When gearbox failure stopped production at the Omagh Minerals site in Cavanacaw, Ireland’s only operational goldmine, it initially seemed that repairs would take weeks if not months. However, in just over a week, motor control expert Ralspeed was able to supply a 250kW variable speed drive system that allowed normal operations at the mine to be quickly resumed, thereby minimising loss of output and productivity

Central to the operations of the Omagh Minerals goldmine is a ball mill – essentially a large drum into which the ore is placed along with heavy balls. As the drum rotates, the balls gradually crush the ore into a powder from which the gold can more readily be extracted. Fully loaded, the ball mill at the Omagh Minerals site weighs over 50 tonnes, and it has been calculated the torque required to start it is in excess of 24,000Nm.

This mill is driven by a 250kW motor, via a 5.8:1 ratio gearbox. Problems had, however, been experienced in the form of gearbox failures and, as a result, Omagh Minerals called in bearing specialist Hayley Group to investigate. The conclusion was that the type of gearbox being used was only just adequate for the application, and that fitting a larger gearbox would improve reliability. Shortly after this conclusion had been reached, the in-service gearbox failed, putting the ball mill out of action. The best delivery available on the larger gearbox recommended as a replacement was, however, eight weeks and Omagh Minerals was reluctant to have the mill out of service for such a long period.

Experts from the Hayley Group knew that the maximum load on the gearbox occurred when the mill was started and that, if this load could be reduced, a smaller gearbox, which was available on a much shorter delivery, could be used.

The key to reducing the starting load was to replace the direct-on-line starter currently in use with a variable speed drive that would give a smooth controlled start up with greatly reduced peak torque. This solution was only attractive, however, if a suitable variable speed drive system could be obtained quickly.

While deliveries on enclosed customised 250kW drive systems are typically several weeks, Ralspeed, a company that specialises in offering a fast response on variable speed drive and softstarter systems, confirmed it could design, build and supply a panel for this application in just one week.

“The Ralspeed guys were as good as their word,” says Mark Jones of Hayley Group, “and the panel, which incorporates a high-performance Vacon drive, was delivered right on schedule. Because Ralspeed had designed specifically for the application, it was quick and easy to install and commission, which meant we were able to get it into service without delay.”

With the replacement gearbox installed – albeit of a smaller size than that originally envisaged – and the Ralspeed variable speed drive system fitted in place of the original direct-on-line starter, the ball mill was back in operation in less than two weeks. It has now been working reliably for several months without any sign of distress to the gearbox or other parts of the drive system.

“This is an application that places great demands on the mechanical components in the drive chain,” says Jones, “but, as we’ve seen these demands have been significantly reduced by the Ralspeed variable speed drive system. This gives us complete control over the rate of acceleration and deceleration of the ball mill drum, thereby eliminating shock loads on the drive system.

“This has turned out to be an effective and convenient solution, not least because of the very fast delivery and excellent support provided by Ralspeed. We are confident that the new arrangement will make the ball mill much more reliable in future and, by eliminating costly breakdowns, it will pay for itself very quickly. In fact, I would have no hesitation in saying that, with this application, Ralspeed has certainly hit the gold standard for Omagh Minerals.”