Brevini enables 20 metre machines for iron ore processing facility

Designed to separate materials by size, a trommel is a key piece of equipment within an ore processing facility, but dealing with hundreds of tonnes of rock every hour means that the design must be very robust. Designing and manufacturing a complete package requires several manufacturers to co-operate successfully in order to deliver a properly integrated solution that is both reliable and efficient.

The Holborne Group is a specialist engineering company that has considerable experience in designing and manufacturing trommels. Based in Shropshire, the engineering team has worked closely with its supply partners to design and construct the latest machine. Brevini was contracted to develop the complete drive package, which included the motors, gearboxes, couplings and electrical control system.

For this particular trommel, the power required for normal running would be 520kW and this would be used to power four drives that would produce a gearbox output speed of 70rpm. Due to the size and weight of the screens, the initial start-up procedure would require 680kW for around 15 seconds in order to accelerate the machine up to working speed. This additional power requirement would affect the specification of the drives and gearboxes being supplied by Brevini.

The gearboxes were designed around the Posired 2 helical drive units that incorporate a two stage reduction, giving an output ratio of 16:1. All of the units were supplied with identical handling of the input and output shafts, which allowed the gearboxes to be installed in any of the drive positions on the rotary screen. This rationalisation of the design simplifies the installation and reduces the amount of essential spares required.

The reduction gears are housed in a bespoke, fabricated steel housing, as opposed to the more usual cast casing. This arrangement ensures that the gearboxes will fit easily into the restricted space of the trommel design and also eliminates the need for an angled base plate, which reduces the material costs and the manufacturing time for the components.

The design of the couplings also posed a challenge to the Brevini engineers since the maximum diameter was limited to 280mm. Considering the substantial torque that has to be transmitted, the designers created a coupling using a special hardened material that formed fully crowned and barrelled gear teeth giving ±2° of misalignment per gear mesh.

The hubs and flanged covers were manufactured from high tensile steel which provided a higher torque capacity and enabled standard couplings to be used for heavy shrink fit applications. All of the components were manufactured to extremely high standards using precision machines; the tight tolerances allowing complete interchangeability.

Each gearbox was powered by a 185kW, six-pole, IE3 electric motor that was fitted with an incremental encoder, thermistors, space heaters and labyrinth seals. The six-pole design gives a running speed of 1,188rpm, which allowed Brevini to improve the overall efficiency of the drive system by using a two-stage reduction gearbox.

The encoders were included in the design to provide feedback and position control in conjunction with the variable speed drives to enable load sharing, and also to prevent any individual drive trying to operate alone. In this way the drives are synchronised in their operation and the design prevents any abnormal loads being applied that could lead to premature gearbox wear.

In addition to the main barrel screen drive units, Brevini also designed and supplied the feed and discharge screw conveyor drives that used 37kW motors connected to Posiplan gearboxes. Again, the design was based on efficiency and reliability, using IE3 motors and the proven Posiplan drive fitted with labyrinth and cassette seals.

The electrical control system was designed and built as a bespoke package that would maximise the effectiveness of the drives and improve efficiency. The system provides complete integration of the control circuits and ensures safe and reliable operation of the barrel screens. This includes a progressive start-up procedure that minimises the load on the motors and achieves normal operating speed within 15 seconds.

The completed trommel was fully assembled in the Holborne premises before being tested to ensure that everything operated as designed. This required the company to bring in five generators in order to provide sufficient power to the machine for the final stages of the project. Once all the systems had been verified the vast machine was then dismantled in preparation for shipping to Trinidad.

Carl Hanson, Managing Director, Holborne, comments: “This has been another great project for us that has also benefited from the expertise and flexibility of the Brevini engineers. Their experience and manufacturing capabilities have helped to deliver a product that will provide many years’ service in a very unforgiving application.”