RECOMMENDED GUIDELINES FOR HYDRAULIC SYSTEM FILTRATION

IN ORDER TO INSURE PEAK PERFORMANCE AND EXTENDED OPERATING LIFE OF THE MACHINE, IT IS RECOMMENDED THAT AN ISO CLEANLINESS CODE OF 16/14/11, OR BETTER, BE MAINTAINED REGARDLESS OF SYSTEM PRESSURE. THE OIL CANNOT BE TOO CLEAN.

WHY FINER FILTRATION IS NECESSARY

Hydraulic oil contamination acts like a sandblaster inside the system, wearing and damaging critical hydraulic components.

Ninety (90) percent of servo system failures are the result of contaminated hydraulic fluid.

Effects of Contamination

A contaminant is any material foreign to a hydraulic fluid that has an unfavorable effect on the fluid's performance in a system.

Material contaminants can exist as gas, liquid or solid. A gas can either be dissolved or entrained in the fluid. A liquid, depending on its compatibility with the fluid, can be free, dissolved, or emulsified.

The above types of contamination aggravate the abrasive wear in hydraulic components. This abrasive wear is more prevalent between sliding surfaces of pumps, motors, and cylinders. It increases the tolerances and results in higher leakage rates. These higher than normal leakage rates in turn decrease pump delivery and increase system operating temperatures. In addition to the effects on components of sliding type surfaces, metal erosion will occur on valve control edges, seating surfaces, restrictions, and throttles.

Solid non-abrasive contamination such as material from seal wear, filter elements, textile fibers, paint chips, etc., cause problems by blocking passages, channels, gaps, lines, and filters. These contaminants over a period of time will cause a condition that will result in eventual component malfunction.

Solid particles that are small or approximately the same as component clearances can cause particularly high levels of wear. Modern hydraulic components are designed fir higher pressures and are more sensitive to contamination because of tighter tolerances between moving parts. These higher pressure rated components increase the loading on the lubricating film between moving parts and make it more critical to maintain proper filtration levels.

Results of components part wear and increased leakage are:

1. Efficiencies of the hydraulic components are lower and component performance is reduced.

2. Loss in control of system components and their ability to maintain system dynamics.

3. Decrease in efficiency causes excess heat to be generated in the system.

4. More energy required cooling and removing the excess heat.

5. Decrease in component efficiency, which decreases machine cycle times.

6. Increase in product costs due to lower output per machine operating hour.

Sludge is not typically abrasive, however, it is a source of resinous and gummy coatings on moving parts and reservoir walls. Sludge will clog passage and cause a loss in heat transfer from the reservoir to the atmosphere, adding to an over-heating condition. Acids in the fluid will coat surfaces and corrode and pit moving parts.

AIR AS A CONTAMINANT

While not filterable contaminants, air causes fluids to become more compressible and impairs proper system operation. Air is a major contributor to the chemical breakdown of fluid additive packages. This phenomenon is commonly referred to as "oxidation."

The majority of air is either self-generated by improper return of fluid to the reservoir or it is ingested into the system through filler breathers and cylinder rod seal.

Symptoms of aeration in fluid include:

• Higher component noise levels
• Erratic system performance
• Premature pump failures
• Increased fluid temperatures
• Fluid viscosity changes
• Loss of fluid lubricity properties
• Incomplete lubrication

Rules of thumb dictate that the filtration rating of the filler/breather should be identical to that of the return line filter.

Higher standards of filtration can be accomplished by specifying a high quality filler/breather for the reservoir. Of equal importance is the filtration rate of the filler cap. Typically, these units have a filtration level of 10u nominal.

More attention in these areas during design stages can help achieve overall filtration cleanliness levels.

Oxidation

The availability of air in the fluid of a system increases the oxidation rate of the fluid. Solid reaction products and acids may be formed causing clogging of passages, rust, and corrosion of the system hardware. This increased oxidation rate and oxidation reaction products can cause premature component failures.

Components in the fluid power system rely upon the fluid for lubrication. Introduction of air into the fluid can cause foaming. Foam in the fluid reduces its ability to lubricate properly. This reduction of lubricity cans premature component failures.

Aggressive fluid maintenance programs and filtration design techniques cannot eliminate oxidation, but they can keep its effects to a minimum. Every 20 F operational temperature above 130 F results in a 50 percent loss in system life of oil.

