TM 9-2350-256-34
INTERMEDIATE MAINTENANCE
RECOVERY VEHICLE, FULL TRACKED: MEDIUM, M88A1
MAIN HYDRAULIC SYSTEM MAINTENANCE
INITIAL SETUP:
References
TM 9-2350-256-10
GENERAL
The main hydraulic system powers and controls the main winch, hoist winch, hoisting boom, and spade. The main winch and
hoist winch are driven by vane-type hydraulic motors, and the hoisting boom and spade are actuated by hydraulic cylinders.
Hydraulic fluid, under pressure for operating the hydraulic motors and hydraulic cylinders, is provided through a closed power
circuit by a fixed-displacement, vane-type, double pump driven by a mechanical transmission (power takeoff) from the main
engine. The hydraulic pump draws its fluid from the hydraulic oil tank, and pump output is controlled by engine speed. The
operation of the hydraulic motors and the hydraulic cylinders is controlled through the control circuit. Each winch hydraulic
motor is provided with a pilot-operated, spring centered, directional combination control valve which directs the fluid in the
power circuit to the winch being operated. These directional control valves are remotely controlled by manually operated
hydraulic control valves, which direct control pressure to them. The directional combination control valves for the spade and
boom cylinders are manually controlled. Any internal leakage through the hydraulic motors, cylinders, or valves is returned to
the hydraulic oil tank by tank return. Description of operation of the basic circuits in the main hydraulic system will be found
below. Schematic diagrams of the main hydraulic system idling, and the hydraulic system operating the main winch, hoist winch,
NOTE
Refer to TM 9-2350-256-10 for operating instructions for the winches, power takeoff, and hoist system.
With the main engine operating at system idling speed, the power control valve is placed in the ON position, directing mechanical
transmission pump pressure to the clutch (port no. 228), thus engaging the clutch which permits the mechanical transmission to
drive the main hydraulic pump. The pump draws filtered fluid from the hydraulic oil tank (port no. 69) and discharges it through
two outlet ports (port nos. 1 and 44) under pressure. The pump discharge pressure is directed to the relief and unloading valve
(port no. 2) and the hoist winch combination control valve (port no. 27). Pump discharge pressure passes through the relief and
unloading valve (port no. 3), through the hydraulic subplate (port nos. 136 and 125) of the main hydraulic control panel, and to
the system selector valve (port no. 45) which is placed in the MAIN position. The pump pressure is then directed to, and blocked
at the spade and boom combination control valves (port no. 33), as they are in their normal spring-centered HOLD position.
Differential pressure across the pilot pressure valve, in the main relief valve, forces the valve open to allow a parallel flow (port
no. 4) of pump discharge pressure to be directed to the hoist winch combination control valve (port no. 22). Differential pressure
across the pilot pressure valve, in the hoist winch combination control valve, forces the valve open, directing the pump pressure
(port no. 20) to the main winch combination control valve (port nos. 12 and 13) where the action duplicates the hoist winch
combination control valve and the hydraulic fluid is returned to the oil tank (port no. 11). Hydraulic pressure is directed to both
sides of the winch motors (port nos. 17 and 18) by the spring-centered directional control valve in the winch combination control
valves.
With the main winch control valve placed in the PAYOUT position, the pilot pressure (port nos. 15 and 16) tends to equal pump
discharge pressure and is directed to both ends of the pilot-operated directional control valve (port nos. 8 and 10) in the main
winch combination control valve. The ends of the directional control valve differ in size, the larger end at port no. 10 and the
hydraulic differential across the valve causes it to move, directing the flow of pump pressure to one side of the main winch motor
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