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B. Parking Brake Linkage Mechanism
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(1)
The parking brake cable attaches to a control arm. When the parking brake is set, the control arm rotates a spring loaded shaft. Which moves two shaft-mounted lockout pawls into position against the captain's brake pedal linkage. The pawls keep the brake pedals depressed. A cam which is mounted on the shaft actuates the parking brake switch to close the parking brake valve.
C.
Parking Brake Valve
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(1)
The parking valve closes to prevent brake pressure return from the brakes, and extend parking brake holding time. The valve is a two-position, motor-operated, ball type valve. The valve closes (POSITION 2) when the parking brake is engaged, and opens (POSITION 1) when the parking brake is released. The valve operates through a gear train, and requires 28 Vdc power from the parking brake switch to actuate. The valve remains in the selected position when power is removed.
(2)
A manual override handle allows the valve to be set manually in either POSITION 1 (open) or POSITION 2 (closed). However, subsequent application of power will drive the valve to the electrically commanded position.
(3)
An external position indicator (manual handle) is provided.
(4)
The motor includes two limit switches to shut off power when the valve reaches the commanded position. A third limit switch in the valve is used to indicate the valve position.
(5)
The valve is located on the forward wall of the right landing gear wheel well.
D.
Parking Brake Accumulator/Pressure Transmitter
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(1)
The accumulator is precharged with nitrogen to provide backup hydraulic pressure for operation of the brake system and parking brake when normal (right) or alternate (left) hydraulic system pressure are not available. The accumulator is continuously pressurized through a one-way check valve whenever the right system is pressurized. Nitrogen and hydraulic fluid are separated by a floating piston inside the accumulator.
(2)
A pressure transmitter is installed next to the accumulator in the RH wheel well. The transmitter provides a pressure signal to the brake accumulator pressure guage on P3 panel in the flight compartment.
E.
Accumulator Isolation Valve
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(1) The accumulator isolation valve is a two-position valve which opens when the right hydraulic system is depressurized, and closes when the right hydraulic system is pressurized. When the isolation valve is open, the accumulator supplies pressure for the parking brakes. The accumulator is isolated from the depressurized right hydraulic system by a one-way check valve.
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F. Close Sense Relay
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(1)
The close sense relay is energized when the parking brake valve is closed. Closed contacts provide signal logic to the antiskid/autobrake control unit. Two other switch contacts interface with the Voice Recorder System (AMM 23-71-00/001). The relay is located on the P36 miscellaneous relay panel in the Main E/E Compartment.
G.
Parking Brake Switch
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(1)
The parking brake switch is a two position switch which is positioned by a mechanism link on the parking brake mechanism (See Fig. 1 for effectivity). When the parking brake is set the link moves the switch to the set position. When the parking brake is released, the switch moves to w the released position. When in the set position , the switch provides 28 vdc power from the hot battery bus to CLOSE the parking brake valve. When in the released position, the switch provides 28v dc power to open the valve.
(2)
Another switch contact provides interface with the CAUTION AND WARNING system (AMM 31-51-00/001). A siren sounds to provide unsafe takeoff configuration warning if the parking brake is engaged, the airplane is on the ground, and takeoff thrust is applied.
(3)
The parking brake switch is located adjacent to the parking brake mechanism below the flight compartment floor.
H.
Parking Brake Accumulator Charging Valve/Gage
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本文链接地址：757 AMM 飞机维护手册 起落架 CHAPTER 32 - LANDING GEAR 3(54)
