TM 5-3895-346-14
HYDROSTATIC SYSTEM
HYDRAULIC CIRCUITS
CHARGE PUMP CIRCUIT
Fluid flows from the reservoir through a filter to the inlet of the charge pump mounted on the main pump which
is driven at pump shaft speed. The purpose of the charge pump is to provide a flow of fluid through the
transmission for cooling purposes, to supply fluid under pressure to maintain a positive pressure on the low-
pressure side of the main pump/motor circuit, to provide sufficient fluid under pressure for control purposes,
and for internal leakage makeup.
MAIN PUMP AND MOTOR CIRCUIT
Fluid from the charge pump is directed to the low-pressure side of the main circuit by means of one of two
check valves. The second check valve is held closed by the fluid under high- pressure on the other side of the
main circuit.
Fluid flows in the main circuit in a continuous closed loop. The quantity of fluid flow is determined by pump
speed and displacement while direction of flow is determined by the swashplate angle from neutral.
A manifold valve assembly, connected across the main circuit, includes elements essential to provide the
proper operation of the transmission. The manifold valve contains two pilot- operated, high-pressure relief
valves which serve to prevent sustained abnormal pressure surges in either of the two main hydraulic lines by
dumping fluid from the high-pressure line to the low-pressure line during rapid acceleration, abrupt braking and
sudden application of load.
Also provided in the manifold valve as- sembly is a shuttle valve and a charge pressure relief valve. The
shuttle valve functions to establish a circuit between the main line that is at low pressure, and the charge
pressure-relief valve to provide a method of controlling the charge pressure level and also a means of
removing the excess cooling fluid added to the circuit by the charge pump. The shuttle valve is spring centered
to a closed position so that during the transition of the reversing of pressures in the main lines, none of the
high-pressure fluid is lost from the circuit.
COOLING CIRCUIT
Excess cooling fluid from the manifold charge pressure-relief valve enters the motor case, then flows through
case drain lines to the pump case, through the pump case and heat exchanger to the reservoir. The heat
exchanger bypass valve is used to prevent high-back pressure at the heat exchanger due to cold fluid or a
restricted heat exchanger.
During periods of operation when the main pump is in neutral, the shuttle valve will be centered and the excess
flow from the charge pump is directed to the cooling circuit by the neutral charge relief valve in the charge
pump. When operating at this condition, cooling flow is not admitted to the motor case, but through the pump
case and heat exchanger to the reservoir.
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