Features The cylinders used for lifting operations through cables (Strand Jacks) are a compact and effective solution for the positioning of heavy loads which operate both in lifting and lowering. Their operating principle is the same of precompressioning and post-tensioning systems, with a bundle of cables (strands) which are guided through wedge grips arranged both at the base and at the rod end of the cylinder, passing through the central hole of the cylinder itself. It is then possible to carry out liftings for serveral metres with repeated strokes of the cylinder. The heads were the wedge grips are located are hydraulically controlled to lock and unlock the wedges so that the cylinders can be used also for lowering operations. A piloted check valve is fitted on the cylinder inlet as a safety provision against any possible hose rupture and as a control of the lowering speed. The cylinders are equipped with position sensors which communicate to the management system the reaching of the points of intervention commands to sequence them appropriately in relation of the desired direction. If requested, the cylinders can be equipped with a linear stroke transducer in order to verify the position of the rod. A block with a clevis and a stud is provided with the cylinder. Thank to the wedge grips which grip the cable, it allows the cables insertion and the locking and unlocking from the outside. The cylinders are designed to be used with standard 0.6″ strands (not supplied) with a safety factor of 2.5. This limits the wearing of the strands and of the wedge grips, which can therefore be used several times. The access to the terminal heads where the wedges are located is however facilitated by the constructive solution adopted for the cylinder. Accessories such as strand recoilers, strand guides or other devices for the guiding and recollection of the strands can be designed and delivered according to the specific needs. Each cylinder is operated by a dedicated control unit, equipped with solenoid valves to control both the cylinder itself and the hydraulic lock and unlock of the terminal heads, of the pressure gauge and of the electrical box for the power of the motor and valves with connector for the control cable, four 5 metre hoses with quick couplings. The control unit specifications for each cylinder are shown in the table below. Two control systems with PLC for the automation of lifting and lowering movements are available: – ZMRJ1 for single cylinders, with a three-position selector (lifting, hold, lowering) and emergency stop; – ZMRJ4 to control up to 4 cylinders simultaneously, with 4 two position switches to include or exclude each cylinder from the operation, a three-position selector (lifting, hold, lowering) and emergency stop. This system performs lifting and lowering cycles operating on the cylinders relating on the contacts arranged on the cylinder itself to determine its position. Each stroke ends when all the cylinders operating have reached the end of stroke, before starting the retraction of the cylinders. Since the cylinders are controlled by control units of the same flow, the movement speed is substantially equal for all the cylinders and it is thus possible to obtain a simple and efficient synchronization system similar to the split-flow systems.
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