# Simplified Design of Magnetic Gear by Considering the Maximum Transmission Torque Line
## 1 Introduction  
Since 1901, magnetic gears (MGs) have been developed as an alternative to the mechanical gear with the use of permanent magnetic force. Unlike mechanical gears, magnetic gears make a contactless speed conversion with a very efficient torque transmission performance, inherent to overload, reduced audible noise, and lubrication-less systems [1, 2]. The transmission torque is an important output to meet the demand of its application, especially from the conversion of high-speed input to low-speed output. In recent years, various magnetic gears have been developed in different configurations and concepts. The motivation lies with the impact of the magnetic gear technology with the capability to enter the industries sector. For example, a magnetic gear system with bigger torque capacity withreduced ripples through the flux modulator structure with linking bridges between them, which is known as modulating ring, is presented in References [3,4]. The triple permanent magnetic gear structure is introduced by [PERSON] [5], with improved magnetic energy density. [PERSON] [6] deployed additional permanent magnet (PM) poles in the stationary parts of the flux modulator, which acts like an extra magnetic gear for the same volume with increased transmitted torque. Another MG configuration that has been studied is the axial-type MG, which comprises a high-speed rotor, flux modulator, and low-speed rotor, which are all axially stacked to create an axial magnetic flux flow. This type of MG is suitable for an application that limits the size of the MG in radial length, since the performance depends on the axial length [7, 8, 9]. A transverse-flux-type MG has also been studied and analyzed by a few researchers [8,10,11]; this type has advantages over the conventional structure. However, the torque density for the transverse-flux MG is lower than that for the radial-type MG, as studied by [PERSON] [10], primarily due to the leakage in flux [11]. The axial-field flux-modulated MG proposed a new configuration where the high-speed and low-speed rotors are sandwiched between two flux modulators. This type of MG offered the advantages on applications that required isolation between input and output shaft, since it provides shorter gaps between the rotors, along with a simplified mechanical structure [12]. Besides using the MG for speed conversion of a motor, some researchers developed a new structure of MG with dual-iron ring which is integrated with a generator. The integration between the MG and generator produced higher power density for low-speed applications, such as wind turbine [13, 14, 15, 16]. In most of the magnetic gears design, the output torque of the MGs is usually unpredictable due to the complications in its approach mathematically. Thus, there are no specific references to design a magnetic gear that can meet the torque required for its application at the very early stage. For instance, if the application requires a gear with 100 Nm of torque to be installed in a small space, the designer does not have a good reference in terms of the volume, size, and thickness of the permanent magnet (PM) to design an MG to meet the desired torque and size. In another case, a researcher needed to study the torque capabilities of a few MGs, to integrate them with a motor [17], or prototyping several number of models and testing their practicality before appropriate design evaluations can be arrived. Among the most common approaches, such as trial-and-error and parametric optimizations, this proposed method is aimed to compute the maximum torque transmission value, using the Maximum Transmission Torque Line (MTTL) value. Therefore, a reference to present a simplified approach towards the design of coaxial magnetic gears is proposed to fulfill the torque requirement for its application, which this work attempts to present. By analyzing various designs of magnetic gears with random parameters, the characteristic of the transmission torque against the total volume of PM is obtained.