We asked a sampling of filter manufacturers what mistakes or omissions were most commonly made in filtration - what should every fluid power system designer know about contamination control? Below, glean some ideas for yourself from the experts...


"In-line oil filtration only treats the symptoms and not the disease. The symptoms are particles in the oil. The disease is oxidation. Oxidation cause tars and varnishes to form and can only be removed by an electrostatic oil filtration system. So don't treat the symptom, cure the disease."

- Ken Anderson, Kleentek Corp., Cincinnati, Ohio

"New oil is never clean. Therefore, the system should generally be filled by using a filling filter. This fine mesh filter should be rated at least B10 “ 75 (10 µm absolute)."

- Mathias Funke, Industrial Controls Div., Moog Inc., Easy Aurora, NY

"Place filters where they can be easily serviced and replaced."

"Record inspection and replacement intervals where they can be observed and compared. As frequency of replacement increases, it's a warning of component failure."

"Check used filter elements for trapped metal. An increase in yellow metal shows a failure in wear plates or slippers. An increase in white and gray metals (ferrous) indicate wear caused from contamination or cavitation. Black, carbonaceous material, sticky gooey varnish, or gray ash may signal time for an oil change and a review to see what is causing the heat."

"Use the largest capacity filter that you can fit into available space, with flow capability at least two times flow."

"Always use new O-rings, lubricated with clean hydraulic oil."

"Consider using a portable filter cart for large mobile systems exposed to dirty environments."

"Return line, pressure filtration, and suction strainers will contribute to extending the life of more expensive components."

"Never allow a hydraulic system to be operated at higher speeds than recommended. Cavitation, lubricant starvation, heat damage, and filter bypass or oversaturation may negate any positive effects of even the best design."

- C. David Reece, Muncie Power Products, Muncie, Ind.

"If you're not sure when to change a filter, put a pressure differential indicator on the filter adapter. All the guesswork is gone!"

- Vicki Kirkham, Central Illinois Manufacturing Co./Cim-Tek Filtration, Bement, Ill.

"Keep in mind that micron ratings of filter elements can be an important consideration in the operation of hydraulic systems outdoors in winter, or in extremely cold indoor environments. If a system can not generate sufficient heat so the oil will begin to flow through the filter as opposed to being on bypass constantly, this can be easily addressed."

"Constant bypass = no filtration."

- Filtrec North America, Sault Ste. Marie, Mich.

"Employ the used filter as a predictive maintenance tool. If metal debris is noticed on the surface of the filter media, good magnification and wear metal analysis can pinpoint the source of the metal. This way, components that will obviously break down soon can be changed out during a scheduled down period. Many thousands of dollars can be saved by employing predictive versus reactive maintenance."

- David Murray, Triple R America Co. Ltd., Toronto

"Don't judge a filter housing by its maximum operating pressure. When sizing a filter, look for the manufacturer's Cyclic Fatigue Rating (NFPA T2.6.1). A cyclic fatigue pressure is lower than the operating pressure and verifies the fatigue pressure rating of the pressure-containing envelope of a metal fluid power component."

- Gus Schroeder Jr., Schroeder Industries, McKees Rocks, Pa.

"All hydraulic filters should be fitted with visual and/or electrical indicators. The visible signals allow easy and clear monitoring of the filter element status, and electrical indicators can be wired remotely to provide information at a central control panel or off-site location. Then, of course, don't ignore your indicators! If you do, and the filters go into bypass mode, you're just dumping dirty fluid back into the system. Filter bypass mode was designed to enable cold starts on mobile equipment until operating temp is reached.

"Frequent oil analysis is important in preventing problems. Don't take oil samples from the bottom drain port of the reservoir because all the big chunks of dirt and gunk fall there. Do take oil samples from a turbulent point in the system - from a working line while the system is running - to get a representative sample."

- The industrial hydraulic filter staff at Donaldson Co., Inc., Bloomington, Minn.

"The question should not be how clean can you get your oil, but how dirty can your oil be and still give you very good pump, valve, cylinder, and seal life?"

"On the inlet side of a pump, always size the filter at least one size larger than the pump inlet port. Keep the inlet line velocity down to 4 ft/sec. Hint: Velocity = Pump delivery (gpm) / Pipe size (in.2) x 0.3208."

- David R. Royer, Flow Ezy Filters Inc., Ann Arbor, Mich.

"When a customer starts up and flushes a new system I ask that they consider a finer grade of media for the first element used. For example, if the system uses 10 µm media, start the flush with 6 µm. At the next element change go back to 10 µm. Benefits are: no 6-µm or larger contaminant, rapid stabilization of system contamination level, minimal element usage, and greatly reduced potential of 'infant mortality.'"

- Greg Farkas, MP Filtri USA, Inc., Norcross, Ga.

"The level of filtration is only as good as the weakest filtration component - a 3 µm absolute return line filter doesn't mean 3 µm oil if the reservoir breather is 50 µm."

- John Stockell, fluid power products, PTI Technologies Inc., Newbury Park, Calif.

"Typical oil additives, such as anti-oxidant agents, will compensate and balance chemical changes from oxidation. If oil oxidation has not been minimized, oil additives and base oil will transform into oxidation by-products - waxy residues on valve surfaces, sludge on the bottom of the reservoir."

- Vichai Srimongkolkul, OilPure Technologies, Inc., Kansas City

"Locating a coarse filter upstream of a fine filter can improve the useful life of the fine filter by preventing larger particles from reaching the fine filter."

- Mike Evans, HYDAC International, Lehigh Valley, Pa.

"When sizing a filter, remember to consider start-up conditions. During start-up or cold operating conditions, fluid viscosity may be higher than during operating conditions. When appropriate, a larger sized filter may be required. Additionally, when an indicator is used, a thermal lock should be considered - this will prevent premature tripping, locking out the indicator until the fluid reaches normal operating temperature."

"Finer filtration is not always better. If equipment requires 10 µm, 3-µm filtration does not necessarily provide a better option. The 3-µm element will clog faster, resulting in more frequent element changes, increased maintenance and production costs, and normally a more expensive element."

- Barry Afflerbach, HYDAC International, Lehigh Valley, Pa.

"It's a common misconception that coarse filters last longer than fine filters. This is not really the case. Once a fine filter has removed particles from the system and stopped wear from occurring, there are few particles left to be removed - and thus, long life. The coarse filter, on the other hand, doesn't prevent wear, and therefore particles will continually need to be removed."

- Darren Nowicki, Pall Industrial Hydraulics, Port Washington, N.Y.

"There is no such thing as 'too many filters.'"

- Al Klimas, Mannesmann Demag Corp., Pittsburgh, Pa.