TRACK DRIVE HYDRAULIC SYSTEM.
The track drive system is a hydrostatic system consisting
of two variable displacement propulsion pumps driven by
the engine through the pump drive gearbox. A pump
pilot control valve with integral electronic displacement
control is mounted on each propulsion pumps pilot
control port. The pump pilot control valve receives an
electric signal from the control handle on the operator
console gauge panel, which increases or decreases the
output of the propulsion pump. Each pump drives a two-
speed, variable displacement motor connected to a
gearbox with a 65: 1 reduction and built-in brake system.
One gearbox is connected to each track drive. Because
both tracks are identical, the following applies to either
The control handles receive 12 VDC from the circuit
breaker panel (CB1). Control handle position in either
potentiometer on a circuit board. Within the control
handle are two cam-activated micro switches which are
closed and opened when the control handles are moved
out of neutral detent. When the control handles are
moved out of neutral in either forward or reverse, a micro
switch closes, passing 12 VDC to bias the output
transistor on internal circuit boards. The signal then
passes though a variable resistor, which varies the
voltage signal, depending on control handle position on
the potentiometer. Output voltages from 3.6 to 1 volts
control reverse propulsion, and voltages from 4.0 to 7
volts control forward propulsion. This voltage is sent to
propulsion pump, controlling pump output.
The propulsion pump is a variable displacement pump,
supplying a constant pressure with variable flow rates.
The output from the propulsion pump is determined by
the position of the control handle. As the control handle
is moved, an increasing voltage is applied to the
displacement control on the pump. As the signal
increases, the displacement control changes the signal
to the propulsion pump. The variable displacement in
the pump increases or decreases the tilt (angle) of the
swashplate in the pump. As the pump turns, the
swashplate causes pistons to stroke in or out, forcing oil
out of the pump.
Hydraulic oil is drawn from the hydraulic reservoir into
the propulsion pump by an internal charge pump. The
charge pump is a fixed displacement pump within the
that supplies cool oil from the reservoir to the pump, and
supplies oil to operate the control system. The charge
pump ensures that the internal oil pressure of the main
pump stays constant at 350 psi (2400 kPa). An internal
relief valve controls this pressure. The charge pump
also circulates charge pressure oil from the pump,
through the charge filters, and back to the pump. During
operation of the pump and motor, a small amount of
hydraulic oil leaks from the main closed circuit and is
used to lubricate the rotating parts of the pump and
motor. A case drain to the hydraulic reservoir provides
relief of this oil. Hot oil flows from the low pressure side
of the hydraulic circuit to allow for cooling. The charge
pump replaces any oil that is lost to the reservoir through
the case drain.
Because the propulsion pump and motor are a closed-
loop system, no return other than the case drain is
required. The main ports of the pump are connected by
hydraulic lines to the input port of the motor. Hydraulic
oil flows in either direction from the pump to the motor
and then back to the pump. Crossover relief valves
protect the high pressure side of the pump by limiting
pressure to 5000 psi (34 500 kPa).
The hydraulic oil from the propulsion pump is sent to the
twospeed propulsion motor. The swashplate in the
motor is limited to two positions, providing two speeds.
As the oil flows through the motor, the volume of oil
against the swashplates forces the pistons to compress,
turning the motor.
The displacement of the propulsion motor is determined
by the two-position speed range switch on the operator
With the speed range switch set to down, "tortoise" pave
position, the charge pressure fluid is routed though the
displacement motor. This sets the motor swashplate to
13° for maximum displacement. When set to the up,
travel "hare" position, charge pressure fluid flow is
reversed in the high speed shift solenoid valve to set the
swashplate at 7 1/2° for minimum displacement.
The propulsion motor is coupled to a speed reduction
gearbox at a 65: 1 ratio. The output of the gearbox is
attached to the spring-actuated, hydraulically released
brakes. The brakes are sprocket-coupled to the paving
machine tracks. When hydraulic pressure is removed
from the propulsion system the brakes automatically
apply, stopping the paving machine.