TM 1-1500-204-23-2Figure 4-155. Disc Brake Assembly Cross Sectionchanges or minor leakage. During normal braking of the aircraft by pedal actuation, the lever of the parking brake valveis held in the off position by means of a spring. The camshaft to which the lever is attached holds the two outer poppetsof the valve unseated as shown here. This permits fluid pressure from the master cylinder or power brake control valveto pass straight through the parking brake valve to the wheel brake assemblies. In this position, the inner poppets areseated and the spring-loaded pistons are inoperative.(b)Setting parking brakes.To set the brakes for parking, the parking brake valve is left inthe off position until the brake pedals are depressed (see figure 4-157). While the brake pedals are still depressed, theparking brake control handle is pulled to the park position. This rotates the camshaft of the valve, causing the outerpoppets to seat as shown in figure 4-158, fluid pressure is now locked within the wheel brake assemblies. At the sametime, the rotating camshaft unseats the inner poppets, thereby opening the compensator cylinders to the wheel brakeassemblies. As a result, the compensator cylinders become charged, the pistons become partially retracted, and thesprings become partially compressed. Spring tension on the spring-loaded pistons maintains constant braking pressureby compensating for volume changes produced by changes in temperature and by minor leaks.(c)Brakes locked.After the parking brake has been set in the park position, the brakepedals are released. The parking brake lever then becomes locked in the park position by means of a locking pin, whichis extended to the locking position by a spring. The locking pin is retracted, permitting release of the lever by fluidpressure, when the brake system is pressurized by operating the brake pedals.c.HelicopterRotorBrakeSystems. Rotor brake systems are Installed on some helicopters as a means ofshortening the time required to bring their rotor assemblies to rest after power has been shut off. They also prevent therotor assemblies of parked helicopters from windmilling. A basic hydraulic rotor brake system is shown in figure 4-159.Newer helicopters have more complex systems, but the same basic principles apply.4-150
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