SOLID CONTAMINATION

The unit of measurement used to describe the size of a contaminant particle is the micron. This is a unit of length equal to one-millionth (106) of a meter - or 0.0000394 inches. As a matter of interest, here are some common items with their size in microns:

Grain of Salt	>100 Microns
Human Hair	75 Microns
Red Blood Cell	7 Microns
Bacteria	1 Micron

SOURCES OF SOLID CONTAIMINATION

Contamination enters hydraulic systems in a variety of ways. The main sources:

Built-in Contamination

Each component (for example, valves, pumps, reservoirs, manifolds, hoses, tube, pipe) subassembly or system will contribute contamination if it is not flushed and tested with fluid of an acceptable cleanliness level. If clean components, subassemblies and systems are not properly sealed after test (for example, protective caps and covers), contamination will enter during storage and transportation.

Typical built-in contaminators include metal burrs, chips, dust sand, and weld slag, rubber and cleaning/flushing solutions from the manufacturing of the manifolds, reservoirs, hoses, tubing, and piping. Dust, moisture, paint, lint from shop rags, sealant, and other contaminants will also enter if a machine or system is assembled in an area that is not kept free of such materials. Lack of care in assembly will also contaminate systems. It is not uncommon for pieces of plastic plug or duct tape not removed at assembly to be found in components, filters, and reservoirs.

New Oil

New oil does not come with a guarantee of cleanliness unless it is specified when ordering. Often samples of new oil have contamination far above acceptable levels. If a reservoir or tank is not adequately cleaned when it is drained after testing or before a fluid change, contamination that has accumulated will be mixed with the new oil and reintroduced into the system. Oil in bulk storage tanks may also have high levels of water plus rust and scale from the corrosion of tanks. A way to prevent this is to pre-filter oil before placing it in the machine.

Environmental Contamination

Airborne contaminants from the molding processes, as well as dust, dirt and moisture typically found in manufacturing environments, may enter a system through air filters. They will also be brought into the system by exposed cylinder rods. Lack of care during maintenance will contaminate a system. Removal of any plug, cap, or cover will expose a system to all containments in its environment. Contaminants from plastic pellets and rags to hand tools have been found in systems.

Generated Contamination

Even systems initially started with fluid meeting cleanliness levels will have contamination introduced from the normal wear of components and degradation of the fluid. Operating systems beyond the recommendations of the producers of the equipment or fluid, as well as exposure to the sources described above will accelerate the generation of contaminants.

DIAGNOSTICS

Constant monitoring and maintenance of systems fluids can improve machine-operating performance more than any other single factor. Scheduled monitoring of the system filtration levels must become part of the standard maintenance program.

Or equal importance is the degree of keeping accurate records of the maintenance data. This allows for proper analysis of component failures and machine performance.

Minimum Recommended Cleanliness Levels

The level of cleanliness to be maintained in a given hydraulic system is dependent upon how the system is being applied, its expected level of performance, and the types of components being used. High-pressure systems require more attention due to the closer fit of moving parts, and the increased leakage and performance loss caused by component wear.

The typical critical clearances in fluid system components:

Component	Critical Clearance (In microns)
Gear Pump (pressure loaded)
Gear to side plate	˝ to 5
Gear tip to case	˝ to 5
Vane Pump
Tip to vane	˝ to 1*
Sides of vane	5-13
Piston Pump
Piston to bore (R) **	5-40
Valve plate to cylinder	˝ to 5
Servo Valve
Orifice	130-450
Flapper Wall	18-63
Spool - sleeve (R) **	1-4
Control Valve
Orifice	130-10,000
Spool - sleeve (R) II	1-23
Disk type	˝ to 5
Poppet type	50-250
Actuators	1-25
Hydrostatic Bearings	1-25
Anti-friction Bearings	˝* -
Sleeve Bearings	˝* -

* - Estimated for thin lubrication film
** - Radial clearance

For the purpose of this guideline, the ISO Code is used as a standard of measurement.

ISO Code

This three-part number is an International Standards Organization Code to illustrate the level of cleanliness of the hydraulic fluid in the unit. The numbers are logarithmic representations of the total number of particles greater than two (2) microns (first number), greater than five (5) microns (second number), and greater than 15 microns (third number) in a one milliliter sample. The larger the ISO Code, the more contaminants, the more potential for wear.

IN ORDER TO INSURE PEAK PERFORMANCE AND EXTENDED OPERATING LIFE OF THE MACHINES, IT IS RECOMMENDED THAT AN ISO CODE CLEANLINESS CODE OF 16/14/11, OR BETTER, BE MAINTAINED REGARDLESS OF SYSTEM PRESSURE. THE OIL CANNOT BE TOO CLEAN.

