plate to tilt. Oil, expelled by the opposing servo
cylinder. returns through the displacement control
valve (FNR valve) to the pump case.
As the swash plate reaches the tilt set by the speed
control lever, the displacement control valve (FNR
valve) spool returns to a NEUTRAL position, trapping
the oil to both servo cylinders and holds the swash plate
in its titled position. The swash plate will remain titled
until the operator moves the speed control lever.
With the pump drive shaft and cylinder block
rotating clockwise and the swash plate is titled to the
rear, it is now time to start pumping. As the cylinder
rotates past the pump inlet port, the inlet check valve
opens: oil is then forced by the charge pump into the
piston bores that align with the inlet port under low
charge pressure. As rotation continues, oil is forced out
of the outlet port at high pressure by the pump pistons
when they align with the outlet port. This flow of oil
drives the motor.
The distance the pistons reciprocate in and out of
the cylinder block depends on the angle of the swash
plate of the pump. This determines the volume of oil
displaced per revolution of the pump. The greater the
angle, the greater the volume and the more oil flows
from the pump. As the angle of the swash plate is
varied so will the volume of oil displaced from the
As pressure oil enters the inlet port of the motor,
the pistons that align with the inlet port pushes against
the swash plate. Since the fixed swash plate is always
tilted, the pistons slide down the inclined surface and
the resulting forces rotate the cylinder block. This, in
turn. rotates the output shaft driving the machine
As the cylinder block continues to rotate
clockwise, oil is forced out the outlet port at low
pressure and returns to the pump where it is
recirculated through the pump and back to the motor.
This is called a closed system because the oil
keeps circulating between the pump and the motor.
The only extra oil comes from the charge pump that
maintains a given flow of oil through the system
whenever the machine is running.
A shuttle valve, located in the motor manifold and
controlled by high oil pressure, prevents high oil
pressure from entering the low-pressure side of the
system. This action keeps the charge circuit open to the
low-pressure valve while the system is running.
The high-pressure relief valve, located in the
motor manifold, monitors the pressure of the forward
flow of oil and protects the system from too high
pressures. If pressure exceeds the rated psi, a relief
valve opens and oil bypasses the cylinder block in the
motor. This will either slow or stop the machine. The
bypassed oil returns to the pump. This action continues
until the load is reduced below the rated psi. Then the
relief valve closes and oil again flows to the cylinder
block, moving the machine forward.
NEUTRAL (fig. 6-16).With the speed control
lever in neutral, free oil flows from the reservoir
through the oil filter to the charge pump. The charge
pump pumps the oil past the high charge pressure
control valve and into the main pump housing. The oil
circulates through the housing and returns through the
oil cooler and back to the reservoir.
Trapped oil is held in the cylinder block of the
pump, in the motor, and in the connecting lines
between the pump and motor by two check valves in
the pump end cap.
When the control lever is in neutral, the swash
plate in the pump is also in neutral and the pistons
within the pump are not pumping. Therefore no oil is
being moved to provide either forward or reverse
The cylinder block in the pump rotates in a
clockwise direction and is driven by the engine of the
equipment. Rotation is viewed from the drive shaft end
of the pump. Because the oil is not being pumped to the
motor, the cylinder block in the motor is stationary and
the output shaft does not move.
With the drive system in neutral, the high
charge pressure control valve, (located at the
charge pump) controls pump pressure. When
the system is activated for reverse or forward,
the low charge pressure control valve located
in the motor manifold controls the charge
pressure at a lower psi.
REVERSE (fig. 6-17).As the speed control
valve is moved to reverse, the spool in the
displacement control valve (FNR valve) moves out of
neutral allowing pressure oil to flow into the lower
servo cylinder, tilting the swash plate forward.
When the swash plate reaches its desired tilt,
which is set by the control lever, the displacement
control spool returns to neutral. This action traps the