Contaminated Fluids: Guilty as Charged


CALGARY, Alberta – The biggest enemy for hydraulic fluids and hydraulic systems is contamination, contended Thelma Marougy, principal engineer for lubricants and fluids at Eaton Corp.’s Hydraulics Operations in Southfield, Mich. Contaminants can cut life in half or more, and controlling contamination is key for successful operation in hydraulics.

It’s a proven fact that 75 to 80 percent of all hydraulic machinery failures can be traced to contamination in hydraulic fluids, Marougy told the Society of Tribologists and Lubrication Engineers’ annual meeting in Calgary last week. In a special forum dedicated to filtration issues in lubrication, Marougy joined experts from Afton Chemical, who described how knowledge gained in the area of hydraulic fluids is leading now to improvements in the cleanliness of gear oils, too.

Eaton’s research shows that contamination occurs in hydraulic systems in many ways. Even new fluid out of the drum may not be clean enough to guarantee the health of a hydraulic fluid system, Marougy warned. And yet, she urged, if we are introducing these contaminants it means we can get rid of them too, before they get in, so the filtration machinery doesn’t have to work so hard to do it.

One common way for equipment to be contaminated is when new fluid is introduced; for example, fluid which began life in a relatively clean state, but was transported in unclean trucks or vessels, or pumped into the equipment using hoses that were not clean. It’s the same thing with drums, they get used over and over and we see rust, scale and water contaminating the new fluid, Marougy said. Careful cleaning, reconditioning and inspection of drums is needed before they are filled, and tank trucks and hoses should be used only to transport like materials, or thoroughly flushed and cleaned if they are used to carry motor oil on one trip, hydraulic fluid on the next, to prevent cross contamination.

Likewise, new equipment often comes with built-in contaminants, she pointed out. Burs, fibers, chips and paint flakes are among the common particles that the initial start-up may find. Welding spatters are another. We’ve even seen rags left inside the fluid reservoir or in pipes, Marougy recounted.

Most contamination, however, is ingressed – sucked into the system as it operates. This is the single thing we most see not paid attention to, Marougy said. Just do not leave the reservoir open after filling, or after a maintenance event. Make sure reservoir vents have filters on them. Remember, every time a hydraulic cylinder pulls out into the atmosphere, when it draws back in, some moisture and abrasive particles can be pulled back into the fluid.

Finally, contaminants may be internally generated. Hard particles may be generated through abrasive wear or corrosion, and lead to further abrasion, and more wear particles in a spiraling cycle.

Michael Reinehr, Afton’s European OEM liaison manager for industrial lubes in Hamburg, Germany, sees these same issues plaguing other industrial lubrication areas. A few years ago industry started to realize that non-hydraulic applications require attention as well, he told the Calgary meeting. Gear box lubrication is a typical example of this.

According to a study by German wind turbine manufacturers, Reinehr cited, the average unscheduled downtime on wind turbines was 280 hours a year, or more than 11 days – a serious problem when trying to guarantee reliable electricity. The largest share of these outages, over 35 percent, were due to gearbox failures from bearing problems, he added, and the German institute FAG says about 20 percent of all bearing failures can be traced to solid contamination in the oil.

Wind gear manufacturers would like offline fluid filtration to screen out particles at least 5 microns in size, and they’re considering lowering that to 1 micron or lower, Reinehr said. However, OEMs are also beginning to suspect that filtration is altering the oil properties. The trouble goes beyond merely taking out additives, Reinehr said, to actually altering their performance. For example, the bonding agent used in some fiberglass filters seems to have a detrimental effect on the oil and its additives.

A standardized gear oil filtration and performance test is urgently needed, he said. Current tests address the filter performance, or the oil’s cleanliness, but not how they interact as a system. Fortunately, a working group in Germany is making good progress on developing a dynamic test to look at the entire gear oil system – operating environment, filtration and fluid together.

Also speaking at the session was Jose Reyes Gavilan, Afton’s marketing manager for metalworking fluid additives in Richmond, Va., who gauged the effectiveness of various additives to control particulates and abrasion. Fluid, to protect, has to provide a film to coat the moving parts of machinery, he pointed out. And that film must be robust enough to suspend particles under both boundary and hydrodynamic lubrication conditions.

As the concentration of abrasive particles increases, Reyes Gavilan explained, it becomes more difficult for the fluid to maintain a film that can hold particles safely in suspension. Using results from tests on a high-frequency reciprocating rig, he showed how quickly a clean oil can form and sustain a film. However, as particles build up the film strength plummets; at 9 percent particulate loading, for example, the film lost almost 80 percent of its strength.

Antiwear additives alone may not be enough to combat this effect under boundary lubrication conditions. For example, if ZDDP is added to the fluid to combat wear, the fluid forms a film very readily — in the absence of particles. But as abrasive particles contaminate the film, there is rapid reduction in the film strength. So polymers can be used to boost this film strength, Reyes Gavilan said.

Under elastohydrodynamic conditions – where metal-to-metal contact occurs – it’s key that the film thickness has to be greater than the size of the abrasive particles, to keep them in suspension. Speed and temperature affect EHD film thickness, but another challenge is to control particle agglomeration and prevent smaller, relatively harmless particles from clumping up and doing damage. To do this, the oil needs to effectively suspend the particles in solution, and keep them from depositing on surfaces. Dispersants and functionalized polymers (including polymethacrylate chemistries) work well to do this, Reyes Gavilan said.

For the end-user, contamination control really boils down to three easy steps, Eaton’s Marougy said. First, set up a cleanliness target, taking into account your pressures, operating conditions and how critical the piece of equipment is. Second, take action to reach that target. And then monitor to make sure the target is maintained.

Market needs are moving end users to invest in more sophisticated and higher-pressure equipment, which demands a cleaner system, she concluded. Finally, practice proactive maintenance – do not wait until the system fails.

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