We hope that this new addition will prove a useful aid to internet users. Any suggestions you may have for our glossary will of course be welcomed at sales@rt-filter.de.
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Abrasive effect Hydraulic elements such as valves or pumps have moving metal parts which are only fit-sealed. These parts have a clearance well below 50 µm, so it is essential to ensure adequate filtration of the hydraulic fluid. The finer the filtration, the more elaborate and expensive it is, and the question of which filter to use in which part of the system is therefore also an economic one. The chosen solutions often fail to allow for micro-particles < 10 µm, even though these can include very hard particles such as silicon. These are carried along in the flow and graze the surfaces they come into contact with. The resulting wear is known as abrasion and shortens the lifetime of the components. In such an environment, it may make economic sense to install additional bypass filters with microfilter elements (≤ 5 µm) in order to prolong the lifetime of the machine. Here again, RT-Filtertechnik offers competent support and a range of suitable products. |
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Absolute filtration rating This term means, for example, that a 10 µm absolute filter would not let a 10 µm diameter glass bead through. It should only be cited in connection with the glass bead test, which is only used by filter manufacturers. It should not be cited when talking to users as it may mislead them. |
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Absolute pressure Pressure in a system as compared with zero pressure in a completely empty space (100% vacuum) (according to DIN 1314) |
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Additive (for lubricants) Chemical additive designed to modify the existing properties of a fluid or endow it with new ones (CETOP RP 100). The natural substance mineral oil lacks a number of properties required by a pressure fluid, and which can only be obtained with the use of additives. These include substances to improve rust protection, age resistance, wear resistance, VT capacity, load bearing capacity (pressure resistance), etc. |
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Additive consumption Pressure fluids which are subject to higher loads require the use of additives. Various additives are decomposed or destroyed by operational stresses. For example, HP (high-pressure) additive molecules may be crushed by the shear effect or detergent additives may be filtered out due to dirt adhesion, thus resulting in their depletion. In the case of more expensive oils, it may be worth replacing the additives. For smaller oil quantities a regular change of oil is the better course of action, however. |
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Aging of fluids Damage to the pressure fluid mainly due to oxidisation (formation of acid residues) and polymerisation (enlargement of hydrocarbon chains); generates sludge or resinous coatings on components as a result of the ageing process. Ageng is accelerated by air (oxygen), water, high temperatures and metallic catalysts. It is measured by the neutralisation index. |
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Air breather The actuators in a hydraulic system (cylinders, swivel drive, motors, etc.) consume different volumes of oil depending on functional status. This means that the oil level in a hydraulic tank is constantly fluctuating, and so air must be allowed to pass freely into and out of the tank. It is also particularly important that this air is filtered. The grade of filtration should be at least as fine as the finest filter in the system. In addition to this, there are also air breather and bleeding filters with a water absorbing function. |
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Air breather and bleeding filter The actuators in a hydraulic system (cylinders, swivel drive, motors, etc.) consume different volumes of oil depending on functional status. This means that the oil level in a hydraulic tank is constantly fluctuating, and so air must be allowed to pass freely into and out of the tank. It is also particularly important that this air is filtered. The grade of filtration should be at least as fine as the finest filter in the system. In addition to this, there are also air breather and bleeding filters with a water absorbing function. |
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Air Cleaner Fine Test Dust Test dust for conducting the multipass test according to ISO 4572. The quantitative composition of the individual particle sizes is predefined, thus ensuring a uniform contamination basis for all multipass tests. Since ACFTD consists of sand with different grain sizes, however, its effect on the filtration result cannot be applied straightforwardly to contamination through abraded matter, which has different geometric shapes. Production of ACFTD ceased around 1992. This was replaced by ISO-MTD dust, on which the new multipass test ISO 16889 is based. |
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Anti cavitation valve In suction/return flow filters, the oil flowing back out of the system is filtered and pressurised by a precharge valve. This enables the pump - usually the feed pump of the hydrostat - can be filled with filtered oil and excess pressure. Should the return flow volume fall below the required pump intake, the pump supply still has to be guaranteed. For this purpose, an anti-caviation valve is integrated in the filter through which the oil from the tank can pass into the filter housing. In the case of RT suction/return flow filter, this valve can also be optionally equipped with a strainer. |
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APC - particle counter The quickest way to analyse particles in the fluid is with an automated particle counter. In this process, the oil flows past a light source, causing the intensity of the light beam registered by a photodiode to decrease according to the size of the individual particles. The change in the light beam intensity triggers voltage changes in the photodiode which provide a direct measure of the particle size. Obtaining a correct reading depends on the particles passing the light beam in succession: with this method of measurement, however, the result may be falsified by air bubbles and water droplets. It is therefore important to prepare the samples correctly. Essential criteria for a meaningful oil analysis are: Regular and representative sampling and correct counting of the particles provide an early indication of any adverse occurrences. That way failures can be prevented at an early stage. |
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Bar Unit for measuring pressure: 1 bar = 100.000 N/m˛ The unit “bar“ is frequently used in modern fluid technology, as that way the traditional numeric values can still be used for pressures. In the SI system, the unit “Pascal“ is recommended for pressure data: 1 Pa = 1 N/m˛ A simple conversion gives: 10 bar = 1 • 1.000.000 Pa = 1 MPa. |
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BAUMA BAUMA is the world's biggest trade fair for building and building materials machinery. It takes place in Munich at 3-yearly intervals, i.e. 2004, 2007, 2010, etc. For more information, visit www.bauma.de. |
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Beta-value Measure of the effectiveness of a filter. It is a proportional value calculated from the number of particles before and after passage through the filter at a certain pressure drop. βx = Particle number > x(µm) before the filter / Particle number after the filter According to this, βx = 1 means that no particles are removed, and βx = 2 means 50% are removed. Today’s target minimum practical value of βx = 200 means that 99% of dirt with the particle size x is filtered out: higher β-values than this thus have little practical significance. The βx-value is not sufficient on its own; rather, all parameters within which the β-value was found must be stated, i.e. the full test report. The test procedure is standardised by ISO 16889. |
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Bio oils Biodegradable fluids are hydraulic oils which biodegrade much quicker than mineral oils. Biodegradation is a chemical transformation brought about by microorganisms under aerobic conditions and in hydrous environments, leading ultimately to mineralisation (OECD screening test). Biodegradability, soil and water safety and the absence of dangerous substances are important criteria for the environment-friendliness of a fluid. To the biodegradable fluids belong: HEES, HEPR, HETG und HEPG. |
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Bursting pressure Static pressure at which a pipe or pipe connection is damaged so severely that the pressure medium escapes (DIN 24312). This definition should also be used with reference to the filter housing, although it does not coincide with DIN-ISO 2941, according to which the permissible bursting/collapse pressure is the pressure at which a filter element can still maintain its full performance. |
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Bypass filter Bypass filters are used for separating either extremely fine particles (< 5 µm) or water from the oil. Specially treated glass fibre or cellulose paper elements are often used for this purpose. The above aim can only be achieved if the filter is operated with a low, non-pulsating volume flow. Bypass filters are supplied either by a separate pump directly from the hydraulic tank or from the system line via a flow regulating valve. RT-Filtertechnik also offers a specially designed combined filter element which allows integrated bypass filtration. |
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Bypass filtration Bypass filters are used for separating either extremely fine particles (< 5 µm) or water from the oil. Specially treated glass fibre or cellulose paper elements are often used for this purpose. The above aim can only be achieved if the filter is operated with a low, non-pulsating volume flow. Bypass filters are supplied either by a separate pump directly from the hydraulic tank or from the system line via a flow regulating valve. RT-Filtertechnik also offers a specially designed combined filter element which allows integrated bypass filtration. |
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Bypass filtration In bypass filtration, a certain flow volume is diverted from the main pipe (usually the return flow pipe) of the system. This is done by means of a flow divider or other flow control valves in order to keep the diverted flow as even as possible. A microfiltration system or filter unit for water separation can be integrated into this bypass flow. |
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Bypass flow In hydraulic terms, a bypass flow is a volume flow which is fed by a separate pump (usually a ZRP) independently of the actual feed pumps of the system. A bypass circuit offers an effective means of microfiltration as the volume flow is individually adjustable and even. Many bypass flow systems also cool the oil, thus additionally ensuring the thermal stability of the system. |
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Bypass valve Valve parallel to a hydraulic device which allows the pressure medium to bypass that device under certain conditions, e.g.: a pre-stressed check valve fitted in the bypass flow of a filter which allows the unfiltered medium to bypass the filter element when a specified pressure difference is reached in the filter. This does impair the effectiveness of the filter, however. Its design is that of a bypass check valve. |
