(1)
The solenoid valve is a 2-stage, 3-way operated shutoff valve. When de-energized, the valve moves to the right most or closed position. The mechanical spring acting on the spool and the presence of supply pressure (admitted via an internal passage of the spool) provides the closing force. This force drives the piston to the right and isolates supply pressure from the rest of the module. The module in turn ports output flow to return.
(2)
Input of 28 v dc to the solenoid valve causes supply pressure to be applied to the piston. This drives the shutoff spool to a fixed, open position and applies pressure to the rest of the module. The valve pressure switch then closes to show high (above 1000 psi) pressure. If the selector switch is at OFF and the solenoid pressure switch shows high pressure, the AUTOBRAKES light will come on.
(3)
The pressure control valve consists of a jet-pipe first stage, an in between pressure-feedback stage and a slide-and-sleeve second stage. Supply fluid entering the pressure control valve second stage also feeds the jet pipe thru a first stage filter. The jet pipe directs a jet of fluid from a nozzle into two ports. The change of kinetic energy of the jet into static pressure in the two ports provides the pressure required to drive the second stage.
(4)
A torque motor in the first-stage electrically controls jet pipe position and the amount of flow to two receiver ports. One ports the pressure to return, and the other controls the pressure.
CONTROL ORIFICE
PRESSURE SWITCH SUMMING PISTON
METERED BRAKE PRESSURE SECOND PRESSURE FEEDBACK STAGE
STAGE
SHUTOFF
VALVE
ELECTROHYDRAULIC PRESSURE CONTROL SERVOVALVE (TWO STAGE)
RETURN
SN SN
SN OPEN
N SUPPLY PRESSURE INCREASE
SOLENOID VALVE CLOSE BRK PRESS
S
DC TORQUE MOTOR
DECREASE BRK PRESS JET PIPE/RECEIVER FIRST STAGE RETURN HYD AMPLIFIER
FIRST STAGEPRESSURE
CONNECTING SPRING SWITCHING FILTER (NON-REPLACEABLE)
PISTONMETERED PRESSURE
SOLENOID PRESSURE SWITCH
(SWITCHES SHOWN IN PRESSURE OFF CONDITION)
Autobrake Module Schematic Figure 10
32-42-00
ALL ú ú 03 Page 18 ú Mar 15/87
A
757 MAINTENANCE MANUAL
(5)
A feedback spring attached to the jet pipe on one end and the pressure-feedback spool on the other counters the input of the torque motor. The spring returns the jet pipe to its steady state position (for that particular pressure) when the commanded pressure is reached. The feedback spool moves and compresses a spring until the spring force equals the first-stage control pressure acting on the area of the feedback spool.
(6)
Brake pressure and the first-stage command pressure act on equal areas at opposite ends of the second-stage spool which ports fluid either in or out of the brake as required until brake pressure equals the first-stage command pressure. Without first-stage command pressure applied, a spring biases the second stage to return. An 0.070 inch diameter orifice in the valve return port limits brake pressure release to ensure smooth brake release during autobrake disarm.
(7)
The pressure control valve pressure switch checks module output pressure. When brake application conditions are met and the commanded deceleration exceeds the actual airplane deceleration by more than one foot per second square for more than three seconds, the system shuts down and the AUTOBRAKES light comes on.
L.
Autobrake Shuttle Valve Assembly (Fig. 11)
________________________________
Figure 11
32-42-00
ALL ú ú 04 Page 19 ú Mar 15/87
A
757
MAINTENANCE MANUAL
(1)
The assembly consists of a valve and a pressure sensing switch. The basic three-way valve allows brakes to work by either a manual or an autobrake system. Two valve assemblies are located one each on left and right wheel well forward bulkhead. Both valves and switches are LRUs. Pressure switch can be replaced without removing valve from the airplane.
(2)
The valve has two input ports (one normal, one auto) and one output port (brake). Under normal operation, the normal port connects to the brake port. When the autobrake system applies pressure, a detented slide located in the valve moves to block the normal port. This slide shift allows fluid flow from the auto port to the brake port.
(3)
The pressure switch, connected to the normal input port, checks pressure downstream of the normal brake metering valves. When manual braking effort exceeds 750 psi on either the left or right pedal, the switch opens to provide an input to the control unit to disarm the system.
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：757 AMM 飞机维护手册 起落架 CHAPTER 32 - LANDING GEAR 2(165)
