Trends in Hydraulic Filtration
Proper filtration, without a doubt, plays an important role in ensuring that hydraulic systems operate trouble-free. High performance filters ensure the cleanliness of the hydraulic fluid over its entire service life. In addition, customers are faced with ever-changing application requirements that require longer filter change intervals, higher operating safety, increased separation efficiencies and increased compatibility with the new generation of hydraulic oils. Below you will find an overview of some important technologies and trends in the industry and their impact on users of hydraulic systems.
Filter performance data
At first sight, one could gain the impression that standard filter elements would have changed little over the years. But even if today’s filters are similar to those of the past filter generation, the performance has changed a lot. The essential parameters are dirt-holding capacity and pressure loss. In the year 2000, a typical ARGO-HYTOS filter element with the fineness 10 µm(c) had a specific dirt-holding capacity of about 6 mg/cm2. Today, this capacity has increased by more than 130 % to about 14 mg/cm2 while the pressure loss has been reduced by about 50%.
There are several reasons for these remarkable improvements. On one hand, research into materials technology has led to improved filter media. Increasing the dirt-holding capacity of glass fiber media at the same pressure drop was an important factor for the improved performance. The pore volume is a major parameter. Finer fibers ensure the greatest possible pore volume and create more capacity for greater dirt absorption.
Such improved filter materials also resulted in a lower pressure drop, enabling the installation of additional layers. In the past, filters typically had a single glass fiber layer to capture and hold contaminant particles. Today, most high-performance filters are double-layered. These layers consist of a coarser pre-filter layer to capture the larger particles and a main layer to trap smaller particles. The combination of the pre-filter and the fine filter layer increases the dirt-holding capacity and improves the oil cleanliness.
The significantly lower pressure drop is, in addition to better filter media, due to an improved design of the supporting and protective fabric. Glass fiber filter media are soft and break under pressure. Wire mesh – typically of steel or stainless steel – provides protection against damage to the internal and external surfaces of the media.
Changes in the tissue structure were also of great importance. In the past, the wires were typically woven in a linen weave. However, there was the risk that, in this type of weave, the wires would become interlocked under pressure and the fold would thus be completely closed. Today, twill bindings ensure that the filter element folds cannot be completely interlaced. Even under load, the element always maintains a minimum clearance in the fold, thus ensuring efficient filtration with low pressure drop.
This optimized filter material structure of the ARGO-HYTOS EXAPOR®MAX 2 element design reduces the pressure loss in the filter element folds by up to 50 % and up to 40 % in the filter element. Conversely, the filter elements can achieve a flow rate up to 65 % with constant pressure loss.
The customer benefits in several respects: through improved dirt-holding capacity and lower pressure loss with constant filter fineness. Filters of the same size thus have longer filter change intervals, and a higher nominal volume flow. At constant filter change intervals, customers can use smaller and more cost-effective filters. This protects the environment and the resources.
Environmentally friendly hydraulic fluids
For some years now, the trend has been toward using environmentally friendly fluids in hydraulic systems, e.g. higher refined base oils because of their improved technical properties, such as aging resistance. However, these oils have a lower conductivity. Newer additive packages also significantly influence the conductivity.
In the past, conventional hydraulic oils often contained zinc dithiophosphate (ZDDP) protecting them from wear and corrosion and acting as an antioxidant. Since this component has now been classified as harmful, users have turned to zinc-free oils. The reduction in the amount of organometallic additives such as ZDDP lowers the conductivity of oil. Therefore, the elimination of this additive, e.g. in environment-friendly oil, reduces the conductivity and increases the risk of electrostatic charging.
If a non- or low-conductive hydraulic oil flows through a system, an electrostatic charge can be generated at the interfaces between oil and non-conductive surfaces such as filter fleece and hoses. This charge is generated by the rapid separation of two non-conductive surfaces. Filter elements have a large non-conductive surface, and charge build-up increases with increasing flow velocity of the oil. As soon as the charge quantity is large enough, discharges occur in the form of sparkovers.
Conventional filter material could be locally destroyed by discharge flashes and associated high temperatures.
This results in holes through which dirt particles can pass unfiltered. This leads to increased wear of hydraulic components and later to malfunctions and to the failure of the machine. However, the high temperatures of the discharge flashes also contribute to an accelerated oil aging, thus to a deterioration of oil properties and to the shortening of the oil life. Oil aging-related byproducts additionally reduce the service life of the filter elements. Also adjacent electronic components can be damaged due to electrical discharges. To avoid such problems, the charges must be balanced.
For this purpose, a special filter element design was developed, which ensures charge balancing and prevents destructive discharge flashes.
Glass fibers in a filter element are themselves not conductive, but, as already mentioned, the inner supporting meshes and the outer protective mesh are made of metal.
