Year of publication: 2008
Modern hydraulic systems, with their low gap tolerances and high operating pressures as well as finer filters, place ever greater demands on the filterability of hydraulic oils. In order to determine this precisely, the filterability test has been developed in accordance with ISO 13357 and included in the DIN 51524 standard for HLP hydraulic oils. Since spring 2008, the OELCHECK laboratory has been operating new test equipment that conforms to this standard.
Today’s hydraulic systems need filters. Whether you install the filter permanently in the system as a fullflow filter or as a, partly mobile, by-pass filter, you remove impurities and ageing products from the oil. By doing this, you safeguard the performance of the entire system and assure the longest possible
service life of components and oil. Modern hydraulic liquids have to be easy to filter. The filterability of an oil describes its characteristic differential behaviour under pressure when flowing through a filter. Even when developing hydraulic oils and filter materials, care is taken to ensure that the liquid
itself generates the lowest possible differential pressure increase.
In practice, this means that the filter or oil service life is not shortened, e.g. by the adhesion of gum-like, non-particulate components from the oil or from their reaction products on the filter, and it means that condensate does not affect the filterability.
Modern hydraulics work with increasingly lower gap tolerances and better surface quality. As a result, higher operating pressures and hydraulic components that work more precisely can be used for smaller tank contents. More effective filters guarantee the availability and reliability of the systems over a long period of time. Their average pore size has been reduced from what used to be 10 to 20 μm to 3 to 12 μm. The retention rate (ß value) has at the same time been increased for the depth filters. These “finer filters” can, however, block up in relatively shorter intervals and must then be replaced by new filter elements. The increase in the pressure differential, which is used to determine when to change the filters, depends on the filterability of the hydraulic fluids as well as the reduced ability of the finer filters to absorb dirt.
Using filtration tests, in which oil flows through a filter, often under intensified conditions, the interaction between the filter membrane and oil is examined in more detail. Most “static” filterability tests are carried out as internal tests, due to the lack of a standard, using different membrane pore sizes and
liquid volumes under a vacuum or pressure conditions. However, the results can vary enormously according to the procedure used and often depend on the individual experiences of the laboratory. Since the early eighties, scientists have been searching for a test capable of becoming a standard. This has
now been passed as the ISO 13357 filterability test part 1 and 2.
The OELCHECK laboratory is already using the corresponding test equipment. Using this equipment, it is possible to sufficiently differentiate the filterability of a new oil even as early as the development stage. In used oil analytics, the test produces valuable evidence regarding the change in filterability compared to the fresh product.