# Experimental Wear Characterization and Durability Enhancement of an Aeronautic Braking System
## 3 Results and Discussion
### Comparison of Results and Discussion  
The results of this work are reported in Table 4, where the three analyzed configurations are compared in terms of wear, durability, and maximum braking torque normalized with respect to the minimum ATP value.  
The NAO configuration shows the best results in terms of durability, wear, and braking torque compared to both the base and eight-spring configurations. This improvement over configurations with carbon fiber as the friction material is due to the low operating temperature of the brake and the wear behavior of the different material structures and reinforcements. Although the heterogeneity of the NAO due to the polymeric structure with reinforcements results in good mechanical properties, the wear behavior is nonuniform.  
Figures reporting the results of the monitoring parameters during the endurance tests performed on the NAO and base configurations demonstrate that the trends of these parameters are similar, although the failure of the NAO configuration occurs after about 35,000 actuation cycles. Furthermore, although the NAO configuration results in an optimized braking system, the failure of the brake is still associated with wear progress and the presence of ferromagnetic particles at the interface.  
This evidence suggests that, even in the optimized NAO configuration, the presence of ferromagnetic wear particles remains related to the failure of the electromagnetic brake, as these particles contribute to the increase in the air gap. Given that the particles are located between the solenoid and the anchor due to the solenoid attraction, the system is presumed to fail due to the incapacity of the solenoid to generate a sufficient magnetic field to guarantee the total aperture of the brake.  
The results of this study are related to a specific industrial braking system for helicopters and are aimed at improving the durability and efficiency of the device, optimizing its tribological behavior and wear resistance. The literature contains few similar studies on electromagnetic brakes for the same application, as the performance of a braking system is strongly dependent on its structure, the chosen materials, the actuation, and other thermal issues. A few works can be taken into consideration for a comparison, mainly in terms of procedures. For instance, the studies in [24; 25] are both focused on the design optimization of brakes for aircraft and helicopters based on the analysis of the tribological behavior, but different architectures, actuation technologies, and materials are considered compared to the work presented here. In [24], the brake is based on a disk and clasping arm arrangement, and the actuation is hydraulic. Different friction materials were experimentally tested--a semi-metallic material and some non-asbestos organic materials--with the aim of meeting a set of established requirements in terms of the holding torque, mass, volumetric constraints, and fast engagement. For these purposes, an NAO material was chosen as the best one (Trimat MR2215). This finding matches the results of this study, which demonstrated the best performance with the NAO configuration. In [25], new asbestos-free friction linings were analyzed considering four materials by calculating the distribution of the contact pressures with numerical models and comparing results with experimental ones. Again, in this case, different materials and configurations were investigated. In [29], an electromagnetic brake for aeronautical applications with a similar structure to that of the brake presented in this paper was prototyped and analyzed, but the authors considered several design issues not specifically concerning tribological aspects.  
\begin{table}
\begin{tabular}{c|c||c c c} \hline
**Configuration** & **Comparative Wear Level** & **Actual Cycles** & **Max Braking Torque** \\ \hline Base & & & 3000 & 278\% \\ \hline
8-springs & & & 4385 & 303\% \\ \hline NAO & & & 35,223 & 354\% \\ \hline \end{tabular}
\end{table}
Table 4: Comparison of results.