How do Magnetic Filters Work?
While a large number of configurations exist, most magnetic filters work by producing a magnetic field or loading zones that collect magnetic iron and steel particles. Magnets are geometrically arranged to form a magnetic field having a nonuniform flux density (flux density is also referred to as magnetic strength)
Particles are most effectively separated when there is a strong magnetic gradient (rate of change of field strength with distance) from low to high. In other words, the higher the magnetic gradient, the stronger the attracting magnetic force acting on particles drawing them toward the loading zones. The strength of the magnetic gradient is determined by flux density, spacing and alignment of the magnets.
Various types of magnets can be used in these filters (see sidebar). Magnets used in some filters can have flux density (magnetic strength) as high as 28,000 gauss. Compare this level to an ordinary refrigerator magnet of between 60 and 80 gauss. The higher the flux density, the higher the potential magnetic gradient and magnetic force acting on nearby iron and steel particles.
While there are many configurations of magnetic filters and separators used in process industries, the following are general classifications for common magnetic products used in lubricating oil and hydraulic fluid applications.
The most basic type of magnetic filter is a drain plug, where a magnet in the shape of a disc or cylinder is attached to its inside surface (typically by adhesion). Periodically, the magnetic plug (mag-plug) is removed and inspected for ferromagnetic particles, which are then wiped from the plug.
Today, such plugs are commonly used in engine oil pans, gearboxes and occasionally in hydraulic reservoirs. One useful advantage of mag-plugs relates to examining the density of wear particles observed as a visual indication of the wear rate occurring within the machine over a fixed period of running time.
The appearance of these iron filings on magnets are often described in inspection reports using terms such as peach fuzz, whiskers or Christmas trees. If one normally sees peach fuzz, but on one occasion sees a Christmas tree instead, this would be a reportable condition requiring further inspection and remediation. After all, abnormal wear produces abnormal amounts of wear debris, leading to an abnormal collection of debris on magnetic plugs.
While magnetic plugs are inserted into the oil below the oil level (for example, drain port), rod magnets may extend down from reservoir tops, special filter canisters or within the centertube of a standard filter element.
These collectors consist of a series of rings or toroidal-shaped magnets assembled axially onto a metal rod. Between the magnets are spacers where the magnetic gradient is the highest, serving as the loading zone for the particles to collect.
Periodically the rods are removed, inspected and wiped clean with a rag or lint-free cloth. A conceptual example of a particular rod magnet filter is shown in Figure 1. When the rod is removed, the sheath or shroud can be slid off the magnet core to remove the collected debris. This debris can then be prepared for microscopic analysis to aid in assessing machine condition.
Flow-through Magnetic Filters
illustrates an example of a commercially available flow-through filter.
In this configuration, sold by Fluid Condition Systems under the MAGNOM trademark, the magnets are sandwiched between metal collection plates that have specific flow slots.
As fluid passes through the slots, ferromagnetic particles accumulate in the gap between the plates. However, they do not interfere with flow (clogging), or risk particles being washed off by viscous drag.
One advantage of flow-through magnetic filters is the large amount of debris they hold before cleaning is required. The cleaning process typically involves removing the filter core and blowing the debris out from between the collection plates with an air hose.
Spin-on Filter Magnetic Wraps
There are several suppliers of magnetic wraps, coils or similar devices intended for use on the exterior of spin-on filter canisters. Spin-on filters are commonly used in the automotive industry but are also utilized in a number of low-pressure industrial applications.
These wraps transmit a magnetic field through the steel filter bowl (can) in order for ferromagnetic debris to be held tightly against the internal surface of the bowl, allowing the filter to operate normally while extending the service life. Unlike the conventional filter element, the magnetic filter wrap can be used repeatedly.
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