A sheet of paper weighs around 2 grammes – the paper reels that the new three-crab double girder overhead travelling crane belonging to Sappi Alfeld AG in Germany has to lift at least every 30 minutes are 10 million times heavier.

South African Pulp & Paper Industries (Sappi Group) has acquired paper factories all over Europe – including, in 1992, the then Hannover Papier, now Sappi Alfeld. Around 1,000 employees produce graphics papers and special papers on five machines – about 340,000 tonnes a year.

Paper making machine No. 2 was converted from 650m/min to 1,000m/min at the end of 2001 and R. Stahl Fördertechnik GmbH supplied a specially designed double girder overhead travelling crane to work with it.

The new 18.6m span crane has three crabs. Two, each with a 16.5t SWL and connected by a beam, tandem lift the paper reels after they have been wound on machine No. 2. The reels are 5m long, 2.4m in diameter and weigh 22t. They are transported either to an interim storage area or to a further processing point in a linear arrangement behind the paper making machine. The centre 30t crab is to assist with maintenance work on the paper making machine.

R. Stahl’s Hannover branch, together with the crane factory in Ettlingen near Karlsruhe, designed an extremely low headroom crane with a high natural frequency of >=4Hz and a deflection of less than 1/1,200 of the bridge length, as demanded by the customer. This was necessary to prevent the crane from oscillating too much, even under full load and at a speed of 40m/min, and to enable the load to be set down accurately. Dimensions of the existing SAPPI bay into which the crane had to be lifted dictated the low overall headroom of 1,465mm. In addition to the new three crab crane three more run on the 170m long crane runway which is 8.7m from the ground – two for unloading the paper making machines and a small maintenance crane. Optical sensors prevent the new crane from colliding with neighbouring cranes.

The end carriages had to be divided and designed with four wheels on each side to cope with the high weight of the crane and the limited carrying capacity of the runway. All three crabs are classified H3/B4 and have the Stahl AS 7080 wire rope hoist rated 2M. Hoisting speeds are 5/0.8m/min. Features of this hoist which has been in production for many years, are compact dimensions, powerful drive and long rope lengths, Stahl says.

Conventional contactor control is fitted which operates in the 60°C ambient temperature, 10m up in the air, without any additional cooling. Integrated with the crane control is a microprocessor-controlled SLE (Stahl electronic load monitor) module for electronic control and monitoring of the hoist motors. Hoist motor starting and braking characteristics are controlled by the SLE, as is the electrically activated hoist brake, and it also monitors hoist motor temperature. Separate smooth start modules are fitted for crab and crane travel.

Operation is by a push button controller via which one of three programs are chosen, depending on the task:

•Program 0: All crabs operated independently.

•Program 1: Joint operation of the two outer 16.5t crabs, for moving the paper reels, with a fixed distance between hooks of 5,400mm. The crabs align themselves automatically when the travel button is pressed.

•Program 2: Joint operation of the two outer 16.5t crabs with a fixed but freely selectable distance between hooks.

As the total hoisted load of the three crabs considerably exceeds the safe working load of the bridge, a cumulative load circuit is integrated into the control. This monitors the loads actually suspended from the crane and prevents more than 30t from being attached to the crabs. At the same time the load cumulation function serves as an overload control and records all operating conditions and load spectra.

Due to the long (20m) hook paths in the paper-making bay, reeving for true vertical lift was chosen, and the wide design of the bottom hook blocks makes it easier to move loads more smoothly.