What is a magnetic field?
A magnetic field is quite simply the area around a magnet in which its magnetism can affect other objects. If you were to be able to see a magnetic field, you would see lines called lines of flux that show the direction of a magnet’s force or field.
So what is gauss?
Gauss is a term used to refer to the flux density, or number of lines of flux measured per square centimeter. The magnetic field strength generally refers to the total flux available in an area of interest. This is foremost determined by the magnetic material used in manufacturing the body or cake of the magnet. For example, we hear the term MGOe (Mega Gauss Oersteds) when describing magnetic material used to manufacture magnets. Each different MGOe level will have a maximum strength the material can be magnetized to. Currently 52 MGOe is the strongest material available. A magnet will only generate a finite amount of flux, depending on the style or geometry, size and material used. For this reason, when we design a magnet, we design it to use the available flux in the most efficient manner to accomplish the desired results.
What is the pull value?
In some industry terms, it is the amount of force that is required to remove a magnet away from a flat piece of iron. However, varying grades of iron make this test inconsistent and it is impossible to conduct in the field with many styles of separation equipment. In our industry when we refer to the pull value of the magnet, we are referring to the amount of force (in pounds of pull) that is required to remove, or pull a certain size piece of metal away from the surface of the magnet. You can also understand this as “hold value.” The larger the piece of metal used to test a magnet, the higher the pull value will be. We typically conduct pull tests with either a ½” ferrous ball or a ¼” ferrous ball. Varying degrees of pull value can be seen from manufacturer to manufacturer utilizing the same level MGOe material, based upon the magnet design and purpose. However, this usually is accomplished by utilizing a much thinner protective sleeve covering the magnet reducing the air gap, or distance between the magnetic material and the surface of the protective sleeve. It should be noted that when this is done, it reduces the durability and possibly the longevity of the magnet.
What is Air Gap?
Air gap is the distance between the actual body of the magnet and the outer surface of its protective cover that your product will flow across. Air gap drastically changes the gauss level and pull value of the magnetic field. Simply stated, the further away from the magnet, the less strength of the magnetic field.
Don’t I always want the minimum amount of Air Gap possible to increase the magnet strength when choosing a magnetic separator?
Not necessarily. If your product you are processing is abrasive or you are experiencing large tramp metal at high processing rates, you will want to weigh the cost of replacing the magnetic separator pre-maturely versus the actual strength necessary to retain the tramp metal you are looking to remove with the separator. When using a thicker protective cover, you can increase the longevity of your magnet while maintaining adequate magnetic field strength and holding value to provide effective tramp metal separation. Another way some manufacturers decrease their air gap, therefore increase their gauss levels and holding value is to eliminate the protective coating on Neodymium magnetic material. Neodymium is very vulnerable to oxidation. Corrosion causes the magnet to crumble into a powder, losing the magnetic field and possibly causing premature replacement of the magnet.
So what is most important, magnetic field, gauss or pull value?
That is a great question and not one that is easily answered. Each application is unique and requires different considerations when choosing the correct separator for you. To be effective in cleaning your process of tramp metal, we need to consider two very simple things: First, how close to the magnet does a piece of tramp need to get for the magnetic field to attract it to the surface of the magnet; we call this reach out. Secondly, we need to understand how tightly we will need to hang on to the piece of metal before it releases it back into your product stream; we call this pull value or holding force. If a magnet is designed with maximum gauss strength at the surface of the magnet but is lacking in the magnetic field reach out, then it will not matter how much gauss or holding force the magnet has because tramp metal will not be drawn to the surface of the magnet to begin with. Likewise, if your magnetic field is adequately strong to reach out and pull your tramp metal to the surface of the magnet, and your magnet is lacking in holding value, then the tramp you have captured simply washes back off into your product stream. Again, when MPI designs your magnetic separators, we design it to utilize the available flux in the most efficient manner to accomplish the desired results. It is always best to contact a knowledgeable partner to help determine what is best for your individual application.