Requirements for hydraulic fluids

Mineral oils are primarily used in hydraulic and lubricating systems, because their properties with regard to aging, corrosion protection, temperature effects on their viscosity, lubrication properties and water-holding capacity can be improved by supplementing a base oil with additives.

Depending on the requirements, mineral hydraulic oils are classified into different quality groups according to DIN 51524.

Non-flammable fluids are used in mining, pressure die casting machines, foundries, and other applications where there is a risk of burning mineral oils due to the high heat levels.

Many HFA fluids have viscosities very close to that of water and are therefore used mainly in fire hazard areas, such as in mining or automated welding. Useable over a temperature range from +5°C to +55°C, these oil-in-water emulsions are similar to cutting oil emulsions used in metal machining. They are prepared by the user himself by mixing an HFA concentrate with the required volume of water. In general, the oil proportion is no greater than 20 %. HFA E mineral oil emulsions are distinguised from HFA S emulsions, which contain mineral oils.

HFB fluids having a nominal viscosity close to that of hydraulic oils, have not become widespread in Germany since they are not recognized as non-flammable fluids. HFB fluids are used in Great Britain and the Commonwealth countries. They can be used from +5°C to +60°C, and their mineral oil content is

The most common examples of these aqueous polymer solutions are polyglycol-water solutions. They are supplied ready-to-use, and can be used at fluid temperatures from -20°C to +60°C, depending on the viscosity requirements. In order to keep the reduced water content resulting from evaporation as low as possible, the operating temperature should not be greater that +50°C. In any case, the water content (< 35 %) and the rust protection reserve of the HFC fluid must be monitored during operation and mutst be maintained at the target value by adding desalinated water or rust protectant as required.

Water-free, synthetic HFD fluids are categorized into fluids based on phosphoric acid esters (HFDR) and other water-free synthetic fluids, such as polyol esters or organic esters (HFDU). Their temperature range (max. from -20°C to +150°C) is determined by the viscosity-temperature curve and viscosity requirements of the drive. It is generally lower that for mineral olis, and must be checked on a case-by-case basis.

These environmentally friendly fluids are based on vegetable, animal or synthetic oils, and have low biotoxicity. They are used as an alternative to mineral hydraulic fluids in agriculture and forestry, and in mobile hydraulics.

• HETG: natural ester based on vegetable oils (rapeseed oil, sunflower oil, etc.), non-water-souble
• HEES: synthetic ester, non-water-soluble
• HEPG: polyalkylene glycole, polyglycols, or polyethylene glycols, water-soluble

Requirements and uses are set forthin VDMA-standard sheets 24568 and 24569.

Lubricating oils based on mineral oils can also be filtered using star-pleated filter elements. The most commonly used Newtonian fluids are lubricating oils for circulatory lubrication, as well as turbine and air compressor oils. Depending on the components to be lubricated, filter ratings of 10 to 25 µm are generally used. The potential flow capacity is dependent on the viscosity of the lubricating oil.

LUBRICANTS AND THEIR AREAS OF APPLICATION

Synthetic hydraulic fluids are designed mainly for special applications (e.g. for aerospace and military). They have similar filtration properties to mineral oils, but have specific advantages over them. Ofter, however, they are aggressive to metals and sealing materials.

The necessary properties of hydraulic and lubricating fluids can be reliably ensured only be supplementing with additives. These are often composed of particles much less than 1 µm in size. This leads to the following limit for the filtration of hydraulic fluid: dirt particles must be filtered out, while additives must remain in the hydraulic fluid with absolute certainty. The manufacturer of the hydraulic fluid must guarantee filterability in this sense.

The filterability, and thus the ability of the hydraulic fluid to floow continuously through a fine filter, depends not only on the viscosity, but also to a large degree, on the components of the oil in the colloidal range in which the additives are present. Contaminants can lead to significant changes in the colloidal structure of the fluid, and thus cause the filter to clog.

Because it is not economically justifiable to remove all contaminants from hydraulic systems with very fine filters, cleanliness classes are defined for hydraulic fluids. They define the permissible number of particles – graded according to operating requirements and sensitivity of the components used.

The most important divisions of cleanliness classes for particle counts are ISO 4406:1999 and the successor standards of NAS 1638 i.e., SAE AS 4059. The classification systems are oriented to the fact that the most commonly used filters today are depth filters with a balanced ratio of filtration quality and service life. Their filter media do not have uniform pore size; rather, they have a spectrum of pores. For example, for a filter element that captures 99 % of all particles > 10 µm, not all particles > 10 µm will be captured, and sometimes even a few significantly larger particles can pass through.

In industrial hydraulics, particle counts are coded according to ISO 4406:1999. Now that ACFTD test dust has been replaced by ISO MTD, particle sizes also have a new definition.

According to ISO 11171:1999, the diameter of the equivalent projected area circle is now the defining dimension (see the overview for the definition of particle size). The ISO 4406:1999 standard was also updated with the new definition of test dust and particle size. This new edition of ISO 4406:1999 now uses a three-digit code for particles > 4µm(c), > 6µm(c) and > 14 µm(c). The number of particles in each class is cumulative.

The sizes >6 µm(c) and >14 µm(c) largely correspond to the particle size >5 and >15 µm previously used under ACFTD calibration. The range of particles >4 µm(c) newly included in the classification, corresponds to about 0.9 µm in the old standard.

In order to distinguish the new standard from the old one, the filter ratings in the new standard end with a “c”.

