Skip to main content

Fluid Cleanliness (ISO 4406) for Proportional and Servo Valves

An ISO 4406 cleanliness code is the number a hydraulic specification uses to say how many solid particles the oil may carry, and on an axis driven by a proportional or servo valve it is one of the acceptance values the valve maker's data sheet sets. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article explains what ISO 4406:2021 covers, how suppliers describe the three-number code, what valve and filter suppliers publish for proportional and servo valves, how contamination fails a metering spool, and how a system is flushed, measured and monitored. The code is set at design, threatened at fabrication and assembly, proven at flushing and commissioning, and held by monitoring, along the chain design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring.

What does ISO 4406:2021 cover, and what does it leave to other documents?​

ISO's catalogue lists ISO 4406:2021 as Edition 4, published 2021-01, 6 pages, from ISO/TC 131/SC 6, last reviewed and confirmed in 2026, replacing ISO 4406:2017. Its abstract is one sentence: the document "specifies the code to be used in defining the quantity of solid particles in the fluid used in a given hydraulic fluid power system." The abstract names no particle sizes, no volume basis and no number of code fields, and it states no cleanliness requirement for any component.

That split matters when reading a specification. The code itself comes from ISO 4406; the required value for a valve comes from the valve maker's data sheet or from a filter supplier's recommendation, and the method for setting a system target is a separate ISO document, ISO 12669:2017. The valve selection article already makes this point for one data sheet: the proportional vs. servo valve article quotes the Rexroth 4WRE/4WREE requirement that the cleanliness class stated for the components must be adhered to.

Supplier documents also describe different editions. Parker's 2012 guide describes ISO 4406:1999, two editions before the current one, and HYDAC's 2024 handbook and Moog's 2024 catalog name no edition. In this article every code rule is attributed to the supplier that states it, with the edition it describes (ISO 4406:2021; ISO 12669:2017; Bosch Rexroth RE 29061/10.05, Technical data note 1; Parker FDCB805UK, 2012, p. 6).

How is a three-number cleanliness code built from a particle count?​

Parker's guide describes ISO 4406:1999 as "a way of summarising the distribution of contaminants in a fluid by counting the particles per 100ml sample of hydraulic fluid: the figures are cumulative." The counts are converted to code numbers for three channels, 4 µm(c), 6 µm(c) and 14 µm(c), which Parker calls the representative sizes "particularly associated with wear and damage in hydraulic systems."

Parker's worked example runs as follows. A sample with 700,000 particles larger than 4 µm(c) per 100 ml scores ISO 20, because 700,000 lies above 500,000 and below 1,000,000. 140,000 particles larger than 6 µm(c) score ISO 18, and 7,000 particles larger than 14 µm(c) score ISO 13. The fluid is reported as 20/18/13.

HYDAC's 2024 handbook gives the same structure in its own words: the particle counts "are determined cumulatively (i.e. > 4 µm (c), > 6 µm (c) and > 14 µm (c)) (manually by filtering the liquid through an analysis membrane or automatically with particle counters) and assigned metrics." Its class table runs from class 0 to class 28 and "> 28" per 100 ml. Class 18, for example, covers more than 130,000 up to and including 250,000 particles per 100 ml, and class 19 covers more than 250,000 up to and including 500,000. In both suppliers' tables each class's upper limit is about double the one below it: 130,000, 250,000 and 500,000 per 100 ml for classes 17, 18 and 19 (Parker FDCB805UK, 2012, pp. 6–7; HYDAC EN 7.603.12/12.24, 2024, pp. 8–9).

What do the "(c)" suffix and the per-ml or per-100 ml basis change?​

Two notation details change what a number means. Parker's guide states that sizes given as "µm" refer to ACFTD (Air Cleaner Fine Test Dust) distributions and sizes given as "µm(c)" refer to ISO Medium Test Dust (MTD) distributions. HYDAC's SAE AS 4059 table sets the ISO 11171 sizes of > 4, > 6 and > 14 µm(c) against the ISO 4402 sizes of > 1, > 5 and > 15 µm, and states that "the determined particle size depends on the measuring method and the calibration."

