Hydraulic fluids are construction elements for complex systems and not an arbitrarily exchangeable product, which is why they must be conscientiously selected, maintained and monitored. In addition to their viscosity, the degree of purity plays a decisive role from the very beginning. Hard but also soft contaminations in the hydraulic fluid are always a risk factor for systems. Depending on the application area, these contaminations cause most hydraulic malfunctions. Especially hard particles such as dust, metal abrasion or sealing materials as well as soft, adhesive parts from aged additive components, oxidation products and water put a strain on hydraulic fluids. Most often, the effect is severe: Particles cause wear in pumps, valves, motors, cylinders, gaskets and hoses. The control edges of valves can adhere and block valves. Filters clog up quicker and must be exchanged more frequently. Deposits in heat exchangers lead to higher operating temperatures and shorter oil life-cycles. Leaks can occur at valves, pumps, cylinders and motors. The effectiveness of hydraulics is also decreased.
Purity classes and contamination degrees
The size of particles in oil is measured in μm (millionth of a metre, micrometre). For illustration purposes: The human eye can easily see a hair, which has a diameter of approximately 75 μm, but can no longer detect objects of less than 50 μm. Specific analysis devices are used to detect fi ne contamination in oil samples. The contamination degree of oil depends on the number and size of contained particles. The more particles with a critical size are contained in the oil, the more likely resulting damage to components that come in contact with the oil. The degree of oil contamination is described by allocation to purity classes, which also provides a basis for comparison. The current ISO 4406:2017-08 describes the number of particles contained in 100 ml of oil. Organised according to size and quantity, the particles are subsequently cumulatively divided into specified particle ranges. ISO 4406:2017-08 provides a classification for particles with a size of ≥ 4 μm, ≥ 6 μm and ≥ 14 μm. The determined purity class of an oil is stated as a composite number, e.g. as 19/17/14.
DIN 51524 – not a measure of all things
DIN 51524 (2017) defines the minimum requirements for hydraulic oils in an unused condition with regard to water separation capacity, filter capacity, sealing compatibility, air separation capacity, oxidation stability and wear protection. Part 1 relates to DIN HL oils, part 2 to HLP oils and Part 3 to HVLP oils. As a minimum requirement, new oil must also adhere to the purity class 21/19/16. It has been taken into account that empty oil containers (primarily drums) are not exactly highly pure vessels prior to being filled with oil. While the minimum purity class 21/19/16 is e.g. sufficient for hydraulic oil for a robust lift, it does not usually fulfil the requirements for servo valves and many other applications. Principally the following applies: The more complex the system, the lower the gap tolerances and/or the greater the operating pressure and the higher the requirements for the purity of the hydraulic fluid. This is why many pump, valve and system manufacturers stipulate specific purity classes that are significantly narrower than the minimum requirements of the new DIN 51524 edition.