Underground » Mining Technology and Production
Evaluation of the downtime statistics of two longwall mines studied as part of the Landmark Automation Project showed that AFC (Armoured Face Conveyor) and BSL (Beam Stage Loader) chain related failures accounted for 27% of all downtime. Virtually all of these failures were caused by inappropriate chain tension. That is, insufficient design factor of safety, and excessive or insufficient pretension. At the same time the longwall industry is demanding longer, higher powered AFCs for new mines and upgrading equipment at existing mines to satisfy the desire for increased production.
Traditionally chain tension is inferred from the measurement of related parameters such as drive motor currents and visual observation or instrumentation of the carry side chain as it is disengaged from the tailgate sprocket. These methods shed no light on the sharing of load between the chains at points of interest around the conveyor, the dynamic tensions created by the drive sprockets, or the overall mechanical and electrical efficiency of the drives and sprockets during heavily loaded starts and normal running conditions. An understanding of the average and peak chain tensions is necessary to establish the real factors of safety for the strength and fatigue life of the chain.
A chain tension meter was designed, certified Ex ia for use in ERZ0 sections of Australian coal mines and successfully field trialled on a longwall face during production. The dynamic behaviour and unequal sharing of load between the chains were recorded and found to be significant. The changing format of tension distribution around the conveyor as it is loaded was recorded and the implications of the change in the location of the point of minimum tension from tailgate to maingate as the conveyor is unloaded emphasised.
The trial was run monitoring AFC chain tensions over a few shifts. During this time the robustness of the tension meter electronics and protection housing was demonstrated. Poor roof conditions and maintenance activities resulted in a maximum production rate of around 30% of nameplate rating. The field trial conducted in this project was aimed at proving the chain tension meter could be used to measure dynamic chain tension in the longwall production environment. Quantitative information on specific AFC design parameters and assessment of the effect on the equipment of different operating practices will require a number of well controlled trials. These trials should be conducted at a sites utilising a variety of equipment and experiencing different operating conditions.
The project did experience a number of delays. Access to the trial face was the major contributor. It is believed that similar delays can be avoided in future face trials by heavily involving the host site in the design of the test program prior to committing to a trial. The demonstrated robustness of the equipment will also assist.
The project has successfully demonstrated that the measurement of AFC chain tensions on a working face can be achieved reliably. Targeted trials are now required to provide the industry with the information necessary to make informed decisions on the procurement and operation of chain conveyor equipment. Measurement of the tensions in a number of AFC chains will provide information to improve the understanding of the equipment, calibrate the more sophisticated chain conveyor design programs available to the industry and assess the effects of operating configurations and practices on the life of the equipment.