SUNDSTRAND HYDROSTATIC TRANSMISSION
ground conditions which establish the demand on the
system. The pump and motor are contained in separate
housings and are connected by high-pressure hoses. All
7-86. This section contains a description of the
valves required for a closed loop circuit are included in
Sundstrand hydrostatic transmission, transmission
either the pump or motor assemblies. A reservoir, filter,
components and their function in their hydraulic circuits.
Service instructions are given that include removal of
34 and 7-35 illustrate the system in neutral, forward and
components, disassembly, inspection, reassembly and
7-87. Checks and adjustments are presented in this
7-95. CHARGE PUMP CIRCUIT.
section. Design and performance specifications are
presented in Section 2. The troubleshooting guide is
7-96. CHARGE PUMP.
7-97. A charge pump is a part of the hydrostatic
pump assembly. When the engine is running, the
charge pump provides oil supply, cooling and
7-89. The closed loop hydraulic system consists of two
pressure for control functions.
subsystems: the hydrostatic transmission subsystem
7-98. The charge pump draws oil from the hydraulic
and the steering subsystem. The two subsystems share
tank, through the suction filter to supply oil, under
hydraulic oil and hydraulic tank. These components are
pressure, to the low-pressure side of the main circuit.
presented in this section. Refer to the steering section
for the remaining coverage of the steering hydraulic
7-99. NEUTRAL CHARGE PRESSURE RELIEF
7-90. HYDRAULIC OIL.
7-91. Specifications for hydraulic oil are given in Section
7-100. When the transmission is in neutral, charge
pump pressure is controlled by the neutral charge
7-92. HYDRAULIC SYSTEM COMPONENTS
pressure relief valve. This valve is an integral part of the
charge pump and has no adjustments that can be made
(see figures 7-33, 7-34 and 7-35).
in the field. When charge pressure exceeds the range of
190 to 220 PSI (13.36-15.47 kg/cm2) above case
pressure, the neutral charge pressure relief valve opens
allowing oil to flow into the pump case, oil cooler and
7-94. The hydrostatic transmission offers infinite control
of speed and direction. Control of the variable
7-101. SUCTION FILTER.
displacement, axial piston pump and the fixed
displacement motor is the key to controlling the unit.
7-102. A low-pressure filter is mounted on the hydraulic
When the variable pump swash plate is tilted, by
tank. It serves as the suction filter for the transmission
movement of the Direction-Throttle Control Bail, a
inlet oil. It is a 10 micron filter. There is no by-pass
positive stroke to the pistons is created. This, at any
valve on the suction filter.
given input speed, produces a flow from the pump. This
7-103. COOLING CIRCUIT.
flow is transferred through high-pressure lines to the
motor. The ratio of the volume of flow from the pump to
the displacement of the motor will determine the speed
7-104. The cooling circuit maintains a flow of oil through
of the motor output shaft. Movement of the Direction-
the transmission for cooling purposes. Excess oil from
Throttle Control Bail to the opposite side of neutral
the low-charge- pressure relief valve enters the motor
causes the flow from the pump to reverse and the motor
housing. Flow is then directed through drain lines to the
output shaft turns in opposite direction. Speed of the
pump housing. The circuit is completed from the pump
output shaft is controlled by adjusting the displacement
housing through the oil cooler to the oil tank. When the
(flow) of the transmission. Load (working pressure) is
hydrostatic pump is in
determined by external conditions such as grade and