REED B50HPG, Technical Manual, Page: 25 / 235

OPERATION and MAINTENANCE
Rev 1.2 24 of 80
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S-TUBE CIRCUIT
Since there is only one outlet for the pumping material, a means is required to transfer
the material from the material cylinder to the outlet and into the delivery line. To
accomplish this, an s-tube is installed in the hopper. Since there are two material
cylinders and one s-tube, the s-tube must be shifted from one material cylinder to the
other, whichever one is loaded with the pumping material.
The s-tube shift hydraulic circuit is of the open center type, meaning that when the
control valves are in the neutral position, the internal passages of the valves are open,
allowing the hydraulic fluid to return to the tank. With the engine running the hydraulic
pump is operating, producing a flow of oil which, with no control energized, will pass
through the shift circuit on its way back to tank.
To meet the flow and pressure requirements of the shift circuit, one section of a tandem
pump is used. Note: a single pump may be used if unit is not required for auxiliary
equipment. The tandem hydraulic pump is of the gear pump design with a fixed
displacement, meaning it is designed to constantly produce the same displacement at a
pre-set maximum, depending on engine rpm. The tandem gear pump is directly
connected to and driven through the main hydraulic pump. In addition to the hydraulic
pump, the s-tube shift circuit consists of a manifold, an accumulator, solenoid valve
cartridges, a solenoid directional valve, and 1 or 2 hydraulic shift cylinders. The
following is offered to describe the function of each in the system.
S-TUBE CIRCUIT MANIFOLD
Like the main hydraulic circuit, the shift circuit is also equipped with a manifold block. It
contains an unloader cartridge, relief cartridge and solenoid valve cartridges. A solenoid
operated directional valve is mounted on top of the block and an s-tube selector control
valve is located on front of the block. Each of these components is designed to perform
a particular function in the swing circuit as explained in the following descriptions:
•
RELIEF CARTRIDGE
This cartridge is used to divert the pump flow from going to the accumulator
once its capacity has been reached, directing it back to tank. It becomes
operational when the unloader cartridge setting has been reached, acting as
a dump valve.
• UNLOADER CARTRIDGE
This pressure sensitive cartridge is used to protect the system from excessive
pressure and to limit the amount of pressure being applied to the accumulator by
hydraulically signaling the relief cartridge to open once the unloader
setting has been reached. The unloader will also redirect the oil
back to the accumulator when it senses a drop-in system pressure,
when the hydraulic cylinder shifts for example.