The bypass valve isolates the hydraulic motor from hydraulic system pressure. The bypass valve also prevents a hydraulic lock and permits backdrive of the hydraulic motor.
A dual-coil bypass solenoid valve controls pilot pressure to the bypass valve, but only one coil of the solenoid is used. When the FSEUs energize the bypass solenoid valve, it removes pilot pressure from the open side of the bypass valve. This closes the bypass valve, which connects the hydraulic lines to both sides of the hydraulic motor.
The bypass valve is spring-loaded and hydraulically controlled to the closed position. The open side of the valve has a larger surface area for the pilot pressure than the close side of the valve. When the center hydraulic system pressure is low, springs automatically force the bypass valve to close. When the center hydraulic system pressure is high, the bypass valve opens.

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LEADING EDGE SLATS -PRIMARY CONTROL VALVE -FUNCTIONAL DESCRIPTION

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777 AIRCRAFT MAINTENANCE MANUAL

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LEADING EDGE SLATS -SLAT POWER DRIVE UNIT (PDU)

Purpose

The slat PDU turns the slat torque tubes during operation in all three modes. The slat PDU has these LRUs:
*
A hydraulic motor

*
An electric motor

*
A limit switch

*
A gearbox.

Information on the hydraulic motor, the electric motor, and the limit switch is shown later.
Location

The slat PDU is in the left wing root. Remove the access panel or open the small access panel under the wing to get access.
Gearbox

The gearbox transmits the power from the hydraulic or electric motors to the torque tubes.
The hydraulic motor or the electric motor rotates gears in the gearbox. These gears increase the torque from the motors. The gearbox turns the torque tube couplings on both sides of the PDU. These couplings turn the torque tubes.
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The gearbox contains a ball ramp torque brake to limit the output torque from either side of the PDU. When the torque brake activates, a torque trip indicator shows. There is a separate torque trip indicator for the PDU output to the left wing slats and to the right wing slats. The torque trip indicators have the labels LEFT WING and RIGHT WING. The indicators are reset manually.
The slat PDU has a rig pin that locks the PDU gearbox in the slats UP position. A ground lockpin next to the rig pin locks the rig pin in the rig position.
The gearbox also contains a stroke control mechanism to stop the PDU output if one of the motors on the PDU fails to stop. The stroke control mechanism stalls the hydraulic or electric motors within two torque tube rotations of the maximum normal slat positions.
During high-speed operation in the primary mode, the gearbox turns the torque tubes at 640 rpm. During low-speed operation, the gearbox turns the torque tubes at 128 rpm.
During operation in the secondary and alternate modes, the gearbox turns the torque tubes at 85 rpm.
Training Information Point
Wrench flats on the torque tube between the PDU and slat 7 let you manually rotate the slat system.
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LEADING EDGE SLATS -SLAT POWER DRIVE UNIT (PDU)

To install the rig pin in the slat PDU, retract the slats to the UP position. See that the alignment mark on the output shaft aligns with the fixed index mark on the PDU. If the alignment mark does not align with the index mark, use the wrench flats on the torque tube to turn the PDU in the extend direction, then in the retract direction until the marks align. Push the rig pin in the PDU and hold it while you lock it with the ground lockpin.
The gearbox contains oil and it is necessary to do servicing at specified times. Use the oil fill plug.
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LEADING EDGE SLATS -SLAT POWER DRIVE UNIT (PDU)

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LEADING EDGE SLATS -PDU HYDRAULIC AND ELECTRIC MOTORS

Purpose

The PDU hydraulic and electric motors supply power to turn the gearbox and the torque tubes.
Location
The motors are on the slat PDU and connect to the gearbox.
Hydraulic Motor

The hydraulic motor operates in the primary mode. It receives hydraulic pressure from the slat primary control valve. During secondary and alternate mode operation, the primary control valve blocks hydraulic power to the motor. It also permits the hydraulic motor to be backdriven.
　
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