747-400
MAINTENANCE MANUAL

VENT
FUSE
RETURN TRUCK POSITIONING ACTUATOR BODY GEAR

TRUCK POSITIONER BYPASS REDUCER VALVE ASSY (WITH CARTRIDGE RELIEF VALVE)

CHECK
VALVE

HYDRAULIC
SYSTEM NO.1

Body Gear Hydraulic Tilt Schematic Figure 6
U23298
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (2)  
The snubber valve on the shock strut inner cylinder moves up and down when the shock strut operates, acting as a one-way restrictor. When the shock strut is compressing, the snubber valve allows an unrestricted flow of fluid from the upper chamber into the annular space between outer and inner cylinders. When the shock strut extends, the snubber valve moves upward, closing off holes in the upper bearing. This action restricts flow from the annular space and thus absorbs shock strut rebound. When the shock strut is serviced with fluid in the compressed position, the outer cylinder contains fluid all around the piston rod and in the space between the inner and outer cylinders. The shock strut is then inflated with nitrogen or air to the specified pressure.

 (3)  
Shocks are absorbed by the flow of hydraulic fluid through the annular space between the orifice and the tapered metering pin. As the shock strut is compressed, fluid flows through the orifice and the movement of the tapered metering pin through the orifice progressively varies the area of the annular space between orifice and pin. The variation in area results in an adjusted rate of hydraulic flow from the inner cylinder chamber to the upper side of the piston, thus providing uniform control of the impact loads on the airplane structure when landing.

 (4)  
Taxi shocks are cushioned by the increasing volume of hydraulic fluid above the piston further compressing the volume of compressed air or nitrogen in the upper end of the outer cylinder. As the shock strut extends, there is a reversal of flow through the orifice. The hydraulic fluid now being forced from the upper to the lower side of the piston must pass through the space around the metering pin, and through holes in the orifice support tube. Also, fluid in the annular space between the inner and outer cylinders is forced through the snubber valve restriction, and through the upper bearing into the upper chamber. The resulting resistance to hydraulic flow dampens the return movement of the shock strut.

 (5)  
The body gear truck positioning actuator tilts the wheel truck to a pitched up position at an angle of 7 degrees after liftoff, thus assuring correct body gear positioning for retraction. The tilting force is supplied by hydraulic pressure acting on a two way piston. The truck positioning actuator consists of a piston and shaft inside a barrel which is locked to a body assembly. A check valve and flow fuse assembly are located upstream of the pressure reducing valve on the pressure line. The pressure reducing valve includes a cartridge relief valve as an integral component. The flow fuse will limit fluid loss if leakage should deveop in the line.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 The hydraulic fuse will also be set by a loss in hydraulic pressure. A set fuse will prevent the body gear truck from tilting to the proper angle for retraction. Should a pressure loss develop in the system, the fuse must be reset by holding the manual bypass lever in the raised position for 2 seconds.
 (6)  Body gear steering is electrically coupled to the nose gear steering to establish a fixed position of body trucks relative to a defined position of the nose wheel. A switch operated by a cam driven by the nose wheel steering tiller arms the body gear steering at approximately | 21 degrees of nose wheel steering. At this angle the control transducer gives a signal to move the main gear to a defined position when the gears are on the ground. This arming circuit is deactivated whenever the gear is in tilted position or when wheel speed is greater than 20 knots. Two hydraulic actuators mounted on the body gear shock strut connect to the lower torsion link for truck positioning to a maximum body gear steering angle of | 13 degrees.
　
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