HOIST WINCH ASSEMBLY
The hoist winch assembly (Figures 4 and 5) is installed in the hull of the vehicle beneath the crew compartment. It is used
for lifting objects and for guiding and hauling the main winch cable. Major components of the assembly are a winch cable,
hoist winch combination control valve, counterbalance valve, reversible hydraulic motor, brake cylinder and winch assembly
complete with an internal brake.
Hoist Winch Combination Control Valve
The hoist winch combination control valve (Figure 5) is mounted on the side of the hoist winch counterbalance valve, which
is mounted to the hydraulic motor manifold. It is a three-position, balanced-center valve, and serves to direct hydraulic flow
through the counterbalance valve and manifold to the hydraulic motor, thus determining the direction of motor rotation.
Hoist Winch Counterbalance Valve
The hoist winch counterbalance valve (Figure 5) is located between the hoist winch combination control valve and the hydraulic
motor manifold. The function of the valve is to prevent a possible overspeed of the hoist winch motor and also prolong the life of
the hoist winch brake.
Hoist Winch Hydraulic Motor
The hydraulic motor (Figure 5) is mounted to the manifold at the rear of the hoist winch. It is a vane-type, reversible motor which
serves to convert hydraulic energy into rotary motion and torque. This torque is then transmitted to the hoist winch. Hoist winch
hydraulic motor operation is the same as main winch hydraulic motor operation.
Hoist Winch Gear Train Operation
When the hoist winch control valve is in the RAISE position, the hydraulic motor rotates in the counterclockwise direction
(Figure 6) and drives the winch bevel gear and pinion gear, causing them to rotate counterclockwise. (Gear direction of rotation
is determined by viewing the hoist winch from the brake assembly side.) The pinion gear is splined to the pinion shaft and thus
drives the shaft in the same direction. As the pinion shaft rotates, the geared end of the shaft engages the low speed gear and the
pinion gear engages the high speed gear, both of which rotate freely around the brake shaft. Between the high and low speed
gears, and splined to the brake shaft, is the gear clutch. The external teeth of the gear clutch are meshed with the internal teeth
of the gear coupling. This coupling is positioned either left or right in order to couple the gear clutch with either the high or low
speed gear, thus causing the brake shaft to rotate in the clockwise direction. The splined end of the brake shaft drives the brake
drum assembly, while the gear end of the shaft meshes with and drives the drum gear in a counterclockwise direction. The drum
gear then rotates the cable drum in a counterclockwise direction to raise the winch cable. Placing the hoist winch control valve in
the LOWER position causes the hydraulic motor to rotate in a clockwise direction. All gears then rotate in the opposite direction
from that described above and the cable drum rotates clockwise to lower the winch cable.
Hoist Winch Brakes
The hoist winch brake cylinder (Figure 4) is located on the brake housing at the front of the hoist winch. Within the cylinder is a
piston which is spring loaded to the applied position. The piston exerts force on the brake drum to prevent drum movement in the
HOLD position. The piston is backed off and the brake released by introducing hydraulic pressure into the brake cylinder. The
hoist winch brake (Figure 6) consists of a circular brake band which makes contact with the brake drum, and a ratchet and pawl
assembly which allows the brake drum to rotate only in the counterclockwise direction. The brake band is held in its normally
applied position by a spring-loaded piston in the brake cylinder. The brake is released by introducing hydraulic fluid into the
brake cylinder to overcome the spring force. When the cable is raised, the brake band is in its normal applied position and the
brake drum which is not secured to the brake shaft, is held stationary. The two pawls, which are secured directly to the brake
shaft, rotate freely in a clockwise direction, thus enabling the cable drum to reel in and raise the cable. When the control valve
is in the HOLD position, hydraulic motor power is cut off, cable drum rotation ceases and the brake band remains in its normal
applied position. Any weight on the cable is held suspended by the pawls which engage the ratchet teeth on the inside diameter
of the brake drum and prevent cable drum movement. The cable is lowered by introducing hydraulic pressure into the brake
cylinder to counteract the piston spring pressure and release the brake band. The entire brake drum assembly is then free to rotate
in a counterclockwise direction even though the pawls within the brake drum are engaged with the ratchet teeth.