metal in short order. Now imagine a heavily loaded shaft rotating at high speed against a journal bearing lubricated with contaminated oil. It’s the same thing, and the shaft and bearing get polished. This is called three-body abrasion.

The Causes of Liquid Sandpaper

The smaller the particles in your oil, the more evasive they become. They are difficult to extract by filtration and centrifuges, and they are very slow to settle in tanks and reservoirs, if at all. Small particles easily remain suspended by turbulent oil movement.

We are all aware that particles create more particles. The number of new particles generated from a single ingressed particle depends on many factors, including the:

  • Type of machine
  • Filtration
  • Settling
  • Number of frictional zones
  • Working clearances
  • Operating speeds

Figure 2. Debris field on a membrane showing the collection of extremely small particles after they were extracted from the oil. Both dust and wear particles are present.

Basically, it relates to how many surface scratches and indentations a particle is allowed to make before being pulverized, settled to the tank floor, or removed by a filter or oil change. If ingressed particles reach the filters quickly, there is less damage, and fewer new wear particles are produced. Conversely, if no filtration or poor filtration is the case, this leads to longer particle residence time in the fluid and, thus, greater damage and production of wear debris.

An average ingressed dirt particle (left in the oil) will generate somewhere between five and 20 new particles (secondary particles). Some of these particles will make more particles (tertiary particles). The situation is self-propagating. Additionally, a single scratch mark from a grain of dirt can produce a corkscrew wear particle long enough to crush into five or more particle segments.

Figure 3. Corkscrew particle formed by abrasive
cutting action (three-body abrasion).
Courtesy of R&T.

Why Small Particles Matter

The most destructive particle in our lubricants can be characterized by the following features and properties:

  • Is roughly equivalent in diameter to be carried into the working clearances of frictional zones (sliding or rolling) of our machines – usually less than ten microns.
  • Is harder than our machine’s surfaces. Particles of sand and ambient dust will scratch a hacksaw blade. Large particles are more friable (crushable) than small particles.
  • Will easily remain suspended due to its small size, turbulent fluid movement, and lack of suitable filtration. 
  • Has sharp angular edges from comminution.
  • Is small enough to bypass seals and enter the headspace of sumps and reservoirs.
  • Is small enough to get sucked into drums and totes of oil by thermal siphoning. 
  • Is small enough to accumulate large amounts of interfacial surface area in contact with the oil and its additives.

All these collectively contribute to liquid sandpaper.

Solving the Liquid Sandpaper Problem

Solving the liquid sandpaper problem starts with preventing contamination from getting into the oil in the first place. Exclusion is far cheaper than removal, by a factor of at least ten.

Exclusion means addressing the problem at the source. Take inventory of contaminant ingression sources. For many machines, the inhaling of airborne contaminants into the headspace of drums, totes, and reservoirs is the primary source. Forced convection of air by thermal siphoning, machine-driven air currents (e.g., movement of gears, plunging oil return-line flow), and cyclical changes in the tank oil level (hydraulic cylinder movement) can escalate the ingress.

Figure 4. Oil drums breathe in dirt and moisture by thermal siphoning due to routine changes in ambient air temperature. When ambient air cools, air, moisture, and particles can be siphoned into the drum's headspace. Over time, contamination keeps building up in the oil. This device, sold by Luneta, enables quick inspection of this siphoning action so that swift corrective measures can be taken.

Air typically enters through:

  • Vents and breathers
  • Shaft seals
  • Unsealed hatches and cleanout covers
  • Other unprotected machine openings.

Dirty transfer containers, hoses, and funnels are also common sources of particle contamination. During storage, dirt and moisture-laden ambient air passes through bungs on drums and totes. In fact, new oil is a major source of liquid sandpaper contamination.

The cost of contaminant exclusion relates to both retrofitted hardware and routine maintenance tactics for blocking contaminant entry. These costs include such things as:

  • Transfer cart filtration
  • Proper breathers on machines and lubricant storage vessels
  • Improved seals (such as labyrinths)
  • Tighter system closures
  • Greater awareness and care during internal inspections and part replacements (education and better procedures/tools)
  • Routine cleaning of machine exteriors

Figure 5. A high percentage of hydraulic system
particle ingression comes into play with the
cylinder wiper seal. When looking at this,
it is easy to imagine the liquid sandpaper
in your oil. Image courtesy of M. Williamson.

Next, quickly remove particles that do touch the oil, so they don’t multiply by crushing and wear debris generation. Fine filtration, especially offline filters, can help. They cost considerably less than full-flow filters per gram of dirt removed. 

You can also use a separation booster to agglomerate particles so they can be more easily separated by gravity, centrifugation, and/or mechanical filtration. These fluid conditioning products can effectively polish your oil even at a submicron level before harm is done to the machine and additives.

As a last resort, if your lubricant is packed with liquid sandpaper and the cost of cleaning the oil is impractical, simply change the oil.

Should You Just Let Machines Fail?

Consider this – you come up with a smart plan to get liquid sandpaper under control, including both exclusion and removal. You’ve carefully studied and engineered the optimum solution. There are costs involved, but as usual, it’s a matter of “pay some now or a lot more later.” However, management and the bean counters are skeptical and have asked for a financial justification study. This is not your skillset, so you dismiss the idea. Things go back to business as usual.

Ask yourself, when was a financial study ever requested to obtain funds to repair a failed machine, especially when plant production was stalled? Sadly, I’ve heard maintenance folks say they’ve quit trying to propose proactive measures to management – they claim it’s easier to just let the machines fail.

This is like saying it’s easier to wait until you have a heart attack rather than proactively make the lifestyle changes needed to avoid heart disease. These differences are often deeply ingrained in management and business culture. Does your organization have the “here and now” folks or those who plan and prepare?

On the bright side, an increasing number of companies are led by managers who do get it. Much of this has been driven by the growing base of documented success stories from organizations and program leaders who have championed change and happily reported their results. They didn’t need to be beaten over the head, but rather, they took the initiative and captured the benefits.

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About the Author

Jim Fitch, a founder and CEO of Noria Corporation, has a wealth of experience in lubrication, oil analysis, and machinery failure investigations. He has advise...