Hydraulic fluids perform all sorts of different tasks:

• transferring forces and movement
• control functions
• lubricating moving parts, including protecting against wear and reducing friction
• protecting against corrosion
• cooling (temperatures > 100°C possible)
• dampening vibrations

As hydraulic systems are optimised, so the requirements placed on hydraulic fluids increase. Systems are becoming ever more compact and designed for smaller oil quantities. The oil consequently spends less time in the oil container, and has less time to cool to the ideal mineral oil temperature of less than 60°C. At a temperature of 10°C higher, however, the increased oxidation tendency of mineral oils means that oil service life is halved. Due to oxidation, a mineral oil-based hydraulic oil that is changed after 10,000 operating hours at 60°C must be changed after only 2,500 operating hours at 80°C. As such, the oxidation or ageing stability of oils is becoming ever more relevant. Temperatures are not only rising because the oil quantities are falling, but also because pressures are increasing. If only a few years ago oil suited to 400 bar of pressure was needed for hydraulic engines and cylinders, today it is not unusual to encounter demands for oil suited to over 600 bar of pressure. With improved base oils and corresponding additives, modern hydraulic fluids are mastering these higher temperature requirements, as well as increased mechanical strains, and are contributing decisively to the continuous operability of the systems involved. These high demands can often only be met by using synthetic multi-purpose hydraulic oils, which are more suited to long-term use. The advantages of synthetic oils over conventional mineral oils can also be clearly proven using oil analysis.

The analysis set 2 is usually recommended for the routine monitoring of mineral oil-based hydraulic oils from plants with filling volumes up to approx. 1,000 liters. 

The scope of analysis of Set 2 includes:

• Wear metals: iron, chromium, tin, aluminum, nickel, copper, lead, manganese.
• Magnetizable iron: PQ index ƒ Additives: zinc, phosphorus, sulfur, silicon (antifoam), calcium, magnesium, barium, boron, molybdenum
• Impurities: Silicon (dust), potassium, sodium, lithium (grease), water (above 0.1% measured by IR spectroscopy and by means of a "splash sample").
• Oil condition: viscosity at 40 °C and 100 °C, viscosity index, oxidation by FT-IR and optical impression (diesel effect)
• Oil cleanliness: cleanliness class according to ISO 4406 (particles >4 µ, 10µ and 14 µ).
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For the evaluation of larger oil fillings or when using synthetic oils, the analysis set 4 should be used.

The scope of analysis of Set 4 additionally includes:

• the exact water content in ppm 
• the acid content as Acid Number (AN)

With additional "individual tests" (note: this requires a further quantity of oil), the compatibility can be assessed even better. Ultimately, however, close observation of the system is always decisive, as the actual conditions cannot be reproduced in detail with a laboratory analysis.

Foam occurs on the surface of oil charges when air bubbles of a few µ to 1 mm in size separate from within the interior of an oil tank charge, rise to the surface, and then fail to immediately burst or disintegrate. These then form a stable foam layer on the oil surface. Foam formation is influenced by the surface tension of the oil, by operating temperature and its effects on viscosity, and by the air intake method. Mixtures of oils of different compositions, impurities, and also oil oxidation can cause an oil to foam more strongly. Oils are often designated as miscible. This does not mean, however, that oils are always truly „compatible“ with one another. When a PAO-based oil is mixed with a detergent HLP hydraulic oil, or a saturated ester-based bio-oil is mixed with an unsaturated ester, the surface tension of the fluids is altered. It is also possible for differently formulated oils in which foaming tendency has been improved by the addition of silicone-containing additives to change their foaming tendency when mixed, which can lead to the emergence of foam from all possible openings. For most systems, a foam layer up to 5 cm high presents no issue. Problems arise, however, when a sudden increase in foaming tendency is observed. In these cases, the surface foam can act as an insulating carpet, reducing heat dissipation, or flow from openings in the system. In addition to polluting the environment, the resulting loss of oil can lead to insufficient lubrication. The more active EP and AW ingredients an oil contains as additives, the more likely it is to form surface foam. Anti-foam additives, which are mostly silicone-based, are added to the oil during production. When mixing oils containing different additives, special caution is recommended in the case of oils with significantly different silicon contents. Even if the oils are approved for the same purpose or meet the same specifications, this does not mean that they will exhibit the same foaming tendency when mixed. Even mixing an oil containing only few additives with an oil that contains many often leads to increased foam formation. In specific cases, the problem can be avoided by adding an anti-foam additive provided by the oil manufacturer. In the majority of cases, however, OELCHECK tribologists are obliged to urgently recommend a complete oil change.

Alongside water, failures in hydraulic systems are most frequently caused by hard contaminants in the form of dust or wear particles. Hydraulic systems therefore require filters in order to function properly. These ensure that the entire system is able to operate, as well as enabling the longest possible service life for its components and oil. Modern hydraulic fluids must demonstrate good filterability. The average pore width of filter media was previously 10 to 20 μ. Today, however, the filters fitted in these systems have a pore width of 3 to 12 μ. Lab data on the filterability of an oil describe its behaviour when flowing through a filter. In the event that excessively short filter lifespans are noted following a change of oil or filter, the test results of the used oil should be compared with those of the fresh oil. In practice and in laboratory tests, the oil responsible for the problem often leaves dark, sticky deposits on the filter medium or has a low oil purity. The cause may lie in a differing additive content, or in the differing base oils of the mixed oil types. Filterability testing will ultimately provide information on the cause of the poor filterability and short filter lifespan.

Hydraulic fluids are miscible with one another – with the exception of PAG-based hydraulic fluids. Whether oils which the manufacturer has designated as miscible are also compatible with one another, on the other hand, can only be established by thorough laboratory testing. To this end, alongside constituent additives and base oils, such characteristics as foaming tendency, air release time, water separation, and filterability must also be examined. In the event that a problem with a hydraulic system caused by mixing incompatible oils does occur, an oil change is generally the only available solution.