The volume basis also varies. Parker notes that "Industry conventionally reports raw particle counts as per 100ml for hydraulic fluids, and per ml for fuel, though this is not part of any standard." HYDAC's own class table is per 100 ml.

HYDAC's table labels the µm(c) sizes as ISO 11171 sizes. At standard level, ISO 11171:2022 specifies procedures for "primary particle-sizing calibration for particle sizes 1 µm(c) and larger" of liquid automatic particle counters that are capable of analysing bottle samples, and it "is not applicable to particle-sizing calibration using NIST SRM 2806b primary calibration suspensions." As engineering reasoning, one failure mode is a comparison across calibrations: an older report in µm sizes set beside a newer one in µm(c) sizes can show a change in code that comes from the counter's calibration, not from the oil (Parker FDCB805UK, 2012, p. 3 and p. 9; HYDAC EN 7.603.12/12.24, 2024, pp. 8–11; ISO 11171:2022).

What happens with low counts or an older two-number code?​

Parker's guide, describing the 1999 edition, states: "When the raw data in one of the size ranges results in a particle count of fewer than 20 particles, the scale number for that size range is labelled with the symbol '>'."

Older documents carry a different code. The Pall Corporation paper by Bensch, Needelman and Lundquist (1996) cites ISO 4406 of 1987 and notes that the "Recent three digit Code is per ISO DIS 4406, 1994." The same paper reports an industrial requirement written as 14/10, a two-number code. As engineering reasoning, a two-number value cannot be rewritten as a three-number one without the original particle counts, and a specification that carries an old two-number value forward should be re-based on a current supplier figure rather than padded with a guessed first number (Parker FDCB805UK, 2012, p. 6; Bensch et al. 1996, p. 5 and ref. 25; ISO 4406:2021).

What cleanliness codes do valve makers publish for proportional and servo valves?​

Moog's 2024 catalog for its D633 and D634 direct drive servo valves, which it describes as "Direct Drive Valves (DDV) equipped with integrated electronics and closed-loop position control of the spool," gives a "Recommended cleanliness class according to ISO 4406" of 18/15/12 "For functional safety" and 17/14/11 "For longer service life." "For functional safety" is Moog's own label in that row; the row cites no safety standard, and this article does not read it as a performance level or a safety-function rating.

Moog's operating instructions for the explosion-protected D633K, D634K and D635K proportional valves (Rev. G, dated December 2011) give 18/15/12 "for normal operation" and 17/14/11 "for longer life," with the footnote "For long life wear protection of metering lands." A second footnote states: "The cleanliness of the hydraulic fluid greatly influences the functional safety and the wear and tear of the valve. In order to avoid malfunctions and increased wear and tear, we recommend filtrating the hydraulic fluid accordingly."

Another valve in this library carries its own figure: the Rexroth 4WRE/4WREE data sheet sets a maximum permissible contamination of 20/18/15, covered in the valve selection article. As engineering reasoning, the code a system must hold is the tightest one among the valves actually installed, read from the edition of each data sheet in hand (Moog CDL 59872-en Rev. D, 2024, p. 3, p. 7 and p. 12; Moog CA49304-001 Rev. G, 2011, p. 7; Bosch Rexroth RE 29061/10.05).

What do filter suppliers recommend for proportional and servo valves?​

HYDAC's handbook tabulates "Target cleanliness class" values by component and pressure band, with a filtration rating for each. For proportional valves it gives 17/15/12 below 140 bar "(moderate conditions)", 17/15/12 at 140 to 200 bar "(low / medium under poor conditions)", and 16/14/11 above 200 bar "(high pressure under poor conditions)", each with a 3 µm filtration rating. For servo valves it gives 16/14/12, 16/14/11 and 15/13/10 in the same three bands, again at 3 µm. For comparison, directional valves are listed at 20/18/15, 19/17/14 and 18/16/13.

