Tight space calls for fine crane engineering

Condra grabbing crane working at the railhead serving Sishen Iron Ore Mine, near Kuruman, South Africa

Grabbing cranes are usually large machines with wide spans and high lifting heights.  Not so the Condra grabbing crane installed in February at the railhead serving Sishen Iron Ore Mine, near Kuruman.

Completed in December and delivered by road in January, the crane is the final link in Sishen’s ore spillage recovery chain, using its 0,5 cubic metre clamshell grab to transfer dumps of ore spilled by the conveyor system and subsequently recovered, into railway cars.

This feature rich and finely engineered double-girder electric overhead travelling grabbing crane is a relatively small machine with a capacity of 1,5 tons, a span of just 7,2m and a lifting height of a fairly standard 5,7m.

Condra has previously manufactured much bigger grabbing cranes for various applications, such as the 25-ton 30m-span machine for a cement factory in Mozambique, but the Sishen specification called for a very high degree of operational precision, resulting in the need for low-tolerance engineering of the crane clamshell grab so that it can move smoothly in and out of the railway cars.

Working speeds are quick for the short distances involved.  Cross-travel speed is 16m/min for the 7,2m end-to-end travel distance.  Long travel speed is 32m/min for a gantry length of just 20 metres.  Hoist speed is 6,2m/minute.

A Condra spokesman explained that the design challenge lay in configuring a crab to operate within the relatively tight travel and lift dimensions.

“For a grabbing crane this is a very confined area in which to work.  Spans and lifting heights are usually much larger,” the spokesman said.

“There was also the design requirement to be able to dismantle the crane beyond the normal requirement of transporting an abnormal load by road, because shipping was scheduled to take place in the second half of December during the road network embargo on abnormal loads which allows free flow of peak seasonal holiday traffic,” the spokesman explained.

Although the dismantling requirement was met and the crane completed on schedule, transport was for various reasons delayed to January.

The spokesman said that manufacture of the crane was relatively straightforward and standard, though the lead time of twelve weeks was tight.

On the factory floor, the challenge was to work with the galvanised grating needed for full-length walkways either side of the hoist.  This material tends to distort when cut because of the internal stress caused by galvanising.   Checker plate could not be used because of its propensity to collect dust.

Sishen’s grabbing crane is feature rich, with variable frequency drives incorporated throughout, a radio remote control with optional pendant control, downlights, four red-and-green proxy lights to indicate movement clearance on the gantry and grab, and a digital scale monitor on the remote control to show the exact grab load on a continuous basis.

There is a second, bigger digital scale read-out on the crane itself, to transmit grab load status at a glance when not reading the remote control.

“As far as crane technology goes this was a very interesting crane to build, with several design and fabrication challenges,” commented the spokesman.

“We are happy to have it in our portfolio of successfully engineered bulk handling products.”