Proper filtration plays an important role in ensuring that hydraulic systems operate trouble-free. High-performance filters maintain the cleanliness of the hydraulic fluid over its entire service life. In addition, designers are faced with ever-changing application requirements — longer filter change intervals, higher operating safety, increased separation efficiencies, and increased compatibility with the new generation of hydraulic oils. Following is an overview of some important technologies and trends in the industry, and their impact on users of hydraulic systems.
Today’s standard filter elements are similar to those of the past filter generations, but the performance has changed a lot. The essential parameters are dirt-holding capacity and pressure loss. For example, 17 years ago, a typical ARGO-HYTOS filter element with 10-μm(c) fineness 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 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 key 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 oil cleanliness.
The significantly lower pressure drop is also 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 woven in a linen weave; however, with this type of weave, there was a risk that the wires would become interlocked under pressure and the fold would 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, which produces efficient filtration with low pressure drop.
Designers benefit in several respects. Filters of the same size have longer filter change intervals, and a higher nominal volume flow. At constant filter change intervals, they can use smaller and more cost-effective filters. This protects the environment and the resources.
Green 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 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 nonconductive surfaces such as filter fleece and hoses. This charge is generated by the rapid separation of two nonconductive surfaces. Filter elements have a large nonconductive surface, and charge buildup 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, leading to increased wear of hydraulic components, and later to malfunctions and to the failure of the machine. The high temperatures of the discharge flashes also contribute to an accelerated oil aging, and eventually to a deterioration of oil properties and the shortening of the oil life. Oil aging-related byproducts 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.
A special filter element design, such as ARGO-HYTOS Exapor®Spark Protect filter elements, can ensure charge balancing and prevent 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. The special 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 buildup through the material.