27-21-00
 ALL  ú ú 01 Page 9 ú Oct 25/84
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 7.  Rudder Trim Control Mechanism
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 A.  The rudder trim control mechanism provides a means for controlling the directional trim of the airplane and indicates the units of rudder trim (Fig. 4). The control mechanism consists of a rudder trim knob and rudder trim indicator on the aft end of the control stand.
 B.  The trim indicator is driven electrically by a transmitter in the rudder trim actuator on the feel and centering mechanism. The indicator shows up to 17 units of left or right trim. Each unit represents approximately one degree of rudder trim. The indicator is located on panel P8 next to the rudder trim knob and is powered by 115 volts ac from bus XFR BUS-1. The RUDDER TRIM control circuit breaker is located on circuit breaker panel P6.
 C.  The rudder trim knob is located on center console panel P8 and activates arming and control switches which direct electrical input to the rudder trim actuator motor. The knob is spring-loaded to return to neutral. The RUDDER TRIM circuit breaker is located on circuit breaker panel P6.
 8.  Rudder Trim Actuator
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 A.  The rudder trim actuator consists of an acme screw driven ram, coupled to a motor-brake and position sensor (RVDT) through a gear reduction, and limit switches enclosed by an aluminum housing. The motor drive is controlled by the rudder trim knob or by activation of an internal limit switch. An electromagnetic brake prevents actuator overrun and internal mechanical stops prevent overtravel in case of a limit switch malfunction.
 B.  Trim commands from the trim knob cause the actuator to extend or retract, which rotates the feel and centering mechanism. This provides a new zero force pedal position corresponding to the trimmed rudder position. The actuator provides the ground point for the feel and centering mechanism.
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 27-21-00
 ALL  ú ú 02 Page 10 ú Mar 15/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 9.  Rudder Feel and Centering Mechanism (Fig. 5)
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 A.  The rudder feel and centering mechanism consists of the feel and centering unit which provides artificial feel to the pilots' pedals and centers the rudder. The feel and centering unit is located below the rudder power unit in the vertical fin (Fig. 4). The feel and centering unit has a support shaft, a feel and centering crank, two frames, an arm and roller with a spring assembly, and a cam. The support shaft is bearing mounted on structure. The feel and centering crank is fixed to the support shaft and is connected through the trim and feel rod to the lower crank on the torque tube. The two frames are bearing mounted on the support shaft. The arm and roller and the spring assembly attach to and rotate with the two frames. The cam is fixed to and rotates with the support shaft. The rod end of the rudder trim actuator is supported by a fitting on the forward end of the two frames. When the rudder pedals are displaced, the torque tube rotates causing offset of the trim and feel rod and in turn rotation of the feel and centering crank, the support shaft, and the cam. As the cam rotates, the arm and roller are displaced out of the detent position to extend the spring assembly and provide artificial feel. Rudder trim input through the rudder trim actuator rotates the two frames with the arm and roller and spring assembly. The force of the spring assembly holds the arm and roller in the cam detent and causes the cam to rotate. Rotation of the cam causes subsequent motion of the support shaft, the feel and centering crank, the trim and feel rod, the lower crank and the torque tube which then causes input to the rudder power unit, repositioning the rudder. At the same time, the lower crank causes input to the rudder control system with movement of the pilots' pedals.
 10. Rudder Power Control Unit (Fig. 6)
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 A.  The rudder power control unit moves the rudder right or left when actuated by rudder pedal input and provides wind gust snubbing when the airplane is parked. The unit is located in the vertical fin (Fig. 1). The body is fixed to fin structure and the power head to the rudder. The control unit is separately powered by hydraulic systems A and B. Either system acting alone will provide full rudder control.
　
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