-two mounting devices are on each side of the gear box; this permits an easier removal,
 1EFF : ALL 1 27-44-00Page 5 1 1 Nov 01/98R 1 1 1CES 1

 -an oil level sight glass to see the level of the oil in the gear box.
 4. Power Supply
____________
 (Ref. 27-90-00)
 5. Component Description
_____________________
 (Ref. Fig. 001, 003)

 A. Hydraulic Motors Two hydraulic motors are installed on the THS actuator. Each hydraulic motor is a fixed displacement type. The hydraulic fluid enters the pressure port and causes the rotating group to turn. The rotating group has nine pistons. These pistons move in their bores in the cylinder barrel when the cylinder turns. Turning of the cylinder barrel gives the necessary torque. The torque is transmitted through a splined drive shaft to the gear box of the THS actuator.
R Two drain tubes are installed below the flanges of the hydraulic motors. R These drain tubes drain the leakage of the hydraulic motors.
 B. Fail-Safe Ball Screw-Jack
 (1)
 Fail Safe System

 A fail-safe tie bar goes through the center of the screw shaft and prevents axial separation of the screw shaft. The fail-safe ball screw-jack has two load paths. The primary load path transmits the load and the secondary path stays free of any load. If the primary load path is axially separated, the secondary path takes the load and prevents damage to the ball screw-jack. Therefore the two ends continue to turn if the screw shaft is fractured.

 (2)
 Ball Screw-Jack

 The ball screw-jack is made up of the ball screw and the fail-safe ball nut. The fail-safe ball nut has three transport guides, a threaded fail-safe unit wiper and ice chipper. The mechanical input shaft limits the range of travel of the screw-jack.

 (3)
 No-Back System

 The THS actuator has a no-back system that has a no-back brake which is of a ratchet and pawl type. The no-back brake holds the ball screw in its last position. It prevents movement of the ball screw under aerodynamic loads. A no-back wear detection device in the form of an indicator (no-back wear indicator) is installed on the lower side of the gear box. The indicator has a cam roller which faces the top edge of the claw-stop. This finds the gap which agrees with the wear limit to be detected. If a high wear does occur, a compressive external load causes the top claw-stop to move up to a higher position. This causes the top claw-stop to touch the cam roller of the indicator. This then makes the red finger of the indicator "pop" out and give a visual wear alarm.

 C. Control Valve Blocks
 The THS actuator has two control valve blocks. The two valve blocks control the operation of the THS actuator. Each valve block has:
 -a high pressure (HP) filter,
 -an inlet and outlet port,
 -two input control shafts,
 -
a control valve,

 -
a shut-off valve,

 -
a POB control valve,

 -
a control device for the shut-off valve.

 The control valve blocks supply pressure to the hydraulic motors and to the brake release piston. The control valve blocks can be installed on the left hand (LH) or on the right hand (RH) side of the THS actuator.
 D. Gearbox
 The gearbox is in a split housing. The two housing parts are made of a light metal alloy. The two housing parts are referred to as the upper casing and the lower casing. The upper casing holds the screw and no-back housing assembly. It also supports the two hydraulic motors, the mechanical input lever and the control position transducer. The lower casing supports:
 -
the PTA,

 -
the monitor position transducer,

 -
the two pressure-off brakes,

 -two hydraulic block valves,
 -the magnetic drain plug.
 In the gearbox is the oil which is necessary to lubricate the internal components. The level of the oil can be checked visually through an oil level sight glass. The oil level sight glass is on the upper casing.

 E. Pressure-Off Brakes
 Each hydraulic motor shaft has a POB. Each POB is located at the output shaft of their related hydraulic motor. The POB is a dry brake with a hydraulic release which is used to lock the shaft of the motor. The shaft of the motor is locked if a failure occurs in the hydraulic system or in a hydraulic motor. It thus lets the second motor fully control the ball screw through the power differential.
 F. Pitch Trim Actuator
 The pitch trim actuator (PTA) has:
 (1)
 Three brushless motors, each with an electro-magnetic clutch. In the normal mode, one clutch will be applied (energized) and the others de-clutched (de-energized). The output of the three motors is connected to move the input shaft through a reduction gear.
　
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