Permanent magnets for 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 under the same volume as the traditional coupling, thus limiting the development of magnetic couplings.


The magnetic properties of the second-generation permanent magnet materials samarium-cobalt and aluminum-nickel-cobalt magnets (AlNiCo) are greatly improved compared with ferrite materials, so that the magnetic couplings made can transmit larger torques. However, the high prices of samarium cobalt and alnico have seriously 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. NdFeB not only has better magnetic properties, but also has a higher price than SmCo, which makes it more competitive in the market. NdFeB has high magnetic energy product, less consumption, good processing performance, can be cut and drilled, and has high yield. Therefore, it can reduce the volume of magnetic couplings, reduce costs, and improve efficiency. It has been widely used in magnetic force in transmission couplings.


Classification of Magnetic Couplings

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


1. Planar magnetic couplings

Structure: On two disks of the same diameter, magnets are installed in the way that NS poles cross. When in use, install the two discs on the driving shaft and the driven shaft respectively, leaving a certain air gap in the middle.


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 transmission has a simple structure and does not require high coaxiality of the two shafts during installation. Due to the principle of plane suction, the smaller the air gap, the greater the torque. In addition, since the magnitude of the transmitted torque is proportional to the area of the disc, the torque of this magnetic coupling cannot be made too large, otherwise it will cause difficulty in installation if it is too large.


2. Coaxial magnetic couplings 

Coaxial magnetic couplings are the most widely used synchronous transmission at present, and the typical application is the magnetic pump.


Structure: The coaxial magnetic couplings are composed of an outer rotor, an inner rotor, a spacer sleeve and a bearing system. Magnets are installed on the outer circumference of the inner rotor and the inner circumference of the outer rotor. The magnets are even-numbered poles. arrangement. Align the working surfaces of the magnets of the inner and outer rotors, that is, automatic coupling. Spacer sleeves and bearing systems are primarily used in the construction of magnetic drive seals.


Air gap and isolation: There is a certain air gap between the inner and outer rotors, which is used to isolate the active and driven parts, and 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 easier the isolation, but the poorer the effective utilization of the magnetic field of the magnet. The radial position of the air gap is the working radius of the magnetic coupling. During design, the required torque of the transmission can be obtained by adjusting the size of the air gap radius.


When the load exceeds the maximum torque, the transmission begins to "slip", that is, the magnets jump from the current coupling state to the next coupling state with a circular stagger. During this slipping process, the magnetic field in the air gap changes rapidly, and the magnets of the inner and outer rotors are charged and demagnetized by each other at the same time, generating heat. The temperature can rapidly rise above 100 degrees Celsius in a short period of time, causing 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.


3. Hysteresis drive

Hysteresis transmission is the way of transmission using the principle of hysteresis. The common hysteresis transmission is generally a coaxial structure similar to a synchronous transmission. The difference is that the inner and outer rotors use different magnetic materials. Generally speaking, the inner rotor (drive shaft) uses materials with high coercive force and high remanence, such as NdFeB. The outer rotor (driven shaft) is made of low coercivity magnetic material, such as AlNiCo. The magnets on the driving shaft are arranged crosswise according to NS poles. When the load is not greater than the rated torque, the driven shaft and the driving shaft rotate synchronously; 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 in the form of heat during the charge and demagnetization process of the inner magnet and the outer magnet.


This kind of hysteresis transmission structure is commonly used in magnetic cap screwing devices, which can ensure sufficient tightening force for the bottle cap without damaging the bottle cap.


4. Vortex drive

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


On the driving disk, high-performance magnets are installed in the way of NS crossing. The driven disc is made of copper with good electrical conductivity. Magnetic field lines pass through the copper disk. The driving disc rotates, and the eddy current drives the driven copper disc to rotate accordingly.


The eddy current drive can be synchronous or non-synchronous. To be precise, synchronous eddy current drives generally have a small amount (5%) of out-of-sync. For example, input 1000rpm, output 950rpm. This out-of-synchronization can be accepted as transmission loss. The typical application of asynchronous eddy current transmission is the tension control system of take-up and pay-off lines. Through special control, the speed regulation function within a certain range can also be realized through eddy current transmission.



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