Hydraulic filter

01

Set the cleanliness target as an ISO 4406 code

Donaldson explains that ISO 4406:2017 cleanliness codes such as 22/21/18 give the particle count range at 4 µm and larger, 6 µm and larger and 14 µm and larger. In its table per 100 mL, code 22 means 2,000,000 to 4,000,000 particles, 21 means 1,000,000 to 2,000,000 and 18 means 130,000 to 250,000; each code step up doubles the particle count and each step down halves it. It notes some organisations count per 1 mL, giving ranges 1/100 of the per-100 mL chart, and that ISO 4406 says nothing about the type of contamination. [2]

Hy-Pro, part of Donaldson, says targets depend on component sensitivity, system pressure and operating environment, that component and bearing makers set ISO code limits as maximum tolerances, and suggests 16/14/11 or lower for sensitive equipment such as servo valves. It lists upgrading elements, correct filter placement, proper reservoir breathers, regular oil sampling and filtering new oil before it enters the system as the steps to reach a target. [4]

Editorial calculation: in the per-100 mL table, 16/14/11 means 32,000–64,000 particles of 4 µm and larger, 8,000–16,000 of 6 µm and 1,000–2,000 of 14 µm; moving from 22/21/18 to 16/14/11 cuts the 4 µm count by six steps, or a factor of 2⁶ = 64. [2] [4]

Give the supplier the target ISO 4406 code and the component with the strictest cleanliness requirement, fluid type and viscosity range, maximum and cyclic flow, system pressure, filter position (pressure, return, suction, off-line), required Beta ratio at a stated particle size and test standard, allowable clean and terminal pressure drop, bypass valve setting, element collapse pressure, dirt-holding capacity, connection and housing material, seals, temperature range, and quantity.

02

Specify Beta ratio, particle size and test

Donaldson says a micron rating alone is of little value: the Beta ratio is the number of particles of a given size upstream divided by the number downstream, measured with particle counters on fluid dosed with ISO test dust. Using ISO 16889 values, a Beta ratio of 2 means 50% efficiency, 10 means 90%, 20 means 95%, 75 means 98.7%, 100 means 99%, 200 means 99.5%, 1,000 means 99.9% and 2,000 means 99.95%. A tighter filter than needed can shorten element life, raise pressure drop or strip fluid additives, while a looser one lets more damaging particles through. [1]

Donaldson notes that ISO 16889 uses a steady flow rate and medium test dust, while the newer ISO 23369 cyclic-flow multi-pass test, intended for elements used in cyclic flow, changes flow at a 2:1 to 4:1 ratio — the standard suggests every five seconds at 4:1 — and uses fine dust; it says elements tested under cyclic flow are challenged harder in efficiency and capacity because captured particles can be released when flow changes. [3]

Editorial calculation: at Beta 200, 1 particle in 200 passes; at Beta 1,000, 1 in 1,000 — so going from 200 to 1,000 cuts the particles passing at that size by a factor of five even though efficiency only rises from 99.5% to 99.9%. [1]

03

Compare what suppliers offer

Suppliers linked to this page list hydraulic filters or elements; each card links to the page reviewed.

Listings include hydraulic filter elements alongside oil, fuel and air filters, and wire-mesh hydraulic oil cartridges; ask each for the Beta ratio at a stated particle size and the test standard, element collapse pressure, dirt-holding capacity, and the original equipment numbers it replaces.

04

Trace the filter construction

This is a functional reference built from the sources cited; it is not one maker's specification.

05

Fill the specification fields with values for your filter

A hydraulic filter quotation needs values for your system. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the target, flow and test basis together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Target: [ISO 4406 code; sensitive component]
Fluid: [type; viscosity; temperature]
Flow: [nominal; cyclic]
Pressure: [system; allowed pressure drop; bypass]
Efficiency: [Beta ratio at µm; ISO 16889 or 23369]
Position: [pressure, return, suction, off-line, breather]
Housing: [connection; seals; material]

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