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Condra is to manufacture two overhead cranes for underground use at a Botswana diamond mine.

The first crane will be a 15-ton suspension crane. It will feature two I-beams bolted to rock-bolt flanges anchored in the roof of a mine chamber following blasting, excavation and preparation to create a service workshop. Usually, the I-beams would be the topmost side components of a fixed gantry constructed from the ground up.

The wheeled 11m span crane girders will then run suspended from the lower flanges of these two long-travel I-beams, with the crane’s crab mounted atop the girders to provide the cross-travel. The whole structure of the crane becomes suspended, running along and across the roof of the chamber.

Condra designed this crane to specifications supplied by the project’s consulting engineers. Suspending the crane will deliver the greatest possible lifting height, reducing the volume of rock needing to be blasted and excavated to form the chamber. After installation, the crane will have a lifting height of 9m.

The second crane will be a conventional overhead design, with a 35-ton capacity, 9.2m span and a lifting height of 12.7m. It will work in a larger, existing chamber.

The cranes will be used for service and maintenance applications, the suspension crane being installed in a chamber close to the ore body, where it will work on machinery used for drilling, blasting and ore removal. Such a location will save the time and cost of moving the machines over a greater distance to workshops closer to the shaft.

Condra said: “For both cranes, Condra’s design team had to calculate balance points for individual components, precisely positioning lifting lugs on the sides of component assemblies so that they will hang precisely vertically below the cage that will lower them down the mineshaft. Similarly, lifting lug positions atop the components will ensure balance during crane assembly and installation within the confined spaces of the service chambers.”

To minimise chipping and rust, Condra will apply an edge finish, grinding all angular steel to circular section before the application of a customer-specified protective coating.

The cranes will feature flame-proofing, stainless steel panels and full weld seams in place of what the company said is the more usual space-welding along the lengths of steel joints, including the girders.

Control of both cranes will be by mobile pendant. The suspension crane will feature a flashing light and siren for safety.

The consulting engineers specified brake logic to avoid overuse of the push buttons on the mobile pendants. Condra explained: “Operators are prone to repeated and rapid use of this control, causing excessive heat and wear on the braking system. With brake logic, as soon as the button is pressed twice in quick succession, a two-second temporary block is activated, overriding the operator and correcting his use of this control.”

Condra to supply overhead cranes for Botswana diamond mine