Published in June 2008 Australian Longwall Magazine

The test at Moranbah was the first time a battery-powered data logging device had been used to measure real-time AFC chain tension on an Australian production face.

The meter was designed and built by Brisbane-based mining research centre CRCMining, jointly funded by CRCMining and the Australian Coal Association Research Program (ACARP).

While the project has taken some time to get off the ground with delays in achieving Intrinsic Safety certification for the battery-powered logger and access to a trial face, the CRCMining team was determined to pursue the project given the amount of money AFC downtime costs the Australian longwall sector.

The root cause of AFC downtime events and accelerated wear in chain and guides is suboptimum chain tension, which eventually results in poor longwall reliability and high operating costs.

During the ACARP-funded Landmark Automation Project an evaluation of the downtime statistics of two longwall mines were studied. Analysis found AFC and Beam Stage Loader (BSL) chain related failures accounted for 27% of all downtime. Virtually all of these failures were caused by inappropriate chain tension.

"Due to the complexity of obtaining chain tension during operation, traditionally AFC manufacturers and longwall engineers alike have had to depend on static tension measurement, observed chain slack at the tailgate and recorded motor currents to make critical design and operating decisions," CRCMining researchers said.

"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."

In answer to this, CRCMining has developed a real-time tool. The AFC Chain Tension Meter consists of a matched pair of five-link chain sets, a total of four strain gauges, a data logger device and a protective enclosure. The strain gauge leads are waterproofed and protected by the enclosure.

CRCMining said the protective enclosure (protection flight bar) was built to the same cross-section of a normal flight bar.

The data logger has enough capacity to log two channels at 150Hz for at least 12 hours. If more data is required, the entire logger is unplugged and removed from the protection flight bar enclosure and replaced with a fully-charged unit with fresh memory.

The logger with stored data is then taken to surface where the data is downloaded to a PC and the batteries recharged.

To test if the meter worked in the harsh underground environment, CRCMining undertook field trials. First of all the researchers had to test the durability of the protection flight bar, and to make sure it would not interfere with any of the AFC system components.

This test was conducted without the data logging system fitted and was carried out in July 2006 by MNM maintenance staff.

With the durability proven, researchers then moved to stage two testing to measure the tension in the chain at all locations around the AFC while it was in production.

The test was conducted in June last year with four shifts of data recorded.

"During this time the robustness of the tension meter electronics and protection housing was demonstrated. The dynamic behaviour and unequal sharing of load between the chains were recorded and found to be significant," the researchers said.

"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 proved the meter was reliable in a production environment and CRCMining has since been awarded further funding by ACARP to measure chain tension at multiple sites.

"Our experience in the field test shows that a specific set of controlled experiments under various operating conditions is required. Without this controlled environment, too many simultaneous changes to the many variables associated with the AFC cloud the relationship between cause and effect. If properly planned, the required testing at any one face should take less than one shift."

The researchers are now looking to gain commitments from interested mines to host the trials.