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Cavitation The formation of cavities in fluids. This occurs when the local static pressure in a fluid falls below a critical value, which is generally roughly equivalent to the steam pressure of the fluid. Manifestations of cavitation: Of particular relevance to fluid technology is flow cavitation. This mainly occurs where the cross-section narrows or widens or the flow direction changes suddenly. In narrower sections, the static pressure falls due to the increased flow speed and the consequent conversion of static pressure to dynamic pressure (flow energy). After a widening of the cross-section, eddy currents usually occur at whose centre the static pressure can fall to the critical value due to the high peripheral velocity of the fluid. A similar effect occurs after sudden changes of direction. A distinction is made between steam and gas cavitation depending on whether the cavitation bubbles contain steam from the fluid or a gas such as air which was dissolved in the fluid. For example, water only dissolves small amounts of air, so cavitation in water will usually be steam cavitation. Mineral oil on the other hand dissolves much larger quantities of air (approx. 9 % by volume under normal conditions according to Bunsen coefficient). Cavitation in mineral oil can therefore be assumed to be a mixture of steam and gas cavitation. The main consequences of cavitation are: |
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Cavitation damage The formation of cavities in fluids. This occurs when the local static pressure in a fluid falls below a critical value, which is generally roughly equivalent to the steam pressure of the fluid. Manifestations of cavitation: Of particular relevance to fluid technology is flow cavitation. This mainly occurs where the cross-section narrows or widens or the flow direction changes suddenly. In narrower sections, the static pressure falls due to the increased flow speed and the consequent conversion of static pressure to dynamic pressure (flow energy). After a widening of the cross-section, eddy currents usually occur at whose centre the static pressure can fall to the critical value due to the high peripheral velocity of the fluid. A similar effect occurs after sudden changes of direction. A distinction is made between steam and gas cavitation depending on whether the cavitation bubbles contain steam from the fluid or a gas such as air which was dissolved in the fluid. For example, water only dissolves small amounts of air, so cavitation in water will usually be steam cavitation. Mineral oil on the other hand dissolves much larger quantities of air (approx. 9 % by volume under normal conditions according to Bunsen coefficient). Cavitation in mineral oil can therefore be assumed to be a mixture of steam and gas cavitation. The main consequences of cavitation are: |
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Closed circuit Circuit of a hydraulic drive in which the fluid is circulated from the pump to the motor and from there directly back to the suction pipe of the pump. Any leakage oil is replaced by a low-pressure feed pump. Since very small oil volumes are circulated in such systems (e.g. approx. 8 l at 300 kW), the oil must be constantly renewed and cooled. The feed pump thus also acts as a flushing pump. The closed circuit is of particular advantage where the load has a pushing function (e.g. vehicles) or pulling function (e.g. cranes). In these phases, the hydraulic motor becomes a pump which is in turn supported by the drive motor (e.g. combustion engine), i.e. a closed circuit can be used for braking. No valves are necessary in the power flow for purposes of motor control. Applications with a closed circuit lend themselves particularly to the use of suction/return flow filters because of their functional advantages for cold starts. |
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Closed circuit Circuit of a hydraulic drive in which the fluid is circulated from the pump to the motor and from there directly back to the suction pipe of the pump. Any leakage oil is replaced by a low-pressure feed pump. Since very small oil volumes are circulated in such systems (e.g. approx. 8 l at 300 kW), the oil must be constantly renewed and cooled. The feed pump thus also acts as a flushing pump. The closed circuit is of particular advantage where the load has a pushing function (e.g. vehicles) or pulling function (e.g. cranes). In these phases, the hydraulic motor becomes a pump which is in turn supported by the drive motor (e.g. combustion engine), i.e. a closed circuit can be used for braking. No valves are necessary in the power flow for purposes of motor control. Applications with a closed circuit lend themselves particularly to the use of suction/return flow filters because of their functional advantages for cold starts. |
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Cold start When designing hydraulic filters for mobile hydraulic applications, it is important to take into account the cold start characteristics. The high viscosity of the fluid at low temperatures (< 0 degrees) increases the flow resistance of a filter element by many times. To protect the element from destruction and ensure that the overall filtration result (oil purity) comes up to expectations, the design of the filter area, grade of filtration, filter materials and valve system must be designed with great care. This takes a lot of experience - something which RT-Filtertechnik, with over 40 years' expertise in mobile hydraulics, can offer its customers in plenty. |
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Cold start conditions When designing hydraulic filters for mobile hydraulic applications, it is important to take into account the cold start characteristics. The high viscosity of the fluid at low temperatures (< 0 degrees) increases the flow resistance of a filter element by many times. To protect the element from destruction and ensure that the overall filtration result (oil purity) comes up to expectations, the design of the filter area, grade of filtration, filter materials and valve system must be designed with great care. This takes a lot of experience - something which RT-Filtertechnik, with over 40 years' expertise in mobile hydraulics, can offer its customers in plenty. |
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Collapse pressure Pressure at which a filter element is damaged so severely that it is no longer useable. According to ISO 2941, however, the collapse pressure only means the pressure at which the filter element can still maintain its full performance under load. |
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Combined element The fine filtration of hydraulic oils is usually performed with glass fibre fabric based deep-bed filters. To filter out microparticles (< 5 µm), a complex additional bypass filtration system is often installed. Now RT-Filtertechnik has developed the combined element, integrating glass fibre fabrics with multiple grades of filtration. That way a single filter element can be used to filter out microparticles and ensure the ongoing purity of the system at the same time. |
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Comité Européen des Transmissions Oleohydrauliques et Pneumatiques The European Fluid Power Committee was established in 1962. CETOP consists of over 700 firms from 13 European countries, represented via the national fluid power associations. The member associations are from Belgium, Denmark, Finland, France, Germany, Italy, the Netherlands, Norway, Sweden, Switzerland, Slovenia and Spain and the UK. The aim of CETOP is to promote and support the European fluid power industry. The Committee plays an important role within the European Union, allowing national interests to be coordinated at European level and a united front to be presented to the EU Commission. CETOP offers its members information which is often unavailable from other sources, e.g. statistics and economic data. It also conducts joint PR work via various publications or via CETOP exhibition stands at approved trade fairs, as well as drawing up recommendations for further training in the hydraulic and pneumatic sector. In addition, CETOP allows opinion shaping and coordination at European level in matters of international standardisation and EU directives. CETOP had already published technical recommendations for fluid power long before the responsible Technical Committee was established at ISO, and still plays a pioneering role in drawing up technical regulations, e.g. in the field of pressure fluids and contamination monitoring. Since the establishment of the European Committee, a total of more than 100 Technical Recommendations have been drafted, some of which have been published worldwide as ISO standards. The preparation of standards and recommendations in cooperation with national standardisation organisations and the relevant Technical Committees of ISO and CEM is thus an important function of CETOP. All non-standardised CETOP recommendations have been withdrawn by CETOP as of 2004. |
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Contamination indicator Indicates when the flow resistance of a filter element has reached the maximum permissible value due to increasing dirt accumulation. This can be implemented via a direct visual display, although this requires careful observation. A better solution is to use electric indicators with lamps or acoustic signals. |
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Cumulative filter efficiency The cumulative efficiency of a hydraulic filter is a measure of how well it filters out solid particles. It is calculated according to ISO 16889 as follows: εx = (Nu - Nd)/Nu – 1/βx εx = cumulative efficiency for particles larger than x µm Nu = Number of particles > x µm upstream of filter Nd = Number of particles > x µm downstream of filter The cumulative efficiency is generally expressed as a %. It is linked to the βx - value from the multipass test. |
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Deep-bed filters Deep-bed filters can be composed of a wide variety of media. In hydraulic filters, it is mostly cellulose paper and glass-fibre tissue that is used. For special applications, synthetic tissues (high strength) and metal-fibre tissues (temperature loads above 130 şC) are used. Thanks to their tissue-like structure, deep-bed filters not only separate particles at the surface of the filter material, but the particles can penetrate the material itself, thus greatly increasing the effective filtration area. Unlike strainers, tissue filters do not have “holes”, but practically consist of labyrinths in which the particles get caught. Consequently, instead of a focused “straining” effect, a wide range of particles are captured. |
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Diesel effect If mineral oil containing air bubbles is compressed very quickly, the bubbles are heated to such a temperature that the gas and air mixture may self-ignite. This leads to a sharp local rise in pressure and temperature – which can also damage seals – as well as to accelerated ageing of the oil. |
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Dirt holding capacity of a filter Also known as effective dirt capacity. Indicates the amount of dirt a particular filter can absorb before a reference pressure (usually the response pressure of the bypass valve) is reached. In other words, the dirt holding capacity is also a measure of the filter element’s service life. The dirt holding capacity can be calculated with a high degree of accuracy from the α value. It mainly depends on the filter element used and – if a bypass valve is fitted – its initial pressure difference ∆pA. The dirt holding test is conducted according to ISO 16889 (cumulative filter efficiency). ISO 3968 contains the tests for determining the pressure difference. |
