Overview:

A study by Dr. E. Rabinowicz was presented to the Society of Lubrication Engineers, which observed that 70% of component replacement (or loss of “usefulness”) is due to surface degradation. In hydraulic c and lubricating systems, 50% of these replacements result from mechanical wear, (link to mechanical wear) with 20% resulting from corrosion (water) (link to Corrosion Page). Properly designed particulate removal (filtration) (link to Filter Selection Factors page) (link to Filter Placement Page) and water removal systems (link to water removal page) can eliminate a majority of these problem.

In the twenty plus years since this study’s debut, all major hydraulic component manufacturers have adopted it’s findings, and have included specific filtration levels (Link to ISO Contamination Codes) needed to be maintained for proper (5+ year) life of all hydraulics.

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Common Results of Contamination Control

• Decrease in premature component failures: 70% of surface degradation troubles stop.
• Increase of system productivity.
• Decrease in energy consumption costs. Clearances stay the same, contributing to higher efficiency.
• General and dramatic system reliability improvements
• Reduced hazardous waste / oil disposal costs.

Contaminant Sources – Where do the contaminants come from?

Built-in contaminants – occur mostly from the component manufacturing processes. Built-in contaminants include:

• Weld splatter,
• core sand,
• metal chips,
• abrasive dust,
• test stand dirt and
• lint.

Induced contaminants – Entering the system through

• defective seals,
• open reservoirs,
• cylinder rods,
• filter tubes,
• top-off,
• breather caps,
• maintenance and
• defective coolers.

System generated contaminants – Coming from:

• Assembly,
• Start-up,
• Break-in,
• And operation.

Sludges and Acids – Are introduced to the system from reactions between water and heat, with the hydraulic oil and metal within the components.

ISO CODE

The International Standards organization (ISO) has quantified your oil’s overall cleanliness level with an international recognized standard. Adapted by ISO, the ISO Cleanliness Code makes it quick and easy to judge the cleanliness of your fluid. Based upon a 1 milliliter (1 ml) sample of your oil, the ISO Code assigns a reference number to the amount of 2, 5 and 15 micron particles counted in your sample. One 91) micron is equal to 1 millionth of a meter, or 0.000039 inches.

ISO Reference # of Particles in 1 ml Sample
10 5 – 10
11 10 – 20
12 20 – 40
13 40 – 80
14 80 – 160
15 160 – 320
16 320 – 640
17 640 – 1280
18 1280 – 2560
19 2560 - 5120

Note: As the Reference Number increases, the number of Particles doubles!

EXAMPLE: ISO 17 / 16 / 14

• # 17 This number equals the number of 2.0 (two) micron particles within the 1 ml sample of oil.
  The quantity ranges from 640 – 1280.
• # 16 This number equals the number of 5.0 (five) micron particles within the 1 ml sample of oil.
  The quantity ranges from 320 - 640.
• # 14 This number equals the number of 15.0 (fifteen) micron particles within the 1 ml sample of oil.
  The quantity ranges from 80 - 160.

Definition of Filtration Terminology:

Service Life vs. Dirt Capacity

Definitions of service life and dirt capacity are given below. Dirt capacity should not be used to predict filter element service life due to many variables that affect direct capacity data.

Service Life:
Service Life is the length of time that a filter will survive in any actual system before the minimum differential pressure (?P) is reached.

Dirt Capacity from the Multi-Pass Test

• Dirt capacity is accurately measured with the ISO 4572 Multipass test, (ANSI/NFPA) T3.10.8.8R1.
• The ISO 4572 Multi-Pass test uses “real-life” scenario hydraulics to effectively measure the incoming quanity of particulate matter to a hydraulic system, and measure the individual filter element’s capability to capture and hold that particulate, up to and beyond the element’s rated flow and pressure drop specifications.
• This is the most accurate means of rating filter elements.

Test variables that can affect capacity data include:

• Flow rate.
• Contaminate
• Contaminant ingression rate.
• Multi-Pass vs. single pass.
• Terminal pressure drop.
• Filter integrity.

Comparing dirt capacity of two elements.

• All variables must be equal.
• Elements must be of equivalent size.
• Elements must be of equivalent efficiency (link to Beta ratio).
• Retained dirt capacity values must be compared.

Dirt capacity would appear to be an easy parameter to measure and understand; however, using dirt capacity to predict service life is quite difficult. Two different filters with the same dirt capacity will almost always have quite different service lives. For example, coarser filters with higher dirt capacities will allow more particle generation because of wear. They generally have shorter service lives than fine filters.

Apparent Dirt Capacity
Apparent dirt cpacity is the amount of dirt that can be added to the filter test stand before the minimum differential (?P) is reached.

Retained Dirt Capacity.
Retained dirt capacity is the amount of dirt that is captured by the filter in a test system before the terminal ?P is reached.

