Magnetic coupling and permanent magnet

Magnetic coupling is one of the important downstream applications of permanent magnetic materials. Today, Actility will systematically introduce the principle, classification and application of magnetic couplings, and also talk about the permanent magnet materials in magnetic couplings.

What is a coupling?

Coupling is an important part of mechanical transmission, which transmits torque through the mutual coupling of the driving shaft and the driven shaft. The following figure shows several common coupling forms, which can help everyone better understand what is a coupling.

The traditional coupling is a contact type, the structure is relatively complicated, and it will wear out during daily operation. If the operation overload occurs, it will cause serious wear to other mechanical parts, which is very detrimental to the stability of the mechanical operation equipment of. If the driving shaft and the driven shaft of the coupling need to work in two different media that are isolated from each other, a sealing element must be used for dynamic sealing, so that there is either an increase in rotation resistance to ensure a reliable seal, or a tight seal There is a leakage problem. In addition, with the wear and aging of the sealing elements, leakage will be aggravated, especially in systems where harmful gases (hazardous liquids) exist. Once leaked, it will pollute the environment and endanger lives.
Magnetic coupling is a non-contact coupling, which is generally composed of two magnets, and an isolation cover is set in the middle to separate the two magnets. The inner magnet and the transmission part are connected to each other, and the outer magnet and the power part are effectively connected. The magnetic field NS poles are coupled and interact to transmit power. The magnetic coupling has the function of buffering and absorbing vibration. In addition, it adopts a new principle of magnetic coupling to realize the transmission of force and torque between the driving shaft and the driven shaft without direct contact, and the dynamic seal can be turned into a static seal. , Achieve zero leakage. Therefore, it is widely used in occasions with special requirements for leakage.

Classification of magnetic couplings

Common magnetic transmission includes three types: synchronous transmission, hysteresis transmission and eddy current transmission. Due to their respective characteristics, they are used in different fields. Synchronous transmission refers to the synchronization of output and input. Common synchronous coupling structures include planar magnetic couplings and coaxial magnetic couplings.

Planar magnetic coupling

Structure: Install magnets on two disks of the same diameter in a way that NS poles cross. When in use, the two discs are respectively installed on the driving shaft and the driven shaft, leaving a certain air gap in between.
Principle: Since the N pole of the A magnet attracts the S pole of the opposite B magnet, while repelling the N poles on both sides of the B magnet, it is ensured that the driven shaft and the driving shaft keep synchronous rotation within a certain torque range.

Torque: This planar actuator has a simple structure and does not require high coaxiality of the two shafts during installation. Since the principle of plane attraction is adopted, the smaller the air gap, the greater the torque. In addition, since the transmitted torque is proportional to the area of the disc, the torque of this magnetic coupling cannot be too large, otherwise it will cause oversize and difficult installation.

Coaxial magnetic coupling

The coaxial magnetic coupling is currently the most widely used synchronous drive, and a typical application is a magnetic pump.
Structure: The coaxial magnetic coupling is composed of an outer rotor, an inner rotor, a spacer sleeve and a bearing system. The outer circumference of the inner rotor and the inner circumference of the outer rotor are equipped with magnets. The magnets are even-numbered poles, and the circumference is in the NS cross mode. arrangement. Align the working surfaces of the magnets of the inner and outer rotors, that is, automatic coupling. Isolation sleeve and bearing system are mainly used in the structure of magnetic transmission seal.

Air gap and isolation: There is a certain air gap between the inner and outer rotors to isolate the driving and driven parts. The air gap is mostly between 2mm-8mm. The smaller the air gap, the higher the effective utilization of the magnet, but the more difficult the isolation; the larger the air gap, the more convenient the isolation, but the worse the effective utilization of the magnet’s magnetic field. The radius position where the air gap is located is the working radius of this magnetic coupling. The torque of the required actuator can be obtained by adjusting the size of the air gap radius during design.
When the load exceeds the maximum torque, the actuator starts to “slip”, that is, the magnet jumps to the next coupling state from the current coupling state and the circular displacement. During this slipping process, the magnetic field in the air gap changes rapidly, and the magnets of the inner and outer rotors are simultaneously charged and demagnetized by each other, generating heat. The temperature can quickly rise to above 100 degrees Celsius in a short time, which will cause the magnet to demagnetize and the actuator to be scrapped. Therefore, although this type of actuator can play the role of overload protection, it is generally not used as an overload protection device.

Hysteresis actuator

Hysteresis transmission is a method of transmission using the principle of hysteresis. Common hysteresis actuators are generally coaxial structures similar to synchronous actuators. The difference is that the inner and outer rotors use different magnetic materials. Generally speaking, the inner rotor (active shaft) uses materials with high coercivity and high remanence, such as neodymium iron boron. The outer rotor (driven shaft) adopts low-coercivity magnetic materials, such as AlNiCo. The magnets on the driving shaft are arranged in a crossed arrangement of NS poles. When the load is not greater than the rated torque, the driven shaft rotates synchronously with the driving shaft; when the load exceeds the rated value, the inner and outer rotors slip, and only the rated torque is transmitted to the driven shaft. The excess energy is released thermally in the process of charging and demagnetizing the inner magnet and the outer magnet.
This kind of hysteresis transmission structure is commonly used in magnetic capping devices, which can ensure that the bottle cap gets enough tightening force without damaging the bottle cap.

Eddy current drive

By replacing the permanent magnetic material of the driven part of any of the above-mentioned magnetic couplings with non-ferromagnetic materials with good conductivity, such as copper and aluminum materials, eddy current transmission can be realized, although the transmission efficiency is not necessarily high. The simple disc eddy current transmission structure is shown in the figure:

On the active disk, high-performance magnets are installed in the NS cross mode. The driven plate is made of copper with good conductivity. The lines of magnetic force pass through the copper plate. The driving disc rotates, and the eddy current drives the driven copper disc to follow the rotation.
Eddy current transmission can be in two states: synchronous or non-synchronous. To be precise, a synchronous eddy current drive generally has a small amount (5%) of asynchrony. For example, input 1000rpm, output 950rpm. This asynchrony can be accepted as transmission loss. Non-synchronous eddy current transmission, the typical application is the tension control system of the retractable wire. Through special control, eddy current transmission can also realize the speed regulation function within a certain range.

Permanent magnets used in magnetic couplings

The invention and development of magnetic couplings are closely related to the continuous progress of permanent magnet materials.
The magnetic coupling originally used ferrite material, but due to its low magnetic performance, it can only transmit a small torque in the same volume as the traditional coupling, which limits the development of the magnetic coupling .
The second-generation permanent magnet materials samarium cobalt and aluminum nickel cobalt magnets (AlNiCo) have greatly improved magnetic properties than ferrite materials, so that the manufactured magnetic coupling can transmit larger torque. However, the high prices of samarium cobalt and aluminum nickel cobalt have severely restricted the development of magnetic transmission couplings.
Neodymium iron boron (NdFeB) permanent magnet material has become the third generation permanent magnet material after samarium cobalt. Nd-Fe-B is not only more excellent in magnetic properties, but also better in price than SMCO, which makes it more competitive in the market. NdFeB has the advantages of high magnetic energy product, less demand, good processing performance, cutting and drilling, and high yield. Therefore, it can reduce the volume of magnetic coupling, reduce the cost, and improve the efficiency. At present, it has been widely used in magnetic drive coupling.

Source: China Permanent Magnet Manufacturer – www.rizinia.com