ARGO-HYTOS Exapor®Spark Protect filter elements connect the two mesh fabrics with a pleated metal film. Thus, electrostatic charge can pass through the conductor without a sudden, violent discharge build-up through the material. Exapor®Spark Protect completely eliminates destructive discharge flashes. The filter elements are compatible with the standard filter elements such as EXAPOR®MAX 2 and therefore do not require any conversion or additional measures on the hydraulic system. Contrary to a retrofitted electrostatic discharge protection, all other filter characteristics remain the same.
Exapor®Spark Protect filter elements are problem solvers. We recommend their application whenever the electrical conductivity of the hydraulic oil in a system is < 500 pS/m.
Copy filter elements
While renowned filter manufacturers undoubtedly have made significant product improvements, this does not mean that the better products are in the hands of the users. The proliferation of filter element copies is becoming an increasingly worrying trend in the industry. Suppliers of copy filter elements refer to well-known manufacturers and claim that their products are original production elements with the same performance. In reality, they are usually only dimensional copy filter elements, have inferior filter media and often have poor quality control. Unfortunately, many users buy such replacement filters because of price and fit, and do not worry about the impact on their machines and equipment.
It is important to note that many filter elements look similar, but in fact represent complex hydraulic components. In addition to parameters such as dirt-holding capacity, filter fineness and pressure loss, the user should consider other decisive features:
The filtration efficiency of a filter element, which is characterized by the filter fineness, is decisive for the oil cleanliness in a system over the entire service life. The flow fatigue strength of the filter material ensures the oil cleanliness also with changing flow load.
A high dirt-holding capacity provides long filter change intervals, provided that the filter unit has the required flow fatigue strength and is compatible with the hydraulic fluid. An excellent differential pressure stability ensures that the filter elements remain intact and functional during frequent cold starts, which strongly stress the material due to the high viscosity of the hydraulic oil.
The filtration performance of copy filter elements does not withstand any comparison with original filter elements at any of these points. To be able to compare original and copy filter elements, one would have to test them on a laboratory test bench under standard conditions. Users often learn the hard way, that copy filter elements have only a fraction of the life span of the original filter elements.
This means more frequent changes, a greater risk of equipment damage and, ultimately, a higher total cost for the user.
To solve this problem, the current trend is moving away from standard filters to customer-specific filters. In this case, by means of clever functional integration into the filters or by system integration of the filters, e.g. in hydraulic tanks, an entry barrier has been created which makes the copying difficult because of the very high technical complexity or prevents it due to protective rights. This ensures that original replacement filter elements are always used and the required oil cleanliness is achieved over the entire service life. This makes it possible to extend guarantees and ensure a superior performance of the devices.
Another continuing trend is that hydraulic-filtration manufacturers no longer just supply individual components, but are developing into suppliers of complete system solutions. Today’s customers want more than a filter housing, they want everything around the filter, from mounting accessories and connection adapters to pressure switches and oil condition sensors.
With the increasing importance of a supply chain management among OEMs, there is an ever-growing demand for more-complex integrated solutions from fewer and fewer main suppliers. This includes the integration of functions and systems with a particular focus on the reduction of the interfaces as well as on the production of preassembled and tested functional units.
To name just one example, ARGO-HYTOS has supplied a customer-specific suction filter, including a pressure control valve for the lubrication circuit, a pressure switch, a temperature sensor and a modular, patented connection system, adapted to the customer’s installation space situation. This significantly reduced installation time, effort and costs.
The Industrial Internet of Things (IIoT) and Industry 4.0 are not major drivers for filtration technology right now. But the potential to network equipment plants such as filters, digitally with the cloud, offers exciting possibilities. Take the example of a clogging indicator of a filter. Today, an on/off indicator in a tractor can determine whether the filter element is operating normally, or if it is clogged.
However, an “intelligent” display could be connected to the electronic control of a machine to monitor parameters such as temperature, flow and motor speed, and possibly track the filter behavior, e.g. during cold start. With a simple algorithm, the user can gain information about whether the system is operating within specified limits; or whether the operation is running outside the specifications, e.g. whether the volume flow is lower or higher than expected. Through these signals, users can gain more complex information and tackle flexible service concepts such as preventive maintenance. Similar to the technology of some new cars, sensors can monitor operating hours, engine speed, oil temperature range, number of cold starts etc., and use this data to develop service life software models. Ultimately, the user receives a service indication for the oil or filter change only if it is actually required, instead of changing the filters at regular intervals regardless of the operating cycle.
This technology will be adapted to future filtration systems. The trend to improve machine reliability will continue, and will be supported by the need for information and sophisticated monitoring and control algorithms. Even machines at remote locations are warned of imminent machine damage to prevent unscheduled downtime and reduce operating costs.