7-119. The variable displacement hydrostatic pump
high-pressure and return functions of the main circuit
consist primarily of a drive shaft, charge pressure relief
valve, replenishing check valves, servo pistons,
7-126. When the transmission is in neutral, the
swashplate, cylinder block assembly and valve-bearing
swashplate in the hydrostatic pump is also in neutral and
the cylinder pistons are not stroking. In this mode no oil
is being pumped into the main circuit, therefore, the
7-120. The pump drive shaft is splined into the center of
cylinder block assembly in the motor is stationary and
the cylinder block. The cylinder block assembly contains
the motor output shaft does not turn.
nine (9) cylinder pistons. When the engine is running the
cylinder block and pistons are always rotating.
7-127. CHARGE PUMP.
7-1 21. Movement of the Direction-Throttle Control Bail
7-128. A charge pump is a part of the hydrostatic pump
causes the servo control circuit to act on the
assembly. Whenever the engine is running, oil is being
displacement control valves. These valves cause one or
drawn from the tank through the suction filter by the
the other of the two (2) servo pistons to move the pump
7-122. The pump swashplate can tilt either side of
7-129. CHECK VALVES.
center. When the swashplate in exactly centered, the
7-130. There are two (2) check valves in the hydrostatic
transmission is in neutral.
pump assembly. With the transmission in neutral, both
7-123. Each cylinder piston has a slipper plate at one
check valves will open allowing charge pressure to
end. The slipper retainer keeps all nine
lubricate the system. With the transmission out of
(9) slippers in contact with the thrust plate which
neutral, one of the valves will be closed and the other
contacts the swashplate. When the swashplate is
open. The valve on the high-pressure side of the main
centered, the cylinder pistons do not stroke in the
circuit will be held closed by high-pressure. The other
cylinders. When the swashplate is tilted, the cylinder
valve will admit oil from the charge pump to the low-
pistons stroke. The degree of tilt determines the volume
pressure side of the cylinder block assembly to maintain
(displacement) of oil pumped by each ,cylinder piston.
adequate oil in the loop.
7-124. The valve-bearing plate contains elon- gated
ports that are in contact with the cylinder block. The
7-131. SERVO CONTROL SYSTEM.
opposite side of the valve-bearing plate mates with the
7-132. Control of swashplate angle (displacement) is
pump end cap. Two ports in the end cap are connected
accomplished in the hydrostatic pump by use of a
through high-pressure lines to a hydrostatic motor.
powered servo system.
7-125. When the swashplate is tilted, each cylinder
piston moves back and forth in its cylinder with each
7-133. DISPLACEMENT CONTROL VALVE.
revolution of the cylinder block. When a piston rotates
7-134. The Displacement Control valve assembly is a
past the inlet ports in the valve-bearing plate, it moves
closed center four-way valve with the servo pressure
out of the block and draws low-pressure oil into the
ports exhausted at the center (neutral) position. The
cylinder. This is the intake stroke of the piston. Further
valve is operated through internal linkage connections
rotation of the cylinder block moves the filled cylinders
with both the swashplate and the external control
toward the outlet ports in the valve-bearing plate. They
(Direction-Throttle Control Bail).
are forced into the cylinder block by the tilt of the
7-135. To put the pump in stroke, the control handle
swashplate. The oil under high pressure ports to the
moves the displacement control spool through a torsion
outlet port in the end cap through the valve-bearing
spring. Once the swashplate angle is set, a feed-back
plate. With the swashplate tilted to either side of center,
link returns the displacement spool almost to neutral
one side will be forward vehicle motion with one main
where it ports just enough oil to the servo cylinder to
circuit line to the motor being high-pressure and the
keep the swashplate at the proper angle.
other being return. The opposite tilt of the swashplate
reverses the vehicle direction and it also reverses the