Pros and Cons of Magnetic Filters
The decision to use magnetic technology in a given application depends on various machine conditions and fluid cleanliness objectives. These include the expected concentration of ferrous particles, type of oil used, operating temperature, surge flow and shock and machine design.
Because of the numerous commercial products, configurations and applications, certain items on the lists of advantages and disadvantages may not apply. Nonetheless, this list can serve as a starting point for making the decision whether magnetic technology is a good choice in a given application:
Reusable Technology – The cost of removing a gram of particles from the oil with magnetic technology is low compared to disposable filters.
Limited Flow Restriction – Unlike conventional filters, most magnetic filters exhibit little to no increase in flow restriction (pressure drop) as it loads with particles. While conventional filters can go into bypass when they become plugged with particles, magnetic filters (including mag-plugs and rods) continue to remove particles and allow oil flow. For instance, most diesel and gasoline engines provide no indication of a filter that has gone into bypass. In such cases, the oil may go for an extended period of time without being filtered. Common causes of premature plugging of engine filters include coolant leaks, poor combustion, poor air filtration and overextended oil drains.
Extended Life of Conventional Filters – When used in conjunction with conventional mechanical filters (Figure 8), an increase in effective filter service life may be experienced. In certain cases, two to three times life extension may be experienced.
Improved Reliability of Electro- hydraulic Valves – Servovalves and solenoid valves are adversely affected by particles that are magnetic (iron and steel) due to the electromagnets deployed when actuating these valves. The continuous and efficient removal of these particles by magnetic filters can substantially enhance the reliability of these valves.
Lower Risk of Oil Oxidation – Iron and steel particles are known to promote oil oxidation by their catalytic properties. Premature oil oxidation can lead to varnish, sludge and corrosion. Everything else being equal, the continuous and efficient removal of iron and steel particle by magnetic filters should have a positive impact on oil service life, and over time, reduce oil consumption if oil is changed on condition.
Enhanced Wear Particle Identification – Traditionally, wear particle identification is performed microscopically by examining particles extracted from oil samples (analytical ferrography). Those particles that have evaded filters have often been reworked (comminution) by traveling through heavily loaded rolling and sliding dynamic machine clearances. Once ground up, crushed and pulverized, they are more difficult to analyze to determine the source location, cause and severity of wear. However, particles removed from mag-plugs, magnetic rods and magnetic filters are often in their original “virgin” state which can greatly enhance the accuracy of machine condition analysis.
Quick Wear Metal Inspections – Mag-plugs and rods can be removed for visual inspection (daily, weekly, etc.) without stopping the machine or removing a filter. They provide a dual service of contaminant removal and condition monitoring (from the density of wear particles observed).
Oil Flow Not Required – Many machines are lubricated by oil splash, bath, flingers, slingers and paddles. Without access to a pump and oil flow, conventional onboard filters cannot be used to keep the oil clean and optimize machine reliability (reduce wear) and lubricant service life (reduce oil oxidation). However, magnetic plugs and rods do not require oil to flow in pipes and lines. They require the oil only to agitate and circulate in a sump, reservoir or oil pan. This movement causes these particles to migrate to a loading surface of the magnetic separator.
Can be Used in Gravity Flow Drain Lines – Most wear metal production comes from the business end of a machine (bearings, gears, cams, etc.). Oil often returns to tank down drain lines and headers (flooded or partially flooded) by gravity. Due to the lack of oil pressure, it is nearly impossible to locate fine filtration on gravity drains to catch wear debris before it enters the reservoir. However, magnetic filters, rods and plugs generally do not restrict flow, enabling these particles to be quickly and conveniently removed directly in oil drains.
Detached Particle Agglomerations – A common risk associated with using magnetic separators is the possibility of particles becoming detached from the magnet and washed downstream in mass, potentially entering a sensitive component. This concern is reduced if the magnetic separator is located on a drain line or if a conventional filter is positioned downstream to trap migrating debris. Risk of debris being washed off is highest under surge flow conditions, cold starts, shock, high oil viscosity and/or high oil flow rates.
Magnetized Transient Particles – Adding to the risk of particle washoff is the chance of these particles becoming magnetized while they were attached to the permanent magnet. After floating downstream, they might adhere magnetically to frictional surfaces such as bearings, causing wear. They could also lodge into narrow flow passages, orifices, glands and oilways, thus restricting flow.
Nonmagnetic Particles Remain Unchecked – Indeed, magnetic separators will have little effect on controlling nonferrous particles composed of silica, tin, aluminum or bronze. Other types of filters and separators must be used.
Cleaning Requirement – Unlike conventional filter elements that are thrown away after becoming plugged, magnetic filters are reusable and therefore must be cleaned. The cleaning procedure varies but typically is messy and involves the use of an air hose. Specific cleaning safety precautions must be taken. Magnetic rods and plugs generally need to be wiped clean only at each service interval.
Separation is not by Size-exclusion Mechanics – As previously discussed, separation is based on physics considerably different from size-exclusion – the method which defines the performance of conventional mechanical filters. Instead, the capture efficiency of magnetic separators is based on many factors including the collective influence of particle size, magnetic susceptibility, flow rate, viscosity and magnetic field gradient.
As such, magnetic filters are not known for having well-defined micronic particle separation capability. Therefore, it is important to determine what micron filter rating is needed by the tribological components in the system, considering the oil viscosity, fluid flow rate through the filter, the properties of the challenge particles, etc.
Experience shows that most modern hydraulic components need protection of at least five microns or greater. Studies conducted some 20 years ago at the Fluid Power Research Center at Oklahoma State University for the Office of Naval Research showed that no magnetic filter at that time could satisfy this requirement when used alone. In such cases, the best choice might be a combination of conventional and magnetic filters.
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