With the global demand for minerals, such as iron-ore and coal, increasing, the expansion of materials handling conveyor sizes has required larger power- transmission equipment that will not only function but also not fail prematurely in mining operations.

Power transmission products manufacturer Bibby Turboflex, a subsidiary of multinational mechanical power-transmission company Altra Industrial Motion, says holdback, or backstop, sizes have become bigger and, therefore, it has become important for the holdback torque arms to be mounted correctly to avoid premature failure of the holdback.

A holdback is an antirunback device used in inclined conveyors to prevent reverse movement of the belts. It is also used in overland conveyors to avoid the unusually severe shock loading on start-up, where the loaded belt sags between idlers.

Bibby Turboflex highlights that large holdbacks are used to help increase the safety of high-capacity inclined bulk conveyors or bucket conveyors in mining applications. In the event of failure, holdbacks help prevent load conveyors from moving backwards. Without holdbacks, conveyors carrying tons of iron-ore or coal, may accelerate backward, causing damage and downtime.

Altra Industrial Motion notes that the largest holdback, which is over 50% more than the previous sizes, has a torque capacity of more than 1.2-million pounds, which is a challenge when the holdback torque arm has to be mounted on the conveyor. Also, as the holdbacks are predominantly situated outdoors, the high contamination environment and extreme temperatures present challenges when mounting the torque arm.

“A torque arm mount for these larger backstops must be able to restrain the unit from significant rotation, should there be a reversal in force on the conveyor shaft. These torque-arm reaction forces can range from just over 1 000 lb to 100 000 lb for larger size models,” says Altra Industrial Motion sales and product training manager Greg Cober.

He adds that, should power be cut off or when the load of the minerals exceeds the drives’ capability in a way that causes the load to back drive, the backstopping action of the clutch is activated. One end of the torque arm is pivotally connected to the head member of the clutch, while the other end of the arm is connected to a stationary structure.The clutch rotates slightly against the force, which enables the torque arm assembly to receive a portion of the system load.

The company mentions that holdback manufacturers provide integrated torque arms that can withstand these forces, but the system assembler must also ensure that the mounting will withstand the forces.

“When mounting, the end of the torque arm must not be rigidly attached to the steel framework. A bracket, or stirrup, positioned at the end of the torque arm, must instead provide clearance to permit the backstop to centre itself in axial and angular positions to prevent the pinching of the bearings, which causes damage or failue of the unit. This top of arm clearance can either be half an inch or one inch, depending on the arm type,” he highlights.

Altra Industrial Motion believes that it has the expertise to correctly mount the torque arms for various mining operations, as it has completed the supply of and the mounting of the unit to South Africa-based mining group Exxaro for the Medupi Grootegeluk expansion, which is yet to be commissioned.

Exxaro’s Grootegeluk mine is one of the most efficient mining operations worldwide and operates the world’s largest coal beneficiation complex, where 9 000 t/h of run-of-mine coal is upgraded at six different plants. Situated 25 km from Lephalale, in Limpopo, this openpit mine has an estimated minable coal reserve of 2.8-billion tons and a total measured coal resource of 4.6-billion tons, from which semisoft coking coal, thermal coal and metallurgical coal can be produced.

About 14.8-million tons a year of production is destined for use by power stations. The coal is transported directly to State owned power utility Eskom’s Matimba power station, also in Limpopo, on a 7 km con- veyor belt. Grootegeluk also produces 2.5-million tons a year of semisoft coking coal, the bulk of which is railed directly to steel company ArcelorMittal South Africa, under a long-term supply agreement.

Grootegeluk’s capacity is being doubled to supply the new Medupi power station that Eskom is building nearby. The $1.3-billion Grootegeluk Medupi expansion project will increase throughput to 14 000 t/h and supply Medupi with 14.6-million tons a year of coal for 40 years.

“In June this year, we also supplied units to integrated energy and chemical company Sasol, where we have completed the mounting and installation of the holdback on the con- veyor shafts and we are also waiting for the commissioning of the project. Not only do we supply the units and mount them, we also offer on-site surveys and inspection of the backstops, as well as staff training on the correct installation and maintenance procedures that should be carried out on site,” concludes Cober.