From dirt trap to system protector
 Prof. Hans Thoma  Filter system based on the Thoma principle  Magnetic core from a wheel laoder after 3 operating hours  Valve assembly showing precharge and anti-cavitation valves of a suction-return flow filter  Filter element with combined fabric structure |
From a contemporary perspective, the filtration systems of the 1930s and 1940s - as displayed outside the head offices of some mobile machinery manufacturers to this day – are best described as dirt traps. Simple coarse strainers were used to protect the components against dirt, and dirt retention capacities and purity classes were still concepts of the future. If you wanted additional protection for your system, you would use magnetic prefiltration. This system was essentially developed by the well-known mentor and pioneer of the science of hydraulics, Prof. Hans Thoma. It was he who experimented with this technology back in the 1930s and obtained the relevant patents. At his instigation, his son-in-law Dipl.-Ing. Friedrich Landwehr used magnetic filtration according to the “Thoma principle” in the filter systems of the then Regeltechnik Friedrichshafen. This technology is still used today in modern filter systems and, though it uses different materials such as aluminium and plastic, it is still every bit as valid, not just as a means of protecting the filter element, but rather as an indicator of the level of contamination. While you can’t see the dirt in a soiled filter element, the particles adhering to the magnetic core are clearly visible. Due to increasingly exacting demands and the consequent refinement of hydraulic systems, it became vital to pay more attention to the purity of the hydraulic fluid. More sensitive dosing and quieter, more effective pumps and motors required smaller gap dimensions. This led to component failures from dirt contamination. Hence the challenge to filter manufacturers to come up with system-supporting filtration systems to ensure cleaner hydraulic fluid. Classical filter systems such as suction filters, return flow filters and pressure filters are constantly being improved on and still have a role to play. Then along came – love them or loathe them - the DIN filters, mainly for stationary hydraulic applications. But the real breakthrough, at least in the field of mobile hydraulics, was without doubt the development of the suction-return flow filter at the beginning of the 1990s. With this system, it became possible for the first time to influence the functionality of a hydraulic system through suitable valve technology and controlled intervention. Suction-return flow filters do more than just keeping the hydraulic fluid clean for as long as possible: through the combined action of precharge and anti-cavitation valves, they actually increase the reliability and uptime availability of the machinery. In the past, mobile machines had to go through a time-consuming and energy-wasting warm-up phase after every cold start: equipped with modern suction-return flow filters, they are now ready for operation straightaway. The motto “time is money” is certainly true of the suction-return flow filter – and in more ways than one. Combining two filters, i.e. a suction and a return flow filter, in a single housing saves an additional tank cut-out with the attendant production and assembly costs, an additional filter element with the associated maintenance costs and in many cases – depending on the overall system – expensive pressure filters. And as a positive side-effect, it keeps the system supplied with filtered oil in all operating states. It is important, however, that the machine manufacturer involve the filter supplier in the hydraulic planning process at a very early stage in order to ensure optimal adaptation of this complex filtration system. Besides the innovative developments in terms of filter systems, the filter elements themselves have undergone consistent improvement and refinement. After all, the best filtration concept is of little use if the technical and cost advantages are cancelled out by frequent changing of the element and inadequate purity levels. The challenge was - and remains - a complex and highly demanding one, i.e. to achieve a better filtration performance, higher purity levels, a higher dirt absorption capacity, longer maintenance intervals, lower flow resistances, small dimensions and smaller energy losses. From simple, metal-only strainer elements whose low flow resistance came at the expense of a very low dirt retention capacity and, by modern standards, an inadequate degree of filtration, there soon arose filter elements with phenolic resin-impregnated paper as the filter medium. The filtration capacity of these was significantly higher and, because it was now possible to increase the area of filtration without enlarging the overall dimensions, the dirt retention capacity was similarly improved, though at the cost of a higher flow resistance. These paper elements are still in use today, mainly due to perceived cost advantages, but should no longer have a place in modern hydraulic systems. They are vastly inferior to glass-fibre elements in terms of dirt retention capacity, deposition rate and universality. Moreover, paper elements swell up on contact with various hydraulic fluids. In the case of glass-fibre elements, a close cooperation exists between the media suppliers and element developers, resulting in some extremely promising designs. New filter materials in a wide variety of combinations are producing remarkable results. Deposition rates are improving all the time, dirt retention capacities are increasing significantly and flow resistances are decreasing. By incorporating additional, water-absorbing layers, it is now possible to create highly effective and user-friendly filter elements. Developments are also in full swing in the field of polyamide fabrics, which are being used in an increasing number of element types. Despite all the promising and innovative developments in the filtration sector, however, there are some critical aspects which should not go unmentioned. The cost pressure to which all of us are ultimately subject will always bring forth some suppliers who are apt to make dodgy claims in order to sell their products. Take suppliers’ statements suggesting that users of their filters will never have to change the oil, for example. Anyone who knows anything about oil and is able to judge the amount of wear to which the oil in high-power hydraulic applications is exposed can only warn against such statements, popular though they may be. Similar caution should be observed in the case of those suppliers who claim to be able to clean glass-fibre filter elements. Glass-fibre elements are depth filters in which the particles are caught in a “labyrinth” of fibres. Once forced under pressure into this position, how are the entangled particles supposed to get out again? Moreover, tests have shown that the connections between the glass fibres are destroyed in an ultrasonic bath. And quite apart from this, filter elements in hydrau¬lic systems are subject to a greater or lesser puslation load. In other words, the lifetime of a filter element is limited not only by the dirt retention capacity, but also by the stability of the supporting fabric. If a filter ele¬ment is used for longer than intended, it risks being destroyed without the user noticing. Costly component damage is thus pre-programmed. Attractive as they may seem, such offers can thus prove highly risky. To single out the so-called niche suppliers for criticism would be unfair, however. There are important aspects which are often neglected even by established filter manufacturers. Take the issue of eco-friendliness, for example: a topic which should be of particular importance to filter manufacturers, and which is given appropriate attention in various areas such as incineratable plastic filter elements or filter elements with a higher dirt absorption capacity and hence fewer element changes. Nevertheless, a soiled filter element is still classified as special waste and needs to be disposed of accordingly. But that’s not the point. Consider for a moment what happens when an element is changed in a machine. In the case of pressure filters at least, which are very widely used, changing the element generally involves unscrewing an oil-filled filter hood from underneath. Anyone who has ever changed an oil filter in their car will know what this is like. The still hot oil gets all over the service engineer’s hands, soils the machine and possibly the environment, and has to be topped up again. Even in the case of suction filters, which are sensibly installed horizontally below the oil level, it is normally impossible to change the element without some leakage. No consideration is given to the resultant waste of resources and environmental pollution. There are however already filter systems in which the filter elements can be removed from the top. Because of this, and the fact that the fluid flows from the inside outwards, the dirt stays in the filter element when it is changed – and the oil stays in the system. Clearly this extra feature doesn’t come free – but then every user should consider the cost to the environment without it. In conclusion, there is no doubt that developments in the field of filtration have been and will continue to be highly promising. Filtration now enjoys a higher priority and is no longer regarded as a necessary evil. Machine manufacturers have an interest in ensuring maximum reliability of their machines. This depends in large part on a clean and therefore trouble-free hydraulic system - something which can only be achieved with good filtration. Author: Frank Philippin |
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