Individual element combinations as an economic solution for suction-return-flow filters
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One of the most interesting developments in recent years in the field of hydraulic filtration is without doubt the principle of suction-return-flow filtration (or return-flow suction filters). Advocates of principle claim it improves the cold-start performance of the system and saves costs. Yet this technology raises problems of its own, as practical applications have shown. However widespread suction-return flow filters are becoming, the technology behind them remains controversial. Let us look at how this type of filter came to be developed in the first place. Hydrostatic drives or other closed-circuit systems have long been a feature of various types of machinery used in structural, agricultural or environmental engineering. These systems contain high-quality components which are subject to a lot of stress. Since only a small amount of fluid exchange takes place in such systems, this area requires special protection against solid impurities. One successful way of doing this is to fit suction filters upstream of the feed pump. An economical method of filtering the entire hydraulic system is that of full-flow return filtration. In this case, the oil filtered in the return pipe is fed to the tank to stabilise and cool down. This “classical“ function of the hydraulic tank function is now regarded as obsolete. Modern solutions such as wheel loaders have to be lightweight and compact in their design and efficient in their operation. From this point of view, a large hydraulic tank creates unnecessary ballast, particularly when we take into account the cost of large oil volumes. Hence the trend – which continues to this day - towards ever smaller tanks. Nowadays a tank volume/flow rate of 0.5 – unthinkable 20 years ago – is no longer a thing of impossibility. As a logical consequence of this trend, designers began – if only to save space - to tackle the problem of filtration. The idea of combining return-flow and suction filtration - later heralded as a great innovation – had not yet occurred to anyone 20 years ago. This quickly changed however, as attention turned to improving the performance of hydraulic systems in critical states. RT-Filtertechnik developed such a filter system at an early stage, and has since accumulated a wealth of experience in this area. The success and advantages of this filter are based on three factors: firstly, one filter can take the place of two; secondly, the feed pump is supplied with extra-finely filtered oil; and thirdly, the suction lines are pre-charged with excess pressure to “fill“ the pump. Earlier doubts regarding the use of this filter system due to anticipated problems with high temperatures have since been laid to rest by effective oil cooling systems designed to increase cooling capacity. The main precondition for trouble-free operation is that the flow rate in the return line should exceed the demand volume in the suction line. Series of tests at a German wheel loader manufacturer‘s have shown that the return flow rate of the working hydraulic circuit can break down intermit- tently due to the kinematic factors of pressure intensification and velocity. As a result, the suction lines of the filter are no longer supplied with excess pressure, and vacuums occur in both the suction and return flow lines. Regular occurrences of this state – even if only for a brief millisecond – can lead to mal-functions in hydraulic components. As an extra precaution, it is a good idea to fit a sufficiently large anti-cavitation valve which allows oil to be sucked directly out of the tank on a temporary basis if a shortage occurs in the return line. Problems can also occur if the calculations on paper do not match the result of the finished pipework of the machine, or if the routing of the suction pipework is too long and complicated. As long as excess pressure is available, the system remains stable: as soon as a vacuum occurs, however, there is a risk of cavitation. This situation can be remedied by the return line of an additional circuit with a (speed-) stabilised flow rate. Suction-return-flow filters are equipped with a bypass valve. If the flow resistance in the filter element reaches a certain level due to heavy soiling or high viscosity, the valve opens and the flow begins to divide as it moves towards the tank. This can in turn lead to an undersupply of the filter, so that the advantage of the filter is lost, particularly in extreme cold-start situations. For this reason, RT-Filtertechnik has developed an individual combination of different filter elements and specially adapted valve inserts. These offer effective protection against resistances in the filter and cooler which, in the form of back pressure, can cause leakages in the housing of the hydrostatic drive. One challenge the designer has to face is that of the undissolved air content. Experience shows that mobile hydraulic systems in particular nearly always contain some undissolved air. The use of suction-return flow filters does not allow the oil to stabilise in the tank and the air to rise to the surface and escape. In machines equipped with such filters, there is thus a “hidden“ risk of undissolved air passing directly into the suction line of the feed pump. RT-Filter therefore has at its disposal the only oil conducting system of its kind on the market which separates the air-enriched oil from the air-reduced oil. This system was developed in collaboration with a major wheel loader manufacturer in Germany. As shown in practical applications, the valves integrated in the filter are exposed to the internal dynamics of the mobile hydraulic system. In situations involving heavy-duty construction machinery, valve units with plastic components often fail to withstand the strain, and the issue of long-term mechanical stability is frequently raised by customers. Tried-and-tested valves which are not designed exclusively with price in mind are used in RT filter series such as SR, SRE and Axx. To take the example of the SR series: this contains an oil feed system with a bottom suction connection, integrated bypass return flow and double chamber system for optimal cavitation protection. This is supplemented by field-proven RT magnetic cores, which give the hydrostatic drive added protection against microparticles up to 0.1 mi-crometers in diameter. To make sure the advantages of this technology can be exploited in all operating conditions, RT-Filtertechnik offers intensive consultation including an analysis of both the overall system and all its components, e.g. cooler, hydrostatic drive or thermal regulator. |
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