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Contaminants: Unseen Killers

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CALGARY, Alberta – The biggest enemy for hydraulic fluids and hydraulic systems is contamination, contends 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.

Its 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 here last month, in a special forum dedicated to filtration issues in lubrication. The session also heard how greater understanding of contamination and cleanliness issues in hydraulics is leading to improvements in gear oils too.

Eatons 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 added, if we are introducing these contaminants it means we can get rid of them too, so the filtration machinery doesnt 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. Its 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 only be used 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. Weve even seen rags left inside the fluid reservoir or in pipes, Marougy recounted.

Most contamination, however, is ingressed. This is the single thing we most see not paid attention to. Just do not leave the reservoir open after filling, or after a maintenance event, Marougy advised. Make sure reservoir vents have filters on them. Remember, every time a hydraulic cylinder pulls out into the atmostphere, 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.

Wider Impact

Cleanliness isnt only a hydraulics problem, however. Similar issues are being seen in other industrial lubrication areas, Richard Kuhlman, OEM liaison for Afton Chemical Corp. in Southfield, Mich., added. For example, theres a lot of research going on with wind tubines and their gear boxes. Not that wind power is ever going to be a big-volume market, but wind turbines can be thought of as a canary in a coal mine, bringing attention to contamination-related issues like micropitting and cleanliness, he told the conference. If we can get a handle on this in wind turbines, it will have a good impact on other areas.

Michael Reinehr, Aftons European OEM liaison manager for industrial lubes in Hamburg, Germany, concurred. 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, 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, Reinehr 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 today recommend offline fluid filtration to screen out particles at least 5 microns in size, and Reinehr said theres talk of lowering that to 1 micron or smaller. However, OEMs also are beginning to suspect that filtration is changing the oils properties, such as by depleting needed extreme pressure additives. 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 to understand this phenomenon, he said. Current tests address the filter performance, or the oils cleanliness, but not how they interact as a system.

These tests include the ISO 13357 filterability test, a one-pass test for low-viscosity oils, but it measures only filter plugging. It also uses a 0.8-micron filter, which is suitable for hydraulic fluids but too fine for gear oils. There are also proprietary, in-house methods at filter and gear makers like SKF, Internormen, Pall and Hydac.

Help may be on the way, though. FVA (Germanys equivalent to the American Gear Manufacturers Association) has a working group on gear oil filtration which is developing a dynamic test using samples from the field – real live gear oils, Reinehr said. This will be a multipass test, circulating the gear oil in a system with repeat passes. As proposed, the test rig even allows the addition of water at any time to the sample, so the effects of water contamination can be evaluated, too. And a Flender foam tester can be added, so you can see that property after the first pass, the second pass, the third or the hundredth pass, Reinehr added.

The Additive Factor

Also speaking at the session was Jose Reyes Gavilan, Aftons 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, Afton research shows, 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, Reyes Gavilan demonstrated 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 gear oil lost almost 80 percent of its film 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.

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.

Call to Action

Contamination control really boils down to three easy steps, 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.

ISO Standard 4406 offers guidelines for fluid cleanliness in low-tolerance components, but theres no single recommendation that covers all equipment types and makes. So Marougy weighed in with some practical advice for end users:

Determine the most sensitive component in your system, like a servo valve, and use that to set your own cleanliness target. As the pressure goes up, so does your cleanliness need to, Marougy said. You may need to adjust the ISO cleanliness code for your fluid type, and if its very cold or very hot, I suggest you keep the fluid even cleaner because of the harsh environment.

Pay attention to the selection, proper sizing and location of filters. There should not be only partial flow going through the filter, Marougy stressed. At the same time, close off the opportunities for contamination ingress, by putting filters on reservoir vents, for example.

Monitoring can be done on-site, using both portable particle counters and a fluid analysis program. Many hand-held laser-type particle counters are now available for on-site checks, with one caveat: They are most accurate for dry fluid. If you have water in the fluid, it needs to be dried first before testing.

Take representative fluid samples and send them to an outside laboratory to test for water content, viscosity, contaminants, wear particles, chemical spectrum, etc. Variability can be a problem with some of these tests from one lab to another, so always use the same lab so you can compare results over time. Learn to read the reports you get from the lab.

Samples must be collected with care. But lots of people dont realize this, said Marougy with a knowing shake of her head. Theyll use an old Coke bottle to collect a sample because its what they have on hand. Instead, use ultraclean bottles that are supplied by the lab. Keep the cap on the bottle until the last minute before the sample is drawn, and put the cap back on right away afterward. Wipe off the the port or valve where the sample is to be drawn, draw off a bit and discard it into your waste oil, then draw a fresh sample. And always use a new piece of tubing to draw the sample.

Take your sample when the machine is running or immediately after it has been shut down at the end of the workday. If you wait until the next morning, after the fluid has been resting overnight, youll get a sample that looks clean, but isnt really whats going through your pumps and valves, Marougy advised.

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

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