The table's footnotes carry its conditions. Poor conditions "can be caused by large flow rate fluctuations, pressure peaks, frequent cold starts, extremely high ingress of dirt or the presence of water," and "Two or more system filters with the recommended filtration rating may be required to achieve and maintain the required target cleanliness class." The handbook's closing note says its information "refers to the described operating conditions and applications."

Parker's 2012 guide gives "Suggested acceptable contamination levels" in a single list without pressure bands: 17/15/12 for "Highly sophisticated systems and hydrostatic transmissions," with proportional valves as the typical component and sensitivity "Critical"; 16/14/11 for "Performance servo and high pressure long-life systems," with industrial servo valves, also "Critical"; and 15/13/09 for a "Silt sensitive control system with very high reliability," with high performance servo valves, "Super critical." HYDAC's below-140-bar servo-valve value (16/14/12) and Parker's industrial servo-valve value (16/14/11) differ by one class in the third field, and both are supplier recommendations, not ISO requirements (HYDAC EN 7.603.12/12.24, 2024, pp. 22–23 and back page; Parker FDCB805UK, 2012, p. 8).

How is a system target set when several components need different codes?​

HYDAC's handbook states: "For system cleanliness, we recommend working at one class better than the required cleanliness for the most sensitive component. Filling filtration / flushing filtration to be at least one filtration rating finer than the system filters."

Applied to an example, as engineering reasoning using HYDAC's table: a press-type handling axis running above 200 bar under poor conditions with a servo valve, a variable piston pump and cylinders has, from the table, targets of 15/13/10 for the servo valve, 16/14/11 for the pump and 18/16/13 for the cylinders. The servo valve is the most sensitive component, and one class better in each field gives a system target of 14/12/9. The same system with only a proportional valve, at the same pressure, would start from 16/14/11 and arrive at 15/13/10.

ISO has its own method for this step at standard level. ISO 12669:2017 "specifies a method of determining the required cleanliness level of a hydraulic system, that is, the most appropriate fluid cleanliness level for an operating hydraulic system based upon the individual requirements of that system." It is applicable where cleanliness is expressed to ISO 4406, "although conversion to other contamination coding systems is possible," it applies to high and low pressure fluid power systems and lubrication systems, and it "does not include the effects of soft deformable particles that can be generated by thermal decomposition of the hydraulic fluid" (HYDAC EN 7.603.12/12.24, 2024, pp. 22–23; ISO 12669:2017).

How does contamination fail a proportional or servo valve?​

The Pall paper, written for commercial-aviation hydraulics, states that the spool/sleeve mechanism, "In almost all cases," is "the most sensitive to contaminant related failures." Its Table 1, which the authors extracted from the ASME Wear Control Handbook (1980), gives a dynamic fluid film thickness of 1–4 µm between a servovalve spool and sleeve, and the paper notes sliding clearances in hydraulic components "down to 0.5 µm."

The authors name three mechanisms. "Mild stiction causes an increase in break-out force resulting in jerky valve movement. Severe stiction can cause jamming failures in some valve designs." Silting "occurs when the valve is stationary and pressurized"; "Even particles smaller than the clearance can form a 'dam' and cause silting," and the valve "can become unstable with a large hysteresis. In severe cases, the valve can become jammed and inoperable, sometimes called contaminant lock." Erosion by high-velocity particles at servovalve metering edges reduces "the pressure gain, increasing null leakage."

The paper also reports earlier studies second-hand. As reported by Pall, Tessmann and Foord (Oklahoma State University) found that "for the particular valve tested, the highest sensitivity was in the 5-15 µm size range," and Black (Moog) found hysteresis rising with dirt concentration and returning to its original value once the oil was filtered clean, with permanent degradation in pressure gain (17% decrease) and null leakage (from 3% to over 20% of rated flow). The authors add that, as of 1996, "there is no standard industry method for evaluating the sensitivity of a servo." Parker's guide states that the most damaging contaminants "are normally between 6 and 14 microns," and HYDAC lists "Loss of control accuracy" and "Control spool blockage" among the consequences of solid contamination (Bensch et al. 1996, pp. 1–4; Parker FDCB805UK, 2012, p. 4; HYDAC EN 7.603.12/12.24, 2024, p. 7).