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Disposable filter element Filter element which is replaced with a new one after a certain operating period or on reaching a certain flow resistance. The soiled filter element must be disposed of in an environment-friendly manner. Most deep-bed filters are disposable. Alternatively, the element can be washed out ultrasonically in the case of strainer filters. |
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Dynamic viscosity The following relationship applies between the shearing strain = F/A and the velocity gradient in a moving fluid: (η • dv)/ds The proportionality factor η is known as the dynamic viscosity. This is a measure of the internal resistance acting against the movement of adjacent layers in the fluid. The dynamic viscosity thus has a strong impact on the friction of the flow medium as well as the leakage losses in gaps. The dynamic viscosity is a variable specific to each fluid which, in the case of Newtonian fluids, depends on temperature and pressure. It is measured in Pa • s. The former unit P (cP) is no longer valid. Conversion rule: 1 cP = 1 mPa • s. |
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Element for pressure filters Filter element which can withstand a differential pressure at least as high as the nominal pressure of the system without damage to its structure or to the filtering material. |
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Feed pump Travelling machines with a hydraulic travelling gear are usually equipped with a closed circuit. In this closed circuit, oil leakages occur which have to be drained off directly into the tank. Consequently, the same volume of oil has to be fed back from the tank into the closed circuit. This is done via the fed pump. Since the fed-in oil remains in the closed circuit for a long time and is exposed to high loads, it is very important to ensure good filtration. To ensure optimal operating conditions for the feed pump, a suction/return flow filter is used. |
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Feed pump circuit Travelling machines with a hydraulic travelling gear are usually equipped with a closed circuit. In this closed circuit, oil leakages occur which have to be drained off directly into the tank. Consequently, the same volume of oil has to be fed back from the tank into the closed circuit. This is done via the fed pump. Since the fed-in oil remains in the closed circuit for a long time and is exposed to high loads, it is very important to ensure good filtration. To ensure optimal operating conditions for the feed pump, a suction/return flow filter is used. |
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Filler filter and air breather The actuators in a hydraulic system (cylinders, swivel drive, motors, etc.) consume different volumes of oil according to their functional status. This means that the oil level in a hydraulic tank is constantly fluctuating. It must therefore be ensured that air can move freely into and out of the tank at all times. It is important that this air is filtered, the grade of filtration being determined by the finest filter in the system. A vent valve with a water absorption function is also provided. In versions with a filling function, the air filter unit can be easily unscrewed to fill the tank with oil. A metal sieve is fitted in the filling filler neck for coarse filtration. |
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Filter area The filter area of a folded filter element is calculated mathematically. The larger the filter area, the lower the flow resistance of the filter element - and the greater the dirt absorption capacity. A minimum distance must be maintained between the folds of the filter element however in order to make effective use of the filter area. If the area is increased any further, this can have adverse effects on all operating parameters. |
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Filter element The filter element is the part of a filter into which solid particles above a certain size are separated. The separation material used in the filter element may be paper, glass fibre, plastic or metal gauze. Paper and glass fibre elements are always disposable elements, while metal gauze elements can be ultrasonically regenerated. |
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Filter lifetime In filtration, the term service life means the number of operating hours before the filter element has to be changed. The service life of a filter element depends on several parameters: dirt absorption capacity, filter area, specific contamination in the machine and initial pressure difference. At the end of its service life, the element must be cleaned (wire mesh elements only) or changed. The maintenance required can be indicated visually or electrically, or it may be defined in terms of a set number of operating hours. When determining the maintenance intervals, the mechanical loading of an element also needs to be taken into account. In the case of mobile machinery in particular, filter elements must be designed with a view not only to their dirt absorption capacity, but also their mechanical endurance strength. |
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Filter material Filter elements are made from various kinds of materials. Non-metallic materials are almost exclusively used for filter tissues: These include: Metallic materials are generally used for special purposes: |
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Filter range All fluid systems (hydraulic systems, cooling circuits, etc.) must be fitted with a filtration unit in order to maximise the lifetime of both the fluid and system components. The filtration grade, type, installation point and size of the filter must be such that the required filtration result is achieved while observing due economy of operation. It is therefore essential to choose the right filter based on expert advice. To help you make a preliminary selection, RT-Filtertechnik also offers an electronic selection guide on its homepage. |
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Filter tissue Filter tissues consist of any number of superimposed layers which are produced by the suspension of irregularly arranged fibres (chopped strands). This results in a large number of small but irregular pores which form a curved path for the flow medium. |
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Filtration grade When defining the filtation grade, it is necessary to distinguish between the different filter materials used. With the wire mesh elements commonly used in suction filters or strainers, the filtration grade can be defined simply in terms of the mesh width. Even here, however, there are some types of mesh fabric (braid and armoured braid fabrics) whose mesh width is not so easy to measure. The wire mesh elements used in hydraulics usually have a filtration grade of 20 - 200 µm. In order to define the filtration grade of deep-bed filter elements, the so-called multipass test (ISO 16889) was developed. In this test, a defined, contaminated oil flow is passed through the filter, and the number of different sized dirt particles are measured upstream and downstream of the filter. The ratio of upstream to downstream particles (of a certain size) is known as the beta value. The larger the beta value, the more particles are retained. The particle size at which the beta value reaches a pre-specified number is known as the defined filtration grade of the element. Typical grades in hydraulics are 3 µm, 5 µm, 10 µm, 15 µm, 20 µm and 25 µm. |
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Flow rate The volume of fluid which flows through a particular cross-section within a unit of time. The flow rate is generated by the pump. Important: the flow rate within the piping system may also exceed the max. output of the pump (area ratio of cylinders). Normal unit of measure: l/min, symbol: Q or qv |
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Flow rate The flow rate of the pressure fluid determines the flow resistance in pipes and hydraulic components. A higher rate leads to more power dissipation. There is also a critical rate depending on viscosity above which the flow changes from laminar to turbulent. A turbulent flow is undesirable in hydraulic systems. When designing filters, the flow rate also has to be taken into account to ensure an optimal separation of the dirt particles. The following flow rates are regarded as optimal in a hydraulic system: |
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Flow resistance Each component through which a pressure fluid flows presents a resistance to that flow. The strength of the resistance can be measured upstream of the component as a pressure (in bars). The resistance is influenced by the following factors: The flow resistance in filter elements also depends on the degree of contamination. The higher the resistance in the components, the greater the power dissipation and hence the heat generated in the system. This has to be set against the cost of all components. When designing filters, it is necessary to consider not only the resistance but also the dirt absorption capacity and hence the achievable . |
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Full-flow return filtration In this case the full-flow filtration system is designed so that the oil flowing back from the system (actuators and valves) is fed through a return flow filter, usually fitted in the tank. This method of filtration is highly effective and cheap to install, but has the disadvantage that leakage oil cannot be passed through the filter, particularly in mobile machinery. The effectiveness of the filtration should always be verified in action in the machine. |
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Full-flow return filtration In this case the full-flow filtration system is designed so that the oil flowing back from the system (actuators and valves) is fed through a return flow filter, usually fitted in the tank. This method of filtration is highly effective and cheap to install, but has the disadvantage that leakage oil cannot be passed through the filter, particularly in mobile machinery. The effectiveness of the filtration should always be verified in action in the machine. |
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Full-flow return flow filter In this case the full-flow filtration system is designed so that the oil flowing back from the system (actuators and valves) is fed through a return flow filter, usually fitted in the tank. This method of filtration is highly effective and cheap to install, but has the disadvantage that leakage oil cannot be passed through the filter, particularly in mobile machinery. The effectiveness of the filtration should always be verified in action in the machine. |
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Full-flow systems Modern hydraulic systems require high standards of oil purity. These can only be maintained if the entire flow volume moving through the system passes at least once through a microfilter (≤ 20 µm). This type of arrangement is known as full-flow filtration. |
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Glass fibre fabric Glass fibre fabrics are one of the main materials used in modern filtration. They are produced by mixing selected fibres 1-5 mm in length and 3-10 µm in diameter according to a fixed ratio. The production process is similar to that of paper, usually involving resin-bonding and impregnation. They are superior to cellulose paper however in that they have a much more homogeneous fibre structure and hence larger open pore surfaces. This serves to reduce the flow resistance. |
