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A magnetic pump coupling is two component magnetic assembly that consists of a drive assembly (so called because it is connected to a motor or other rotating device) and a driven assembly which is not mechanically connected to the drive assembly (called driven because it is being rotated by another source). Often the driven assembly is within a hermetically sealed chamber and the driven assembly is usually connected to an impeller (a fan blade) to push along or stir or agitate the contents of the sealed chamber.
The driven assembly is made to rotate as the drive assembly rotates through the magnetic interaction between the drive and driven assemblies. When the drive assembly starts to turn, the driven assembly is rotationally displaced and this creates a torque. This torque will pull the driven assembly around to make it follow the drive assembly. If the driven assembly impeller is within a liquid which is thick (viscous), the torque between the drive and driven assemblies will have to be greater before the impeller can start to turn. There will be a point where the torque required to turn the impeller is higher than the maximum torque that can be generated between the drive and driven assemblies. The result of this is that the drive assembly turns and the driven assembly does not follow it (it slips). This condition is useful in preventing drive motor burn out but is not of any use if such a high torque is required in the application as the application would never work. The torque at which this slipping occurs is called the pull out torque (some manufacturers may introduce a safety factor to this value). The value of the pull out torque depends on the design of the magnetic pump coupling and the magnet materials used.
There are two main styles of magnetic pump coupling.
First is to have an assembly ring onto which magnets are placed on the inner diameter in multiples of -N-S- configuration and to then have a smaller assembly ring onto which magnets are placed on the outer diameter in multiples of -N-S- configuration. The smaller ring fits within the larger ring; one is the drive assembly, the other the driven assembly – it does not matter which is the drive or driven. The picture shows this arrangement.
Second is to have an assembly plate onto which magnets are placed on a pitch circle diameter (PCD) in multiples of -N-S- configuration and to then have another assembly plate onto which magnets are placed on a pitch circle diameter (PCD) in multiples of -N-S- configuration. The assembly plates are put together such that the magnets on each plate directly face each other; one is the drive assembly, the other the driven assembly – it does not matter which is the drive or driven.
There are variations in the design but the main variation is that is the choice of material(s) used to assemble the magnets onto impacts on the performance. Using ferromagnetic materials (e.g. mild steel, magnetic stainless steel) will improve the magnetic circuit by boosting the magnitude of the magnetic field which in turn boosts the pull out torque of the design. The pull out torque may also be increased by using stronger magnets, using bigger magnets, using more magnets and by reducing the physical distance between the drive and driven magnets (eliminating the ‘effective air gap’ improves the total magnetic circuit which increases the pull out torque. The design impacts on the performance attained.
We can provide assistance with the design of magnetic pump couplings and we can produce the magnets for your pump coupling applications. We have magnetic FEA capability to assist you with your designs. Please feel free to contact us on 0114 276 2264 for technical support or for a free and without obligation quotation. Or simply supply a drawing or sketch of your requirements by email to email@example.com