747-400 AMM 飞机维护手册 起落架 LANDING GEAR 3(97)

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 B.  During extension and retraction the nose gear pivots on a trunnion rotating in bearings secured to brackets in the aft part of the wheel well. A hinged drag strut keeps the nose gear stable longitudinally when the gear is extended. Nose gear locks are included in the drag brace and the lock actuator. The nose gear is retracted or extended at the same time as the main gear when the landing gear control handle is moved into the UP or DOWN position.
 C.  The nose gear includes a drag strut, shock strut, and torsion links. The nose wheel steering system has mechanical linkage and cables to connect it to the rudder pedals, when the shock strut is compressed by the weight of the airplane. The steering is hydraulically powered using a steering metering valve. The metering valve contains a bypass valve which allows the airplane to be towed at steering angles up to 70 degrees without disconnecting the steering at the torsion links. Refer to 32-51-00/001 for nose gear steering operation.
 D.  The nose gear doors are clamshell type, and consist of four doors that align with the fuselage contour when closed. One pair of doors closes over the wheel well. The second pair of doors closes over the shock strut opening in the fuselage when the gear is retracted. The wheel well doors are normally open only when the gear is in transit. However, they can be opened to permit access to the nose gear area for maintenance. This is done by operating the nose gear release handle. After servicing, the wheel well doors should be closed. The shock strut doors open for gear extension and remain open when the gear is down. See 32-34-00/001 for information on nose gear door retraction. See 32-36-00/001 for information on the mechanical latching mechanism and door ground release mechanism.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
 NOSE GEAR - DESCRIPTION AND OPERATION
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General (Fig. 1)
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 A.  The nose landing gear is a steerable dual wheel assembly. It supports the forward end of the airplane and rotates to control the direction of taxi while on the ground. The nose gear installation consists of wheel and tire assemblies, shock strut assembly, torsion links, steering and metering valve assemblies, retraction actuator, lock mechanisms, and shock strut doors. The gear assembly is hydraulically actuated to retract forward and up into a wheel well recessed into the lower nose section of the airplane. The nose gear is retracted or extended simultaneously with the main gears, when the landing gear control handle in the control cabin is positioned to the UP or DOWN detent.
 B.  Shocks and bumps during taxi, takeoff and landing are absorbed by the shock strut which contains oil and is charged with compressed air or nitrogen. Longitudinal stability is provided by hinged drag strut which folds upward and aft during nose gear retraction (Fig. 2). During steering, the inner cylinder rotates within the outer cylinder. The upper end of the upper torsion link is connected to the nose gear forward steering collar. The lower end of the lower torsion link is connected to the shock strut inner cylinder. Hydraulically actuated steering cylinders supply input force to the steering collar to transmit a turning moment to the torsion links.
Nose Gear Steering Collars
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 A.  Nose gear steering collars are bolted together around the shock strut outer cylinder. They are located in an annular recess just below the tripod attach lugs. The steering collar is split in two halves. Forward steering collar contains the upper torsion link attach lug. Aft collar steering actuator attach lugs slip over the forward collar lugs. The two sections are locked together on each side by actuator attach pins. These pins also hold the rod end of the steering actuators.
 B.  When force is applied by the actuators, these pins transmit a turning moment to the collar. The force is transferred through the torsion links to turn the inner cylinder to the right or left. This causes the axle to rotate and give steering action to the nose wheels.
 C.  The forward steering collar is provided with an angular indication Metal-Cal. The zero calibration coincides with a slot on collar bushing so steering angular displacement may be directly read on the Metal-Cal.
Nose Gear Steering Actuators Supports
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 A.  The actuator supports attach to the aft side of the shock strut outer cylinder, just below the tripod attach lugs. The lower actuator support is a wing-shaped forging, with circular mounts at each end to support the steering actuator trunnions. The upper actuator support is identical to lower, except that it has four additional lugs on the trailing edge for metering valve support attachment and summing lever mounting support.
　
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