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Because they are directly upstream of the arming and control valves, the manual valves have first priority of control over the trim up and down, arm and control solenoid valves. This permits the pilot to manually override any electrical command to the stabilizer.
When the arm and control input cranks move to the trim up position, they command the manual valves to move to the trim up position. The manual valves stop hydraulic pilot pressure to the trim up side of the arming and control valves and port it to the system return line. Both arming and control valves move to the trim up position and permit pressure to the brake and motor.
When the input cranks move to the trim down position, the same process occurs. But in this case, the arming and control valves move to the trim down position and permit hydraulic pressure to the brake and motor.
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HORIZONTAL STABILIZER CONTROL -STCM -FUNCTIONAL DESCRIPTION

Brake Bypass Valve

The manually operated brake bypass valve permits maintenance personnel to test the effectiveness of the hydraulic brake. When activated, the valve blocks hydraulic pressure to the brake and ports it to the system return line. This prevents brake release when the hydraulic motor is pressurized.
Pressure Switch

The pressure switch closes at 1000 psi. It sends a discrete signal to the ACEs to indicate the pressure status to the brake.
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HORIZONTAL STABILIZER CONTROL -STCM -FUNCTIONAL DESCRIPTION

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777 AIRCRAFT MAINTENANCE MANUAL

THIS PAGE IS INTENTIONALLY LEFT BLANK

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HORIZONTAL STABILIZER CONTROL -STABILIZER BALLSCREW ACTUATOR

Purpose

The stabilizer ballscrew actuator uses the hydraulic pressure from the STCMs to move the horizontal stabilizer assembly.
Physical Description

The stabilizer ballscrew actuator attaches to the aircraft structure with a gimbal. The gimbal is at the top of the actuator. The stabilizer ballscrew actuator includes:
*
Two hydraulic motors

*
Two hydraulic brakes

*
A differential gear box

*
A primary brake

*
A ballscrew.

 

Each hydraulic brake is on the same shaft with each hydraulic motor. The two hydraulic motors turn the motor pinions and the motor gears. The two motor gears are part of the differential gearbox. The bull gear pinion of the differential connects with the bull gear and the ballscrew.
The ballscrew also connects to a primary brake. The primary brake has two brakes with brake discs, ratchets, and pawls. The two brakes are similar but each one operates in one direction of rotation.
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Location
The stabilizer ballscrew actuator is in the stabilizer compartment, just forward of and below the STCMs.
Functional Description
Hydraulic pressure from the STCMs pressurizes the hydraulic motors and brakes. The hydraulic pressure releases the hydraulic brakes and permits the motors to turn the motor pinions. They move the differential mechanism in the gearbox.
The differential mechanism permits operation with one or two motors at a time. The differential mechanism adds the speed of both hydraulic motors. When one hydraulic motor operates, the speed of the ballscrew is 50 percent of the speed when both hydraulic motors operate. The bull gear pinion of the differential mechanism drives the bull gear and the ballscrew.
The primary brake makes sure airloads on the stabilizer do not backdrive the ballscrew. The primary brake uses the axial load in the ballscrew caused by an airload on the stabilizer. An axial load in the ballscrew sets one brake and its ratchet pawls make sure the ballscrew cannot turn in one direction. An axial load in the opposite direction sets the other brake. If there is no airload when one or both hydraulic motors operate, the primary brake releases and the ballscrew turns freely. If there is an airload on the stabilizer when one or both hydraulic motors operate, it may cause some drag of the ballscrew on one brake.
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HORIZONTAL STABILIZER CONTROL -STABILIZER BALLSCREW ACTUATOR

The ballnut moves vertically on the ballscrew. The direction of movement depends upon the direction of ballscrew rotation.
The front structure of the stabilizer assembly attaches to the gimballed ballnut assembly.
A safety rod inside of the ballscrew holds the ballscrew in case of failure.
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