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HEES For many manufacturers, the next step – following the negative experiences with rapeseed oils in the engineering sector – was to develop synthetic esters, or HEESs (Hydaulic Oil Environmental Ester Synthetics) for short. Although saturated synthetic esters had already been on the market for years, they had been regarded as too expensive. For economic reasons therefore, mainly unsaturated TMP esters (trimethyl propane esters, also known as oleic acid esters or trioleates) were used. TMP esters have a better low and high-temperature stability than natural rapeseed oil. These differences are due to the different distribution of fatty acids in the esters: the low polyunsaturated fatty acid content of oleic acids means that TMP esters have a better resistance to oxidation and a lower pour point than rapeseed oils or triglycerides. The main advantages of unsaturated synthetic esters over rapeseed oils are thus: One major problem with trioleate esters, however, is that products from different suppliers – despite having the same chemical name - vary considerably in terms of their technical properties. Apart from obvious differences in product quality which are due to the base oils used, both the esterification technology and additive process are critical to the quality of the product. It is always difficult for the user to judge the quality of an unsaturated synthetic ester product from its name, as the abbreviation HEES is used universally for these products according to VDMA 24568 (German Machine and Plant Engineering Association). To make matters even more confusing, the same abbreviation is also used to refer to saturated synthetic ester oils. Environment-friendly lubricants and pressure fluids based on saturated synthetic esters (HEES) have been available on the market since 1985. They are produced when water is separated by the reaction of alcohols with carbonic acids. The base substances used here are derived from the petrochemical industry. Saturated synthetic esters are regarded as environment-friendly due to the following characteristics: They also have a number of positive characteristics from a technical point of view, however: The disadvantage of saturated ester oils is their high price in comparison to mineral oils or even rapeseed and unsaturated ester oils. This price disadvantage has to be compensated for by the above technical advantages and the possibility of using the oils over a longer period. |
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HEPG Polyalkylglycols, or polyglycols for short, have been used for nearly 50 years due to their effective lubricative properties. For a long time, polyglycol oils were mainly prized for their performance under extreme conditions, e.g. very low or high continuous temperatures. Following the introduction of environment-friendly lubricants and pressure fluids, however, polyglycols (Hydraulic Oil Environmental Polyglycols, HEPGs) also began to gain importance in this field. They are all (depending on their structure) environmentally acceptable. Thanks to their solubility in water, for example, they have played a major role in the food industry for over 25 years. It was therefore logical to consider using polyglycols with low toxicity and sufficient biodegradability as a basis for environment-friendly lubricants and pressure fluids. Among the various types of polyglycol, only polybutylene glycols and an ethylene oxide/propylene oxide copolymer are used for this purpose. These are cold-water soluble types, i.e. substances which do not inhibit the exchange of oxygen at the surface of the water while having an oxygen-depleting effect at the bottom; only these types conform to the required ecotoxicity values. Besides the above environment-related characteristics, polyglycols also have good technical properties, e.g.: There are also some properties which make them less suitable as lubricants and pressure fluids, however: Despite these disadvantages, the technical properties of polyglycol-based lubricants are too useful to be forfeited. For this reason, they are still frequently used in the industrial sector, given the right machine configuration and seals/hoses made of suitable materials. |
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HEPR fluids (PAO) The new group abbreviated to HEPRs (Hydraulic Oil Environmental Polyalphaolefines and Related Products) is used to classify fluids which are mostly synthesised from polyalphaolefines (PAOs) and related hydrocarbons. Only low-viscosity PAOs, which are more readily degradable than other mineral oils, may be used here however. The so-called hydrocrack base oils also come under the heading of HEPR fluids. According to ISO DIS 15 380, these are generally understood as “polyalphaolefines and related hydrocarbons including the base oil components of other readily biodegradable base fluids“. In other words, there is much scope for variation among HEPR fluids. Empirical information on the advantages and disadvantages of HEPR fluids is not yet available, and the range of possible formulas makes it impossible to speculate with any accuracy. In practice, the quality monitoring of HEPR fluids should prove a major problem for analysts. On the one hand, mineral oil based products in the widest sense are used as base oils; on the other hand, base oil components of other readily biodegradable base fluids, i.e. rapeseed oils, oleates and synthetic ester oils can be added. Consequently, it will not be possible using the simpler oil analysis methods normally used for routine checks to determine the extent to which the HEPR product has been mixed with other products (biodegradable or otherwise) during use. On the face of it, this may even be welcomed by machinery and lubricant users, as a ban on mixing is always inconvenient. So is the creation of a product which can be mixed so freely such a bad thing? The answer is clearly “yes“, as in this case there is no certainty as to how mixing will affect the fluid’s compatibility with seals, hoses, non-ferrous metals, paint and coatings. The properties of polyalphaolefines and hydrocrack products are sufficiently well known and can thus be reliably assessed. If they are infiltrated by base oil components of other biodegradable base fluids, however, it must be assumed that these properties will undergo fundamental changes. |
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HETG The end of the 80s saw a virtually universal breakthrough of biological oils including rapeseed. Since there were no test methods specially geared to these products at the time, their technical properties were established using test procedures originally designed for mineral oils. In these tests, the rapeseed oils fared well across the board. In practice, however, they proved to have a number of disadvantages from both a technical and ecological point of view, e.g. |
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HFB fluids Flame-retardant, “water-in-oil“ based fluids, i.e. an emulsion of approx. 40% water in mineral oil. Not used in Germany (but in other countries such as England). |
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HFC fluids Flame-retardant fluids based on a solution of polymers (polyglycols) in water in a ratio of approx. 60:40. HFC fluids have a viscosity of 30 to 45 mm˛/s at 50şC and can be used instead of mineral oils without the need for equipment modifications. The only limitation is that roller bearings have a shorter lifetime due to the water content of the HFC fluids, and the working temperature is limited to a range of -20şC to +60şC. The water content must be monitored. |
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HFD fluids Water-free, synthetically produced, flame-retardant fluids based on the following compounds: Common to all HFDs are an incompatibility with most sealing materials and a high density of 1.15 to 1.4 g/cmł (suction characteristics!), making subsequent conversions difficult. |
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High Water Based Fluids (HWBF) Flame-retardant fluids based on water with 2 to 5% concentrate to give the water minimum protection against corrosion and wear. The following fluids are obtained depending on the composition of the concentrate |
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HL oils Abbreviation for hydraulic oils with ageing and corrosion protection properties for systems with high thermal loads. Viscosity index: 95 to 105 (standard DIN 51524, Part 1). |
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HLP oils Abbreviation for hydraulic oils with ageing, corrosion and wear protection additives. Used in environments with a high level of wear, e.g. due to high pressures. Viscosity index: 95 to 105 standard DIN 51524, Part 2. |
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HLPD oils Non-standardised abbreviation for hydraulic oils with ageing, corrosion and wear protection additives as well as an emulsifying (dispersing) additive which keeps dispersed solid impurities and water droplets in suspension. Emulsifying capacity approx. 2% of water content. Viscosity index: 95 to 105. |
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HVLP oils Abbreviation for hydraulic oils with ageing, corrosion and wear protection additives as well as viscosity improving additives. Viscosity index: type 1: 125 to 190; type 2: 240 to 400. (standard DIN 51524, Part 3) |
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Hydraulic fluid Pressure fluids are the fluids used for power transmission in a hydraulic system. It is important that the fluid, as part of the system, be designed or selected as a machine element in its own right. Fluids are classified under ISO 6743 as mineral-oil based, flame-retardant or environment-friendly. The main type designations are HLP (mineral oil with wear and high-pressure additives), HFA (flame-retardant oils in water emulsion) and HEES (bio-oils with a synthetic ester base). |
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Hydrostatic traction drive Hydraulic drive for infinitely variable vehicle propulsion which generally operates in a closed circuit with primary (pump) adjustment or primary/secondary (pump and motor) adjustment. With a suitable drive circuit (e.g. automotive control), the hydrostatic traction drive can be adapted to the performance characteristics of the internal combustion engine so that its torque is fully utilised. That way virtually the full torque is available even at a low engine speed (= minimum gas pedal position f1). The same applies to the braking torque. Overloading of the IC engine is prevented by using load limit sensing control. |
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Initial contamination Contamination of the pressure fluid in a hydraulic system before the first flushing. It consists of the contamination contained in the pressure medium itself before it is poured into the hydraulic system plus the dirt particles from the production process which were not sufficiently rinsed out of the various components before assembly (“inherent dirt“). It is essential to filter this dirt out first before start-up by flushing the system. This is often done using a flushing system with its own pump and large filters. |
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Initial pressure difference ∆pA of filters Pressure difference generated in a new, uncontaminated filter when a certain volumetric flow is passed through it. It depends on the design of the filter element and the grade of filtration as well as the viscosity and size of the volumetric flow. The dirt holding capacity and hence the pressure difference of a filter have an upper limit. Both increase as the initial pressure difference decreases. For this reason, it is an important parameter to consider when selecting a filter. This value has little significance on its own, however: it is also necessary to know the dirt holding capacity of the filter, as a filter with a low ∆pA can clog up more quickly than one with a higher ∆pA. |
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International Organisation for Standardisation (ISO) Worldwide association of national standardisation institutes which deals with all standardisation tasks relating to fluid technology through its Technical Committee TC 131. The resulting ISO standards are either adopted directly (DIN-ISO …) or incorporated into the relevant DIN standard. |
