3-15
PROPULSION SYSTEM
The propulsion system is a closed-loop hydrostatic transmission. The variable volume
piston pump is driveline driven from the engine crankshaft. The pump can deliver up to
34 gpm (129 L/min) or 4,000 psi (276 bar) to the wheel drive motors. The flow from the
wheel drive motors returns directly to the pump inlet, not the reservoir. The built-in
charge pump maintains approximately 220 psi (15 bar) of pressure to the intake side of
the piston pump whenever the engine is running. Excess flow from the 9 gpm (34 L/min)
charge pump combines with case drain flow and goes through the cooler and returns to
tank.
When the transmission control switch in the operator compartment is in FORWARD, and
the accelerator pedal is depressed, the pump starts delivering oil to the wheel drive
motors for forward travel. When the accelerator is released, the pump blocks the flow
and the wheel drive motors stop. If this is done on level ground, the crane will stop and
stand still. If this is done on an incline, the crane will stop, but then creep down the
incline, due to oil leaking through the wheel drive motors. ALWAYS apply the parking
brake when the crane is stopped. When parking on a slope, also set the outriggers or
place wheel chocks on the downhill side of the wheels.
A positraction flow divider is located in the line between the pump and drive motors.
When a wheel is lifted off the ground, is on ice, or for any reason starts to spin, it will
momentarily take more oil than the wheel that has traction. When this happens, a spool
inside the positraction valve will shift and restrict the high flow to the spinning wheel,
forcing flow to the wheel with traction. If the unit stops moving, the engine speed will
have to be increased to resume travel.
The crane is equipped with an oil cooler that will allow it to operate properly when the
ambient temperature is slightly over 100
F (38°C). An oil temperature gauge is provided
just ahead of the steering wheel. The sensor for this gauge is located in the return line
to the oil tank. If this gauge reads above 200
F (93°C) , the unit should be shut down
and the reason for the excessive temperature located and corrected. The fins on the oil
cooler and engine radiator may be dirty and need cleaning.
The transmission speed is controlled by an electronic circuit that senses engine speed
and signals the transmission pump to deliver a proportional flow of oil to the wheel drive
motors. There is a manual operator lever on the control coil of the pump for testing.
A manual bypass valve is built into the piston pump. It appears like a shaft with a cross
hole in it on the side of the pump. If the machine cannot run under its own power, it can
be towed for very short distances at 5 mph (8 km/h) or less by turning the bypass valve
counterclockwise 4 turns. It must be turned back again as far as possible to run
normally.
CAUTION: The crane will roll freely down inclines with the bypass valve open. Keep
the valve closed except when towing and the crane is secured to the tow vehicle. An
operator must be in the crane to operate the brakes.
Courtesy of Crane.Market
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