Applications and Benefits of Magnetic Filtration

Bennett Fitch, Noria Corporation
Tags: contamination control, oil filters, wear debris analysis

Oil filtration in automotive and industrial machinery is essential to achieving optimum performance, reliability and longevity. Lubricant cleanliness is highly important and lubrication practitioners are provided with numerous options for filtering and controlling contamination, including disposable filters, cleanable filters, strainers and centrifugal separators. This article discusses the mechanism of particle separation and reviews the many applications of magnetic filters and separators in the lubrication industry today. A brief guide to commercial filtration products is also presented.

From its origin in the beneficiation of iron ores, the magnet has played a prominent role in the separation of ferrous solids from fluid streams. Even in the control of contamination from in-service lubricants and hydraulic fluids, magnetic separation and filtration technology has found a useful niche. Currently, there are a number of conventional and advanced products on the market that employ the use of magnets in various configurations and geometry.

Role of Magnetic Filters
Car owners, car mechanics, equipment operators, maintenance technicians and reliability engineers know the importance of clean oil in achieving machine reliability. Tribologists and used oil analysts are also aware that in some machines as much as 90 percent of all particles suspended in the oil can be ferromagnetic (iron or steel particles). Typically, one or both lubricated sliding or rolling surfaces will have iron or steel metallurgy. These include frictional surfaces in gearing, rolling-element bearings, piston/cylinders, etc.

While it is true that conventional mechanical filters can remove particles in the same size range as magnetic filters, the majority of these filters are disposable and incur a cost for each gram of particles removed. There are other penalties for using conventional filtration, including energy/power consumption due to flow restriction caused by the fine pore-size filter media. As pores become plugged with particles, the restriction increases proportionally, causing the power needed to filter the oil to escalate.

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) (Figure 1).


Figure 1. Magnetic filter showing pattern
of flux distribution and the collected dirt.

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.

Magnetic Plug. The most basic type of magnetic filter is a drain plug (Figure 2), 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.


Figure 2. Drain Plug Filter

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.

Rod Magnets. While magnetic plugs are inserted into the oil below the oil level (for example, drain port), rod magnets may extend down from reservoir tops (Figure 3), special filter canisters (Figure 4) or within the centertube of a standard filter element.

Figure 3. Tank Magnet
Figure 4a. Canisters


Figure 4b. Low-efficiency Collection Pot

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.
Figure 5 illustrates an example of a commercially available flow-through filter.

Figure 5. 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 (Figure 6).


Figure 6. Collection Plates

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.

Supplier
Plug
Rod
Flow-through Filters
Spin-on Filter Wraps
C.G. Enterprises Automotive Inc.
x
 
 
 
Control Power Co.
x
 
 
 
General Plug and Manufacturing
x
 
 
 
Great Lakes Hydraulics Inc.
x
 
 
 
Halex Development and Distribution, LLC
 
 
 
x
Hydro-Craft Inc.
 
x
 
 
Kebby Industries, Inc.
 
x
 
 
Lisle Corporation
x
 
 
 
Magna-Guard, Inc.
 
 
 
x
Parker Hannifin
 
x
x
 
MAGNOM
 
 
x
 
S.G. Frantz Company
 
 
x
 
One Eye Industries, Inc.
x
x
x
x
Tiger Mag / FilterMag
 
 
x
 
Turbo-mag
 
 
 
x
Twinmagnet / SynLube
 
 
 
x
Vescor Corporation
x
 
 
 

 

Spin-on Filter Wraps.
There are several suppliers of magnetic wraps, coils or similar devices intended for use on the exterior of spin-on filter canisters (Figures 7a-c). 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.


7a. Combo Mechanical and Magnetic Filters
7b. Combo Mechanical and Magnetic Filters
7c. Combo Mechanical and Magnetic Filters


8. Combo Mechanical and Magnetic Filters

Factors Influencing Magnetic Separating Action
There are a variety of magnets and ways in which magnetic filters and separators can be configured in a product’s design. In fact, there is much more to their performance than simply the strength or gradient of the magnetic field. For instance, the size and design of the flow chamber, total surface area of the magnetic loading zones, and the flow path and residence time of the oil are all important design factors. These factors influence the rate of separation, the size of particles being separated and the total capacity of particles retained by the separator.

The magnetic force acting on a particle is proportional to the volume of the particle, but is disproportional to the diameter of the particle (magnetic force varies with the cube of the particle’s diameter). For instance, a two-micron particle is eight times more attracted to a magnetic field than to a one-micron particle. This means large ferromagnetic particles are disproportionately easier to separate from a fluid compared to smaller particles.

The separating force is proportional to the magnetic field gradient and also to the particle magnetization (magnetic susceptibility). Particle magnetization relates to the degree to which the particle’s material composition is influenced by a magnetic field. The most strongly attracted materials are particles made of iron and steel, however, red iron oxide (rust) and high-alloy steel (for example, stainless steel) are weakly attracted to magnetic fields. Conversely, some nonferrous compounds such as nickel, cobalt and certain ceramics are known to have strong magnetic attraction. Materials that cannot be picked up with a magnet (such as aluminum) are called paramagnetic substances.

