The OELCHECK standard analysis sets cover all parameters relevant to statements regarding the further retention of a hydraulic fluid in the system. But sometimes a customer wants to delve deeper into the subject matter. In case of special questions and problems, our diagnostic engineers will be pleased to advise you. They will recommend individually selected, additional special tests. An entire palette of tests is available for your use.
Brugger test (DIN 51347)
While DIN 51524 defines the minimum requirements for wear protection of the hydraulic fluids, their load capacity values can be subject to extreme fluctuations in practice. If, for example hydraulic cylinders stick-slip or rattle, and hydraulic pumps, especially vane pumps, show a decreased output, this indicates a fluid of inadequate performance.
The Brugger test was developed to better evaluate the adhesion and wear protection of an oil on moving components. In the loaded pairing of a turning ring against a cylinder, over which oil is poured, an abrasion indentation is formed. The smaller this is, the better a hydraulic fluid is able to wet the paired surfaces, thereby suppressing squeaking noises and stick-slips.
KRL, tapered roller bearing test (DIN 51350-6)
Many hydraulic fluids, such as type HVLP, contain other VI improvers in addition to a good quality natural VI. These "thicken" the oil, improve the viscosity temperature behaviour, and lend the fluid an above average multigrade characteristic. VI improvers consist of very long chain molecules, which can be severely sheared under load. At the same temperature, the used oil then becomes much thinner than the fresh oil. The loss of viscosity is irreversible. The effects on the hydraulic system of an oil that has become too thin at operating temperature are accordingly severe.
One method to determine the change in viscosity as a result of the destruction of the VI improvers is the KRL test (tapered roller bearing). This variation of the four ball test (German: VKA) examines lubricants whose viscosity under high load should not change for a long time. The result provided by the KRL test is the viscosity before and after the test, as well as the relative decrease in viscosity at 100°C.
Filterability (DIN ISO 13357-2)
The filterability of an oil describes its behaviour while flowing through a filter. This test, which was originally developed for clean oils, provides the opportunity to check hydraulic oils that are still in development, to ensure that no prematurely blocked filters occur in practice. For used oils the test is used, for example, after an oil or filter change, filter service life has been shorten. Problems are often found as sticky deposits on the filter medium, or in the form of unsatisfactory oil purity. The cause of this can be another additivation, a different oil type, or the detachment of tribopolymers and the oil‘s own ageing products. In the case of reduced filter service life, testing the filterability of the used oil in comparison to that of the clean oil will quickly show whether the composition of base oil and additives is the cause of the problem, provided that all other technical values are equal.
The filterability of an oil is given as a simple numerical value. If, for example, an HLP 46 clean oil achieves a value of F=98 in the filterability test, its filterability is excellent. If F values are lower than 50, problems and decreased filter service life can be expected.
LAV, Air separation characteristics (DIN ISO 9120)
Just like water or other liquids, every oil contains air. Since it is „dissolved“ air, it cannot be seen in the form of bubbles. How much air a clean oil can absorb depends on its saturation behaviour. This is influenced mainly by the oil temperature, the oil type, the viscosity, the additivation, and the pressure in the system. The air absorption capacity also changes in the course of operation, by mixing oils that contain different additives, as well as through contaminants or oxidation products. Under the effects of temperature, the air dispersed throughout the oil may be released in the form of visible air bubbles. These are the cause for the „diesel effect“, or cavitation. A deterioration of the LAV in comparison with clean oil is often the reason for system malfunctions. The air output characteristic can be improved neither with additives nor by mechanical means.
Because the exact air content in oil is difficult to determine, there is no standard for this. The LAV value determined in the OELCHECK lab indicates by means of density how long it takes until the air dispersed in oil is separated up to a residual content of 0.2 volume percent.
Foam behaviour (ASTM D892)
Surface foam is formed when air bubbles with a diameter of more than 15 µm up to a few millimetres float up from the oil and do not immediately disintegrate. The walls of the gas-filled foam cells are formed by thin lamellae of liquid. Especially oils with a high content of additives tend to an increased foam formation. In contrast to the LAV test, the foam behaviour can be improved by foam inhibitors – mostly by those containing silicone. However, silicone oil may markedly deteriorate the air separation characteristic of oils. Therefore, caution should be exercised during a subsequent addition! The foam behaviour may deteriorate: When inhibitors are filtered out, oils age strongly, or oils with different surface tensions are mixed. An excessive foam formation may lead to an oil foam leak, and thus to environmental problems.
In the lab, air-perfused high temperature insulating bricks (HTI bricks) in the oil are used to determine how long it takes until the surface foam disintegrates after stopping the air flow.
WAV, demulsification capacity (DIN ISO 6614)
Other than dust, hydraulic fluids are often contaminated by water, which may enter the system in the form of condensate but also during the high-pressure cleaning process. Water accelerates the formation of corrosion. If it is heated in the points of contact due to friction, steam bubbles may form, which are the cause of cavitation in hydraulic pumps. A quick separation of the oil from the water is desirable. Hydraulic oils according to DIN 51524 should have a demulsifying effect.
However, the opposite effect can also be useful. For hydraulic oils of the non-standardised category HLP-D, the water is not to be separated, but to be neutralised by emulsification.
The WAV test, in which oil and water are mixed in a ratio of 1:1 and then stirred, indicates after an idle period if, and how quickly, water separates from the oil. Frequently, the formation of an emulsion can also be observed through an intermediate layer, and this emulsion is responsible in practice for muddy deposits.
TOST test (DIN EN ISO 4263)
In large industrial facilities, hydraulic fluids must remain in use over several years. The oxidation stability plays an important role for the oils used in such facilities. For the assessment of used oils, the oil oxidation, or ageing, is determined with FT infrared spectroscopy and the change of NZ or of AN. A prediction, regarding which oil is better suited for long-term use, can be obtained with the TOST test.
In the TOST test, pure oxygen flows through the warmed oil, to which water is added, for a period of about 3 months and in the presence of a copper coil. The increase of the acid reaction product formed during this process is regularly measured. The longer the process lasts, until the oil becomes "acid", the more suitable it is for long-term use.