Applications & benefits of magnetic filtration

Machinery Lubrication Magazine 9/12/08 2:51 PM

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Applications and Benefits of Magnetic
Filtration
J. Bennett Fitch, Noria Corporation

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).

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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.

Spin-
Flow-
on
Supplier Plug Rod through
Filter
Filters
Wraps

C.G. Enterprises
x
Automotive Inc.
Control Power Co. x

General Plug and
x
Manufacturing

Great Lakes Hydraulics
x
Inc.

Halex Development and
x
Distribution, LLC
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

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.

Possible Disadvantages

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.

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:



Gearboxes (including final drives, differentials, etc.), both forced-
circulating and splash-fed

Large diesel engines, especially where the full-flow filter may prematurely
go into bypass without indication

Any machine with ferrous frictional surfaces but no forced oil circulation
with filtration

Applications where the use of magnetic filters will substantially extend the
life of conventional filters already in use

Applications where iron particles are known to be a major contributor to
oil oxidation problems (particularly hot running machines)

Hydraulic systems, particularly those using electrohydraulic valves

In situations requiring better precision in recognizing abnormal wear
particle generation (and wear particle type)

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 http://212.240.121.32/new/index.asp
(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.

Suppliers



C.G. Enterprises Automotive Magna-Guard, Inc.
Inc. 4401 Twain Ave., # 27
3 Royce Avenue, Unit #6 San Diego, CA 92021
Orillia, ON Canada L3V 5H8 (619) 284-7608
(800) 565-9743 Fax: (619) 282-7608
Fax: (705) 327-7790 ron@magna-guard.com
info@cgenterprises.com www.magna-guard.com
www.cgenterprises.com
MAGNOM
Control Power Co. 910W West Buren St. # 159
310 Executive Dr. Chicago, IL 60607
Troy, MI 48083 (312) 738-1147
(248) 583-1020 Fax: (312) 893-2096
Fax: (248) 583-9496 keith.day@fluidcs.com
sales@jem-cp-r.com www.fluidcs.com
www.controlpowercompany.com
One Eye Industries, Inc.
FilterMag, Inc. D16 6020 2nd St. SE
13260 W. Foxfire Dr. #7 Calgary, Alberta T2H 2L8
Surprise, AZ 85374 (403) 242-4221
(800) 431-944 (623) 556-4201 Fax: (403) 242-4249
Fax: 623-546-1277 info@oneeyeindustries.com
bfowler@filtermag.com www.oneeyeindustries.com
www.filtermag.com
Parker Hannifin
General Plug and 16810 Fulton County Road #2
Manufacturing Metamora, OH 43540-9714
455 North Main (800) 253-1258
Grafton, OH 44044 Fax: (419) 644-6205
800-BUY-PLUG hydraulicfilter@parker.com
Fax: (440) 926-3305 www.parker.com
sales@generalplug.com
www.generalplug.com S.G. Frantz Company
1507 Branagan Drive
Great Lakes Hydraulics Inc. Tullytown, PA 19007
4170 36th St., SE (800) 227-7642
Grand Rapids, MI 49512 Fax: (215) 943-2931
(800) 968-0188 sales@sgfrantz.com
Fax: (616) 949.6598 www.sgfrantz.com
glh@glhydraulics.com
www.glhydraulics.com Tiger Mag / FilterMag
Lake Havasu City, AZ 86405
Halex Development and (800) 345-8376
Distribution Fax: (928) 680-6933
LLC P.O. Box 1542 sales@filtermag.com
Portsmouth, NH 03802 www.filtermag.com
(603) 235-3000
jbarrett@magneticfiltration.com Turbo-mag
www.magneticfiltration.com P.O. Box 91067
Toronto, ON Canada M2K 2Y6
Hydro-Craft Inc. (416) 899-7032
1821 Rochester Industrial Drive Fax: (416) 512-0464
Rochester Hills, MI 48309 info@turbo-mag.ca
(248) 652-8100 www.turbo-mag.ca



Fax: (248) 652-0343
hhydro@aol.com Twinmagnet / SynLube
www.hydro-craft.com 2961 Industrial Rd, # 300
Las Vegas, NV 89109-1134
Kebby Industries, Inc. (800) SYN-LUBE
4075 Kilburn Ave. Fax: (702) 683-8292
Rockford, IL 61101 synlube@aol.com
(815) 963-1466 www.synlube.com
Fax: (815) 962-3490
Vescor Corporation
Lisle Corporation 50 North River St.
807 E. Main Street South Elgin, IL 60177
Clarinda, IA 51632-0089 (847) 742-7270
(712) 542-5101 Fax: (847) 742-5187
Fax: (712) 542-5691 sales@vescor.com
info@lislecorp.com www.vescor.com
www.lislecorp.com

Please reference this article as:
J. Bennett Fitch, Noria Corporation, "Applications and Benefits of Magnetic Filtration".
Machinery Lubrication Magazine. September 2005

Issue Number: 200509
Machinery Lubrication
Contamination Control and Filtration


Applications & benefits of magnetic filtration

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