Beta Ratio:
Developed by the International Standards organization (ISO), the ISO 4572 Multi-pass Effieincy Testr (Beta Ratio) effectively compares the contaminant catching ability of Filter “A” to Filter “B”. Contaminant (Air Cleaner Fine Test Dust, or ACFTD) is put into a hydraulic test stand and per the ISO test standards, flow, pressure and ingression rate are controlled. The amount of contaminant before and after the filter element is measured and a ratio is docuemented. The ratio of contaminant enterning the element and leaving is called the BETA RATIO (ß). The higher the Beta Ratio, the more dirt the element will capture on one pass.

EXAMPLE: Beta (ß) 6 = 1000.

#6 = A six (6) micron particle size is being addressed.
#1000 = ratio of particles (6 micron or larger) In vs. Out. If 1 million particles pass through this filter, only 1,000 would exit. If this number was 75 (not 1,000), and 1 million particles pass through the filter, 13,3333 would exit past the filter element. Obviously, the element with the higher Beta Ratio (1,000 vs. 75) captures and holds the largest amount of particulate.

Percentage of Efficiency:
This terminology is still used by some filter manufacturers and can be very misleading to the end-user. A filter that is “98% efficient” versus a filter that is 99.2% efficient may sound close to identical. However the 98% filter actually allows almost 3 times more dirt downstream than the 99.2% filter. Compare filter elements with BETA RATIO Only. (link to Beta ratio definition)

Effects of Contamination:
Each microscopic particle inside the hydraulic system acts as an “abrasive seed, resulting in eventual component failure. The microscopic particle bridges the dynamic (or running) clearances within the component (link to Typical Dynamic Clearances table page). The microscopic contaminant quickly grinds and erodes away internal pump, valve, motor and cylinder metal. This literally creates more microscopic and highly abrasive contaminant particles, grinding away more and more of your system. Component failure is inevitable. This action is called the “Chain Reaction of Wear” .

Installing effective, contaminant grabbing filtration is the solution to solve this problem. The key is to capture and hold the particle. Once the particle is removed from the active system, abrasive and erosive wear generation ceases. Hydraulic system life increases, productivity continues uninterrupted. Catastrophes are avoided.

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Abrasive Wear:
This is the primary wear mechanism within a hydraulic system. Particles enter the clearances between two moving surfaces, bury themselves into one of the surfaces, and act as a metal cutting tool upon the other moving surface. The particle sizes causing the most abrasive wear are those that are equal to, and slightly larger than the clearances within the system. To protect your system from abrasive wear, particles of approximately the dynamic (running) clearances within the component must be removed.

Abrasive Wear Affects include:

• Dimensional changes.
• Leakage.
• Lower efficiency.
• Generation of more particles – more wear = Chain Reaction of Wear.

Erosive Wear:
A secondary wear mechanism within the hydraulic system is erosive wear. As you know, Abrasive Wear generates more particles by acting as a metal cutting tool, literally gouging away metal from the moving surfaces.

Erosive wear is the result of the large metal particles being ground into smaller particles; ones that do not bridge the gap between the moving surfaces (usually 1-3 microns in size), but due to their vast numbers (in the hundreds of thousands per sample), literally act as a “sandblaster” within our system. Hydraulic pressure and flow blasts these small particles against all sharp edges within the system, and literally erodes them away.

Sludge and Acids
Sludge and acids can also form within hydraulic and lubricating systems. This happens when the fluid chemically reacts to water, dirt, metal, air, heat, pressure and incompatible fluids. Sludge is not generally abrasive, but can generate heat due to loss of lubricating film. Sludge is recognized as the gummy coating on moving parts, slowing or halting their operation. Acids corrode and pit the critical moving parts, destroying the running clearances and causing 20% of total component replacements.

By eliminating the water, dirt and heat within the systems, assuming the oil has not been “burned” or improperly mixed, the oils additive package should remain consistent with new oil. The oil can last indefinitely.

Filter Selection Factors

Degree of Filtration
All hydraulic and lubrication systems shold have filtration. Studies, such as those conducted by Oklahoma State university, the U.S. Navy, S.A.E., A.S.M.E., the Fluid Power Research Center and many others, conclude that the cleaner the hydraulic system’s oil, the longer the system will last. The degree of filtration your systems shoulde have is directly related to it’s operating characteristics.

In selecting filters, the following conditions need to be carefully evaluated, including:

• Beta Ratio – The ratio of contamination entering the filter, vs. contamination passing through the filter (link to detailed Beta Ratio noted above)
• Pressure Drop – All system components produce a resistance to oil flowing through them. This is pressure drop. The drop is the net pressure required for the oil to flow from the filter’s inlet port to the outlet port. In all filters, this includes the pressure drop across the housing and the element. Pressure drop varies with flow rate, viscosity and specific gravity of the fluid. When making a filter selection, the maximum allowable system pressure drop must be considered: operating temperature, highest and lowest temperature, and dirty element bypass valve setting. Care must be taken when sizing a filter in a cylinder circuit or accumulator circuit to size for pressure drop at peak flows.
• System Pressure – Return line filters must withstand the maximum return line pressure, while pressure filters must withstand maximum system pressure. This includes induced pressure spikes due to operations. The housing, bypass valves and element must withstand fatigue from cycling and surges.
• Temperature – Operating temperature effects the viscosity (resistance to flow) of the fluid. Filters must be selected and sized for cold or ambient temperatures, as well as system operating temperature.
• Other factors – The environment in which the equipment operates is very important, especially if the reservoir is vented to the atmosphere. It is quite common to install Beta 3 > 1000 air breathers on the reservoir, removing any possibility of ingressing foreign, airborne particles from entering the reservoir.
• With the increased emphasis on preventing machine downtime, the need for much higher standards of fluid cleanliness is evolving. Since pumps and valve shave clearnances on the order of 1 – 5 microns, it is easy to understand the need for cleaner oil. To meet this demand, filters are now capable of filtering down to the 1 (one) micron level (Beta 1 > 1000), with long service lives and low pressure drops.

