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degrees.
Gear Movement: 0 - 23 Degrees
One actuator gets pressure to the head end and the other
actuator gets pressure to the rod end. This causes one actuator
to extend and the other to retract. This turns the nose landing
gear wheels through the torsion links.
Gear Movement: 23 - 78 Degrees
When the wheels turn to 23 degrees, the swivel valve for the
actuator that is pressurized to retract sends pressure to both
sides of that actuator.
The actuator that was pressurized to extend continues to extend
to steer the wheels. The actuator that was pressurized to retract
is now pressurized to extend. Both actuators continue to extend,
this lets the nose landing gear turn to the 78 degree limit.
Feedback
When the wheels get to the commanded position, the summing
mechanism moves the metering valve back to neutral. This
stops hydraulic pressure to the actuators. The actuators hold
the wheels at the current position.
Compensator
The compensator is a spring-loaded piston type accumulator.
The compensator keeps 220-290 psi pressure in the return lines
of the steering system. This makes sure the steering actuators
stay in their current positions when there is no input to the
system.
Bypass Valve
The bypass valve opens to connect both sides of the piston in
the actuators when an external force turns the nose wheels.
This gives protection to the internal parts of the steering
system.
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Dynamic Load Damper
The dynamic load damper is a hydro-mechanical, dynamic
damping device that decreases shimmy in the nose steering
gear. The towing lever also operates the damper to provide a
fluid connection to both sides of the steering actuator pistons
during an airplane tow.
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NOSE WHEEL STEERING SYSTEM - FUNCTIONAL DESCRIPTION - HYDRAULIC
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
General
The rotary actuator moves to the engage or disengage the
rudder pedal steering mechanism from the nose wheel steering
system. The landing gear transfer valve controls B system
hydraulic pressure to the nose wheel steering metering valve.
Rotary Actuator
The rotary actuator moves to the engage position when the air/
ground relay is energized. A nose on ground signal from the
proximity switch electronics unit (PSEU) energizes the air/
ground relay. The PSEU removes the nose on ground signal
when the nose landing gear moves to the not compressed
position. The air/ground relay moves to the air position 5
seconds after the nose on ground signal is removed. This sends
power to move the rotary actuator to the disengage position.
Landing Gear Transfer Valve
When the airplane is on the ground and with normal quantity in
the hydraulic system B reservoir, the landing gear transfer
valve moves to the alternate position when you move the
alternate nose wheel steering switch on the P1 panel to the ALT
position.
The system B low quantity latch prevents operation of the
landing gear transfer valve when the alternate nose wheel
steering switch is in the alternate position, and the system B
hydraulic quantity momentarily decreases to less than 21
percent. The transfer valve moves to the normal position if the
latch sets during alternate operation.
The low quantity latch stops intermittent operation of the
transfer valve if the system B quantity increases more than and
decreases less than 21 percent again and again. The latch sets
when the nose air/ground system is in the ground mode and the
system B quantity is low. The latch resets when the alternate
　
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