The SAE AS 4059 standard defines 6 cleanliness classes:

>4, >6, >14, >21, >38 µm and >70 µm(c). As with the ISO standard, the values are counted cumulatively. The numbers therefore cannot be directly compared to the old values under NAS 1638. New maximum permissible particle counts have been determined. A new class “000”, is provided for extremely high requirements.

SAE AS 4059 like ISO 4406:1999, ist based on calibration with MTD dust according to ISO 11171:1999.

The NAS 1638, replaced by SAE AS 4059, defines cleanliness classes for 15-25, 25-50, 50-100 µm and >100 µm. Only the particle counts that are actually counted in a class are indicated (differential counts). A cleanliness class (00, 0.1 to 12) was assigned to each size range.

A complete specification under NAS 1638therefore consisted of 5 numbers. Often, however only 2 values of a selected range were given, or the worst of all 5 NAS numbers was given as an overall rating. NAS 1638 is no longer up to date, because fine particles

NAS 1638 was replaced by SAE AS 4059

If an investigation of the contaminants in 100 ml of hydraulic oil, the following particle sizes were measured:

• 210,000 particles > 4 µm (reference number 18)
• 42,000 particles > 6 µm (reference number 16)
• 1,800 particles > 14 µm (reference number 11)

The key for the designation of solid contaminants according to ISO 4406:1999 is then as follows: 18/16/11.

Reference values for the determination of filter rating x (µm) and the cleanliness class found in the hydraulic oil

Prerequisite for filter elements with the best filtration properties: materials that meet quality requirements and high production quality. Standardized tests provide valuable reference points for testing. Only manufacturers that perform them regularly can guarantee consistent standards and achieve the requirement of ßx > = 200 in all cases. Together with other important international testing standards, such as the multipass test, this guarantees the necessary reliability that you require for smooth operation in practise.

Because each type of element can be assigned a minimum pressure value, the bubble-point test can be used as an excellent monitor of the consistency for the production quality of filter elements.

The filter element is dipped in the testing fluid (isopropanol) with its main axis parallel to the fluid’s main axis, rotated 360° after five minutes, and subjected to the indicated minimum pressure. If no permanent bubble stream occurs, then the element passes the test. The test does not, however, provide any information on measuring the filter performance or rate of separation.

The permissible collapse pressure is defined as the pressure differential in the flow direction, which the filter element must withstand.

To this end, a defined quantity of any chemically neutral, particulate contaminant is added to the test circuit, until the pressure differential across the filter element corresponds to the permissible collapse or burst pressure. The pressure differential curve is drawn, and the filter element is passed only if there is no indication of failure and no drop in the slope of the pressure differential curve is registered.

An important aspect in designing hydraulic filters is the differential pressure (also called flow resistance). This value is derived from the total pressure drop from housing inlet to outlet, and results from houding and filter insert losses.

Factors that affect the flow resistance of a clean filter are the viscosity of the fluid, its specific weight, volume flow rate, filter insert medium, and flow paths.

A test stand, consisting of a pump, tank, heat exchanger, and measurement equipment for pressure, temperature, and volume flow is used to determine the flow resistance. p1 is the pressure at the filter inlet, p2 is the pressure at the filter outlet, and ∆p is the flow resistance of the filter. When performing ∆p volume-flow measurements on a filter, a test stand with high system pressure is not necessary. It is sufficient to maintain p2 at a positive pressure value.

The test is used to determine the ability of a filter element to withstand deformations due to changing differential presures (flow volumens) without a change in its burst resistance. A test stand, as shown schematically at the bottom right, is used to perform the test.

The multipass test is the most important test for evaluating retention efficiency, dirt-holding capacity, and service life of a filter element. It is also known as the filter performance test, multi-pass test, or ßx test. A very complex test stand, divided into three parts, is required to perform a multipass test:

• In system 1, the test fluid (MIL-H-5606I) is contaminated with test dust (ISO MTD) to a defined level.
• In system 2, the test filter is installed and the cleaned test fluid is circulated.
• In system 3, the fluid samples taken from system 2 are continuously counted in high-precision particle counters, and the information is displayed visually using a special PC program.

The multipass test is very close to the real-lfie contamination process. Differences include, however, the greater range of contaminants and thereby the very short test duration, relative to the filter service life.

Any changes to the filter element with increasing ∆p, such as can occur during cold starts or other operating conditions, can be clearly demonstrated, however, with conclusions about the effectiveness and lifespan of the filter.

The test equipment and test procedure are very complex, and cannot be performed by an operator. This means that you are all the more dependent on the accuracy of the manufacturer’s data.

Contaminated fluid from system 1 is continuously injected into the circuit of system 2. The constant circulation causes dirt to be fed into the test filter until the maximum permissible differential pressure of the element or the test system has been reached. In the mean time, samples are continuously automatically tested in system 3, and the temperature and pressure curves are recorded. As the differential pressure increases, the progression of the element’s retention efficiency can be determined. The test result is expressed in the form of the ß value, which represents the following ratio:

The following values should always be presented:

• ßx(c) value relative to the Dp, at which the value was measured
• ßx(c) values at the switching level of the maintenance indicator and at the final Dp of the test stand, or the permissible ∆p for the affected element
• The apparent dirt collection at the switching level of the maintenance indicator, and at the final ∆p
• Actual bubble point of the test element, prior to start of test

Only these data, altogether, truly allow for a comparative evaluation of the performance of filters. In order to better evaluate the significance of the ß value, the comparison must be made with the separation rate in % on hand. The separation rate is calculated as follows:

A ß value of 200 thus corresponds to a separation rate of 99.5%.