This photograph of milling plant under construction at a mine in Angola shows the mill, the associated overhead crane atop its gantry, and conveyors nearing completion on both sides.
The overhead crane, manufactured by Condra and already commissioned, is assisting with construction before assuming its primary role of mill maintenance. The project is being managed by specialist engineering company Consulmet.
Condra designed the overhead crane and gantry specifically to overcome functionality constraints of portal crane alternatives tendered by rival firms. Portal machines would have been limited in operation to this mill only, unable to service units planned for the future because of conveyors interrupting their passage.
By contrast, the Condra design has an overhead crane mounted on a gantry with legs situated between the conveyors, allowing uninterrupted crane movement as well as later gantry extension to service future mills when they are built.
The 28-metre-span crane and gantry incorporated special design to allow for delivery by road. Access routes to the site cannot be navigated by conventional tractor and dolly, so box girders and gantry legs had to be designed to fit within standard 12‑metre containers.
The girders were manufactured in three sections to fit these containers. After delivery, they were spliced, the steel plates welded to the four inside faces of the male box sections delivering a friction grip at the splice that reinforces girder strength beyond that delivered by the splice bolts alone. Bolt holes were reamed during manufacture, and the bolts themselves machined for an exact fit to retain camber.
As can be seen in the photograph, additional gantry height beyond that afforded by the containerised legs was achieved by concrete plinths cast as part of the milling area foundation, and by steel pedestals incorporating the crane’s end-carriages fastened below the crane girders. Effective crane lifting height is 16 metres.