There are also competing forces which resist particle separation from the fluid. One such force is oil velocity which imparts inertia and viscous drag on the particle in the direction of the fluid flow. Depending on the design of the magnetic filter, the fluid velocity may send the particle on a trajectory toward or away from the magnetic field or perhaps in a tangential direction.

The competing viscous force is also proportional to both the particle’s diameter and the oil viscosity. If the particle’s diameter or the oil’s viscosity doubles, then the hydrodynamic frictional drag doubles accordingly (resistance to separation). Complicating the situation further, as mentioned above, the magnetic attraction increases by a factor of eight when a particle’s diameter doubles, while the competing viscous drag sees only a 2X multiple. This further emphasizes the fact that larger particles are more easily separated than small particles, even in an environment of considerable viscous drag.

Particle capture efficiency by magnetic technology can be narrowed down to these fundamental factors:

  1. Particles that are the easiest to separate are large (100 microns vs. 5 microns) and highly magnetic (for example, iron and low-alloy steel).

  2. The fluid conditions that best facilitate the separation of magnetic particles are low oil viscosity (ISO VG 32 vs. ISO VG 320 for instance) and low oil flow rate (2 GPM vs. 50 GPM). Even extremely small, one-micron particles can be separated from the oil if both of these fluid conditions exist concurrently.

  3. The most effective magnetic filters employ high-flux magnets and are arranged in such a way that a high-gradient magnetic field develops.

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:

Possible Advantages

Possible Disadvantages

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.

Types of Magnets

NdFeB (Neodymium-Iron-Boron)
This is the strongest in magnetic strength of all the magnets known to mankind. Neodymium, with a number 60 on the periodic table, was first thought to be a rare earth element, due to its inclusion in the “rare earth” elements between 57 and 71 on the periodic table. NdFeB was first developed and commercialized in the mid 1980s. Over the years, the strength of this composition has increased due to new developments.

SmCo (Samarium Cobalt)
Also being one of the “rare earth” elements, Samarium Cobalt can produce magnetic strength near that of NdFeB. It became available in the 1970s but was rarely used. Due to its expensive composition, fragility and difficulty to manufacture, it is used only for its benefits of being able to withstand high temperatures and corrosion.

Ferrite (Ceramic)
Today’s refrigerator magnet - ceramic magnets with Barium or Strontium Ferrite - is the most common of all magnets. It is considerably inexpensive but it contains a lower strength compared to the other magnets. Developed in the 1960s, it was the “useful” magnet, used everywhere. This type of magnet is cost-effective and resistant to corrosion and demagnetization.

AlNiCo (Aluminum-Nickel-Cobalt)
One of the first magnets developed after plain steel, this magnet has a lower strength rating. It is sensitive to demagnetization and can be destroyed if stored incorrectly or if it comes in contact with Neodymium-Iron-Boron. It has excellent machinability and has about half the strength of a ceramic magnet. Reference: www.wondermagnets.com

Best Applications for Filters and Separators
It is logical that the leading applications for magnetic separators are those where a high percentage of the particle contamination is ferromagnetic and the conditions favor a successful performance of a properly selected and installed magnetic filter or separator. As previously discussed, low oil viscosity combined with low flow rate help to facilitate the separation process (where applicable). It’s a good idea to review the lists of advantages and disadvantages in regards to each application and separator type (mag-plug, rod, flow-through, wrap) considered. Possible uses for magnetic technology include the following:

Many commercial products and suppliers of magnetic technology for contamination control of lubricating oils are listed in the sidebar. Specific questions regarding applications and these products should be directed to these suppliers.

Editor’s Note:
The author wishes to thank his father, Jim C. Fitch and his grandfather, Dr. Ernest C. Fitch, for their help in writing this article.

References:

  1. Purslow, Neil. “Advances in Magnetic Oil Filtration.” Diesel Progress, December 2002.
  2. Langton, William G. "Removal of Wear Particles from Oils Using High - G gradient Magnetic Separation.” AD-A036 270, MAE Associates, Inc., January 1977. Distributed by NTIS, U.S. Dept. of Commerce.
  3. Thoma, Jean. “Magnetic Filter. ” Applied Hydraulics, August 1958.
  4. Tyrreil, A.J. “Magnetic Filtration and Separation.” Filtration & Separation, March 1973.
  5. Wells, R.M. “Magnetic Filtration in Hydraulic Systems.” IMechE, 1976.
  6. Reference material taken from  (Magnom, Fluid Condition System) June 6, 2005.
  7. Reference material taken from www.magneticfiltration.com, May 12, 2004.
  8. Hemeon, J.Russell. “Magnetic Plug Assemblies. ” Applied Hydraulics, March 1967.
  9. Dickenson, T. Christopher. Filters and Filtration Handbook, 4 th Edition. Elsevier Science Ltd, 1997.
  10. Reference material taken from www.lenzinc.com, 1/ June 12 / 2005
  11. Reference material taken from www.wondermagnets.com 6/ June 20 / 2004.