Cast off the sway

Two 5t cranes have recently been commissioned for ladle handling in the castings hall of Laycast in the north of England. Each crane uses a Street TVX open barrel hoist and is used to handle ladles of molten metal on a single hook. Street says that the sway control system that is used, developed in conjunction with Finnish specialist Innocrane, maximises load stability in the long and cross travel modes with an optimum travel speed.

The stability question

A crane's operating speed and therefore its productivity is usually restricted by the need to ensure load stability. Greater stability is key to maximising safety and throughput, but it also has technical implications. A swaying load imposes more dynamic forces that will affect crane performance and service life so there is a constant search for load stabilisation technology.

Skilled operators, using careful judgement and close hand/eye co-ordination can achieve good load stability but these skills take time to acquire. Because the feedback loop is never perfect, manual methods will still induce load sway by over-correction and hunting during final positioning. Also, manual control demands a high level of concentration leading to increased operator fatigue and possible error.

Inverter motor technology has introduced stepless acceleration and deceleration which has contributed to load stability by removing much of the harshness of motor operation. Control developments such as Street Crane Smartdrive have enabled skilled operators to exploit this technology while still relying on human judgement.

There are other techniques for achieving greater stability. Laser based stability systems monitor load swing at the hook directly and modulate the speed. These systems are effective, but costly. They are also difficult to set up and maintain.

Replacing the wire rope with a fixed beam removes the pendulum effect and ensures load stability, but this is achieved with the penalty of losing the inherent 100% access flexibility of an electric overhead travelling crane with wire rope hoist. Fixed beams also transfer forces directly to the crane and impose greater loads on the structure which means higher engineering specifications and therefore costs.

At the foundry

Laycast is a modern foundry specialising in high volume components for the automotive industry. Hot metal is tapped from the furnace and the ladle is moved by crane to the casting machines in the casting bay. This process had been under the control of operators on the floor, and ladle stability was assured by the use of a fixed mast system. The aim of crane renewal was to take operational control from the shop floor to a cabin and to replace the fixed mast with wire rope for greater flexibility. This raised stability questions for which Street was asked to provide a solution.

The solution is a PLC based load sway prevention system (LSPS). It works in much the same way as ABS braking or traction control on a car. There are two sources of data input, the total rope and sling length and the operator speed command. Given this data the system calculates the required speed profile for each load to achieve zero swing. It delivers precise movement without the need for the operator to compensate.

LSPS is user friendly because while it does respond to the user's commands, so they retain control, the system optimises the resulting action. This means that the operator concentrates on delivering the load without the burden of controlling sway which makes crane movements quicker and less stressful. More benefits appear as differences between operators are removed and the time to learn the job is reduced.