Grove Published 11-10-2014, Control # 524-00 2-5
RT880E SERVICE MANUAL HYDRAULIC SYSTEM
THEORY OF OPERATION
Three different control systems are used in the hydraulic
system; load sense (LS), constant pressure unloaded
(CPU), and open center (OC).
Hoist, lift, telescope, cab tilt, and counterweight removal are
constant pressure unloaded (CPU) functions. They are
supplied oil flow via a variable displacement supercharged
piston pump. CPU functions have directional spools that
meter the required flow and pressure to the function. The
pump delivers only the required flow at the pumps
compensator setting.
Outriggers, rear steer, service brakes, oil cooler motor,
telescope hose reel motor/brake controller supply and swing
brake release are load sense (LS) functions.They are
supplied oil flow by the same pump as the CPU functions.
The LS functions are in parallel with the CPU valves. LS
signal shuttle valves send the highest load signal of all the
functions that are activated. The function’s spools or orifices
meter the required flow to the function. The pump will deliver
only the required flow up to the compensator setting. When a
load sense function is operated at the same time as a
constant pressure unloaded function, the pump stays at it’s
compensated setting.
The swing function is the only open center (OC) function.
Swing is supplied with oil from a single section gear pump.
Front steer is also supplied with oil from this gear pump and
is prioritized to take any required flow before swing. Front
steer acts like a load sense function because of the priority
valve.
Both load sense and constant pressure unloaded functions
utilize a signal control line that loops through all valves
common to the pump by the shuttle valves. When using a LS
function, the highest signal PSI up to the pumps
compensator setting is sent to the variable displacement
piston pump. When a CPU function is activated, the signal
line being drained across the directional control valve spool
is blocked, causing the pump to rise in pressure to its
compensated pressure setting.
A load signal dump valve prevents the pump from coming on
stroke during engine start. It is activated by the ignition key
switch during the starting mode. This ensures the engine
does not start against a load in the event the accumulators
are charging or the fan motor is activated.
MAINTENANCE
Hydraulic Oil Recommendations.
For the hydraulic oil specifications, refer to Lubrication, page
9-1.
Draining and Flushing
If a component has been changed because of a failure that
might allow metal or abrasive particles to enter the system,
all systems must be thoroughly checked, drained, and
flushed.
1. Remove the reservoir drain plug. Allow about three
minutes after hydraulic oil stops flowing from the drain
port for the side walls to drain.
2. Clean and install the reservoir plug and fill the reservoir
with a 50/50 mixture of fuel oil and clean hydraulic oil.
3. Cycle the crane through all functions several times.
Then return the crane to its stowed position and turn the
front and rear wheels to the extreme left. Shut down the
engine.
4. Remove the reservoir drain plug and drain the reservoir.
Clean and install the drain plug and fill the reservoir with
clean hydraulic oil.
NOTE: Draining the various components will be aided by
connecting a drain line in place of the disconnected
return line.
5. Disconnect the return line from the lift cylinder and raise
the boom to maximum elevation.
6. Connect the cylinder return line and lower the boom to
its stowed position. Replenish the reservoir hydraulic oil
level as required.
7. Disconnect the return line from an outrigger extension
cylinder and fully extend the outrigger.
8. Connect the outrigger return line and retract the
outrigger. Replenish the reservoir hydraulic oil level as
necessary.
9. Repeat Steps 7 and 8 for the remaining outriggers.
10. Disconnect the return lines from a pair of outrigger jack
cylinders and activate the cylinders to their maximum
down positions.
11. Connect the return lines and raise the outrigger jack
cylinders to the stowed position. Replenish the reservoir
hydraulic oil level as necessary.
CAUTION
Hydraulic oil supply lines must be connected to the
cylinders when flushing the system.
CAUTION
When draining the outrigger jack cylinders, always
operate either both front or both rear cylinders together to
prevent twisting the crane.
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