Individual combination of single elements as an economic solution for suction-return flow filters
 Wheel loaders and all-terrain forklifts: examples of applications for suction-return flow filters.  Suction-return flow filter example 1: tank-mounted version of 'SRA' servies  Suction-return flow filter example 2: 'SRC' series version with thermal bypass valve  'SRK'-type combination filter 3: suction-return flow, oil leakage, filling and ventilating filter |
One of the most promising developments of the past few years in the field of hydraulic filtration is without doubt the principle of suction-return flow filtration, resulting in improved cold-start characteristics in hydraulic systems as well as cost savings. So how did this type of filter come to be developed? Hydrostatic drives or other closed-circuit systems have been widely used for some time in construction, agricultural and environmental engineering machinery. As there is only a small amount of fluid exchange in these systems, this part of the system requires special protection against solid contaminants. Suction filters fitted upstream of the feed pump have proved useful in this context, for example. One economic method of filtering the entire hydraulic fluid is that of full-flow return filtration. Here, the oil volume filtered in the return system is fed into the tank to let it settle and cool down. This 'classic' function of the hydraulic tank is now considered outdated. Modern systems such as wheel loaders have to be light and compact, so a large hydraulic tank came to be seen as unnecessary ballast. This conclusion led to the use of small tanks. A tank volume/flow rate ratio of 0.2 to 0.5 unthinkable 20 years ago is now no longer taboo. As a logical consequence of the trend towards smaller tanks, engineering designers began to turn their attention for reasons of space alone - to the problem of filtration. Emphasis now began to be placed on the performance of the hydraulic system in critical states. RT-Filtertechnik was therefore quick to develop a combined filter system for return flow and suction filtration. The success and advantages of this filter are based on three factors: two filters are replaced by one, the feed pump is supplied with ultra-finely filtered oil, the suction lines are pre-stressed with overpressure, the pump is filled. By now, earlier doubts about using the filter system due to anticipated thermal problems had been allayed. This was because additional cooling capacity had been achieved through effective oil cooling systems. The main condition for trouble-free operation is considered to be a flow rate surplus in the return line in relation to the volume required in the suction line. Test series conducted by a German wheel loader manufacturer have shown that the return flow volume of the working hydraulics can collapse for brief periods due to kinematic ratios and velocities. Anti-cavitation valve for greater reliability The suction lines of the filter are no longer supplied with overpressure, and vacuums arise both in the suction and return flow line. Regular recurrence of this operating state even if only for a matter of milliseconds can lead to malfunctions of the hydraulic components. An adequately dimensioned anti-cavitation valve therefore brings added security. This valve allows oil to be drawn directly from the tank for a brief period in case of an under-supply in the return flow line. Problems can also occur if the calculations on paper do not tally with the result of the actual pipework in the machine. A further potential for error is the length and complicated routing of the suction line. As long as there is overpressure, the system remains stable. However, as soon as a vacuum arises, cavitation can occur. This problem can be remedied by an additional return circuit with a (speed)-constant flow rate. Oil conductor system tackles undissolved air in mobile hydraulic systems For this reason, suction-return flow filters are equipped with a bypass valve. When the flow resistance in the filter element reaches a certain level due to heavy soiling or high viscosity, the valve opens and the flow is divided in the direction of the tank. This can in turn lead to an under-supply in the filter. In extreme cold-starting conditions in particular, the benefit of the filter is thus lost. To address this problem, RT-Filtertechnik developed an individual combination of different filter elements and corresponding valve inserts. These offer effective protection against resistances occurring in the filter and cooler which can generate banking-up pressure in the hydrostat housing and lead to sealing problems. One challenge facing hydraulic engineers is that of undissolved air content a problem affecting mobile hydraulic systems in particular. The use of suction-return flow filters does not provide an opportunity for the oil to settle in the tank and the air to escape via the surface. This leads to the hidden problem of undissolved air getting directly into the suction line of the feed pump. To combat this, RT-Filter offer the only oil conductor system on the market which separates air-enriched oil from comparatively air-free oil. The system was developed jointly with a leading wheel loader manufacturer in Germany. The valves integrated in the filter are subject to the internal dynamics of the mobile hydraulic system. Where building machinery is used intensively, valve units with plastic components often fail under the strain. Tried-and-tested valves (with high fatigue strength) are installed in the RT filter series SRA, SRC, SRE and SRK for example. Universal magnetic pre-filtration In addition, a magnetic pre-filtration system is found in all structures. This not only filters out ferromagnetic (and other) microparticles measurements have revealed adhesions of iparticles ≥ 0.1΅m but also acts as an indicator of the degree and type of contamination in the system. All the technologies described above are found in the suction-return flow filter series of RT-Filtertechnik: in the classic tank-mounted SRA version, equipped with multiple return flow and suction connections; in the tank-integrated, built-in SRE version with low-lying suction connection; in the SRC series as a tank-mounted version with thermal bypass valves integrated in the filter head; and in the SRK series, a combination of suction-return flow filter, oil leakage filter and filling and ventilating filter. |
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