Longer face conveyors mean a greater mass of material to move, longer and heavier chains to move it, and of course, more powerful drive systems. DBT has recognised that face conveyor technology is the most critical factor in the success of these operations, and has stepped up to the challenge of these longer conveyors with several installations in Germany, the US and Australia.

DBT has looked extensively into the issues associated with the chain, sprocket and drive train interface.

Over the past five years DBT has been involved with seven longwall installations with face lengths of more than 400m, with the maximum face length currently 500m at Mine West in Germany.

All DSK mines in Germany have face lengths greater than 300m, and several are over 400m. A number of longer faces have 3 x 1000kW drive systems.

In the US, two mines – one with 2m shields and another with 1.75m shields – have delivered long-term high-production performance with very wide faces. The latter US longwall is 442m wide, and the company has already mined more than 11 million tonnes with its DBT PF6 AFC and 48mm chain system.

This installation also has the highest installed AFC power worldwide with 3 x 1220kW DBT CST 65 drives. A similar arrangement is being installed at another US mine in August 2007. The DBT AFC system has produced consistent peak loads of over 6000 tons per hour from the 330m face at one US operation.

The first Australian DBT CST 65 gearboxes rated at 1200kW will be installed during 2007 at BMA’s Broadmeadow operation, initially with 2 x 1200kW drives.

Once the longwall face is extended to 320m, Broadmeadow will have total installed power in excess of 3000kW, offering sufficient reserve to start the conveyor in a heavily overloaded condition. The mine will utilise DBT’s proven 48 x 152mm chain system which is typically achieving a service life of 11 million tonnes.

“Longer longwalls mean greater stress and strain on all components of the face conveyor – and present a number of challenges,” DBT said.

“Considering the installed power of over 3000kW, a single failure can lead to catastrophic results from the release of stored energy. And production comes to a complete standstill until the problem is solved.

“Similarly, with such power and associated inertia, wear on components increases if the total system is not correctly designed, all contributing to an increased cost per tonne.”

So what limits the length of a face conveyor? And what challenges had to be overcome?

For DBT, firstly, it is the maximum available start-up torque for a fully loaded conveyor. DBT’s Controlled Start Transmission (CST) drive system delivers maximum start-up torque and safe start-up even of a fully loaded 400m long conveyor.

The intelligent CST drive system is simply a planetary gearbox with an integrated multi-disc clutch with an infinitely adjustable slip and the new PMC-D drive control unit featuring no-load motor start-up, AFC soft start, and synchronised heavy-load start-up.

According to DBT, it also allows accurate load sharing between up to four drive motors with “excellent and extremely fast overload protection in case of chain jams”.

The CST 65 model is rated for 1200kW (1630hp) per gearbox at 50Hz (1450kW/1950hp at 60Hz), and the clutch delivers maximum efficiency with minimum operational slip.

The use of multiple drives without load sharing leads to the inefficient use of energy and results in greater chain and sprocket wear, DBT said. Precise load sharing substantially improves running chain tension variance.

“Only a totally reliable automatic chain tensioning system – with a sufficient stroke at the tail drive – prevents the dangerous build-up of slack chain. One substantial benefit of the CST controlled clutch system – which was only understood when faces over 380m began to operate – was the dampening of chain oscillations,” DBT said.

Oscillations, or natural harmonics, arise due to the varying friction conditions of the AFC path. This has not been so evident in shorter, lower-powered faces due to the increased stiffness of the chain system.

In the past, DBT had experienced several mines in which drive frames had a vertical harmonic lift of over 200mm, and where the chain/flight system had a natural horizontal harmonic or oscillation. Failure to dampen these oscillations results in severe damage to components of the AFC.

Other key factors include optimum geometry between the chain strand and the sprocket and a large running surface between flight bars and pan profiles to minimise wear and friction. A successful combination of these measures results in significantly longer service life of the chain and drive sprockets, especially for long faces.

DBT has recently developed the new PowerChain aimed at even higher-capacity face conveyors for longer faces, particularly in lower seam heights. First underground installations at stage loaders in two US operations are underway, with the start of the first such chain system in a full longwall scheduled to take place by 2008 at the latest. BMA Broadmeadow will also start using the PowerChain for its BSL from 2007.

DBT is moving towards a future where total installed power on the face conveyor can be well in excess of 4500kW.

“A careful step-by-step approach is being adopted, with detailed total system analysis and low-risk field testing along the way to ensure that customers get what they want: a reliable very high-production conveyor, with no risk of catastrophic failure – and of course the lowest overall cost of ownership,” DBT said.