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 TM 55-1925-273-10-1
0039 00
CAPSTAN
The capstan (figure 6) receives its hydraulic power from the central Hydraulic Power Unit (HPU).
Both the supply (figure 6, item 1) and return (figure 6, item 2) lines flow through the capstan control valve
(figure 6, item 3). When the capstan control valve is in the NEUTRAL position, the supply line is CLOSED, and no supply
flow is permitted to enter the capstan motor (figure 6, item 4). At the same time, both sides of the capstan motor are OPEN
to the return piping. When the capstan control valve is shifted toward the return spring, the supply is routed to the right side
of the capstan motor (figure 6, item 5) and return flow comes from the left side of the capstan motor (figure 6, item 6),
causing capstan motor rotation. When the capstan control valve (figure 6, item 3) position is reversed, flow to the capstan
motor is reversed, reversing the direction of motor rotation.
Supply flow is controlled by a flow control valve (figure 6, item 7) that permits a maximum of 15 gal/min (57 L/min) to enter
the capstan control valve (figure 6, item 3). This flow control valve prevents overspeed operation of the capstan.
Counterbalance valves (figure 6, item 8) are installed between the capstan control valve (figure 6, item 3) and the capstan
motor. The counterbalance valves act as a hydraulic brake to prevent the capstan from overrunning the hydraulic motor. The
counterbalance valves also ensure that the capstan motor (figure 6, item 4) will only turn when pressure is applied from the
capstan control valve.
A fail safe brake (figure 6, item 9) is fitted between the capstan's gear train and the capstan motor (figure 6, item 4). This brake
is a spring-applied, pressure-released design. In this design, an internal spring assembly applies the brake until hydraulic pressure
is applied to release it. In the capstan circuit, hydraulic pressure is supplied to the fail safe brake assembly any time that the
capstan control valve (figure 6, item 3) is moved away from the NEUTRAL position. When the capstan control valve (figure 6,
item 3) is returned to the neutral position, the pressure is removed from the fail safe brake and the springs apply the brake,
providing a mechanical braking action to the capstan motor output shaft.
A shuttle valve (figure 6, item 10) is provided to prevent fail safe brake pressure from returning directly to the reservoir at the
counterbalance valves (figure 6, item 8).
The capstan motor (figure 7, item 1) drives the capstan through a double reduction planetary gear assembly (figure 7, item
2). The fail safe brake (figure 7, item 3) is fitted between the capstan motor and the double reduction planetary gear
assembly. The double reduction planetary gear assembly decreases the speed and increases the torque of the capstan motor
output shaft and transmits that torque to the double reduction planetary gear assembly output shaft (figure 7, item 4) and its
attached pinion gear (figure 7, item 5). The pinion gear, in turn, drives the capstan ring gear (figure 7, item 6), further
reducing output speed and increasing output torque. The ring gear is directly connected to the capstan drum (figure 7, item
7), thus ring gear rotation yields capstan drum rotation.
0039 00-10
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