Selecting magnets

Magnets are of great significance in industrial applications. They are used in a variety of applications, from electric motors to sensors and actuators. However, selecting the right magnet for a specific application requires a deep understanding of the magnetic properties, materials and design features.

Prerequisites for magnetism

The prerequisite for magnetism is the special physical characteristic of electrons to rotate around their own axis. However, the direction is decisive for this so-called spin. Magnetism is only possible if the electrons are aligned in the same direction. For this reason, metals are particularly suitable for being magnetised, because the atoms are arranged here in a grid-shaped pattern. This allows the electrons to move freely and adapt more easily to a direction of rotation. Magnets always have a north pole (N) and a south pole (S), which repel each other. Unlike plus and minus charges in electricity, it is not possible to have a separate positive or negative pole. When a magnet is split, a new, smaller magnet is always created. The smallest indivisible quantity is also called the elementary magnet. These are found in iron, for example – in an unaligned form.

Overview of magnets

Depending on their capability to be inherently magnetic or to be magnetised by external energy supply, a differentiation is made between the following magnet types:

• Permanent magnets: In permanent magnets, the electrons move around their own axis already in an aligned pattern, thus generating a permanently magnetic field. Iron, cobalt and nickel, for example, are magnetic by nature. Permanent magnets are divided into hard magnets and soft magnets. Hard magnets have a high remanence (resistance to demagnetisation) and coercive field strength (the magnetic field strength required to fully demagnetise a magnet), while soft magnets can be easily magnetised and demagnetised.
• Electromagnets: In electromagnets, magnetism is generated by supplying electrical current. As soon as electrons flow through an electrical conductor, the current around them generates a magnetic field. This is usually a wire with windings, where each winding loop serves as a circular conductor. Overall, this results in a very strong total magnetic field exceeding that of permanent magnets.

Basic magnetic properties

The basic properties of magnets include:

• The magnetic flux density (Tesla): The surface density of the magnetic flux that passes vertically through a certain surface element. The term magnetic induction is often used synonymously and also describes the same physical phenomenon, but in a different context (electrodynamics and electromagnetic induction).
• The magnetic field strength (amps per metre): Assigns a strength and direction of the magnetic field generated by the magnetic voltage to each space point.
• Magnetic permeability (also magnetic conductivity): It determines the capability of materials to pass magnetic fields or adapt to a magnetic field.

These properties affect the performance of a magnet in certain applications.

Demagnetisation of permanent magnets

It is possible to demagnetise permanent magnets. However, there may still be a low residual magnetisation. For demagnetisation, the alignment of the atomic spins must be disturbed. This can take place, for example, by external influences such as heat, strong impacts or other magnetic fields. For example, each magnetic material has a maximum application temperature, also called Curie temperature. Above this temperature, the magnetic properties change. This temperature is approx. 769°C for iron, approx. 1127 °C for cobalt and approx. 358 °C for nickel. Electromagnets can be demagnetised by switching off the power supply.

Magnet manufacture and materials used

There are various manufacturing processes for magnets. However, powder metallurgy, in which the materials are first pulverised and then mixed and compressed, is most common. The compression process takes place under heat and is also called liquid phase sintering. Finally, the magnetisation process takes place, in which the elementary magnets in the blank are aligned in one direction using a large magnet or electromagnets. The acting magnetic force should be about three times as high as the magnetic force that the final magnet should have. Some of the common materials are neodymium-iron boron (NdFeB), samarium-cobalt (SmCo), AlNiCo (aluminium-nickel-cobalt) and ferrites.

The materials to be selected depend on factors such as operating temperature, magnetic performance and cost. It is now also possible to manufacture plastic-bonded magnets such as rubber magnets.

Selecting magnets

To be able to select the right magnet, it is important to know the parameters that affect the performance of a magnet:

• Remanence: The flux density that a magnet holds within a closed loop.
• Coercive field strength: The measure of the demagnetisation resistance.
• Maximum energy product: Remanence (Br) of a magnet multiplied by the coercive field strength (Hc).
• Open-loop flux density: The intensity of the magnetic field (measured in Tesla, formerly Gauß). Describes the density of the generated magnetic field (flux density). The magnetic field is visualised as magnetic lines along the magnetisation direction. The field strength is the density of these lines over a certain area and the total number of lines describes the magnetic flux density.
• Adhesive force: The attraction force of a magnet measured in Newton. The material selection, surface texture and magnetic attraction angle have an influence on the attraction force.

The following table shows reference values for the selection of magnets based on the adsorption force and the flux density.

For more information, see product catalogue.

The environment in which the magnet will work also affects performance and durability. In addition to the temperature already mentioned, moisture (rust formation), mechanical stress or corrosion influence the magnetic properties. Therefore, all circumstances should be taken into account for selection and magnets with special properties, such as high moisture resistance, should be used if necessary.

At MISUMI, we offer magnets for every use. The range includes both highly heat-resistant neodymium magnets as well as flexible rubber magnets or prefabricated magnetic hooks. For example: round magnets, magnets with holders, rubber magnets, magnets (screwable), magnets (rectangular) or magnet holders (switchable).

Magnet quality as an important unit of measurement

The magnet quality is an important unit of measurement for magnets. It consists of a letter followed by a number, e. g. UH45:

• Letter: The letter indicates the maximum operating temperature. Magnets usually have a maximum operating temperature of 80°C, which is marked with the letter N. Further information can be: M up to 100 °C, H up to 120 °C, SH up to 150 °C, UH up to 180 °C and EH up to 200 °C.
• Number: The number indicates the stored magnetic energy per volume. It is the product of the magnetic field strength H and the magnetic flux density B.

Industrial use of magnets

Magnets are indispensable in industry. For example, electromagnets are used in the automotive industry wherever electric motors are at work. The rotation of the motor is effected by the attracting and repelling forces of the magnet. Magnets are also used for relays. Here, an electromagnetic switch is installed in a circuit and then a magnetic field is built up via a weak power supply. The switch closes when the power is supplied and opens as soon as the current is switched off, thus dissipating the magnetic field. In some high-speed trains and magnetic levitation trains, strong magnets are used to lift vehicles off the ground and allow them to glide smoothly and quickly. In addition, magnets and their properties are specifically used in conveyor belt systems. The transported material can thus be easily separated or sorted, which is particularly important in the recycling and waste processing industries.

Permanent magnets are for example used in the following applications:

• Conveyors: Permanent magnets are used to separate ferromagnetic materials from non-ferromagnetic materials, e. g. in the recycling industry. The magnetic materials are collected and removed by the permanent magnet.
• Safety devices: In mechanical engineering and custom machine construction, permanent magnets are used in door stays or protective enclosures. For example, they keep doors or flaps closed that allow access to dangerous machine components.
• Access control systems: Permanent magnets are used here in conjunction with electromagnets. The permanent magnet is permanently mounted while the electromagnet regulates the locking mechanism by overcoming the force of the permanent magnet when activated (e.g. by means of an access card), thus unlocking the door.

The application options are very versatile and are reflected in an extensive product portfolio at MISUMI.