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Construction machinery consists of technically complex systems which make tough demands on their components. Installing components of a high technical standard bumps up the price of the machinery. At the same time, economy is a critical factor when buying construction machinery: the acquisition costs have to be weighed against the running costs and expected lifetime. Potential machine failures are na- turally very difficult to calculate in advance, but they are responsible for pushing up the running costs. Much of this is down to hydraulics, as nearly all construction machines have a more or less complex hydraulic system which can be costly to repair. Numerous long-term studies have shown that 75 % of machine failures caused by hydraulic components are attributable to solid impurities. It is therefore extremely important to keep the hydraulic fluid clean. This function is fulfilled by hydraulic filters which can now be installed in various forms. Once frowned upon as a ne- cessary evil, machine filtration systems are now strategically advertised as a means of reducing running costs and prolonging service life. In order to establish which of the claims made by machine manufacturers really do bring advantages for the user and which are merely glib advertising, we need to take a closer look at modern filtration. In terms of materials, glass fibre fabrics are state of the art. These materials have a highly homogeneous fibre structure – i. e. a uniform pore size – which enables them to absorb large quantities of dirt. This kind of filter is known as a deep-bed filter. However, glass fibre elements are always disposable, and cannot be regenerated. Dubious claims as to the “cleanability“ of such elements should always be taken with a pinch of salt. We have to remember that as soon as the glass fibre fabric is exposed to excess pressure or indeed ultrasound, the dirt may well be eliminated but the fibre structure is destroyed in the process. This naturally raises the question of economic viability. Very fine filters below 10 µm may keep the fluid very clean but, since they also remove lots of microparticles, their dirt holding capacity is quickly exhausted. This is not necessarily a good idea in construction machinery, quite apart from being expensive. Better results have been obtained with full-flow systems pro- viding grades of filtration between 10 µm and 20 µm. As an option, 3-5 µm micro- filtration can be used for bypass purposes, although such systems do add to the running costs. For this reason, RT-Filtertechnik GmbH, in cooperation with other renowned manufacturers, offers a system which integrates the microfiltration function in the actual operating element. Such combined elements offer long service times and excellent filtration properties. Some machine manufacturers also offer similar multi-function filters combining glass fibre fabrics with cellulosic paper. Cellulosic papers have an extremely inhomogeneous fibre structure, i. e. they do not have a clear pore size. Paper filters are also deep-bed filters with a high dirt absorption capacity, but they are now regarded as outdated due to their inde- terminate grade of filtration. Their only advantage is that paper is cheap in comparison with glass fibre. There is little point in combining paper material and extra-fine glass fibre fabric in the same element. When machine manufacturers start claiming that such filters alone will guarantee a longer oil life-time in the machine, however, it is a clear sign that advertising gimmics have won over technical accuracy. It is not only the filter materials that are important, but also the filter strategy. Suction filters are ideal in theory, but are invariably problematic in practice due to the risk of cavitation. In some construction machines – those with closed circuits – there is no alternative, however. In such cases, RT-Filtertechnik has obtained excellent results with modern, high-permeability fabrics with high grades of filtration. For closed circuits such as hydrostatic drives, suction-return flow filters have proved successful in recent years. Their use does depend on certain technical conditions, however – i. e. the volumetric balance of the hydraulic system – which means that they are not universally applicable. These filters provide full-flow return filtration and supply selected pumps – usually feed pumps – with filling pressure and clean oil. A full-flow pressure filtration system after the pump is good, but the cost of operating this kind of filtration system reliably, including pressure-resistant housing and differential-pressure-resistant elements, pushes up the price of the filters considerably. An optimised and widespread solution is that of full-flow return filtration. This offers cheap filters which can be easily integrated into the system. The filter elements can also be made larger at a lower cost than in the case of pressure filters. This allows long maintenance intervals to 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 worth € 100 could totally destroy a hydraulic motor worth several thousand euros. The important thing, therefore, is to protect sensitive components. Good results have been ob- tained with compact gauze filters, which are also known as “last chance“ or “police“ filters. These have a mesh width of 60 – 200 µm and can be designed for lifetime use. They provide reliable protection against the consequences of a crash in other components. Once such damage has occurred, however, they must be changed, as metal gauze filters have do not have a high dirt absorption capacity, but are rather surface filters. Again, the user should be wary of dubious claims of “no more filter changes”. This implies that, once the glass fibre elements have been replaced with metal gauze elements, these can be cleaned over and over again. In fact, mesh widths below 40 µm can only be cleaned at all in an ultrasonic bath, and this puts a great strain on the wire material. What with this and the alternating stresses of the machine, the fine filtration effect is lost after 4-5 cleaning cycles at the most. One final point: the experience of filter manufacturers shows that the efficiency of a filter varies from one machine to the next. Machine manufacturers are therefore well advised to work closely with the filter manufacturer in order to find the optimal so- lution rather than installing the cheapest available “off the peg“ filter. The company RT-Filtertechnik GmbH from Friedrichshafen has been working with renowned ma- chine manufacturers in this way for decades – an arrangement which has proved highly flexible and successful. |
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