SHOCK L0CK STRUT
STRUT
(DOWNLOCK)  SIDE STRUT
GEAR IN TRANSIT
Main Gear Lock Mechanism Schematic Figure 4 (Sheet 1)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
DOWNLOCK  UPLOCK
UPLOCK UPLOCK
ACTUATOR  CRANK
ACTUATOR LINK STOP LINK
DOWNLOCK

UPLOCK
SPRING SPRING
UPLOCK HOOK
ROLLER
REACTION
LINK SHOCK
STRUT

L0CK STRUT SIDE STRUT
(DOWNLOCK)
GEAR IN TRANSIT (UPLOCK HOOK TRIPPED)
UPLOCK UPLOCK CRANK DOWNLOCK
UPLOCK  STOP LINK
ACTUATOR
ACTUATOR
LINK DOWNLOCK
UPLOCK SPRING
SPRING
UPLOCK HOOK
ROLLER
REACTION
LINK

SHOCK
L0CK STRUT

STRUT
(DOWNLOCK)  SIDE STRUT
GEAR UP AND LOCKED
Main Gear Lock Mechanism Schematic Figure 4 (Sheet 2)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 7.  Main Gear Modular Package
_________________________
 A.  The main gear modular package contains all the valves required to modify hydraulic flow directed to the main gear retraction and lock actuators (Fig. 6). Two flow limiting valves in the main gear actuator up line are arranged to limit the rate of flow entering or leaving the up port of the actuator. A priority valve and a flow limiting valve are installed in parallel in the downlock actuator up line. A second pair of identical valves are installed in the uplock actuator down line. Each pair of valves controls the hydraulic flow leaving the respective lock actuators when retracting. Any pressure surges exceeding 3500 psi, when actuators are retracting, are released by the priority valve. The net effect of all the valves in the modular package ensures smooth, even, synchronized movements of gear retraction and extension. A modular package is located in the wing root above each main gear.

Main Gear Lock Actuator
Figure 5

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 8.  Main Gear Transfer Cylinder
___________________________
 A.  The main gear transfer cylinder momentarily equalizes hydraulic pressure on both sides of the main gear actuator piston. Pressure is equalized at the start of each extension cycle to relieve gear of actuator force until the uplock is released. The transfer cylinder contains a piston, two springs, and two spring retainers (Fig. 7). One end of the cylinder is connected to the down line. The other end is connected to the up line of main gear actuator. At the end of gear retraction, the cylinder spring retainer is bottomed on the down side of the transfer cylinder. When the control handle is moved to DN for gear extension, pressure is directed to the down side of main gear actuator and the down side of the transfer cylinder. Piston movement directs pressure from the up side of cylinder to the up side of actuator. A flow limiting valve in the main gear modular package delays the pressure loss to return. Therefore, momentarily, pressure exists on both sides of the actuator piston and actuator force is reversed. During the reversal period, the lock actuator unlocks the gear and starts the side strut folding. When the spring retainer bottoms, the counter pressure drops on the up side of the main gear actuator and gear extension proceeds. After the spring retainer has bottomed and hydraulic pressure is off the line, further movement of the piston is available acting against the spring tension. The extra piston movement is provided to damp out pressure surges generated in the system by landing impacts and so prevent pressure fluctuations from reaching the lock actuator and possibly unlocking the gear. The cylinder is mounted in the wing root on the main spar.
　
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