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ISO 16889 This standard is entitled "Multi-pass method for evaluating filtration performance of a filter element", and describes the exact procedure for determining the retention rate, filtration grade and dirt absorption capacity of a filter element. It also draws on other standards such as ISO 2942 (bubble point test) or ISO 4406 (determination of oil purity class via APC). ISO 16889 was issued at the beginning of 2000 following a thorough revision of the standard for the multi-pass test in the 90s. The previous standard was known as 4572. |
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ISO 2941 Filter element collapse pressure/bursting pressure test This standard was first published in March 1974 and belongs to the group of test standards for hydraulic filters. It describes the procedure for testing a filter element for pressure resistance and evaluating the results. The idea is that the tested element should remain integrated even after reaching the collapse pressure/bursting pressure specified by the manufacturer. The prescribed differential pressure is obtained by adding dirt to the oil flow. |
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ISO 3724 This standard belongs to the group of test standards for hydraulic filters. One of the toughest tests for the filter element is the flow fatigue test according to ISO 3724. In this test, a defined differential pressure is set and the flow varied. This establishes the capacity of the element to withstand a fluctuating volumetric flow without damage. Documentation produced under this standard is extremely important for presses, injection moulding machines, construction machinery and many other mobile applications. Successful completion of this test is also a sign of the quality of an element's production. Despite the high costs involved, RT-Filtertechnik conducts this test for its customers on every new product it develops. |
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ISO 4406 This standard describes the purity of a hydraulic fluid in terms of the number of solid particles it contains. According to this, a code number is assigned as shown in the adjacent diagram. The table shows the particle numbers in detail. |
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ISO MTD (Medium Test Dust) This test dust consists of sand particles with a defined size distribution. The dust is used to calibrate APC devices for hydraulic systems or to conduct tests on filter elements. The ISO MTD dust was used in the revision of filter test standards including ISO 16889 and ISO 4406. |
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Karl Fischer The Karl-Fischer titration process is a method of determining the water content in technical products, foodstuffs, chemical substances, etc. (after Karl Fischer, German petrolchemist, 1901-1958). |
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Kinematic viscosity Ratio of dynamic viscosity to fluid density: υ = η / ρ mm˛/s The kinematic viscosity is the normal viscosity indicator in hydraulics. υ was formerly expressed in degrees Engler (şE), then in Stokes or centistokes (cSt) and now in m˛/s or mm˛/s. The conversion rule from cSt is: 1 cSt = 1 mm˛/s |
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Line filter Line filters are hydraulic filters which are either screwed directly into the pipeline or fitted in the machine with a mounting device. They can be used as suction filter, pressure filter, return-flow filter or bypass filter, and are available in a wide range of filtration grades and materials, from 1 µm microfilters to 600 µm coarse protective filters. |
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Magnetic core RT-Filtertechnik offers magnetic cores for use in magnetic pre-filtration or as a filtration solution in their own right, for integration in the housing. All magnetic cores consist of permanent magnets which, when pressed together at like poles and separated by steel plates, generate a strong magnetic field with a maximum radius. |
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Magnetic pre-filtration Typical applications of RT filters are hydraulic systems in mobile and stationary machines. Many large production lines are also equipped with RT filters for purposes of coolant filtration, however. In both cases, the fluid contains a not insignificant amount of metal particles. The size of these particles ranges from 0.5 µm to 100 µm or even larger in unfavourable cases. For over 30 years, magnetic pre-filtration has proved successful in RT filters and has been consistently refined and improved upon. The typical flow direction of an RT element from the inside outwards makes it possible to fit a magnetic core in the stabilised, laminar portion of the flow. The magnets are arranged in a specific way and separated with steel discs, thus creating a particularly strong magnetic field around the core. Measurements have shown that particles as small as 0.1 µm, or up to 30 % of non-ferromagnetic particles are retained. As well as protecting the main filter element, the magnetic core also allows the amount and type of metal impurity to be readily visually identified subject to regular maintenance. Consequently, it can also be used as an early indicator of damage, e.g. in the region of bearings or pistons. |
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Magnetic pre-filtration Typical applications of RT filters are hydraulic systems in mobile and stationary machines. Many large production lines are also equipped with RT filters for purposes of coolant filtration, however. In both cases, the fluid contains a not insignificant amount of metal particles. The size of these particles ranges from 0.5 µm to 100 µm or even larger in unfavourable cases. For over 30 years, magnetic pre-filtration has proved successful in RT filters and has been consistently refined and improved upon. The typical flow direction of an RT element from the inside outwards makes it possible to fit a magnetic core in the stabilised, laminar portion of the flow. The magnets are arranged in a specific way and separated with steel discs, thus creating a particularly strong magnetic field around the core. Measurements have shown that particles as small as 0.1 µm, or up to 30 % of non-ferromagnetic particles are retained. As well as protecting the main filter element, the magnetic core also allows the amount and type of metal impurity to be readily visually identified subject to regular maintenance. Consequently, it can also be used as an early indicator of damage, e.g. in the region of bearings or pistons. |
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Micro-diesel effect The hydraulic fluid is exposed to constantly changing pressures in the system, and the behaviour of the air in the fluid varies accordingly. At high pressures, the air dissolves, whereas at low or negative pressures it escapes from the solution in the form of air bubbles. In certain flow situations - e.g. in the case of cavitation - free air bubbles are released which also contain oil vapour. If these free air bubbles are pressurised, this may cause the air-and-oil mixture to self-ignite. This is known as the micro-diesel effect. It can also cause localised heating of the oil to extremely high temperatures, and produces combustion residues such as soot. |
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Mixed-friction environment Mixed friction occurs in lubricated frictional pairings at the beginning of the movement or as a result of inadequate lubrication. In this case, there is only localised contact between the fixed surfaces, thus generating a fine, so-called abrasive dust. This abrasion contaminates the pressure fluid. |
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Multipass test Test method to establish the filter’s effectiveness (retention rate). For this purpose, a defined mineral oil contaminated with quartz sand (ISO-MTD dust) of various grain sizes is passed through the test filter several times. The retention rate is measured in terms of the β (beta) value, i.e. the ratio between the number of particles of a particular, identical size upstream and downstream of the filter respectively. The multipass test is standardised in ISO 16889. |
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NAS1638 This standard also describes the purity of the pressure fluid used in the system. In contrast to ISO4406, the particles are not added up (cumulative approach) but divided into groups by size and coded accordingly. There are 5 groups. The lowest code number is indicated as the purity code for the fluid in question. NAS 1638 has not yet been adapted to the changed conditions of 1999, however. At that time, the particle size definition, test dust used for measurements, test method, etc. were changed. Consequently, an NAS1638 rating is not currently comparable with an ISO 4406:1999 purity rating. This situation is set to change, as the NAS standard is to be replaced by another American standard which has already been revised to account of the new conditions. This standard is known as SAE AS4059 (Aerospace Standard). |
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Neutralisation number The neutralisation number is an important characteristic of the pressure fluid. The pH value of a hydraulic fluid always differs from neutral and changes over its lifetime and/or according to the load. The NN describes the chemical state of an oil. In order to assess this, however, it is important to know the NN of the oil when new. The NN is normally determined and assessed by a qualified laboratory. |
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Oil analysis The following factors are highly important when using pressure fluids in hydraulic systems: The oil purity is critical for the operational reliability of a hydraulic system and should therefore be regularly inspected. RT filters guarantee a consistently high degree of purity throughout the system and throughout the lifetime of the filter. Since hydraulic oils can have widely differing formulas, it is important to know the initial state of the fresh oil. Water content and air release property are also important indicators for reliable operation which can be determined simultaneously during chemical analysis by independent laboratories. |
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Oil leakage area Hydraulic components containing moving parts are equipped with either a mechanical elastomer seal or a diaphragm gland. The latter can have a volumetric efficiency of up to 98 %. A small quantity of oil is deliberately diverted from the main flow however - particularly in pumps and motors - in order to keep the components lubricated. This so-called leakage oil has to be fed through a separate conductor. The same applies to the leakage oil flows generated in the pilot control stages of some pressure or directional control valves. For functional reasons, these leakage oil flows have to be fed into the tank with as little resistance as possible. For this reason, the filtration arrangement is not uncritical, particularly in mobile hydraulics. Here, RT systems combining strainer filters with main flow microfilters have proved highly successful in the field. |
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Oil life-time The hydraulic oil serves to transmit power in the form of flux (volumetric flow) and pressure. The pressure (shearing action) and thermal load accelerate the ageing process of the oil. Water contamination (especially in the case of HEES fluids) and air intake likewise tend to reduce the oil lifetime. The additive content of the oil is also depleted over time. Topping up the additives is not generally recommended. Given regular inspection, modern hydraulic fluids can be used for many thousands of hours, depending on the configuration and operating conditions of the system. |
