If the brake assembly is not to be installed immediately, install any attaching hardware that is part of the assembly, fill with preservative hydraulic fluid, and cap or plug all openings to prevent contamination. TRIMETALLIC DISC BRAKES Figures 12-53 and 12-54 show a typical trimetallic brake assembly. The trimetallic brake assembly consists of a brake housing subassembly, a keyed torque tube and torque tube spacer, a housing backplate, stationary and rotating discs, and a pressure plate subassembly. Description The brake housing subassembly, keyed torque tube and spacer, and the housing backplate are bolted together to form the basic brake assembly. The remaining components of the brake assembly are mounted over the keyed torque tube and between the brake housing and the housing backplate. The metallic-faced rotating discs have keyways that engage drive keys in the wheel so that they rotate with the wheel. The rotating discs are separated by the stationary discs, which are keyed to the torque tube. The mating surfaces of these rotating and stationary discs constitute the major friction-braking surfaces of the brake. Additional friction surfaces exist between the outer face of one rotating disc and the housing backplate, and between the outer face of the rotating disc at the opposite end and the pressure plate subassembly. The pressure plate subassembly consists of the pressure plate, replaceable wear plate, and wear plate insulator. These three parts are riveted together. The pressure plate serves as a seat for the self-adjusting pins of the self-adjusting mechanism, and rests against the insulators installed in the outer ends of the brake pistons. It is the component through which force is directly transmitted during application and release of the brakes. The wear plate is keyed to the torque tube to prevent rotation of the complete subassembly, and serves as the friction surface for the outer face of the adjacent rotating disc. The wear plate insulator prevents brake heat from being transferred to the pressure plate and the brake pistons. The brake pistons transmit hydraulic pressure through the pressure plate subassembly to the brake discs. Standard O-rings and backup rings around each piston prevent hydraulic fluid leakage and entry of contaminants. The pistons are further protected against heat transfer from the pressure plate subassembly by individual insulators installed in the ends of each piston where it contacts the pressure plate. Self-adjusting mechanisms are located around the brake housing. They accomplish normal release of the brake and provide a continuing adjustment action to compensate for broke wear. Each mechanism consists of a self-adjusting pin, a spring housing and bushing, a return spring guide, a retaining ring, a grip and tube subassembly, and a self-locking nut. The grip and tube subassembly mounts over the self-locking pin, with the grips being installed firmly on the tube. As disc wear occurs automatic adjustment is provided by movement of the adjusting pins through the split collar grips. The retaining ring inside the spring housing serves as a stop and retainer for the spring guide, which, in turn, holds the return spring in position. The head of the self- adjusting pin engages the pressure plate subassembly to allow brake release when pressure is removed. Operation When the landing gear wheel is rotating, the metallic–faced rotating discs of the brake assembly rotate freely between the stationary steel discs. When pressure is applied to the brake assembly pistons, the rotating and stationary discs are forced together, creating friction between their surfaces. The amount of hydraulic pressure applied to the brake pistons is controlled by the aircraft’s brake metering system in response to the operating of the brake pedals. Braking action applied to the wheel brake is proportional to the pressure exerted on the brake pedal. Pressure applied to the brake actuates all of the pistons within the brake housing. These pistons, in turn, force the pressure plate subassembly laterally against the discs and against the housing backplate. As the pressure is applied and the brake starts to actuate, the lateral movement of the pressure plate subassembly pulls the self-adjusting pins, the split collar grip and tube subassemblies, and the return spring guides against the return springs, compressing them until the spring guides bottom in the housings. When the hydraulic pressure is relieved, the return spring mechanisms, acting through the heads of the self-adjusting pins, pull the pressure plate subassembly back to the released position. The pistons also return to their deactuated positions. The extent of the return motion is limited by engagement of the spring guides with the retaining ring stops inside the spring housing. As the discs wear, self-adjusting pins and tubes are pulled through the split collar grips by the force exerted 12-61