State of the art
 Tank-integrated version of return flow filter with upper inlet chamber, suitable for large return flow volumes, with integrated magnetic separator.  Suction-return flow filter with multiple return flow and suction connections plus integrated magnetic separator.  Suction filter for horizontal installation below oil level, with valve system for leak-free element changeover and integrated magnetic separator.  Combination filter - integrated suction-return flow filter, leakage oil filter, filling and ventilating filter, with integrated magnetic separator.  Suction-return flow filter with thermal bypass valve and multiple return flow and suction connections, plus integrated magnetic separator.  Pressure filter with upward element removal for leak-free changeover and integrated magnetic separator. |
Long-term studies have shown time and time again that 75% of machine failures involving hydraulic components can be traced back to solid impurities. Consequently, the importance of keeping the hydraulic fluid clean cannot be overestimated. This task is performed by hydraulic filters, which can now be installed in a wide range of forms. Once frowned upon as a “necessary evil”, filtration systems have become a selling point of the machine, helping to cut running costs and prolong machine lifetime. In order to establish which of the claims by machine manufacturers really do bring advantages for the user, and which are just advertising hype, we need to begin by taking a closer look at modern filtration. In terms of filter materials, glass-fibre fleece is currently ”state of the art“. These fleece fabrics have a highly homogeneous fibre structure – and hence a uniform pore size – which enables them to absorb large amounts of dirt. They are known as deep-bed filters. Glass-fibre elements are always disposable, however, and cannot be regenerated. Adverts which talk of “cleaning” such elements should be treated with caution: exposing glass-fibre fleece to excess pressure or even ultrasound may remove some of the dirt, but it also destroys the fibre structure. This also poses the question of economy, of course. While ultra-fine filters of less than 10 μm can keep the fluid very clean, they also contain numerous microparticles, so that their dirt absorption capacity is quickly exhausted. This is not ideal – or indeed cost-effective - in the full-flow filtration system of a building machine. A better performance is obtained from systems offering a full-flow filtration of between 10 and 20 μm. A 3 to 5 μm bypass microfiltration can also be optionally used, although such systems do push up the running costs. In collaboration with renowned manufacturers, RT-Filtertechnik GmbH is able to offer a system in which the microfiltration is integrated in the actual operating element. Combination elements of this kind offer a long service life with excellent filtration properties. Some machine manufactures offer similar comnbination filters in which glass-fibre fleece is combined with cellulose paper. Cellulose papers have an extremely inhomogeneous fibre structure, however, so that there is no uniform pore size. Paper filters are also deep-bed filters with a high dirt absorption capacity, but they are now regarded as outmoded because of their undefined grade of filtration. The only advantage paper has over glass fibre is its cheapness. Combining paper material with ultra-fine glass-fibre fleece in the same element is already a rather pointless exercise. When machine manufacturers go on to claim that the service life of the oil can only be extended by the use of such filters, then advertising hype has finally triumphed over technical correctness . It isn't just the filter materials that matter, however, but the filter concept too. Suction filters are ideal in theory, but in practice they are subject to the problem of cavitation. In some building machines however – those with closed circuits – there is no alternative. For these, RT-Filtertechnik's modern, high-permeability fleeces with their effective filtration grade have an excellent track record. For closed circuits such as hydrostatic drives, suction-return filters have become increasingly popular in recent years. Their use is dependent on certain technical factors (hydraulic mass balance), which means they are not always appropriate. These filters provide full-flow return filtration and supply selected pumps – generally feed pumps – with filling pressure and clean oil. While it is a good idea to have full-flow pressure filtration downstream of the pump, operating this type of filtration reliably – pressure-stable housing, differential pressure-stable element – does make the filters very expensive. A widely used, optimised solution is that of full-flow return filtration. This allows the use of cheaper filters which are easily integrated in the system. The filter elements can be generously dimensioned at a lower cost than with pressure filters: that way, long maintenance intervals can be achieved without risk. The disadvantage of this type of filtration is that components are not protected against system-internal damage. In other words, a material defect in a directional control valve costing just EUR 100 has the potential to completely wreck a hydromotor worth several thousand euros. It is therefore important to provide sensitive components with extra protection. Good results are obtained with compact strainer filters known as ”last chance“ or ”police” filters. These have a filtration grade of 60 to 200 μm and can be designed for lifetime use. They provide reliable protection against the consequences of a crash in other components. After damage of this kind, they must be replaced however, as wire mesh filters are surface filters and do not therefore have a very high dirt absorption capacity. Once again, dubious offers promising that you'll “never have to change the filter again” should be taken with a pinch of salt. The suggestion is that once the glass-fibre elements have been replaced by strainer filters, you can clean these again and again. The fact is however that filter grades below 40 μm can only be cleaned in an ultrasonic bath. This puts a good deal of strain on the wire material, and the alternating load of the machine does the rest, so that the filter quality is lost after four or five cleaning cycles at the most. As a final postscript, experience in filter production has shown us how the performance of a filter can vary from one building machine to another. Machine manufacturers are therefore well advised to work closely with the filter manufacturer to find the optimal solution rather than using the cheapest “off the peg” filter. RT-Filtertechnik GmbH in Friedrichshafen has been working with major machine manufacturers in this way for decades with a high degree of flexibility and success. |
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