FILTER SELECTION HINTS

1. The higher the Beta Ratio (link to Beta ratio portion here), the quicker the contaminant is removed from the system.
2. The lower the initial sized clean pressure drop, the longer the element will last in service.

Recirculating Filtration Systems - With longer and longer “uptimes” being demanded by production managers, “downtime” for regular maintenance is becoming increasingly scarce. To allow continual equipment operation, while still being able to service a dirty filter, fixed recirculation filtration systems are being installed on the full-time production machines.

These low-cost units are being bolted to the machine, continuously filtering the reservoir oil. If an element becomes dirty, the production machne continues to operate, while the independent, fixed recirculation filtration system can be shut down, to change the dirty element.

Filter Placement Suggestions

• Solenoid valves controlled hydraulic system. 1800 psi, 30 gpm
  • Pressure line filter assembly, installed after the system’s pump, in the pressure line. (Beta 3 > 200)
  • Return line filter assembly, installed in the return-to-tank line, prior to entering the reservoir. (Beta 3 . 200)
• Reservoir Air Breather (Beta 1 > 200)
  • Servo and Solenoid valve controlled hydraulic system.
  • Pressure line filter assembly, installed after the system’s pump, in the pressure line. (Beta 1 > 200)
  • Remote, non-bypassing, pressure line filter installed directly prior to the servo valve. (Beta 1 > 1000)
  • Return line filter assembly, installed in the return-to-tank line, prior to entering the reservoir. (Beta 3 . 200)
  • Reservoir Air Breather (Beta 1 > 200)
• Closed Loop Hydrostatic Transmission Hydraulic System
• • High pressure (6000 psi usually), reverse-flow pressure filters intalled directly into the two main pressure and flow circuits. (Beta 3 > 200).
  • Reservoir Air Breather (Beta 1 > 200)
• Bearing Lubrication Circuit (Turbines, Paper Machines, etc.) (120 gpm, 45 psi)
• • High flow, duplex pressure line filters (Beta 5 > 200)
  • Reservoir Air breathers. (Beta 3 > 200)

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Water and Air Contamination

Water Contamination in fluid systems causes:

• Fluid breakdown, such as additive precipitation and oil oxidation.
• Reduced lubricating film thickness.
• Accelerated metal surface fatigue.
• Corrosion
• Jamming of components due to ice crystals formed at low temperatures.
• Loss of dielectric strength in insulating fluids (Transformer oils).

Dissolved air and other gases in oils cause:

• Foaming.
• Slow system response with erratic action.
• A reduction in system stiffness affecting servo / proportional valve response.
• Higher fluid temperatures.
• Pump damage due to cavitation.
• Inability to develop full system pressure.
• Acceleration of oil oxidation.

Means of Water and Oil Removal

Type Removal Capability Operation

• Absorption Free and dissolved water Absorbs and retains water. Good for very small amounts (1-3 quarts) of water. The used element must be disposed of as hazardous waste.
• Gravity Precipitation Unit Free water; nominal removal of contamination Settling out of gross contamination, water and particulate, by holding oil, water and particulate, in detention chambers. Also small water droplets are coalesced onto water-repelling wire mesh screens.
• Centrifuge Free water, nominal removal of particulate contamination Cone shaped disks rapidly rotating inside a chamber generating centrifugal force to separate water and oil based particles, based on their mass. Centrifuges are inefficient at silt size particulate removal, and require frequent cleaning, maintenance and rebalancing.
• Water Coalescer Free water, removal of particulate contamination varies Oil is passed through a pre-filter to remove particulate, and is then passed through coalescing elements to combine water droplets. Combined water droplets settle out of the oil by gravity.
• Coalescer elements need periodic replacement, and are considered hazardous waste.
• Flash Distillation Purifier Free and dissolved water; dissolved gases and solvents. These large power consuming units pass heated oil through a vacuum chamber where the water is boiled off. Because they use high vacuum and high heat to drive off water, flash distillation purifiers can alter the chemical or physical properties of the oil.
• Vacuum Dehydration (mass transfer) purifier Free and dissolved water; dissolved gases and solvents. Employing the process of mass transfer, water, air and solvents are removed by exposing oil to low relative humidity air drawn up through a chamber by lower pressure maintained by a vacuum. Automatic, computer controlled units run 24 hours a day, 7 days a week. Virtually maintenance free and self-diagnosing ease servicing if ever needed.