Why is new oil not clean enough, and how is a system flushed before the valve goes in?​

Moog's D633K–D635K instructions state: "New oil is never clean. Therefore the system should generally be filled by using a filling filter," a fine mesh filter that "should at least comply with the following requirement: β10 ≥ 75 (10 μm absolute)." Parker's guide lists 20/18/15 as "Typical cleanliness of new hydraulic oil straight from the manufacturer," and HYDAC states that "According to DIN 51524, a cleanliness of ISO 21 / 19 / 16 must be ensured for fresh hydraulic oil"; that is HYDAC's statement of DIN 51524.

Moog's flushing procedure runs in five steps. Before first start-up, "also after modifications," the system "has to be flushed carefully according to the manufacturers instructions of the plant/machine." Flushing elements replace the high-pressure filter elements, and the system is brought to operating temperature. A flushing plate or, "if the system allows," a directional valve is mounted in place of the proportional valve, with Moog's warning that the directional valve "can lead to unpermissable movements in the load (i.e. with parallel drives)." "The flushing process can be considered completed when a system cleanliness of 18/15/12 according ISO 4406 is achieved," and "A long life of the metering lands of the proportional valve can be expected for a cleanliness of 17/14/11." The high-pressure elements and the valve then go back in.

As engineering reasoning, the flushing plate keeps the oil that carries assembly and installation debris away from the valve's metering edges, and the flush acceptance code belongs in the commissioning record (Moog CA49304-001 Rev. G, 2011, p. 13; Parker FDCB805UK, 2012, p. 8; HYDAC EN 7.603.12/12.24, 2024, pp. 22–23).

What does a filter's β rating tell a buyer, and what does it not?​

ISO 16889:2022 is the multi-pass test for hydraulic filter elements at standard level. Its abstract describes "a multi-pass filtration performance test with continuous contaminant injection," "a procedure for determining the contaminant capacity, particulate removal and differential pressure characteristics," and a test "currently applicable to hydraulic fluid power filter elements that exhibit an average filtration ratio greater than or equal to 75 for particle sizes ≥ 25 µm(c), and a final reservoir gravimetric level of less than 200 mg/L," using ISO medium test dust. It applies to three test conditions, with base upstream gravimetric levels of 3, 10 and 15 mg/L.

Supplier documents use the ratio without defining it. Moog's filling-filter requirement reads "β10 ≥ 75 (10 μm absolute)," and the Pall paper reports that the typical industrial specifications it lists for systems containing servo components or rolling contact bearings "generally" recommend a system filter rating of "3 µm or finer based on the industrial standard multi-pass filter test [31] for β=75 to 200," where its reference 31 is ISO DIS 4572 (1996), a predecessor of ISO 16889.

NASA's Engineering and Safety Center, writing about spaceflight propulsion and pressurant systems rather than industrial hydraulics, notes that "Basic filtration terms, such as 'nominal' and 'absolute' filter ratings, have different meanings from vendor to vendor." As engineering reasoning, a filter rating in a specification should name the test method and edition behind it, and a quoted β value from older literature may rest on a different test dust and calibration than a current one (ISO 16889:2022; Moog CA49304-001 Rev. G, 2011, p. 13; Bensch et al. 1996, p. 5; NASA NESC TB 22-02, 2022).

What did US standards require for servo systems, and what applies now?​

The Pall paper states that "the industrial standard NFPA/JIC T2.24.1-1990 [26] states that filtration shall be provided for hydraulic systems with servo components to limit the in-service particulate contamination level to an ISO 4406 Code of 14/10 (NAS 1638 Class 4)." That is a 1990 edition and the old two-number code, quoted here as history only.

