No two hydraulic oils are the same
…or “Does hydraulic-oil mixing also mean hydraulic-oil compatibility?”
The user of hydraulic oils must know: “No two oils are the same!”
Practical experience shows that even hydraulic oils from different manufacturers that meet the same specifications differ quite significantly from one another.
1. cavitation in the valve due to mixing
If hydraulic fluids from different manufacturers or different types from the same manufacturer are mixed, sludging, sticking, deposits due to additive reactions or even a deteriorated air separation capacity can occur in practice. This may lead to foaming or an increased risk of cavitation. Malfunctions and damage to the hydraulic system are inevitable. For this reason, many manufacturers of hydraulic systems generally prohibit any mixing with other hydraulic fluids, including hydraulic fluids of the same classification, because modern lubricants are made from a variety of base oils and “ingredients”, i.e. complex combinations of active ingredients (additives). This additive cocktail can enhance the existing properties of the base oil or give the end product a required property for the application. The list of different base oils and additive components is very long and experience has shown that hydraulic oil manufacturers are not willing to show their cards.
As a general rule, miscibility does not automatically mean compatibility!
2. tank with deposits due to mixing
3. precipitation due to mixing
The experience of independent oil laboratories has shown time and again in their daily work with hydraulic problems that hydraulic oils with the same base oil, similar viscosity and comparable density mix very well with each other, but due to the different additives, incompatibilities occur during operation, which is then the reason for the hydraulic malfunction of the system.
Note: What is chemically miscible can have a highly negative effect on the physical properties of the hydraulic fluid.
Attention! Some oil suppliers advertise miscibility in their technical data sheets and only confirm this. However, the question from the user to the oil supplier must be: “Are the hydraulic oils compatible with each other after mixing?”
Miscibility describes a chemical property and is only given when two liquids easily dissolve completely in each other.
Compatibility, on the other hand, means that two fluids or two hydraulic oils can be easily mixed together, but must retain their individual properties. The different additive packages must not react against each other. This happens if it is not clarified with the oil supplier before mixing hydraulic oils with different designations whether the same or approximately the same additive performance is used in addition to the same base oil properties. Mixing oils with different base oils and different additives should be avoided as far as possible. Information on the compatibility of lubricants is often difficult or impossible to obtain from the oil supplier.
The following hydraulic fluids should not be mixed under any circumstances:
- Hydraulic oils of different classifications (HLP/HLPD/HVLP)
- Zinc-free and zinc-containing hydraulic oils
- Hydraulic oils and engine oils
- Oils with detergent and non-detergent properties
- Oils of the same classification and brands with different viscosities
- Biodegradable oils (HEES/HEPG/HETG/HEPR) and mineral oils
- Biodegradable oils of different brands despite the same classification (e.g. HEES)
- Biodegradable oils of different classifications (e.g. HEES with HETG)
- Glycol-based oils with all other synthetic oils or mineral oils
4. loss of lubricity due to mixing
The following problems may occur:
- Increased risk of surface foaming
- Deteriorated air separation capacity
- Increased risk of cavitation
- Changed behavior towards water
- Seal and hydraulic hose line wear
- Change in friction characteristics
- Shortened hydraulic oil service life
- Poorer filterability or reduction in filter service life
- Increased system contamination due to dissolved deposits
- Increased formation of sludge, deposits / varnish due to additive reactions
- Sticking, e.g. due to zinc soap
Fresh oil top-ups should only be carried out if the existing oil quality in the system still meets the requirements (viscosity, acid content, oxidation). Please also note that fresh oil is only filled using filling pumps with the appropriate filter fineness (absolute filtration). Otherwise there is a risk of dirt entering the system.
Avoid mixing oils, as there is no guaranteed 100% problem-free combination.
5. varnish by mixing base oil I and base oil II
Attention! Depending on the type of fluid and application, the incompatibilities can manifest themselves in rapid and significant hydraulic disturbances. However, these may only become apparent indirectly or in the long term.
Reputable manufacturers of hydraulic machines or systems do not permit oil mixtures and point out that failure to do so will invalidate any warranty claims.
Measures to avoid mixing
- Introduction of fluid management
- A close inspection of incoming goods, as the different types of lubricant from the same oil manufacturer differ only slightly in the long columns of letters and numbers.
- A well-organized oil store with marked storage locations for the various lubricants used reduces the risk of confusion.
- Empty or flush the oil filling pump, which can be used for different types of lubricant, after use and only use filter elements specifically marked with the type of oil.
- Training and sensitization of maintenance and servicing personnel
- on the subject of “Hydraulic oils in practical application”
Conclusion
- Yes, hydraulic fluids are generally miscible with each other – with the exception of PAG-based hydraulic fluids. But whether the oils for which the manufacturer has issued a miscibility declaration are also compatible with each other can either be determined by the oil manufacturer’s technical support (hopefully in writing) or, if not, this can only be determined by a detailed and elaborate compatibility analysis in the laboratory. In addition to the additives and the base oil, properties such as air and water separation capacity, foaming behavior, acid content and filterability at different mixing ratios must also be considered.
Thank you for the guest post:
Ulrich Hielscher – Managing Director – International Hydraulics Academy GmbH
https://www.hydraulik-akademie.de/home
DIE HYDRAULIK AKADEMIE also offers the seminar “Hydraulic oils in practice” on this topic, which can also be booked in-house, with the following content:
Legal basis
- Significance of the relevant legal regulations
Hydraulic fluids
- Hydraulic oils (e.g. mineral & organic oils) and their tasks
- Hydraulic oils according to DIN/ISO and their requirements
Enemies of hydraulic fluids
- Causes of failure, backgrounds and their effects
- Meaning of purity classes
- Resistance of hydraulic hose lines and sealing materials
Hydraulic filter
- Types, structure and function
Monitoring of hydraulic fluids
- Fluid management
Representative oil sampling and analysis
Analysis options
What do the values mean in practice and what
Measures are required
Oil condition monitoring?
Which techniques can be used?
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.
