Ruppel Hydraulik has developed a number of systems for these duties, including compact hydraulic units for sway control, as well as complete hydraulic units for heavy-lift harbour cranes and automated cranes.
Because they operate under harsh environmental conditions, moving large loads or load configurations, harbour cranes are a challenging field of application for hydraulics systems developers. One such is the German company, Ruppel Hydraulik, which has developed a hydraulic sway control system for harbour container cranes, increasing their cargo handling capacity (a description of this system can be found at the end of the article).
Beyond these specialised systems, however, Ruppel Hydraulik is also noted for its design of complete hydraulic systems for harbour cranes. A recent project involved a mobile harbour crane of 150-tonne lifting capacity. Cranes of this kind run freely across the port or dockyard area on a multi-axle chassis. A lattice-type derricking boom is located more or less in the centre of a short vertical mast. The guying is attached to the top end of the mast, and, unlike the usual types of mobile crane, the derricking cylinders are above the boom. Hefty brace supports ensure crane stability when handling heavy loads.
Control circuits
The power output of the hydraulic unit for this crane is provided by two 160kW axial piston pumps with a maximum volume flow of 270litre/min, load-sensing control and pressure cut-off. One of the items to which these units supply hydraulic power is a proportional directional control valve that regulates the in/out movement of the derricking cylinders. A control circuit composed of pressure regulating valves on the pump control blocks, plus pressure sensors in the cylinders, ensures that the pressure in the cylinder chambers always remains the same during their simultaneous operation.
The brace supports are initially swung out to the sides and then extended vertically – always all four of them together. Here too, a control circuit with pressure monitoring ensures the operating sequence is optimised. The support cylinders are extended at reduced pressure until they make contact with the ground or, more precisely, until equal pressure is achieved on all four cylinders. It is then up to the crane operator to ensure the crane is levelled off. During this operation, too, the pressure in all four cylinders is continually monitored.
The drive system includes a comprehensive array of safety functions, such as brake valve blocks for the derricking cylinders and brace supports, as well as an accumulator and electric brake for the winch. The hydraulic unit has now been installed on this 150 tonne capacity unit, and the crane is reported to be working to the complete satisfaction of the customer.
Crane modifications
As an owner-managed, engineering-driven company, Ruppel is also an important point of contact for crane manufacturers and operators interested in single-batch retrofit projects. A recent example involves ship-to-store handling of bulks such as fertilisers. A port authority had ordered a rail-borne crane capable of performing automatic unloading of ocean-going vessels, but it was not achieving the agreed performance values. Unfortunately, before these shortcomings could be addressed, the original manufacturer went into liquidation.
After an analysis of the crane’s current operating condition, Ruppel’s engineers came to the conclusion that the electro-hydraulic control system of the crane’s boom and grab was not capable of achieving the required accuracy. The actual value for the grab position was calculated based on the output of rotary encoders located on the boom. Moreover, hydraulic positioning using solenoid valves was based exclusively on the volume flow control of the pumps, which at around 850 litre/min is extremely powerful.
Ruppel Hydraulik drew up an alternative proposal, which is currently in the process of being implemented. Positioning of boom and grab is controlled via pressure regulation in combination with a directional control system on the cylinder. The pump oil flows are controlled directly by a proportional directional control valve fitted to the cylinder.
An additional two-way pressure-maintenance valve ensures the hydraulic control is pressure-compensated, allowing it to work largely independently of temperature and load. Based on Ruppel’s calculations, this set of measures will result in an improvement in repeat accuracy for grab position from a previous ±10 percent or more to under ±3 percent - a value more than sufficient for bulk goods transfer operations.
Customer-specific valves
Harbour cranes often use powerful hydraulic drives so there is a need for highly sensitive positioning of items such as the crane boom; as a result, pilot-operated proportional valve technology using solenoid valves is often deployed in the control blocks. While Ruppel obtains these valves from outside suppliers, the company does sometimes adapt them to meet specific end-use requirements. As an example, they may need re-equipping with customised pistons.
Hydraulic sway control
Many port container terminal operators use Ruppel’s hydraulic sway control system for container cranes, as they save time and berth dues on every transfer operation. The principle of operation is relatively simple. Four lifting cables are guyed diagonally to the direction of sway. A cylinder is used to perform the duties of actuator, and a compact control block acts as the system ‘brain’. The cylinder, at 190cm in length, tensions the anti-sway cables.
The latest generation of this sway control system is fitted with electronic control to provide more accurate, proportional adjustment and monitoring of the system pressure. The tendency to sway is prevented more effectively as a result; the crane not only works faster, but also uses less energy because swaying motions inject unwanted kinetic energy into the sub-system of lifting gear, load-carrying unit and load, which the crane has to suppress by applying its brakes.