The National Fluid Power Association's "List of Active NFPA Standards" does not include NFPA/T2.24.1; this article does not suggest that a current US systems standard sets a servo-system code. ISO 4413:2010 "specifies general rules and safety requirements for hydraulic fluid power systems and components used on machinery as defined by ISO 12100".

NAS 1638, the class system in the 14/10 equivalence, is described by HYDAC as follows: "Although this standard is no longer valid as a norm, it is often used in practice as it is easy to use (only one metric)." Unlike ISO 4406, it counts particle size ranges, not cumulative sizes (Bensch et al. 1996, p. 5; National Fluid Power Association 2024, Find a Standard; ISO 4413:2010; HYDAC EN 7.603.12/12.24, 2024, p. 12).

How is cleanliness measured and monitored on an operating machine?​

Two ISO documents separate a calibrated count from a trend. ISO 11500:2022 specifies an automatic particle counting procedure for "hydraulic-fluid bottle samples of clear, homogeneous, single-phase liquids using an automatic particle counter (APC) that works on the light-extinction principle," applicable to monitoring circulating fluid, "the progress of a flushing operation," support equipment and test rigs, and packaged stock fluid; its note allows dilution "when APC coincidence error limits are exceeded." ISO 21018-1:2024 covers monitoring "in hydraulic systems that cannot be calibrated in accordance with ISO 11171," and its note states that such instruments "are not considered as or claimed to be particle counters, even if they use the same physical principles as particle counters."

The valve itself is a sensor. Moog's D633/D634 valves "offer analog interfaces for command signal and spool position feedback." As engineering reasoning, a PLC that trends the spool feedback against command, the filter differential-pressure switches, oil temperature, and an online contamination monitor's output can flag a rising code or a sticking spool before the axis faults; the valve selection article describes the slower settling and growing position error that contamination produces.

UTEC Industrial builds this layer on Allen-Bradley ControlLogix and CompactLogix controllers, with PanelView and FactoryTalk operator interfaces over EtherNet/IP, in control panels built to UL 508A (ISO 11500:2022; ISO 21018-1:2024; ISO 11171:2022; Moog CDL 59872-en Rev. D, 2024, p. 3).

Where is fluid cleanliness decided in the chain from design to monitoring?​

NASA's bulletin, written for spaceflight propulsion and pressurant systems, found "only four sources of particulate contamination": particulate loaded with the fluid; particulate built into parts and components at the vendor; particulate introduced by manufacturing processes, "including welding and cutting at the sub-assembly and final assembly levels"; and self-generated particulate "produced by moving parts and soft-good/material degradation within the system." HYDAC's list of causes for an industrial system is similar: assembly contamination, environmental pollution, topping up hydraulic fluid, internal wear processes and oil ageing.

As engineering reasoning, each step of the chain carries a cleanliness task:

  • Design and engineering: the target code from the most sensitive valve, filter locations and ratings, and sampling points.
  • Parts machining, fabrication and assembly: deburring and cleaning of manifolds, tubes and reservoirs after cutting and welding, and capped ports until assembly.
  • Drives and controls: filter-indicator and temperature inputs wired to the PLC.
  • Tuning and commissioning: a flush to the valve maker's acceptance code with a flushing plate in place before the servo valve is installed.
  • Monitoring: a sampling or online-monitor schedule tied to the target code.

UTEC Industrial performs FAT (factory acceptance testing) and on-site commissioning, and a flush acceptance code is one of the values those records can carry (NASA NESC TB 22-02, 2022; HYDAC EN 7.603.12/12.24, 2024, p. 6; Moog CA49304-001 Rev. G, 2011, p. 13).

