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Overview
SP 500 / 750-15 / 750-18 / 1000 / 1250 Operation Manual
Twin-circuit switching vs Single-circuit
Twin-circuit systems have a hydraulic circuit to move
the dierential cylinders, and a separate circuit to
switch the Rock Valve. On single circuit machines, the
main hydraulic pumps moved both the main dierential
cylinders and the Rock Valve.
There are advantages to having a separate hydraulic
circuit to switch the Rock Valve. For instance, with a
sti, lean mix and high pressure, a single-circuit sys-
tem does not always have enough pressure available
to move the Rock Valve. Generally, this decit occurs
when the mix is being pumped straight up. The Rock
Valve must dislocate a column of concrete that still has
pressure on it from the stroke. The next stroke cannot
begin until the Rock Valve has completed its travel. In
a twin-circuit system, as soon as the S-3 spool chang-
es direction, oil is routed to the Rock Valve so it can
change direction, as well as to the valve that chang-
es the direction of the dierential cylinders. If the Rock
Valve cylinder has enough pressure available to switch,
it will do so. If it has insucient pressure, it will wait.
When the dierential cylinders change direction, one of
the following happens:
• If the Rock Valve has already switched, con-
crete is drawn immediately from the hopper and
pushed into the pipeline with a normal stroke, or
• If the Rock Valve has not already switched,
concrete begins to be drawn from the pipeline
and pushed into the hopper. This reduces the
pressure in the pipeline to zero. As soon as pipe-
line pressure is relieved, the Rock Valve moves
across. Once the Rock Valve has moved, con-
crete accelerates out of the pipeline in its normal
fashion.
The concrete is not pumped in reverse, because as
soon as the pressure in the pipeline drops to zero, the
Rock Valve switches. This system has the added ben-
et of reducing wear on the cutting ring, the spectacle
plate, the kidney seal and kidney plate, and the Rock
Valve slewing cylinder.
One advantage of single circuit machines is a lack of an
accumulator. When the engine is shut-o, no hydrau-
lic pressure exits in the system, hydraulic lines can be
safely opened. This is not the case with twin-circuit ma-
chines. Even with the engine shut-o, pressure could
be stored the accumulator. If a line is removed, oil could
be discharged, raising the possibility of injury.
Whenever you work with accumulators, be sure to fol-
low these simple rules.
Accumulator Safety Rules
1. Never assume that the accumulator has no stored
pressure. Check the gauge before removing any
lines, valves, or ttings. Verify zero pressure, and
periodically check that the gauge is in good work-
ing order.
2. Recharge an accumulator (i.e., add dry nitrogen)
only after you read and understand all the instruc-
tions for recharging accumulators. Accumulators
are charged with dry, not liquid, nitrogen. Com-
plete recharging instructions can be found in the
Maintenance section of this manual.
3. Never charge an accumulator with oxygen or
compressed air. Under pressure, oxygen can
contact some of the molecules of hydraulic oil
and lower the ash point of the oil to below room
temperature. If the oil ashes, the accumulator
will explode.
4. Never work on the accumulator, the Rock Valve
cylinder, the cutting ring, or any other connected
component with the engine running. After shut-
ting o the engine, remove the key and place it
in your pocket. Then verify zero pressure on the
accumulator gauge before opening or working on
any of these items.
5. Never improvise the tools or equipment needed
to recharge the accumulator. The correct tools
are shown in the Maintenance section of this
manual.
6. Do not operate the machine, if the accumulator
develops a gas leak, an oil leak, or a ruptured
bladder. Replacement bladders can be ordered
through Schwing’s spare parts department or can
sent in for repair. If you any have questions, call
Schwing America’s Service Department at (888)
292-0262 for assistance.
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