8-7
RT770E UNDERCARRIAGE
Published 11/26/2014, Control # 447-05
STEERING SYSTEMS
Description
To maximize maneuverability, the crane can be steered by
the front axle, the rear axle, or by the front and rear axles
simultaneously. The crane utilizes two separate steering
systems, one to control front axle steering and one for rear
axle steering.
Front Steering System
The front steering system consists of a hydraulic pump, load
sense steer priority flow divider valve (part of the swing
directional control valve), load sense steering control valve,
and two steer cylinders. The hydraulic pump is driven by the
engine and supplies a hydraulic flow of 122.6 L/min
(32.4 gpm) to the load sense steering priority flow divider
valve. The load sense steer priority flow divider valve
provides 34.1 L/min (9 gpm) to the load sense steering
control valve and 80.2 L/min to 114.3 L/min (21.2 gpm to
30.2 gpm) to the swing system. When the steering wheel is
turned, the load sense steering control valve sends a load
sense signal to the load sense steer priority flow divider. As
the load sense pressure increases, the priority flow divider
spool shifts to direct oil from the hydraulic pump to the
steering control valve, and to direct oil from the L port and R
port of the steering control valve to the steer cylinders.
Rear Steering System
The rear steering system is controlled through a section of
the integrated outrigger/rear steer valve and consists of the
control valve and two steer cylinders. A rear steer indicator
system is provided to indicate when the rear wheels are not
centered. This system consists of an indicator light located
on the front console in the cab and a switch located on the
left side of the rear axle. When the rear wheels are turned to
the left or right, the amber indicator light will illuminate.
Secondary Steering System (CE Units)
The secondary steering system is provided to back up the
normal front steering system if loss of hydraulic flow occurs.
The system consists of an hydraulic accumulator, two check
valves, a pressure switch, and a pilot operated, 2 position 3-
way valve used in conjunction with the service brake dual
accumulator charge valve and the load sense steering
control valve.
Theory Of Operation
Front Steering System
A hydraulic oil flow from pump number 2 flows through port 5
of the hydraulic swivel to the steer/swing directional control
valve. Regulated flow from the steer priority flow divider
valve is routed to the steering control valve upon turning of
the steering wheel and pressurizing of the load sense line.
The steering control valve routes the oil to the appropriate
ends of the front steer cylinders to turn the wheels in the
desired direction.
Rear Steering System
Hydraulic flow from the second section of pump number 1 is
directed through the boost selector valve to the integrated
outrigger/rear steer valve. When the Rear Steer Control
Switch in the cab is positioned to left (L) or right (R), a signal
is sent to the rear steer solenoid, shifting the control valve
spool, routing the supply pressure to the appropriate ends of
the rear steer cylinders to turn the wheels in the desired
direction.
Secondary Steering System (CE Units)
When the engine is running, the load sense steering priority
flow divider valve maintains a constant 862 kPa (125 psi)
standby pressure in the steering load sense line. When a
minimum 862 kPa (125 psi) load sense pressure is
maintained in the pilot circuit, the pilot operated, 2 position,
3-way valve ports are aligned to charge the secondary
steering hydraulic accumulator. The hydraulic accumulator is
charged from the switch (SW) port on the service brake dual
accumulator charge valve when the engine is running, A
check valve is installed in the pressure line to prevent back
flow from the secondary steering hydraulic accumulator to
the service brake dual accumulator charge valve and the
service brake pressure switch.
When flow from the steer pump stops for any reason, the
load sense pressure will drop to zero. When the load sense
pressure drops below 862 kPa (125 psi), the pilot ports so
hydraulic fluid stored in the secondary steering hydraulic
accumulator will flow to the load sense steering control
valve. The secondary steering hydraulic accumulator charge
will allow the operator to safely steer the crane to a safe stop.
When the load sense pressure drops below 689 kPa
(100 psi), the secondary steer pressure switch will close its
contacts and energize the red Engine Distress Indicator and
sound the warning buzzer.
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