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B.  Each segment of the airstair consists of built-up side beams with permanently attached risers with removable treads. Handrails, guardrails, and stanchions are attached to the side beams of the airstair. The handrails raise to their erect position as the airstair unfolds to its down or extended position and retract as the airstair folds to its up or stowed position.
C.  A rotary hydraulic damper is installed at the upper end of the middle segment of the airstair. The damper is connected to the upper segment through a link and crank assembly. The damper prevents the airstair segments from bouncing during extension and retraction by restricting the velocity of the folding mechanism. The damper is a self-contained unit consisting of a housing, a rotary shaft splined at the drive end, a 2-cubic inch reservoir with an integral capacity indicator and a fill port. The reservoir is serviced with hydraulic fluid, MIL-H-5606.
D.  An emergency lock is installed to restrain the airstair from unfolding when sudden G-loads may be encountered. The locking device consists of a hook that is operated by a cam on the flange of the airstair hinge, and a pin attached to the middle segment of the airstair. The hook engages the pin when the airstair is retracted.
4.  Drive Mechanism
A.  The drive mechanism consists of a reversible three-phase 115 volt ac electrical power unit, which drives two interconnected gearboxes centrifugal governor and transmission units. (See figure 2.) The control of the drive mechanism electrical functions is accomplished by 28 volt dc power. (See figure 3.)
NOTE:  Both 115 volt ac and 28 volt dc electrical power are required to operate the drive mechanism.
500 
May 15/68  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  52-14-0 Page 3 

Aft Entry Door and Airstair Drive Mechanism  500 
52-14-0  Figure 2  Feb 20/85 
Page 4 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

500  Aft Entry Door and Airstair Circuit 
May 15/68  Figure 3  52-14-0 
Page 5 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  Power Unit
(1)  
The power unit consists of a three-phase ac induction motor and a rotary actuator, which comprises a planetary gear set, torque limiting device, and a disconnect clutch. These components are housed within an aluminum casing.

(2)  
AC Motor

(a)  The ac motor is a bi-directional squirrel cage, induction type. The motor operates at 965 (+125/-100) RPM and is capable of driving the actuator under all operating-load conditions. The motor housing is vented to the atmosphere to eliminate possible moisture condensation.

(3)  
Rotary Actuator

(a)  The rotary actuator is powered by the bi-directional, ac motor. Motor output is through a torsion bar pinion shaft. The pinion serves as the sun gear in a three-planet gear system. Equally spaced around the sun gear, and meshing with it, are three planet gears, which are supported by a driven cage. Meshing with all planet gears is a common, fixed ring gear. The driven cage drives the input shaft of the torque limiting device. All planetary gears are mounted on needle bearings.

(4)  
Torque Limiting Device and Disconnect Clutch

(a)  
The torque limiting device is functionally designed to protect the aircraft structure from stalled ac motor loads. The device is comprised of a number of clutch discs. One set of discs are made of steel and splined internally to mate with the output of the planetary gear system cage. The other set of discs consist of steel plates with sintered friction material facings, and are splined externally to the output clutch element. This clutch disc pack is held under spring pressure. When the load torque is greater than the clutch setting, the spring pressure is relieved by a ball detent sensor; and the clutch discs slip.

CAUTION:  THE TORQUE LIMITING DEVICE IS FUNCTIONALLY INOPERATIVE WHEN OPERATING THE AIRSTAIR IN THE MANUAL MODE OR WHILE AIRSTAIR MOVEMENT IS RESTRICTED IN TRANSIT DURING POWERED OPERATION. DO NOT ATTEMPT TO RESTRICT AIRSTAIR MOVEMENT WHILE OPERATING OR DAMAGE MAY OCCUR TO THE AIRPLANE STRUCTURE OR THE AIRSTAIR.

(b)  
The output of the torque limiter is coupled to the rotor of an electrically-operated, single disk, dry operating friction clutch. When power is applied to the ac motor, the clutch rotor and armature engage to drive the gearbox. The clutch is engaged by an electromagnetic field generated by rectified ac. One phase of the three-phase ac applied to the motor is used to generate the magnetic field. Therefore, as long as power is applied to the ac motor, the clutch will remain engaged. When power is interrupted, the clutch is disengaged by spring force.
　
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