SUMMARY OF FILTRATION SYSTEM REQUIREMENTS

1. It must be capable of reducing the initial contamination to the desired level within an acceptable period of time, without causing premature wear or damage to hydraulic components.

2. It must be capable of achieving and maintaining the desired level, and allow a suitable safety factor to provide for a concentrated ingress that could occur, for example, when oil is added to the system.

3. Filters must be easily accessible for maintenance.

4. Indication of filter condition to suit the end user's requirements must be provided.

5. In continuous process plants, facilities must be provided to allow changing of elements without interfering with plant operation.

6. The filters must provide sufficient dirt holding capacity for an acceptable interval between element changes.

7. The inclusion of a filter in the system must not produce undesirable effects on the operation of components, for example, high back pressures on seal drains.

8. Sampling must be provided to monitor initial and subsequent levels of contamination.

9. Removal of moisture from oil is preferred.

DEFINITIONS AND TERMS

Contaminant - any material foreign to a hydraulic fluid that has an unfavorable effect on the fluid's performance in a system

Contamination Monitoring Methods:

Automatic Particle Counting - automatic particle counting is a method used to determine particle concentrations by size range utilizing a calibrated light source. The calibrated light source may either be a lamp or laser beam.

Gravimetric Evaluation - a gravimetric analysis determines the total solid contamination in a system without regard to particle size. Filtering the sample through a known mille pore filter patch and weighing the dry patch perform the analysis. The difference in weight represents the total solid contamination (usually in milligrams/liter) in the fluid.

Microscopic Evaluation - microscopic particle counting is a method whereby particle concentrations are determined by visual inspection of a patch grid. A known volume of the fluid is filtered through a 0.8-micron grid filter patch. The patch is then examined under a microscope at a known magnification and the particles are then counted.

Foaming - condition created in the oil by introduction of air reducing the oil's ability to lubricate properly.

Gas (dissolved) - passed into solution in the fluid, not emulsified

Gas (emulsified) - suspended in the liquid but not dissolved

ISO Code - this three-part number is an International Standard Organization Code to illustrate the level of cleanliness of the hydraulic fluid in the unit. The numbers are logarithmic representations of the total number of particles greater than two (2) microns (first number) greater than five (5) microns (second number) and greater than 15 microns (third number) in a one milliliter sample. The larger the ISO Code, the more contaminants, the more potential for wear.

Lubricity - the quality of slipperiness

Maintenance Filtration - filtration (off-line or by-pass) designed to maintain oil in continuously good condition in order to prevent catastrophic component failure.

DEFINITIONS AND TERMS

Oxidation - combination of a substance with oxygen. (Examples: slow oxidation - rust; rapid oxidation - fire). Every 20 F operational temperature above 130 F results in a 50 percent loss in systems life of oil.

Preventive Filtration - filtration (usually in-line) designed to protect vital components from contamination created from catastrophic failures

Sludge - high molecular weight products that are oil insoluble and usually the result of excessive oxidation

Viscosity - the measurement of resistance to flow

Sitting - sitting is the accumulation of ultra-fine dirt (five microns or smaller), which is the most evident cause for valve sticking

TAN (Total Acid Number) - a measurement of oxidation that has occurred

REFERENCES

Evans, W.B. "How Important Is Clean Hydraulic Oil?" Hydraulic and Pneumatics, October 1991

Filtroil North America, Inc. "How Filtroil Solves Oil Contamination Problems," Charlottesville, Virginia, 1986

Filtroil North America, Inc. "Oil Analysis As A Maintenance Tool," Charlottesville, Virginia, 1991

Fitch, James C. P.E. "Proactive Maintenance Can Yield More than A 10-Fold Savings Over Conventional Predictive/Preventive Maintenance Programs, "STLE Annual Meeting, May 1992

Fluid Power Handbook and Directory, Hydraulics and Pneumatics

Fluid Technologies, Inc. "ISO Chart," Stillwater, Oklahoma

Frankenfield, T.C. "Using Industrial Hydraulics", Hydraulics and Pneumatics, 1984

Handler, Laurence H., Industrial Lubricant Technology, Cherry Hill, New Jersey 08003

Industrial Air and Hydraulic Inc., a subsidiary of Edward Engineering Corporation, P.O. Box 5327, Arlington Texas 76005