How is the tear-off force of a magnet measured?
The tear-off force is the force required to pull a magnet away from a steel (magnetic) plate under the following conditions:
• The plate must be made of pure steel and be at least 10 mm thick.
• The entire surface area of the magnet touches the given object, so the magnet must be in perfect contact with the plate.
• The magnet is pulled perpendicular to the plate.
What reduces the magnetic force of the magnet?
• The entire surface of the magnet does not touch the object because the given object’s surface is uneven.
• The magnet can slide sideways if it is not pulled perpendicular to the surface, so it will tear off much more easily.
• The magnetic object is not made of pure (magnetic) steel, there is probably a coating of paint, a layer of rubber or plastic or other material on the surface that increases the distance between the magnet and metal plate.
• The magnet does not touch the object, the greater the distance between the magnet and metal plate, the smaller the magnetic force.
• The object is corroded.
Magnet Surface Finish
Nickel is applied to the magnet surface to protect the magnet from corrosion. Caution, if used outdoors for long periods of time or if the finish is damaged, the magnet will begin to oxidise.
Neodymium magnets are very susceptible to corrosion, so they are usually coated with nickel (10-20 µm), zinc (5-10 µm), gold (5-10 µm), epoxy resin (20-30 µm) or passivated (0 µm). A copper layer is usually used as an intermediate layer under the final surface finish.
Neodymium Magnet vs. Ferrite Magnet
Advantages (ferrite magnet vs. neodymium magnet)
- The most powerful neodymium magnets can lift up to a thousand times their weight
- The minimum size of a neodymium magnet outperforms a ferrite magnet by several times
- Neodymium magnets are made by cutting, so almost "any" shape can be custom made. To produce ferrite magnets, a press mould is usually required. For some dimensions, cutting can also be chosen, but only for bigger quantities
Disadvantages (ferrite magnet vs. neodymium magnet)
- Neodymium magnets are more expensive
- Neodymium magnets are prone to corrosion
- Ferrite magnets all have the same temperature resistance, so there is no need to choose different types of materials as with neodymium magnets, whose temperature resistance is determined by the type of material
Neodymium magnets are produced by sintering and their anisotropy (direction of magnetisation) is usually already preset during the production of blanks, which are then cut to specific final dimensions. Therefore, except in exceptional cases, it is not possible to select the type of magnetisation after the final dimension of the neodymium magnet is completed. In contrast, ferrite magnets, their isotropic version, can be magnetised in different variations and directions using the selected magnetising agent. Ferrite anisotropic magnets also have the magnetisation direction set during production, because they are wet-moulded, i.e. in a pre-set magnetisation direction.
FERRITE VS. NEODYMIUM MAGNETS: Powerful neodymium magnets can cause a change in the direction of magnetisation or complete demagnetisation (loss of magnetic force) in ferrite magnets. Therefore, do not put neodymium magnets and ferrite magnets close to each other or together. Store them at least 10 cm apart.
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