Couplings - Magnetic couplings

Magnetic couplings are innovative connecting elements that transmit torque without contact by means of magnetic forces. The power transmission is mechanically wear-free because the drive and output sides are hermetically separated from each other. To learn how to use this property, read our article. You will also learn about the design and function of magnetic couplings, as well as the various uses and advantages they bring. We also look at the limits of their use.

What is a magnetic coupling?

The term magnetic coupling refers to various magnetic coupling types that function by means of a magnetic field. This includes, for example: Electromagnetic couplings, magnetic powder couplings or non-contact couplings. Electromagnetic couplings are electrically controllable and transmit motion by frictional or positive engagement. By applying a voltage to a solenoid coil, a magnetic field is generated, which then closes the clutch. The actual transmission of the torque is therefore not carried out via the magnetic field in this case. Electromagnetic couplings are used in vehicles, e.g. for switching compressors or pumps. Magnetic powder couplings are also friction-locking couplings in which an electromagnet magnetizes metal powder between the input and output pressure plates to form a force-locking connection. Here, too, the connection is controlled by applying a voltage that aligns the metal particles with the magnetic field and thus stiffens. It is suitable as a starting clutch and requires effective heat dissipation in slip mode. A magnetic powder brake is based on the same principle.

Non-contact magnetic couplings are non-contact couplings, i.e. they transmit torque without contact, e.g. between the drive shaft and the output shaft. The transmission can be realized by an electromagnetic or permanent magnetic field. For electromagnetic couplings, a magnetic field is generated using a power source. This allows the torque to be controlled very precisely. Permanent magnet couplings have long-lasting and low-maintenance permanent magnets, whose magnetic field remains constant and without an external energy source. Both variants can also be used together in hybrid couplings.

The hysteresis coupling is another non-contact magnetic coupling. It works with two permanent-magnet rings and a hysteresis disk, in which a torque is generated by the superposition of magnetic fields, which is continuously adjustable via the angle of the poles. Maximum torque is generated with like poles, minimum with unlike poles, without the faces touching. Benefits include silent operation, no break off torque and wear-free power transmission without friction.

Design of a magnetic coupling

A magnetic coupling typically consists of an inner rotor and an outer rotor. Both rotors are equipped with magnets. The magnets are located on the inside of the outer rotor and on the outside of the inner rotor. The input side and output side are often hermetically separated from each other by means of a can, thus enabling non-contact torque transmission. The can is usually firmly attached to the housing.

There are several options for permanent magnet construction:

So how exactly does torque transmission work? At rest, the magnetic fields and their poles are symmetrical. As soon as the rotors start to twist, the magnetic field lines also deflect. Torque is transferred through the air gap. As soon as the otherwise constant twist angle or the maximum clutch torque is exceeded, the magnetic field lines break away to the opposite poles, and the transmission is interrupted.

To learn more about magnets and magnetism, check out our article on the selection of magnets.

Calculating the torque of a magnetic coupling

The torque depends on the strength of the magnets and the distance between the rotor and the stator. The following formula can be used to calculate the torque M:

Benefits of Magnetic Couplings

The key advantage of non-contact magnetic couplings is that they completely separate the input and output sides. This becomes relevant in systems that handle critical liquids or gases, where leaks can cause serious damage to the system. In these cases, magnetic couplings can be used as an alternative to shaft seals. Magnetic couplings can be permanently magnetically coupled (that is, not switchable) or can be switched, for example, using an electromagnetic field.

The non-contact function of magnetic couplings has the advantage of reducing wear on the surrounding components. Greases are not needed. Magnetic couplings also penetrate through non-magnetic components, so that feedthroughs and seals can be avoided.

The material of the can may also be adapted to the appropriate application:

• Oxide ceramic: For example, oxide ceramics are ideal for use in corrosive or very warm environments. Dry running is also possible. In addition, magnetic couplings with ceramic cans are energy efficient. You can also find more details on the properties of technical ceramics in our article Technical ceramics in practice – Ceramic screws, ceramic bearings and hybrid bearings.
• Metal: Metallic cans are the most common variant. They cover the most comprehensive power range and are suitable for applications with many liquids.
• PEEK: PEEK cans are particularly suitable for dry running and are also energy efficient.

Other advantages of magnetic couplings include:

• Overload protection: The magnetic strength and resulting magnetic field determine the maximum transferable torque. At higher torques, the magnetic field becomes overloaded and the clutch slips. In this way, the components are safely protected from damage and are also protected against overheating.
• Low noise: Without direct contact between the components, a non-contact magnetic coupling works comparatively quietly.
• Low maintenance: Because wear on surrounding components is reduced by non-contact force transmission, magnetic couplings are generally considered low maintenance. Greases are not required.
• Precise control: Magnetic strength affects torque and can be precisely controlled.
• Use in cleanrooms: Magnetic couplings do not generate abrasion, which leads to problems in clean rooms.

Different variants and applications

Magnetic couplings can be distinguished, for example, according to the type of their energy source and the direction of power transmission.

Permanent magnet couplings are non-contact couplings and can also be used for a deflection or change of the direction of rotation. When the input side is moved, the magnetic field generates a corresponding movement on the output side depending on the orientation of the output shaft. The use of permanent magnets enables maintenance-free and low-wear power transmission.

To see how power transmission generally works in couplings, read our article Transmission of rotational movements – Basics of couplings.

Various application examples are shown below:

Magnetic Coupling Limits

In magnetic couplings, the transferable torque capacity depends on the magnetic field strength, the separating air gap and the frame size or Area of the magnetic field.

Permanent magnets are usually temperature-sensitive. Some materials, such as the neodymium commonly used for the production of magnets, lose some of their magnetic force as low as 80°C, and ferrite magnets at about 250°C. However, since a neodymium magnet produces the strongest magnetic force compared to permanent magnets made of other materials with the same dimensions, this material is still used. To learn more about the features, benefits and applications of neodymium magnets, read our article Neodymium magnets. When higher temperature and corrosion resistance is required, samarium-based magnets are a good alternative. These operate reliably at temperatures up to 350°C.

Another limitation of magnetic couplings is transferable power. In the standard version, this is lower than mechanical couplings with comparable dimensions. Special designs are required for applications with higher performance requirements.

When metallic containment shells are used, eddy currents can be generated by the rotating magnetic field, which not only lead to energy losses and heating, but also significantly impair the efficiency of the magnetic coupling. In addition, the resulting heat and energy losses may require additional cooling measures, which may further complicate the use. One possible solution is to use containment shells made of materials such as oxide ceramics that reduce such problems but are often more expensive and mechanically less resilient.

Using multiple magnetic couplings or other magnetic drives in close proximity may also result in magnetic field overlaps. These interactions can reduce transferable power and cause undesired torque fluctuations. Therefore, to ensure reliable operation, sufficient safety distances between each drive should be maintained and the system design adjusted accordingly.