# Experimental Wear Characterization and Durability Enhancement of an Aeronautic Braking System
## 2 Materials and Methods  
### The Studied Electromagnetic Brake and Tribological Issues  
The braking system of the helicopter is composed of two electromagnetic brakes, one for normal service and one for an emergency, with two different supply circuits to ensure the service of at least one brake in case of a malfunction. The studied brake is a power-off system, and therefore, the braking action is exerted when the power supply is interrupted on purpose or voluntarily. The system is designed to perform static braking; short transient phases are possible with the stick-slip contact between the disks. Random dynamic contact can also be favored by short distances between the disks, which can come into contact if subjected to vibrations.  
The main components of the electromagnetic brake are:  
* The solenoid, which generates the attractive magnetic field and the magnetic force \(F_{m}\) on the anchor, which opens the brake.
* The springs mounted on the solenoid, which generate a repulsive elastic force \(F_{el}\) on the anchor to compress the disk stage when the solenoid is not supplied.
* The anchor, which is the mobile element determining the opening and closure of the brake.
* Stator disks, which act as counter-faces.
* Rotor disks with antifriction pads that rotate with the pinion connected to the gearbox.
* The brake frame, which represents the external counter-face and keeps the stator disks in the right position.  
The performance of the braking system depends on the following main parameters:  
* Air gap, which represents the gap between the anchor and the solenoid when the brake is not powered and the magnetic field is absent, as represented in Figure 1. Once the solenoid is supplied, it engages the anchor, and the air gap previously located between the solenoid and the anchor is distributed between all the braking surfaces, which are free to rotate, and the system does not exert any braking torque. Therefore, the air gap must be properly dimensioned to avoid dynamic contacts and wear, even when the brake is not working [10].  
Figure 1: Air gap.  
Materials and thickness of the coupled rotor/stator, which determine the tribological behavior of the brake.
* Number of disks \(N_{d}\), which determines the number of friction surfaces and the braking torque. The correct determination of this parameter allows not only an adequate braking performance but also proper wear resistance.  
The studied electromagnetic brake has a base configuration based on the design parameters and materials shown in Table 1.  
The rotor friction material was a carbon material with long interlaced fibers. From a tribological point of view, the described and actual configurations of this kind of brake ensured good braking performance, but the duration was too limited, and the system could not properly work as early as before the Time Before Overhaul (TBO), which represents the number of working cycles before the aircraft component requires an overhaul.  
This finding was confirmed during the qualification test in compliance with the standard for the environmental testing of avionics hardware, DO-160 [30].