TECHNICAL REFERENCE
Proper Filter Application When selecting a new filter assembly or replacement filter, it’s important to first answer some basic questions about your application. Where will the filter be used? What is the required cleanliness level (ISO code) of your system? What type of oil are you filtering? Are there specific problems that needed to be addressed? It’s also important to think about the viscosity of the fluid in your system. In some machinery lubrication applications, for example, the oil is very thick and has a tougher time passing through the layer of media fibers. Heating techniques and the addition of polymers can make the liquid less viscous and therefore easier to filter. Another option is to install a filter with larger media surface area, such as the Donaldson W041 or HRK10 low pressure filters, that can accommodate more viscous fluids. (see Catalog No. F112100) Next, think about duty cycle and flow issues. Working components such as cylinders often create wide variations in flow – also called pulsating flow – that can be problematic for filters with higher efficiency ratings. On the other hand, dedicated off-line filtration (also called “kidney loop”) produces a very consistent flow, so it makes sense to use a more efficient filter. Filters used in applications with steady, continuous operation at lower pressures will last longer than filters that must endure cycles of high pressure pulsating flow. Generally, the lower the micron rating of a filter, the more often it needs to be changed since it is trapping more particles. Finally, it’s wise to ask yourself, “How much is my equipment worth?” Calculate how much it would cost to replace the equipment in your system, in case of component failure, and make sure those areas are well protected with proper filtration. (For example, high performance servo valves are very sensitive, costly components that need to be protected with finer filtration media.) Minimizing maintenance costs through good contamination control practices requires proper filter application based on the specific contamination problems. Good contamination control means cost-effective filtration. When looking for a filter, first assess the needs of your system and any problem areas.
Characteristics to Consider When Specifying a Filtration System 1. Oil Viscosity 2. Flow 3. Pressure
Fluid Properties Lubricity The property of the fluid that keeps friction low and maintains an adequate film between moving parts. Viscosity The thickness of the fluid as measured by resistance to flow. The fluid must be thin enough to flow freely, heavy enough to prevent wear and leakage. Hydraulic fluids thicken when they cool and thin out as they heat up. Because some hydraulic systems work under wide temperature extremes, viscosity can be an important factor. Viscosity Index (VI) The rate of viscosity change with temperature: the higher the index, the more stable the viscosity as temperature varies. VI can sometimes be improved by additives, usually polymers. Rust Resistance Rust inhibiting chemicals in hydraulic fluids help overcome the effects of moisture from condensation. Oxidation Resistance Oxidation inhibitors delay the sludgy/acidic effects of air, heat, and contamination in the system. Foaming Resistance Although control of foaming depends largely on reservoir design, anti-foaming additives in the fluid also help. 4. What Components will be protected by the filter 5. Cleanliness level required (expressed in ISO code 6. Type of oil/fluid 7. Environment (the system, the surrounding conditions, etc.) 8. Duty cycle 9. Operating Temperature
Hydraulic Filters & Accessories • 19
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