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Stabilizer Trim Drive Mechanism
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 A.  The stabilizer trim drive mechanism comprises two hydraulic motors driving a linear ball screw actuator through a gear train, with two hydraulic brakes for secondary anti-creep control. The motors drive a ball nut and screw through differential and bull gear trains. The hydraulic brakes act upon shafts driven directly by the motors. An upper gimbal mounted on the actuator ball nut connects the mechanism to the stabilizer forward attachment fitting. A lower gimbal anchors the trim drive mechanism to the bottom of the actuator bulkhead. Two lubrication fittings in the lower gimbal retaining pins and four in the upper gimbal provide servicing points for the moving parts.
 B.  The two hydraulic motors convert hydraulic pressure to rotary motion to drive the ball screw. The two hydraulic brakes lock the motor shafts to provide redundant braking when the motors are not driving. Each combination of motor and brake is activated by its associated STCM, and can control stabilizer movement independently through the differential. In normal operation, however, the two combinations act in unison.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 C.  The differential, on which the hydraulic motors are mounted, is bolted to the bull gear housing, which contains a spur gear splined to the ballscrew. A safety rod inserted lengthwise through the screw, retained by a safety nut at the top and a safety sleeve and shaft at the bottom, prevents failure of the stabilizer front attachment if the ballscrew fails. A housing bolted to the lower face of the bull gear housing contains the primary brake and incorporates the lower gimbal attachment.
 D.  The trim drive mechanism is equipped with a primary brake contained in an oil-filled primary brake housing. A replaceable lip seal at the bottom of the housing prevents oil leakage. The primary brake consists of two brake disks, located one on each side of a flange on the ball screw. Each disk acts in conjunction with an irreversible pawl-controlled ratchet to prevent the stabilizer from creeping under the influence of aerodynamic loads.
 E.  The ball screw drives a ball nut consisting of a primary and a secondary nut splined together and joined by a bolted mating flange. Ball bearings circulating within the primary nut and through two external ball return tubes, transmit the drive from the ballscrew to the upper primary nut. The lower, secondary nut, carries loads only in event of primary nut or gimbal yoke failure. The nut is internally threaded for drive transmission. An internal scraper at the upper end of the primary nut removes ice and coarse particles from the screw and prevents their entry into the nut during upward travel. A wiper adjacent to the scraper excludes fine particles and retains lubricant. A scraper at the lower end of the secondary nut and a wiper at the lower end of the primary nut perform identical functions during downward travel. Integral stops at each end of the ball nut assembly contact the stops on the actuator shaft and bull gear at the travel extremities.
 F.  Two diametrically opposed trunnions on the primary nut engage the upper gimbal, which connects to the stabilizer. Two trunnions on the secondary nut, oriented perpendicular to the primary trunnions, also engage the upper gimbal. A lubrication fitting and a lubrication vent on the primary nut are provided for packing the nut with grease. Each of the trunnions on the primary nut also provides a lubrication fitting.
 7.  Trim Drive Mechanism Hydraulic Motors
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 A.  The two hydraulic motors are identical units, configured for left or right mounting by the arrangement of hydraulic port adapters during installation. The motors can easily be changed with the trim drive mechanism installed in the airplane. Each motor is a nine-cylinder, piston-type, reversible unit. The motors are mounted side by side on the differential gear assembly casing, which is bolted to the bull gear housing. Besides driving the ball screw, the motors act as back-up anti-creep brakes by hydraulically locking the trim drive mechanism gear train when idle. The motors drive the gear train through separate shafts to permit single-motor operation if one should fail.
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