Germany's leading longwall equipment manufacturer Deutsche Bergbau Technik continues to refine its AFCs using state-of-the-art design and testing methods and facilities. By Uli Paschedag*
Modern DBT face conveyors in operation today are up to 450m long with carrying capacities of more than 5000 tonnes per hour. Sophisticated 3D CAD software is used for the AFC design and this ensures compatibility between all components and also between DBT and third party components. The current state-of-the-art line pan is the PF 4.
The split-profile design of the PF 4 line pan permits design and manufacture of different types of pans with various widths and deckplate thickness to best suit the application. The first PF 4 pan had a width of 832mm which has since been extended to 1332mm. Deckplate thickness (30- 50mm) has typically been 40mm.
A major advantage of this design is that of the four welds on each side of the pan only one is located in the wear area, leaving the other three welds in a virtually wear-free environment. This increases pan stability significantly, a considerable advantage particularly in the latter stages of the pan's life where high-horsepower shearers are employed. Typical bending capabilities of the line pans are ±6° vertical and ±0.8-1.2° horizontal, which allow the AFC to follow undulations in the floor easily.
The split-profile design also enables dogbone pockets to be welded in from the outside without weakening the pan. Profile strength is greater than any pan designed by DBT previously. This unique method of welding in the dogbone pockets from the outside means they do not protrude beyond the pan profile, resulting in the pan being the same width from pan end to pan end.
The dogbone has a typical breaking strength of 3000kN and the pockets are designed and tested for a safety factor of at least 1.5 at the weakest point. In other words, the connector always breaks first. The profiles are normally made out of rolled material to provide the smoothest possible surface. Extensive tests (both at DBT's own test facility and underground) have shown a lower friction factor than cast material.
Unlike the typical Sigma profile, the PF4 pan profile was rounded according to results of a research study that had been carried out by the Clausthal University in Germany. Different profiles were tested for friction between profile and flight bar, and the rounded profile showed minimum friction and consequently lower power requirements due to a maximum contact area between flight bar and profile.
Extensive underground tests in Germany were also undertaken to determine the ideal material for profiles and deckplates. In deference to the common thought in the early 1990s, the hardest material was not the best for wear resistance. Instead a high-strength manganese-based steel showed minimum wear especially as the production rate increased over time. This material's surface hardened as more and more coal was conveyed. Therefore this type of material is referenced as "self-hardening" or "wear-hardening". The maximum pan life in very good conditions has been in excess of 20 million tonnes (and still in operation) with a 40mm deckplate.
The pan connection is designed to take in excess of maximum forces from the coal face induced through the mining machine where transversal forces of more than 4000kN can be endured. In addition to the dogbone pockets being used to restrain this force transfer, the deckplates have a key interlock which further stabilises the pan. The bottom plate underneath the pan with a typical thickness of 25mm is rounded at the connection between the pans to reduce noise during the conveyor operation.
Inspection doors are installed every five or six pans — based on customer needs — to provide access to the bottom chain strand. The doors can be withdrawn either from the gob side (majority of installations) or face side, depending on the application.
Increasingly difficult mining conditions being encountered by longwall operators have been the catalyst for further AFC developments. In particular, the need to convey much larger rock content triggered the development of the PF 5 pan in 1997. The basic design principles and material characteristics are the same as for the PF 4 pan, but the profiles are bigger and the typical deck- plate thickness is 50mm. The dogbones have a breaking strength of more than 4500kN each. This type of pan delivered to date has typically been 1342mm wide, but is available in different widths. PF 5 conveyors are successfully being used in the USA and Australia.
DBT has many successful longwall systems operating with shearers and has developed shearer haulage systems which are integrated into the AFC system. The former standard system is the Dynatrac utilising a set of cast guides and a special chain. This system has been improved and upgraded for high-horsepower shearers to the Rhinoride system in response to the requirements of higher haulage forces. As for heavy-duty forged rackbar systems, DBT offers the "Jumbotrack" system, which allows shearer pulling forces in excess of 1000 kN.
Head-gate drive frames in operation today are typically of cross-frame design. Different capacity ratings are available depending on the total required installed horsepower. This results in different frame widths and shaft diameters and widths for each specific application. The frame side plates are especially thick to accept the high torque that may be generated and to ensure adequate safety factors. Flange plates are used to mount the transmission units and to enable easy rehanding.
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.
* Dr. Uli Paschedag is senior chief engineer projects/technical acquisition for the DBT Group of Companies. He is based at Lünen, Germany.