What should a specification for a proportional- or servo-valve system state about cleanliness?​

As engineering reasoning, drawn from the sources above, a specification for a closed-loop hydraulic system should define:

  • The target code and its source: the three-number ISO 4406 code, the edition cited, and whether the value is the valve maker's requirement, a filter supplier's recommendation, or a value set by the ISO 12669:2017 method.
  • The counting basis: µm(c) sizes, the volume basis (per ml or per 100 ml), and the counting method, either bottle samples to ISO 11500:2022 or an online monitor described to ISO 21018-1:2024.
  • Filtration: filter locations, the rating and the test method behind it (ISO 16889:2022 for the multi-pass test), and any second filter that HYDAC's footnote says may be needed.
  • Filling and flushing: filling through a filter (Moog: β10 ≥ 75, 10 µm absolute, in its D633K–D635K instructions), flushing with a plate in place of the valve, and the acceptance code (Moog: 18/15/12 for its D633K series).
  • Monitoring: sampling intervals, alarm levels set against the target code, and the PLC inputs that trend them.

Each of these items can be checked at factory acceptance and commissioning. The documents quoted here date from 1996 to 2024 and describe different ISO 4406 editions, and the specification should name the edition of each (ISO 4406:2021; ISO 12669:2017; ISO 11500:2022; ISO 21018-1:2024; ISO 16889:2022; Moog CA49304-001 Rev. G, 2011, pp. 7 and 13; HYDAC EN 7.603.12/12.24, 2024, p. 22).

Related Articles

References​

  • ISO 4406:2021: Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles. International Organization for Standardization, 2021.
  • HYDAC EN 7.603.12/12.24: Fluid controlling: Contamination handbook. HYDAC Filter Systems GmbH, 2024.
  • Parker FDCB805UK: Guide to Contamination Standards. Parker Hannifin Manufacturing (UK) Ltd., Hydraulic Filter Division Europe, 2012.
  • Moog CDL 59872-en Rev. D: Direct Drive Analog Control Servo Valves D633 Size 03, D634 Size 05. Moog Inc., 2024.
  • Moog CA49304-001 Rev. G: Operating Instructions for Proportional Valves D633K, D634K and D635K Series. Moog Inc., 2011.
  • Bosch Rexroth RE 29061/10.05 (2005): 4/2 and 4/3 Proportional Directional Valves, Direct Operated, with Electrical Position Feedback, Types 4WRE and 4WREE. Bosch Rexroth, 2005.
  • Bensch, L. E., Needelman, W. M., and Lundquist, J. (1996). "Improving Commercial Airliner Hydraulic System Reliability Through Contamination Control." Presented to the FAA Hydraulics Task Force, SAE A-6 Committee, Orlando, FL, 15 April 1996. Pall Corporation.
  • NASA Engineering and Safety Center. Revisiting Filtration Standards and Definitions for Spaceflight Propulsion and Pressurant Systems, Technical Bulletin No. 22-02. NASA, 2022.
  • ISO 16889:2022: Hydraulic fluid power — Filters — Multi-pass method for evaluating filtration performance of a filter element. International Organization for Standardization, 2022.
  • ISO 11171:2022: Hydraulic fluid power — Calibration of automatic particle counters for liquids. International Organization for Standardization, 2022.
  • ISO 12669:2017: Hydraulic fluid power — Method for determining the required cleanliness level (RCL) of a system. International Organization for Standardization, 2017.
  • ISO 11500:2022: Hydraulic fluid power — Determination of the particulate contamination level of a liquid sample by automatic particle counting using the light-extinction principle. International Organization for Standardization, 2022; amended by ISO 11500:2022/Amd 1:2026.
  • ISO 21018-1:2024: Hydraulic fluid power — Monitoring the level of particulate contamination of the fluid — Part 1: General principles. International Organization for Standardization, 2024.
  • ISO 4413:2010: Hydraulic fluid power — General rules and safety requirements for systems and their components. International Organization for Standardization, 2010.
  • National Fluid Power Association (2024): Find a Standard. National Fluid Power Association, 2024.

Ready to Discuss a Material Handling System?​

UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for heavy industry, from the stress-relieved structure and drives to the Allen-Bradley PLC controls, tuning, and monitoring that run them, at its Spokane Valley, WA facility. Send UTEC the application, loads, and duty cycle to start a system review.

Request a Quote →

Questions? Call (509) 922-1832 or email sales@utec.co

Last updated on