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 TM 55-1925-273-10-1
0039 00
TOW PINS
During operation under usual conditions, the tow pins are powered by the central hydraulic system.
Each tow pin is raised and lowered by its own hydraulic cylinder (figure 11, item 1). The cylinders are extended or retracted
by supplying pressurized hydraulic fluid to the rod side (retract) or to the piston side (extend) of the hydraulic cylinder.
Movement of the hydraulic cylinders (figure 11, item 1) is controlled by solenoid operated directional control valves
(figure 11, item 2), one valve for each cylinder. When the directional control valve is in the NEUTRAL position, the
supply line is blocked and both sides of the hydraulic cylinder are OPEN to the reservoir (return). When the hydrau-
lic control valve is shifted to the right (right solenoid energized), hydraulic pressure is directed to the piston side of
the hydraulic cylinder and the cylinder extends. During extension, the hydraulic fluid forced out of the rod side of
the hydraulic cylinder passes through the directional control valve and back to the reservoir.
When the directional control valve (figure 11, item 2) is shifted to the left (left solenoid energized), hydraulic pressure is
directed to the piston side of the hydraulic cylinder (figure 11, item 1) and the cylinder retracts. Retraction of the cylinder is
further controlled by the holding valve (figure 11, item 3). Holding valve operation is explained below.
In order to prevent uncontrolled or undesired retraction of the tow pins, each hydraulic cylinder (figure 11,
item 1) is provided with a holding valve (figure 11, item 3). The holding valve prevents the cylinder from retracting until
pressure is supplied to the rod side of the cylinder. When no pressure is applied to the rod side of the cylinder, a check valve
(figure 11, item 4) inside the holding valve prevents return hydraulic fluid from exiting the piston side of the cylinder. If this
return hydraulic fluid cannot exit, the cylinder cannot retract. When pressure is applied to the rod side of the cylinder, a
portion of this pressure is routed through the internal pilot line (figure 11, item 5) to OPEN the holding valve. Once the
holding valve is OPEN, return flow is permitted from the piston side of the cylinder and retraction occurs.
Supply to each pair (port and starboard) of tow pin cylinders (figure 11, item 1) is restricted by a flow control valve
(figure 11, item 6). The flow control valve ensures that the tow pin cylinders extend and retract at a controlled rate
of speed.
0039 00-18
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