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Oil purity The purity of a hydraulic fluid is defined by the number of solid particles per ml of fluid. The number of particles is normally measured with an automatic particle counter. The purity class code is then determined from the number of particles of different sizes. The procedure for counting the particles and determining the purity class code is described in the standard ISO4406:1999. |
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Oil purity class The purity of a hydraulic fluid is defined by the number of solid particles per ml of fluid. The number of particles is normally measured with an automatic particle counter. The purity class code is then determined from the number of particles of different sizes. The procedure for counting the particles and determining the purity class code is described in the standard ISO4406:1999. |
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Oil-air release property Capacity of a pressure fluid to separate itself from impurities by rising (air) or sinking (foreign bodies). |
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Original dirt The so-called original dirt largely consists of coarse particles attributable to the production process. While coarse particles often lead to sudden failures, an excessively high concentration of fine particles leads to more insidious defects. Small particles have a strongly abrasive effect, resulting in increased power losses and lower efficiency. Valve bodies, pump and motor housing often have complex inner contours (particularly blind holes and undercuts) in which residues are deposited during production. Metal chips, moulding sand and other particles are rarely eliminated completely during cleaning, however. Preservatives used to protect the components during storage or transportation absorb even more dust which is virtually impossible to remove completely. Dirt also enters the hydraulic system during assembly of the individual components, especially if the assembly conditions are unclean. The most common impurities are wheel swarf, scale, rust, machining chips, paint particles, welding beds, textile fibres and rubber particles. Residues of flushing fluid also contribute to the contamination of the hydraulic system. |
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Particle count The quickest way to analyse particles in the fluid is with an automated particle counter. In this process, the oil flows past a light source, causing the intensity of the light beam registered by a photodiode to decrease according to the size of the individual particles. The change in the light beam intensity triggers voltage changes in the photodiode which provide a direct measure of the particle size. Obtaining a correct reading depends on the particles passing the light beam in succession: with this method of measurement, however, the result may be falsified by air bubbles and water droplets. It is therefore important to prepare the samples correctly. Essential criteria for a meaningful oil analysis are: Regular and representative sampling and correct counting of the particles provide an early indication of any adverse occurrences. That way failures can be prevented at an early stage. An alternative method of counting particles is to pass a certain quantity of the oil sample through a diaphragm and then conduct a microscopic evaluation of the particle counts. This evaluation can be performed by a manual count or with the aid of image analysing software. The procedure for evaluating the results of the particle count for hydraulic fluids is described in the standard ISO4406:1999. |
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Particle counter The quickest way to analyse particles in the fluid is with an automated particle counter. In this process, the oil flows past a light source, causing the intensity of the light beam registered by a photodiode to decrease according to the size of the individual particles. The change in the light beam intensity triggers voltage changes in the photodiode which provide a direct measure of the particle size. Obtaining a correct reading depends on the particles passing the light beam in succession: with this method of measurement, however, the result may be falsified by air bubbles and water droplets. It is therefore important to prepare the samples correctly. Essential criteria for a meaningful oil analysis are: Regular and representative sampling and correct counting of the particles provide an early indication of any adverse occurrences. That way failures can be prevented at an early stage. |
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Permanent magnet A permanent magnet is a piece of magnetisable material, e.g. iron, cobalt or nickel, which retains its static magnetic field without the need for an energy supply. Modern permanent magnets of the brand groups Oxit (barium-ferrite), Oerstit (aluminium-nickel-cobalt) and Secolit (rare earths-cobalt) can exert very strong forces on magnetisable particles. RT-Filtertechnik is the only manufacturer to offer magnetic prefiltration in hydraulic filters as standard. For this, we have been using the highly efficient RT magnetic cores for over 40 years, during which time they have been developed to their present perfection. |
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Permanent magnet A permanent magnet is a piece of magnetisable material, e.g. iron, cobalt or nickel, which retains its static magnetic field without the need for an energy supply. Modern permanent magnets of the brand groups Oxit (barium-ferrite), Oerstit (aluminium-nickel-cobalt) and Secolit (rare earths-cobalt) can exert very strong forces on magnetisable particles. RT-Filtertechnik is the only manufacturer to offer magnetic prefiltration in hydraulic filters as standard. For this, we have been using the highly efficient RT magnetic cores for over 40 years, during which time they have been developed to their present perfection. |
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Polyglycols (HEPG) Polyalkylglycols, or polyglycols for short, have been used for nearly 50 years due to their effective lubricative properties. For a long time, polyglycol oils were mainly prized for their performance under extreme conditions, e.g. very low or high continuous temperatures. Following the introduction of environment-friendly lubricants and pressure fluids, however, polyglycols (Hydraulic Oil Environmental Polyglycols, HEPGs) also began to gain importance in this field. They are all (depending on their structure) environmentally acceptable. Thanks to their solubility in water, for example, they have played a major role in the food industry for over 25 years. It was therefore logical to consider using polyglycols with low toxicity and sufficient biodegradability as a basis for environment-friendly lubricants and pressure fluids. Among the various types of polyglycol, only polybutylene glycols and an ethylene oxide/propylene oxide copolymer are used for this purpose. These are cold-water soluble types, i.e. substances which do not inhibit the exchange of oxygen at the surface of the water while having an oxygen-depleting effect at the bottom; only these types conform to the required ecotoxicity values. Besides the above environment-related characteristics, polyglycols also have good technical properties, e.g.: There are also some properties which make them less suitable as lubricants and pressure fluids, however: Despite these disadvantages, the technical properties of polyglycol-based lubricants are too useful to be forfeited. For this reason, they are still frequently used in the industrial sector, given the right machine configuration and seals/hoses made of suitable materials. |
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Precharge valve The precharge valve is a simple non-return valve which is usually integrated in suction/return flow filters. It has a precharge pressure of between 0.04 and 0.8 bar depending on the application and keeps the pump filled with excess pressure. |
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Pressure The pressure p is the quotient of the normal force FN acting on an area and the area itself A (DIN 24312). The derived unit for pressure in the SI system is the Pascal: 1 Pa = 1 N/m˛. In fluid technology, p is usually expressed in bars (1 bar = 100.000 N/m˛) although MPa (1.000.000 N/m˛) is also used. Unless otherwise specified, pressure in fluid technology is understood as overpressure, i.e. atmospheric pressure is used as the reference value. The pressure of a medium is one of the main parameters in fluid technology. It occurs in many variations, each of which has its own specific significance for the system. |
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Pressure filter Filter which is installed in the high-pressure pipe, i.e. downstream of the pump. The advantage of this is that only the pump output has to pass through it and the subsequent components are protected. The disadvantage is the higher price. If a special type of pure control oil is required, particularly in the case of servovalves, a microfilter is fitted directly in the control oil circuit. |
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Pressure filtration Pressure filtration is performed with line filters which are installed between the pump and consumers. Given a suitably filtration grade (at least 10 µm), this type of filtration offers good protection for the components. It is however relatively expensive depending on the level of pressure. |
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Pressure fluids Pressure fluids are the fluids used for power transmission in a hydraulic system. It is important that the fluid, as part of the system, be designed or selected as a machine element in its own right. Fluids are classified under ISO 6743 as mineral-oil based, flame-retardant or environment-friendly. The main type designations are HLP (mineral oil with wear and high-pressure additives), HFA (flame-retardant oils in water emulsion) and HEES (bio-oils with a synthetic ester base). |
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Pressure tank Pressure tanks or vessels are components for holding pressurised fluids. The design, construction and inspection of such vessels are governed by the Pressure Equipment Directive (97/23/EC). Hydraulic filter housings are not generally classified as pressure vessels. Classification is by output per litre of pressure fluid (V x p) or according to the fluid in question. RT suction gas filters for chillers are pressure vessels for example, and are thus subject to a highly complex inspection and approval procedure. |
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Primary contamination Solid impurities in the hydraulic fluid are classified in terms of primary and secondary contamination. Primary contamination means all dirt particles already present in the system when it is put into operation (see also initial contamination). This dirt often consists of welding sparks, chips, welding residues, casting sand, rubber abrasion and other particles generated by production. The important thing is to eliminate this dirt as quickly as possible from the system through filtration, so as to minimise the amount of secondary contamination arising within the system. By secondary contamination, we mean particles generated in the components through abrasion, cavitation or pitting. It also includes particles entering the system from outside during operation. This can happen through piston or shaft seals or through the tank ventilation. |
