Tail drives are normally the tensionable type, especially where long faces and/or high horsepowers are installed. The tensionable tail drive normally comes with a stroke of 500mm, which allows 1m of slack chain to be controlled. For very long faces or extremely high installed horsepower conveyors a tail frame with an effective stroke of 1m was designed. A fully automatic chain tensioning system (pressure differential) is used to keep the AFC chain at optimum tension during operation at all times. The benefit for the operator is maximum life for the chain and minimum downtime for the face conveyor.
Other important elements in the face conveyor system are sprockets, chain and flight bars. DBT principally uses the so-called GR-sprocket, which ensures a maximum contact surface between the chain links and the sprocket teeth during operation. Three teeth are in contact with the sprockets at all times to carry the load, compared with regular sprockets, where only one tooth is carrying the entire load. Contact of only one tooth at any given time results in greater wear and shorter life of the sprockets and chain.
DBT began the process of base-design of chains for high-performance face conveyors some years ago to match chains and sprockets for optimum performance. One result has been the 42mm x 146mm combination chain, the standard chain for applications in the USA and Australia since 1991. The latest development, created by the need for the highest horsepower installation worldwide at the time (4 x 800kW on a single longwall face conveyor in Germany), lead to development of a 48mm chain. The know-how of DBT in the design of a face conveyor with sprockets and chain generates an optimised system working together. DBT’s flight bars are designed to minimise contact pressure while maximising contact areas. Special design consideration was given to the conveying aspect of different kinds of materials and various mounting arrangements between flight bars and chain. Reverse operation of the face conveyor is of course possible.
An important component to maximise the performance of high horsepower AFCs is an intelligent drive system for soft start, load sharing and overload protection. The Controlled Start Transmission (CST) drive system fulfills these requirements by providing a user-friendly drive control unit. It was developed specifically for application with a chain conveyor in a joint venture between the Westfalia Becorit group of companies (now DBT) and Dodge Reliance (now Rockwell Automation Dodge) commencing in 1991. The main advantages are:
* Synchronized soft-start and heavy-load start-up.
* Staggered no-load motor start-up to reduce voltage drop for all motors, AFC start-up only after the last motor is at full speed independent from motor start.
* Maximum total start-up torque available at all times through synchronisation.
* Accurate load sharing.
* Maximum utilisation of installed power without danger of overheating motors.
* Excellent overload protection.
* Rapid de-clutching of the system when oncoming overload is sensed.
* Maximum chain force well below “danger zone”
* Optimum protection of chain and flights, long life for all components.
Up to 1999, two classes of CST were available: the smaller series 30 system for up to 500kW and the bigger series 45 system for 800kW at a gearbox ratio of i = 33:1. More than 130 CST drive systems have been sold since 1995 and more than 30 longwalls are currently in operation worldwide. The CST is without doubt today’s most flexible and powerful drive system for high-capacity face conveyors.
The latest development is series 65 technology for both drive frames and CSTs. The intended use is for long face conveyors with extremely high carrying capacity (up to 1200kW per drive) as well as for AFCs for plow systems (DBT’s reliable and fully automated longwall mining system for medium and thin seams) which have to run slow (50:1 ratio) because of plow speeds of up to 3 m/s, yet must allow up to 800kW per drive to be installed. For these tasks DBT has developed class 65 drive frames and CSTs. While the series 65 CSTs are a bigger version of the series 45 CSTs, the series 65 drive frames include some new features.
While the drive frame side walls are thick enough for the higher torque of bigger drives, the typical split sprocket design can’t be used anymore. The necessary single-piece sprocket led to a design with a mounting unit consisting of this sprocket, the shaft, the bearings with their housings and distance rings to fill the space between the sprocket and the bearing housings (depending on the drive frame width).
Both series 65 drive frame and CST 65 are in successful operation at DSK’s Lohberg/Osterfeld mine in Germany. Here the AFC operates with 2 x 800kW at a 50:1 ratio.
This technology will allow higher installed horsepower for face conveyors in shearer faces by increasing the rating of each drive or a reduced number of gearboxes and motors using the same total horsepower (two drives instead of three).
The PF 4/PF 5 system with the intelligent CST drive system and state-of-the-art drive frame technology continue to set the standard for high-capacity, high-horsepower face conveyors in underground longwall mining worldwide.
*Dr. Uli Paschedag is senior chief engineer projects/technical acquisition for the DBT Group of Companies. He is based at Lünen, Germany.