Magnetic filters are able to remove all kind of ferritic particles from process liquids like cooling water, coolant, grinding oil or washing water and so on. Principle of each magnetic filter is the magnetic force of the magnetic field. The magnetic field force attracts ferritic particles like iron, steel, hard metal, stainless steel and so on.

Working principle of magnetic field
Magnetic field of magnet

Ferritic particles lining up along the magnetic field lines. The magnetic south pole of a particle shows along the magnetic field line towards the north pole of the magnetic field.

Unlike poles attract each other while correspondent poles reject themselves. In order to achieve the highest force the magnetic field has to be closed. The magnetic field lines have to show from one pole to the other.

Closed magnetic field at a magnetic filter rod

Permanent magnets are made of different materials; normally we use magnets made of AlNiCo ceramic or rare earth. Magnets made of rare earth (Neodymium) offer the highest magnetic flux and the highest force in order to attract ferritic particles. The disadvantages of neodymium are that the material is not corrosion resistant and not heat resistant. Magnets made of neodymium have to be protected by coating or stainless steel pipe. For continuous use the maximum temperature should be limited to 60 °C only. 

magnetic materialAINiCoceramicneodymiumsamarium cobalt
maximum temperature550 °C180 °C100 °C300 °C
corrosion resistancehighmediumlowmedium
max. magnetic flux9000 G3000 G14500 G11000 G
pricemediumlow highvery high

In general the structure of a magnetic filter is very simple. The flow against the magnetic rods inside the housing has to be in a way that particles will be easily attracted. The magnetic fields of the magnetic rods have to overlap in a certain relation so that smallest ferritic particles are attracted with highest possible field force.


FRIESS magnetic filter made of housing with cover and build in magnetic rod

Inside the magnetic housing the flow is slightly turbulent, so that the particles are flushed from the wall of the housing towards the magnetic rods. Each particle should remain as along as possible within the magnetic field. This guarantees enough time in order to attract the particle out of the flow towards the magnetic filter rod.  The higher the magnetic flux of each magnetic rod the faster is the attraction and the better is the cleaning of the fluid.  The magnetic flux inside the filter rods should be as high as possible in order to attract smallest particles.

The field force, which attracts the particles, is reduced proportional to the size of smaller particles. Parameters as follows have influence on the filtration performance of a magnetic filter:

  1. Material of the particle
  2. Size of the particle
  3. Viscosity of process fluid
  4. Magnetic flux of magnetic rods
  5. Time that the fluid remains in the magnetic field
  6. Distance between particle and magnetic filter rod

When selecting the magnetic filter system we have to watch principles as follows:

  1. The higher the viscosity of the process fluid that has to be cleaned the longer the liquid has to remain in the magnetic filter system.
  2. In order to remove ferritic material with weak magnetic properties like stainless steel or hard metal, the time the fluid remains in the magnetic filter has to be longer.

3. The smaller the particles are the longer the liquid has to remain in the filter.

4. The higher the magnetic flux of the magnetic filter rods, the better the separation.

In order to extend the time the liquid remains in the magnetic filter system, the flow through the magnetic filter has to be reduced compared to the nominal flow. In case of a given flow you have the possibility to use the magnetic filter with a higher nominal flow.

Friess magnetic filter are equipped with magnetic filter rods with selected Neodymium magnets with the magnetic flux of approx. 11000 Gauss (guaranteed min. 10000 Gauss).

Control of magnetic flux at a magnetic filter rod of FRIESS magnetic filter (Value shown on the display 10840 Gauss. Maximum value 11440 Gauss.)

Magnetic rods with a magnetics flux lower than 10000 Gauss should not be used in magnetic filter systems for industrial use.

In order to achieve best results the maintenance and cleaning of the magnetic filter rods has to be as easy as possible. The cover of the Friess magnetic filter series FMF can be easily opened with a quick fastener. After opening the quick fastener, you can pull out the cover with the magnetic filter rods and the collected particles out of the housing. The particles collected on the magnetic filter rods can be removed with a simple tool and the magnetic filter can be closed with the quick fastener.

Easy cleaning of the FRIESS magnetic filter system FMF 420/3

Friess magnetic filters are used in many different production processes. Typical examples are:

  • Cleaning of coolant used in grinding or honing process
  • Cleaning of a cooling water in forging process
  • Removal of fine particles in cooling systems
  • Removal of fine particles from cutting oil
  • Removal of fine particles from drawing oil

Most applications use a combination of belt filter, hydrocyclone or centrifuge together with a magnetic filter. Finest ferritic particles, which are not removed by conventional cleaning systems, will accumulate in the process liquid. Magnetic filters are able to remove these finest particles and prevent contamination of the system with fine particles. Companies who already use Friess magnetic filter systems report improvements as follows:

  1. Less defective parts because of better grinding results. The coolant contents more than 95 % less metal particles.
  2. Better washing results and less particles on the surface of the parts after washing process.
  3. Drastically reduced filter costs because the lifetime of filter elements is increased
  4. A longer lifetime of process fluid.

In many cases the payback time of a new Friess magnetic filter system was 3-9 months only.

© FRIESS GmbH The content of this information sheet is protected by copyright. Graphics, texts, logos, pictures etc. may only be reproduced, copied, changed, published or used in any other form with the written permission of FRIESS GmbH. Unauthorized use may lead to claims for damages or injunctive relief.