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Primary contamination Solid impurities in the hydraulic fluid are classified in terms of primary and secondary contamination. Primary contamination means all dirt particles already present in the system when it is put into operation (see also initial contamination). This dirt often consists of welding sparks, chips, welding residues, casting sand, rubber abrasion and other particles generated by production. The important thing is to eliminate this dirt as quickly as possible from the system through filtration, so as to minimise the amount of secondary contamination arising within the system. By secondary contamination, we mean particles generated in the components through abrasion, cavitation or pitting. It also includes particles entering the system from outside during operation. This can happen through piston or shaft seals or through the tank ventilation. |
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Proportional valve Continuously adjustable pressure, flow and directional control valves which convert a variable input signal to a proportional hydraulic output signal and are usually actuated by a proportional magnet. There are no clear specifications or binding standards for differentiating these from other types of continuously adjustable valves. Proportional valves are highly suitable for use as actuators in the control circuit. The position of the control element is reported either by springs or electrically, via inductive position encoders. This electrical feedback greatly improves the performance of the proportional valves by enhancing static characteristics, shortening hysteresis and ensuring greater rigidity, responsiveness and reproducibility. Proportional valves are – in contrast to servo valves – relatively unsusceptible to dirt (e.g. recommended filtration grade 25 µm as opposed to 3…5 µm) and are therefore more robust. They can be divided into the following groups according to their stationary characteristics: |
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Purity The purity of a hydraulic system is primarily defined in terms of oil purity. Thanks to the efforts of machine manufacturers, a standard for classifying component purity is now also being drafted (as of 2004). |
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Purity class The purity of a hydraulic fluid is defined by the number of solid particles per ml of fluid. The number of particles is normally measured with an automatic particle counter. The purity class code is then determined from the number of particles of different sizes. The procedure for counting the particles and determining the purity class code is described in the standard ISO4406:1999. In industry, the American standard NAS1638 or SAE AS4059 is also used as an alternative. |
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Rapeseed and other native oils The end of the 80s saw a virtually universal breakthrough of biological oils including rapeseed. Since there were no test methods specially geared to these products at the time, their technical properties were established using test procedures originally designed for mineral oils. In these tests, the rapeseed oils fared well across the board. In practice, however, they proved to have a number of disadvantages from both a technical and ecological point of view, e.g. |
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Readily biodegradable fluids (hydraulic oils) Biodegradable fluids are hydraulic oils which biodegrade much quicker than mineral oils. Biodegradation is a chemical transformation brought about by microorganisms under aerobic conditions and in hydrous environments, leading ultimately to mineralisation (OECD screening test). Biodegradability, soil and water safety and the absence of dangerous substances are important criteria for the environment-friendliness of a fluid. To the biodegradable fluids belong: HEES, HEPR, HETG und HEPG. |
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REM REM (Scanning Electron Microscope) This technique is used for material analyses. When investigating major cases of damage in hydraulic systems, it can be useful to analyse the solid particles retained in the filter not only optically/by microscope, but also by determining the precise material of individual particles. For this purpose, the particle can be membranised and then subjected to atomic emission spectroscopy under the REM. |
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Return flow filter The return flow filter is installed in the return flow pipe of the fluid system before the fluid returns to the tank. It enables all volume flows from the system to be combined and cleaned in a single filter unit. This is known as full-flow return filtration. Return flow filters are relatively cheap and provide ideal conditions for establishing the purity of the system. |
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SAE AS4059 SAE AS4059 was introduced to replace the above mentioned purity class standard NAS 1638. There is now a new "000" class and the size classes are again calculated cumulatively, as in ISO 4406:1999. The particle sizes are also being adapted and are as follows: >4µm >6µm >14µm >21µm >38µ and >70µm. This means that ISO and SAE AS oil purities are now once again comparable. |
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Saturated esters (HEES) Environment-friendly lubricants and pressure fluids based on saturated synthetic esters (HEES) have been available on the market since 1985. They are produced when water is separated by the reaction of alcohols with carbonic acids. The base substances used here are derived from the petrochemical industry. Saturated synthetic esters are regarded as environment-friendly due to the following characteristics: They also have a number of positive characteristics from a technical point of view, however: The disadvantage of saturated ester oils is their high price in comparison to mineral oils or even rapeseed and unsaturated ester oils. This price disadvantage has to be compensated for by the above technical advantages and the possibility of using the oils over a longer period. |
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Secondary contamination Solid impurities in the hydraulic fluid are classified in terms of primary and secondary contamination. Primary contamination means all dirt particles already present in the system when it is put into operation (see also initial contamination). This dirt often consists of welding sparks, chips, welding residues, casting sand, rubber abrasion and other particles generated by production. The important thing is to eliminate this dirt as quickly as possible from the system through filtration, so as to minimise the amount of secondary contamination arising within the system. By secondary contamination, we mean particles generated in the components through abrasion, cavitation or pitting. It also includes particles entering the system from outside during operation. This can happen through piston or shaft seals or through the tank ventilation. |
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Secondary contamination Solid impurities in the hydraulic fluid are classified in terms of primary and secondary contamination. Primary contamination means all dirt particles already present in the system when it is put into operation (see also initial contamination). This dirt often consists of welding sparks, chips, welding residues, casting sand, rubber abrasion and other particles generated by production. The important thing is to eliminate this dirt as quickly as possible from the system through filtration, so as to minimise the amount of secondary contamination arising within the system. By secondary contamination, we mean particles generated in the components through abrasion, cavitation or pitting. It also includes particles entering the system from outside during operation. This can happen through piston or shaft seals or through the tank ventilation. |
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Secondary contamination Solid impurities in the hydraulic fluid are classified in terms of primary and secondary contamination. Primary contamination means all dirt particles already present in the system when it is put into operation (see also initial contamination). This dirt often consists of welding sparks, chips, welding residues, casting sand, rubber abrasion and other particles generated by production. The important thing is to eliminate this dirt as quickly as possible from the system through filtration, so as to minimise the amount of secondary contamination arising within the system. By secondary contamination, we mean particles generated in the components through abrasion, cavitation or pitting. It also includes particles entering the system from outside during operation. This can happen through piston or shaft seals or through the tank ventilation. |
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Separation capability Capacity of a pressure fluid to separate itself from impurities by rising (air) or sinking (foreign bodies). |
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Service life In filtration, the term service life means the number of operating hours before the filter element has to be changed. The service life of a filter element depends on several parameters: dirt absorption capacity, filter area, specific contamination in the machine and initial pressure difference. At the end of its service life, the element must be cleaned (wire mesh elements only) or changed. The maintenance required can be indicated visually or electrically, or it may be defined in terms of a set number of operating hours. When determining the maintenance intervals, the mechanical loading of an element also needs to be taken into account. In the case of mobile machinery in particular, filter elements must be designed with a view not only to their dirt absorption capacity, but also their mechanical endurance strength. |
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Solid particle Particles which are suspended in the fluid as solid matter and do not bond with it chemically are classed as solid particles. In hydraulics, particles become significant from a size of approx. 1 µm. They may consist of metals, elastomers, silicon, slag or textile materials. |
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Solubility of gas Fluids have the capacity to dissolve certain quantities of gas. In mineral oil, this capacity rises with pressure to approx. 300 bar linear. Dalton’s Law states that: VG = VÖl • α • p/pş VG = dissolved gas volume at reference pressure VÖl = fluid volume α = Bunsen coefficient in normal mineral oils 0.09 for air and 0.08 for nitrogen p = relevant absolute pressure pş = reference pressure (atmospheric pressure). The degree of saturation which this represents does not of course always have to be reached. 1 l oil contains approx. 90 cmł air at atmospheric pressure, but 9 l at 100 bar. Attention: as long as gas is dissolved in the fluid, it will not cause any disruption in the circuit! |
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Specific pollutant status If we compare the dirt accumulation in hydraulic systems of similar designs, we note that, in any specific application, comparable operating and environmental conditions – with regard to the delivery output of the working pump – produce a characteristic value. This value is known as the ”specific pollutant status“ (SPS value) and is measured in g/l/min based on 1000 operating hours. Plastic injection moulding machines have an SPS value of 0.06, for example, while that of machine tools is significantly higher. In construction machinery, the values tend to be relatively low due to continual improvements in piston rod seals and tank ventilation. |
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Strainer filter Strainer filters, also known as "police filters", are installed at exposed points within a hydraulic system to protect the components. Such filters are usually simple structures for mounting in-line and are designed for lifetime service. For a detailed survey of possible applications of the RT strainer filter range, see our article "Playing safe". |
