Playing safe
	To design a good filtration system, we have to analyse the given operating con- ditions closely. Factors to be taken into account include the type of system (mobile or stationary), dynamic load (pulsation frequency), environmental influences and - of course - the nature of the fluid. Finally, the purity class according to ISO4406:99 is established based on the most sensitive component in the system. This information is then used to determine the grade of filtration and a suitable filter is designed for the required service life. One solution which has proved both cheap and efficient is full-flow return filtration. This separates the dirt effectively while ensuring optimal flow conditions and allows the use of large, long-life elements. A disadvantage of this solution is that, although virtually all particles from the system eventually end up in the filter, a certain amount of damage may already have been done by this stage. Consequently, the more sen- sitive components have to be protected by additional filters. The “higher quality“ method is to use pure pressure filtration with line filters. A full-flow solution is quite expensive, however - even with lower pressures. What’s more, the particles generated in the system – secondary contamination caused by wear – are not reliably removed and can get into the pump via the tank. Another solution in special cases is to use suction filters or the innovative suction-return flow filter, though this is not suitable for all systems. None of the above filters is capable of protecting all components simultaneously to the same degree, however. It is therefore important when designing the machine to determine the filtration result, i. e. the oil purity, at several points in the system by means of a particle count. This analysis shows up the main weak points in terms of secondary contamination and allows suitable precautions to be taken. Tests show that the oil purity decreases steadily after filtration due to wear pro- cesses and other secondary contamination sources. A particular threat is posed by those particles which are of a similar size to the valve gaps. Through their abrasive effect, these can cause the volume of contamination to go on rising continually. We also know that approx. 70% of all hydraulic failures are triggered by solid im- purities. The actual cause may be various wear processes, the surface pressure of microscopic cracks or corrosion. This generally leads to the destruction of individual components, thus creating lots of metal fragments which are swept into the system. By the time these particles can be eliminated from the fluid by a filter, it is often too late to prevent damage to other sensitive and expensive components. Triggered by a directional control valve worth a mere 50 euros, such a crash situation can go on to cause damage to a hydraulic motor worth several thousand. To make sure this doesn’t happen, RT-Filtertechnik offers a tried-and-tested solu-tion. This involves fitting strainer filters at exposed points. These protective gauze filters, which are also known as “police filters“ or “last chance filters“, are available in various forms. The protection of sensitive components must be reliably guaranteed, however. It is therefore important that these filters be designed and planned in collaboration with the manufacturer from the word go. The following criteria are vital to a successful performance: Grade of filtration Filter area Service life Price Since the filters act as a kind of insurance for your valuable equipment, the first thing to consider is the price factor. Particularly in the case of heavy-duty mobile construction or agricultural machinery, awareness of the costly consequences of simple defects has led to a much greater readiness to use protective filters in recent years. Of the technical parameters, the first thing to decide on is the grade of filtration. This depends on the anticipated type of contamination and the components to be pro- tected. Values commonly range between 60 µm and 200 µm. Here again, however, there are financial considerations: the finer the filter, the larger the necessary area and the overall size of the filter. Then we have to think about the degree of strength required. Choosing the right wire mesh is an important task of the filter manufacturer. Stainless steel has proved a good material for this purpose. The wire thickness and weave have an impact not only on mechanical stability, but also on the specific open area. Consequently, the wire mesh, design and overall size have to be mutually co- ordinated and the suitability of the filter proved in a series of tests (e. g. according to ISO3724 - Flow Fatigue Charac-teristics and ISO2941 - Collapse Pressure/Bursting Pressure). The filter area is also critical. Since the filters can only be used in the pressure sec- tion, a small area with a high inherent resistance results in a significant power loss, accompanied by a higher risk of blockage. In safety-related components such as steering systems, couplings with protective filters are fitted in front of the valve. Caution is advised in the case of types with a built-in transverse filter screen in the flow direction. The only advantage of these couplings is their low price. Better results are obtained with solutions based on an exchangeable filter element. Such filter couplings, although similar in size, provide approx. 6 -7 times the filter area. The fatigue strength and flow resistance are also much better in filter inserts than built-in screens. A further possible application of these filters is in heavy construction machinery with multiple rotary drives. Whole batteries of such filters are used in bulldozers, for example. The filter elements are designed for life-time use and are only changed in the event of a crash. This solution calls for larger elements, however, due to the greater stresses they are exposed to in terms of volumetric flow pulsation, viscosity range or contamination level. For this purpose, pleated (folded) filter elements are used, as their sinusoidal shape gives the elements greater strength, good flow characteristics and a relatively compact overall size combined with a maximum filter area. Modern machines are characterised by compact design and low hydraulic fluid volumes. This makes it vital to ensure adequate cooling, and we are accordingly seeing an increasing use of hydraulic cooler fan control systems. These have a wide performance range and are easy to regulate. Typical drive units consist of constant motors with fitted proportional valves. Here again, reliable operation is critical. For this application, RT-Filtertechnik has developed a further protective filter format for pressures up to 220 bar. Thanks to the compact shape and light weight of this filter, it can be mounted directly on the connecting coupling of the hydromotor. As usual with RT-Filtertechnik, allowance is made for any special requirements the customer may have, hence the development of models with a 5 µm wire mesh or contamination filters with a 3 µm glass fibre element. The range of protective filters available from RT-Filtertechnik GmbH now covers a wide area and allows flexible adaptation to the given requirements.