Functions of Hydraulic and Lubricating Fluids
What functions do hydraulic and lubricating fluids perform?
Hydraulic fluids transfer energy from the pump to cylinders and motors. At the same time, they protect the system from corrosion, dissipate heat, and lubricate moving components. Lubricating oils perform comparable functions in lubrication circuits. These functions can only be reliably fulfilled if the fluids do not age prematurely and their properties remain stable over the long term.
Filterability of Hydraulic and Lubricating fluids
What determines the filterability of hydraulic and lubricating fluids?
Filterability depends primarily on the viscosity and the colloidal composition of the fluid. Impurities can alter the structure and lead to filter blockages. Additives must remain in the oil – therefore, the manufacturer must ensure reliable filterability.
Types of Hydraulic fluids
What types of hydraulic fluids are there?
Mineral oils:
Standard in hydraulic and lubrication systems, improved by additives. Classified according to DIN 51524.
Flammable liquids:
For high-risk areas such as underground mining, die casting, and welding machines. Subdivided into:
- HFA: Oil-in-water emulsions (max. 20% oil)
- HFB: Water-in-oil emulsions (rare in Germany)
- HFC: Polyglycol-aqueous solutions
- HFD: Anhydrous synthetic fluids (HFDR / HFDU)
Biodegradable hydraulic fluids:
For agriculture, forestry, or mobile machinery. Types: HETG, HEES, HEPG
Lubricating oils and filter fineness
What are lubricating oils and what filter finenesses are used?
Lubricating oils (e.g., for turbines, circulating lubrication systems) are usually Newtonian fluids based on mineral oil. Filter finenesses of 10–25 µm are typically used for filtration, depending on the viscosity and the components.
Types of contamination
What types of contamination are there?
Contamination in fluid and hydraulic systems directly affects the performance, service life, and availability of machines. Three main types are distinguished:
Solid particles (wear and dirt particles): These are formed by abrasion or external influences and lead to increased wear. Consequences: Component damage, impaired control behavior, failures, and reduced machine availability. Action: Targeted filtration for particle separation.
Liquid contaminants (water): Water is present in the fluid, either free or dissolved, and impairs its properties. Consequences: Corrosion, wear, oil aging, reduced lubrication, reduced filter life, and decreased machine availability. Measures: Ventilation filters, water-absorbing filter elements, and vacuum dryers.
Gaseous contaminants (air): Air enters the system through leaks or insufficient venting. Consequences: Foaming, energy loss, pump damage, and oxidation. Measures: Venting, sealing, and use of vacuum dryers.
Recommended oil cleanliness
What oil cleanliness values are recommended for which filter material?
The required oil cleanliness is crucial for the reliability and service life of hydraulic systems. The more sensitive the components are to contamination, the higher the requirements for oil cleanliness according to ISO 4406 and filter fineness.
For servo systems and systems with very high availability, a particularly high oil cleanliness of ≤ 16/12/9 is recommended, achieved with 1-µm filters.
Servo valve technology and systems with contamination-sensitive components generally require an oil cleanliness of ≤ 18/13/10 with 3-µm filters.
For proportional valves and higher pressures, oil cleanliness values of ≤ 18/14/11 with 6-µm filters are sufficient.
Pumps, motors, and modern industrial hydraulics operate reliably with oil cleanliness levels of ≤ 19/16/13 to ≤ 20/16/13 and 10 µm filters.
For hydraulic systems with larger tolerances, an oil cleanliness level of ≤ 21/17/14 with 20 µm filters is sufficient.
The right combination of oil cleanliness and filter fineness ensures reduced wear, increased system availability, and economical operation.
Oil cleanliness classes according to ISO 4406
What oil cleanliness classes exist according to ISO 4406?
The ISO 4406 standard measures the number of particles in oil per 100 ml in three size classes: ≥ 4 µm, ≥ 6 µm, and ≥ 14 µm. Contamination levels are indicated by codes from 5 to 21 – the higher the code, the more particles are present.
Example: Code 18/16/11 represents 190,000 particles ≥ 4 µm, 58,600 particles ≥ 6 µm, and 1,525 particles ≥ 14 µm.
ISO 4406 is the modern standard and replaces older standards such as NAS 1638, which are no longer valid. It ensures a precise and internationally recognized assessment of oil cleanliness in hydraulic systems.
Achievable oil cleanliness according to ISO 4406
Which filter material is used for which oil cleanliness class?
The actual achievable oil cleanliness depends on many factors, such as particle size, shape, material, operating conditions, and filter maintenance. Therefore, oil cleanliness classes cannot be guaranteed but can only be given as guidelines.
The following typical cleanliness ranges apply to glass fiber filter materials for common operating fluids:
Filter type ISO 4406 Oil cleanliness class
Glass fibre 20 µm 19/16/12 - 22/17/14
Glass fibre 10 µm 17/14/10 - 21/16/13
Glass fibre 6 µm 15/12/10 - 19/14/11
Glass fibre 3 µm 13/10/8 - 17/13/10
Glass fibre 1 µm 10/6/4 - 14/8/6
For low-flammability fluids such as HFA and HFC types, a contamination level approximately one ISO class higher should be expected. The precise determination is made via microscopic particle counting.
Overview of the most important filter parameters
What filter parameters are there?
Filter efficiency is determined by various measurements:
- Nominal efficiency (manufacturer-specific)
- Absolute efficiency according to ISO 16889
- Mesh size for wire mesh
The evaluation of filter performance also includes dirt holding capacity according to ISO 16889 using ISOMTD test dust, as well as the differential pressure according to ISO 3968, measured at 30 cst/25 µm.
These three parameters – filter efficiency, dirt holding capacity, and differential pressure – are directly related.
Filter design
What data is needed for filter design?
For the design of a hydraulic filter, precise information about the system, the operating conditions, and the fluid used is required. This includes details about the hydraulic fluid, pressure, flow rate, temperature, and viscosity, as well as the installation situation and filter type.
Functional requirements such as bypass valves, contamination indicators, and maintenance intervals are equally important. Based on this information, the filter housing, filter material, filtration rating, and service life are optimally matched.
Only with complete operating and system data can an economical, high-performance, and reliable filter solution be implemented.
Filter element replacement
What needs to be considered when replacing a filter element correctly?
Proper filter element replacement is crucial for the safety, performance, and lifespan of your system. Legal regulations must be observed and defined work steps followed – from depressurizing the system and replacing seals to properly disposing of the filter element. Maintenance indicators help identify the correct replacement time. The replacement should always be carried out by trained specialists to ensure safe and smooth operation.
Checklist for Short Filter Element Lifespan
What needs to be done?
- Check operating data: flow rate, pressure, temperature, cold start
- Check filter design: size, filter fineness, filter material
- Analyze sources of contamination: abrasion, water, air, external contaminants
- Evaluate oil condition: oil cleanliness, aging, top-up quality
- Check installation: correct assembly, seals, leaks
- Check maintenance: contamination indicator function, change intervals
- Optimize: adjust filter settings, use pre- or bypass filtration
💡 Tip:
Short filter lifespans usually indicate high contamination or a suboptimal filter design.