### Design of Disc Spring for TRUIM  
The disc spring has strong buffer absorption capacity and can withstand large loads with small deformation [12; 13; 14], so as a buffering component it is very suitable for TRUM with very limited axial space working under high-overload conditions. In particular, the disc spring is a non-linear spring. During the loading process, a working area with zero stiffness will occur in the force characteristic of the disc spring, where, the load force keeps almost unchanged but the spring still deforms. This is called the \"quasi-zero stiffness\" characteristic. [15; 16]. Considering this, the disc spring as a buffering element is used to protect the rotor in the motor and the contact interface. The disc springs for TRUMs should be specially designed. Here, the slotted disc spring is selected, which is usually applied in situations where the axial dimension is limited and the outer diameter is large. Based on the experimental experience, the best performance of the PMR40 motor is generally obtained at the pre-loading force of \(135\pm 5\) N. Therefore, when designing a slotted disc spring, it is desirable that the zero-stiffness section occurs at the load of about 135 N and the span of it is expected to be longer, which plays an important role in stabilizing the TRUM's pre-pressure and performance. The calculation formula for load P of slotted disc spring [16] is shown in Equation (1). \[\mathrm{P=}\frac{E}{1-\mu^{2}}\times\frac{\delta^{3}}{D^{2}}K_{1}f\left[1+( \frac{H}{\delta}-\frac{f}{\delta})(\frac{H}{\delta}-\frac{f}{2\delta})\right] \left[1-\frac{D_{m}}{D}/(1-\frac{d}{D})\right] \tag{1}\]  
where \(E\) is the Young's modulus of the slotted conical disc spring; \(\mu\) is the Poisson's ratio; \(\delta\) is the thickness; \(f\) is the deformation; \(H\) is the cone height; \(D\) is the outer diameter; \(D_{m}\) is the slotted bottom diameter; \(K_{1}\) is the constant, which can be calculated by Equation (2). \[K_{1}=\frac{2}{3}\pi\frac{(D/D_{m})^{2}\ln(D/D_{m})}{\left[(D/D_{m})-1\right]^ {2}} \tag{2}\]  
According to references [17; 18; 19], when, \(\sqrt{2}<\frac{H}{l}<2\sqrt{2}\) the designed slotted disc spring will have the force characteristic of \"quasi-zero stiffness\". Figure 2 illustrates the geometry of the designed slotted disc spring and the structural parameters are summarized in Table 1. Figure 3 shows the displacement-load mechanical characteristic of the designed slotted disc spring. From Figure 3, it can be seen that the section in which the disc spring deformation is 1.4-1.9 mm can be regarded as the \"quasi-zero stiffness\" section. In this section, deformation of the disc spring increases, but the load is keeping at about 135 N. When the deformation of the disc spring exceeds 1.9 mm, the lower surface of the disc spring will come into contact with the fixed surface of the test platform. As the load continues to increase, the contact area becomes larger and larger, thus, the mechanical characteristic of the disc spring changes and the stiffness increases instantly. When the disc spring is flattened, the lower contact surface of the disc spring is completely in contact with the test platform's surface, and then the stiffness reaches maximum. The designed disc spring is used to improve the structure of the rotor as a rigid rotor system composed of an upper rotor, a disc spring and a lower rotor. During assembling, the disc spring is compressed to the initial position of the  
The four TRUMs are defined as follows:  
1. The TRUM with the original rotor and new base is labeled as TRUM-1, as shown in Figure 4a. 2. The TRUM with the original rotor and the disc spring disposed above it as a cushioning element is marked as TRUM-2, as shown in Figure 4b. 3. Based on the original rotor, thickness of the rotor's web is reduced and an \"I-Beam\"-shaped ring groove is cut at the edge of the rotor. The rotor with such structure characteristic is called the modified rotor which is less rigid and more flexible than the original rotor.