Ask a rebuilder what killed a pump and the honest answer is usually contamination. Particles too small to see, circulating for months, lap open the clearances that generate pressure. The failure appears as wear on the plunger pair or the valve plate, but the cause was decided long before, at the filtration and maintenance stage.
This article translates oil cleanliness into decisions a maintenance team can act on: which ISO code to target, which filter rating actually achieves it, and how to flush a system so that a rebuilt pump is not destroyed in its first week.
ISO 4406 in Plain Terms
Cleanliness is reported as three numbers, for particles above 4, 6 and 14 micrometres per millilitre of oil. A code of 18/16/13 means the oil contains a quantity of particles in each size band corresponding to that scale number; each step up the scale is a doubling of the count.
That doubling is what makes the code commercially important. Moving from 18/16/13 to 16/14/11 is not a marginal improvement — it is roughly a fourfold reduction in the particle population in each band, and it changes pump life from months to years.
| ISO code | Particle population | Typical application | Expected effect on a piston pump |
|---|
| 22/20/17 | Very dirty | Failed system, post-breakdown | Rapid wear; unacceptable for any piston pump |
| 20/18/15 | Dirty | Neglected mobile hydraulics | Accelerated wear, shortened component life |
| 18/16/13 | Moderate | Common target for general mobile equipment | Acceptable for many machines; not for high-pressure pumps |
| 16/14/11 | Clean | Standard target for piston pumps and servo valves | Design life achievable |
| 14/12/9 | Very clean | High-pressure, high-duty and precision systems | Extended life; recommended for 350 bar duty |
For a piston pump running at 300 bar and above, 16/14/11 is the sensible minimum and 14/12/9 is defensible for machines on continuous duty. Accepting 18/16/13 on a high-pressure pump is a decision to shorten its life, whether or not anyone says so out loud.
Particle Size Versus Oil Film
The reason cleanliness targets are expressed in micrometres is that damage is caused by particles comparable in size to the film that separates moving surfaces.
| Particle size | What it can do |
|---|
| Below 4 µm | Passes through most clearances; limited mechanical damage but contributes to varnish and oil degradation |
| 4–14 µm | The damaging band for piston pumps: comparable to film thickness, causes three-body abrasion and silting of control orifices |
| 14–25 µm | Blocks fine control passages, scores valve plates, jams spool lands |
| Above 25 µm | Visible damage, but far less common than the finer bands and easier to capture |
This is why a filter described as "10 micron nominal" is not the same as a filter that delivers 16/14/11. Nominal ratings describe an arbitrary capture percentage; only the beta ratio describes performance.
Beta Ratio: The Only Filtration Number That Means Anything
The beta ratio at a given particle size is the number of particles upstream divided by the number downstream. A beta ratio of 200 at 6 micrometres — written β6 = 200 — means 99.5 percent of particles at that size are captured in a single pass.
| Rating | Efficiency | Practical meaning |
|---|
| βx = 75 | 98.7% | Minimum credible specification for a pressure line filter |
| βx = 200 | 99.5% | Standard for pump protection; common in return and pressure filters |
| βx = 1000 | 99.9% | High-efficiency elements, often used on critical circuits |
Filter placement matters as much as rating. A pressure filter protects the components downstream but not the pump itself; a return filter cleans oil on its way back to the tank; a suction strainer protects only against gross debris and should never be used as the sole filtration. Offline kidney-loop filtration is the most effective way to raise cleanliness on an existing machine without changing its plumbing.
A Contamination Control Plan
Cleanliness is maintained, not achieved once. The following plan is deliberately simple enough to survive a busy workshop.
| Element | Action | Frequency |
|---|
| Target | Set ISO code per machine class; write it on the service sheet | Once, then review annually |
| Reservoir | Check breather, seal openings, clean filler necks | Every service |
| Filters | Change on differential pressure or schedule, never on appearance | Per hour interval or gauge reading |
| Oil sampling | Draw from a live line at operating temperature, mid-stream | Quarterly, or monthly on critical machines |
| Transfer equipment | Filtered dispensing only; dedicated clean containers | Every top-up |
| Repair work | Cap all open ports, clean fittings before assembly | Every intervention |
The two most common failures of this plan are topping up with unfiltered oil from an open drum and judging filter condition by looking at it. Both undo months of good practice in minutes. A third, less obvious failure is water. Water enters through breathers, condensation in a partly filled reservoir and wash-down, and it does more damage than most solid particles: it destroys film strength, promotes corrosion pitting on the block face, and accelerates oil oxidation. Sampling should therefore include a water content figure alongside the ISO code, with anything approaching 0.1 percent by volume treated as a fault rather than a curiosity.
Flushing After a Repair
Fitting a rebuilt pump into a contaminated system is the fastest way to create a repeat failure. Flush before the new pump goes on.
- Remove or bypass the new pump; connect a flushing rig or use the machine's own charge circuit with a temporary return filter.
- Use warm, low-viscosity flushing oil and circulate at turbulent flow, exercising all functions and cylinders through full stroke.
- Change the flushing filter whenever differential pressure rises, and continue until the element stays clean.
- Sample at the end of the flush and confirm the ISO target before installing the pump.
- Install the pump, fill with filtered oil, and sample again after the first 50 hours.
Skipping step four is common and expensive. A flush that "looks clean" in the reservoir can still be at 19/17/14 in the lines, which is enough to destroy a new rotating group within weeks.
Components Worth Protecting
Cleanliness pays back most on high-pressure rotating groups and precision travel motors. Rexroth A10VT travel motors are covered by A10VT28 to A10VT71 spare parts, where a scored valve plate from contamination is the most frequent failure mode. Sauer Danfoss HMV280 motors use HMV280 motor spares with tight internal clearances that silt up quickly when cleanliness slips. Kawasaki K7V excavator pumps are served by K7V63 and K7V125 repair parts, and their regulators are particularly sensitive to fine particles in the control orifices.
When ordering, give us the pump model and nameplate data, the part numbers required, the machine model and the quantity. We will also tell you the cleanliness level we recommend for that frame size, because the filter decision and the parts decision belong in the same conversation.
Frequently Asked Questions
Is 10 micron filtration enough for a piston pump?
Only if the element carries a credible beta ratio at that size. A nominal 10 micron filter may deliver 20/18/15. Specify β6 = 200 or better and verify with an oil sample.
How often should oil be sampled?
Quarterly is a reasonable minimum for mobile equipment; monthly for critical machines or after any repair. Sample from a live line at operating temperature, not from the reservoir top.
Can I improve cleanliness without rebuilding the system?
Yes. An offline kidney-loop filter unit run for a few days will raise cleanliness significantly and is the least disruptive route on an existing machine.
Does new oil arrive clean?
Not reliably. New oil is often around 18/16/13 or worse as delivered. Filter it on transfer, or accept that the first top-up contaminates the system.
Your message must be between 20-3,000 characters!