## 3 AR Content Development Process of the Air Spring
### Air Spring 3D Modeling  
For railway vehicles, the air spring is installed at both ends of the car frame center, as shown in Figure 2. It receives compressed air at 4-6 bar pressure from the air compressor, which is applied through the brake operating unit. The air spring serves as a suspension device with smooth damping performance by compressing the gas and applying fluid resistance. Optimizing damping performance with pneumatic systems requires meticulous design of the air spring as a 3D model, tailored to its stiffness characteristics. The stiffness characteristics are analyzed using innovative methods, including advanced 3D finite element analysis techniques. These techniques comprise the alternating direction implicit finite difference scheme for the three-dimensional time-fractional telegraph equation, and the second-order backward differentiation formula alternating direction implicit Galerkin finite element method for handling evolution equations with nonlocal terms in three-dimensional space [36; 37]. It improves ride comfort through vertical and lateral motion damping and adjusts vehicle height through leveling valves.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Division** & **Mobile Device** & **HMD Device** \\ \hline \multirow{2}{*}{Interaction} & Touch gesture and & Hand tracking-based \\  & button-based interaction. & interaction. \\ \hline \multirow{2}{*}{Point of view} & Experience with perspective & Experience with perspective \\  & applied like an actual camera. & provided like actual glasses. \\ \hline \multirow{2}{*}{Usability} & Adjustable display brightness, & Low display brightness, \\  & no restriction on field of view. & narrow field of view. \\ \hline \multirow{2}{*}{Deployment environment} & Easy distribution on personal & Difficult distribution due to \\  & smartphones and tablets. & high cost. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Examples of overseas research content.  
Figure 1: Procedures of air spring AR contents development and design.  
The maintenance process for air spring assembly and disassembly is straightforward and involves external and dimensional inspections. Maintenance personnel conduct visual inspections of the external surfaces and contact areas of bellows, upper plates, lower plates, and skid plates for cracks and leaks caused by friction. Additionally, dimensional inspection measures the standard height between the bottom of the car body frame and the bogie frame to check whether it meets the specified tolerance and performs height adjustment. To measure the height of these air springs, information such as the tools provided in the maintenance manual, height measurement criteria, and height adjustment procedures are required. Since such external inspections and dimensional checks are difficult to identify visually, relying solely on 2D drawings poses a problem. Therefore, it is important to develop AR content that displays the hierarchical structure of the air spring and the air diffusion due to leakage in a 3D format for the workers.  
Figure 3 shows the process of developing a 3D model of the air spring for the AR maintenance training content. To provide accurate model information for the 3D model's shape, the manufacturer's 2D drawings and prototypes were gathered using a 3D scanner (Shinging 3D, Hangzhou, China), as shown in Figure 4, in ASC format. The 3D CAD model was generated using the Geomagic Design X program [38] and CATIA V5 [39; 40] based on the scanned shape data, saved as an XRL extension file. The XRL format 3D model was converted to STP for integration into Unity [41], and the final model file was produced as an FBX format Unity 3D model file.  
Figure 4: Process of using a 3D scanner.  
Figure 3: Process of creating a 3D model of the air spring: (**a**) ASC format; (**b**) XRL format; (**c**) FBX format.  
Figure 2: Railway vehicle bogie frame and air spring.