VARIABLE DISPLACEMENT PUMP
FIGURE 2
the excess cooling fluid added to the circuit by the
CHARGE PUMP CIRCUIT
charge pump. The shuttle valve is spring centered to a
Fluid flows from the reservoir through a filter to the inlet
closed position so that during the transition of the
of the charge pump mounted on the main pump which
reversing of pressures in the main lines, none of the
is driven at pump shaft speed. The purpose of the
high pressure fluid is lost from the circuit.
charge pump is to provide a flow of fluid through the
COOLING CIRCUIT
transmission for cooling purposes, to supply fluid under
Excess cooling fluid from the manifold charge pressure
pressure to maintain a positive pressure on the low
relief valve enters the motor case, then flows through
pressure side of the main pump/motor circuit, to provide
case drain lines to the pump case, through the pump
sufficient fluid under pressure for control purposes, and
case and heat exchanger to the reservoir. The heat
for internal leakage makeup.
exchanger by-pass valve is used to prevent high back
MAIN PUMP AND MOTOR CIRCUIT
pressure at the heat exchanger due to cold fluid or a
Fluid from the charge pump is directed to the low
restricted heat exchanger.
pressure side of the main circuit by means of one of two
During periods of operation when the main pump is in
check valves. The second check valve is held closed
neutral, the shuttle valve will be centered and the
by the fluid under high pressure on the other side of the
excess flow from the charge pump is directed to the
main circuit.
cooling circuit by the neutral charge relief valve in the
Fluid flows in the main circuit in a continuous closed
charge pump. When operating at this condition, cooling
loop. The quantity of fluid flow is determined by pump
flow is not admitted to the motor case, but through the
speed and displacement while direction of flow is
pump case and heat exchanger to the reservoir.
determined by the swashplate angle from neutral.
DISPLACEMENT CONTROL OPERATION
A manifold valve assembly, connected across the main
The Sundstrand Heavy Duty Variable Pumps
circuit, includes elements essential to provide the proper
incorporate a powered servo system to control the
operation of the transmission. The manifold valve
swashplate position with a correspondingly low operator
contains two pilot operated high pressure relief valves
effort.
which serve to prevent sustained abnormal pressure
The valve assembly is a closed center four-way valve
surges in either of the two main hydraulic lines by
with the servo pressure ports exhausted at the center
dumping fluid from the high pressure line to the low
position. The valve is operated through internal linkage
pressure line during rapid acceleration, abrupt braking
connections with both the swashplate and the external
and sudden application of load.
control handle. Refer to figure 3, page 51.
Also provided in the manifold valve assembly is a
shuttle valve and a charge pressure relief valve. The
shuttle valve functions to establish a circuit between the
Cont'd. on page 51
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
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