Determination of the purity of oil, the purity classes…
In addition to the basic properties of “viscosity” and “wear protection”, the purity of lubricating and hydraulic oils is particularly crucial for the safe operation of machines and systems. The degree of contamination of the oil is determined by the size and number of particles present in the oil.
Particle counting determines the number of particles per size class per 100 ml of oil, which can be displayed as shadows in the laser sensor. No distinction is made between the type (soft or hard) and shape of the particles (long or round).
To simplify the assessment of the degree of contamination, a classification is made into so-called “cleanliness classes”. First, the number and size of the particles are determined. The number of particles is then used to assign the product to a cleanliness class. The methods for determining oil cleanliness and the assignment of cleanliness classes are defined in ISO 4406 and NAS 1638.
(Source: Oelcheck)
Purity classes according to ISO 4406
Since 1999, ISO 4406 has specified three classes >4µ, >6 µ and >14µ when evaluated with a counter. If the particles are counted manually on a filter, only 2 cleanliness classes (>5µ and >15µ) can still be specified. In ISO particle counting, the number of particles is cumulative, i.e. the number of particles >5µ also includes the particles > 15 µ. The particles counted in an oil sample are related to 100 ml according to the table and assigned to a purity class for each size class.
The ISO purity class is given as a composite number, e.g. 21/18/17. The first number refers to particles >4µm, the middle number to particles >6µm and the right-hand number to large particles >14µm. (Source: Oelcheck)
Purity classes according to NAS 1638
The determination of cleanliness classes according to NAS (National Aerospace Standard) is based on the requirements of modern aircraft hydraulic systems. Compared to the ISO standard, NAS 1638 also differentiates between particles larger than 15 µm.
According to NAS 1638, particles are counted in 5 size classes and a purity class from 00 to 12 is assigned for each size range.
However, only the largest (worst) of the 5 individual classes (e.g. 9) is specified as the NAS class. The counting method is differential, i.e. the number of particles actually present in a class is specified. Particles are counted per 100 ml of oil in the sizes 5-15, 15-25, 25-50, 50-100 µm and > 100 µm. (Source: Oelcheck)
Specified cleanliness classes of the component manufacturers
For components such as pumps, valves, etc: pumps, valves, etc., manufacturers usually specify cleanliness classes. These can be found on the product data sheets, in operating instructions or other enclosed documents.
If a component fails, in most cases, depending on the cost of the damage, an oil sample is taken and the individual component parts are examined in detail under a microscope. In many cases, the manufacturer can detect increased contamination and the customer must bear the costs for the component and the associated inspection (depending on the product and manufacturer).
Guide value purity classes
(Source Argo-Hytos)
| ComponentPurity | class according to ISO 4406 |
| Axialkolben- Pumpen und Motoren | 21/18/15 |
| Radialkolben- Pumpen und Motoren | 21/18/15 |
| Zahnrad- Pumpen und Motoren | 21/18/15 |
| Flügelzellen- Pumpen und Motoren | 20/17/14 |
| Wegeventile (Magnetventile) | 21/18/15 |
| Druckventile (regelnd) | 21/18/15 |
| Stromregelventile | 21/18/15 |
| Rückschlagventile | 21/18/15 |
| Proportionalventile | 20/17/14 |
| Servoventile | 17/14/11 |
| Zylinder | 21/18/15 |
Purity classes in connection with the operating pressure
(Source Argo-Hytos)
| Operating | pressureChange in oil purity |
| 0-100 bar | + 3 |
| 100-160 bar | + 1 |
| 160-210 bar | 0 |
| 210-250 bar | – 1 |
| 250-315 bar | – 2 |
| 315-420 bar | – 3 |
| 420-500 bar | – 4 |
| 500-630 bar | – 5 |
The higher the working pressure, the cleaner the medium must be!
If you have any questions about oil cleanliness and off-line filtration, please contact us: lars.benk@schuhmacher-tech.de or +497424/9495663
Lars Benk: Profile of a field-tested expert
Basic training: Tool mechanic, specializing in stamping and forming technology.
Further training: State-certified technician with a focus on mechanical engineering.
Additional qualification: Degree in technical business administration, combining technical know-how with business expertise.
Professional experience: More than a decade of active work in the field of hydraulics.
Specialization: Certified specialist in oil analysis (MLA II – Machine Lubrication Specialist), with a specific focus on the maintenance and optimization of machines through professional lubricant analysis. Certified specialist for hydraulic fluids – specialist knowledge for optimal hydraulic systems.