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Suction filter Once installed in a hydraulic circuit, the suction filter keeps the pump supplied with ready filtered oil. In mobile applications, however, suction filtration has the disadvantage that high viscosities (at low temperatures) may lead to cavitation damage. For this reason, suction filters should only be used where absolutely necessary in terms of design. |
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Suction filtration Once installed in a hydraulic circuit, the suction filter keeps the pump supplied with ready filtered oil. In mobile applications, however, suction filtration has the disadvantage that high viscosities (at low temperatures) may lead to cavitation damage. For this reason, suction filters should only be used where absolutely necessary in terms of design. |
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Suction-return flow filter Suction-return flow filters are tank-integrated return flow filters whose housing is sealed from below with a valve plate. Integrated in this plate is a pre-charge valve which only admits the oil into the tank once a certain overpressure (normally 0.5 bar) is reached. This causes the filtered oil to rise back into the filter head, where it can be used to supply a feed pump in a closed circuit, for example. This eliminates cold-start problems in mobile production machines with hydrostatic travelling mechanisms and integrates the dual functions of suction and return flow filtration in a single filter. |
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Suction-return flow filter system Suction-return flow filters are tank-integrated return flow filters whose housing is sealed from below with a valve plate. Integrated in this plate is a pre-charge valve which only admits the oil into the tank once a certain overpressure (normally 0.5 bar) is reached. This causes the filtered oil to rise back into the filter head, where it can be used to supply a feed pump in a closed circuit, for example. This eliminates cold-start problems in mobile production machines with hydrostatic travelling mechanisms and integrates the dual functions of suction and return flow filtration in a single filter. |
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Surface filter Filter which only eliminates impurities at the surface of the filter element. Surface filters are designed with uniform pores (gaps) so that they can retain virtually all particles of a certain size. They do have a low dirt holding capacity, however, which means that they have a much shorter lifetime than deep-bed filters. Surface filters are made from thin fabrics mostly consisting of metal threads (strainer filters). Other surface filters are metal edge filters. |
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System return flow In a hydraulic system, a pump moves a column of oil through pipes and valves to the actuators, where the output power is made available (to move loads, drive vehicles, etc.). Once the work has been performed, the oil flows back from the actuator into the hydraulic tank. In this area, the oil is only under a slight pressure and flows at a relatively low flow rate (2 – 3 m/s). The various return flow pipes from a system can be easily combined, thus allowing all the oil to be fed to one filter. This is known as full-flow return filtration. RT Filtertechnik offers a wide range of ideal solutions for this purpose - ask our experts! |
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Test bench filters Hydraulic components or gears are often put into operation or tested on a test bench. The presence of ever changing components on the test bench means that dirt is constantly entering the fluid circuit. A suitable fine filtration system is therefore urgently recommended. To ensure economic operation, test bench filters are equipped with filter elements with a very high dirt absorption capacity. RT-Filtertechnik has many years of experience in this field and offers a wide range of suitable filters. |
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Test on ISO 3724 This standard belongs to the group of test standards for hydraulic filters. One of the toughest tests for the filter element is the flow fatigue test according to ISO 3724. In this test, a defined differential pressure is set and the flow varied. This establishes the capacity of the element to withstand a fluctuating volumetric flow without damage. Documentation produced under this standard is extremely important for presses, injection moulding machines, construction machinery and many other mobile applications. Successful completion of this test is also a sign of the quality of an element's production. Despite the high costs involved, RT-Filtertechnik conducts this test for its customers on every new product it develops. |
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Ultrasonic bath In an ultrasonic bath, ultrasonic waves are generated in a fluid (usually heated water with a gentle detergent). These ultrasonic waves are suitable for removing micro-particles from surfaces to which they cling by adhesive force. Ultrasonic baths are therefore suitable for cleaning wire gauze elements or filtering out solid contaminants from glass-fibre elements for investigation purposes. They are not suitable for cleaning glass-fibre elements, however, as this would damage the fibre structure of the filter element. |
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Unsaturated esters (HEES) For many manufacturers, the next step – following the negative experiences with rapeseed oils in the engineering sector – was to develop synthetic esters, or HEESs (Hydaulic Oil Environmental Ester Synthetics) for short. Although saturated synthetic esters had already been on the market for years, they had been regarded as too expensive. For economic reasons therefore, mainly unsaturated TMP esters (trimethyl propane esters, also known as oleic acid esters or trioleates) were used. TMP esters have a better low and high-temperature stability than natural rapeseed oil. These differences are due to the different distribution of fatty acids in the esters: the low polyunsaturated fatty acid content of oleic acids means that TMP esters have a better resistance to oxidation and a lower pour point than rapeseed oils or triglycerides. The main advantages of unsaturated synthetic esters over rapeseed oils are thus: One major problem with trioleate esters, however, is that products from different suppliers – despite having the same chemical name - vary considerably in terms of their technical properties. Apart from obvious differences in product quality which are due to the base oils used, both the esterification technology and additive process are critical to the quality of the product. It is always difficult for the user to judge the quality of an unsaturated synthetic ester product from its name, as the abbreviation HEES is used universally for these products according to VDMA 24568 (German Machine and Plant Engineering Association). To make matters even more confusing, the same abbreviation is also used to refer to saturated synthetic ester oils. |
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Vacuum membranisation In order to assess the filtration process, it is important to know what type of solid particles the fluid is contaminated with. For this purpose, the filter material with the retained particles or the contaminated fluid is treated in an ultrasonic bath. The fluid in which the separated particles are suspended is then sucked through a filter membrane under vacuum. Typical membrane sizes in hydraulic systems are 0.5 to 3 µm. The solid particles are retained by the membrane during the vacuum suction process, and can then be analysed under a microscope. |
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Vacuum switch Vacuum switches are used to monitor the degree of contamination of suction filters. When a certain flow resistance is reached in the filter (usually -0.18 bar rel.), an electrical contact is closed or opened to signal that the filter needs changing. |
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Viscosity Viscosity is a fundamental property of fluids and describes the friction between the moving fluid layers, i.e. the flow. In other words, viscosity is a measure of the flowability or lubricity of hydraulic fluids. A basic distinction is made between dynamic viscosity and kinematic viscosity. For hydraulic oils, the critical parameter is kinematic viscosity. The viscosity of oils decreases as the temperature rises. |
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Viscosity improvement additive Macromolecular additives designed to obtain a flatter VT curve. Due to their long length, the molecules of viscosity improvement additives are shear sensitive, so their effect diminishes over time. |
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Viscosity index The viscosity/temperature characteristic of a fluid. The less the viscosity changes with temperature, the higher the VI (CETOP RP 100). The VI is calculated theoretically (DIN ISO 2909) and is 95 - 100 for “normal” mineral oils and approx. 160 and above for oils with a flat VT curve (HV oils). |
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Volumetric flow pulsation All feed pumps which work on the displacement principle have a finite number of pistons, gear wheel flanks or other feed components. The volume flow thus generated has a pulsating character at the pump output, which causes pressure fluctuations in the pipe. This factor has to be taken into account when designing line filters. However, many mobile machines have a full-flow return filtration system. These return-flow filters are exposed not only to the pulsation of the volume flow, but also to the fluctuations of the kinematic process. Important factors here are the work cycle, the area ratio of the cylinders and the number of cylinders. The volume flow pulsation thus produced in the return-flow pipe can generate very high dynamic flow forces. It is therefore important when designing the filter elements to ensure sufficient mechanical fatigue strength. |
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Water contamination When mineral oil or diester oil based hydraulic fluids mix with water, they are said to be contaminated. The water usually gets into the system via the hydraulic tank ventilation. It speeds up the oxidation of the hydraulic oil and, from a certain concentration onwards (approx. 1000 ppm), causes diester oils to go off. Since the water in the oil can also have a damaging effect on the components, measures should be taken to prevent water contamination. This can be done with RT ventilation filters with moisture-absorbing granules or also bypass filters. |
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Wire mesh Filter elements with a wire mesh fabric are often used as a low-cost, re-usable solution in strainers, suction filters or return flow filters. Depending on the requirements (filtration grade, pressure, dynamics), different types of fabric such as twill, linen or braid are used. Wire mesh elements are always surface filters, i.e. they become soiled more quickly than disposable elements. When regenerating filter elements, it should be noted that anything finer than 40 µm can only be cleaned in an ultrasonic bath. RT-Filtertechnik offers a wide range of filter elements with gauges from 5 to 900 µm, as well as individual filter housing solutions. |
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ZRP ZRP is the abbreviation for the German term Zahnradpumpe (gear pump) - a low-cost, fixed-displacement-type pump often used in hydraulic engineering. |
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