Project measures life spans of staples in longwall mines

AN ONGOING project by ACARP aims at providing a method to measure the life expectancy of a wide range of staple types and sizes and permit more informed assessment of staple replacement strategies.

Lou Caruana

A typical longwall installation might use in excess of 10,000 staples, with every one potentially subject to fatigue failure. Industry wide expectation for staple fatigue life is approximately 50,000 cycles.

The project carries on from a previous project which aimed to accurately determine the fatigue life of just one staple configuration. Due to the prevalence of DN20 staples in Australian longwalls, and perceptions that this staple size fails frequently, DN20 square section spring steel staples were adopted for the initial test program.

This project was aimed at repeating the initial tests for D-section 420 stainless steel staples, which are sometimes regarded as a better product. DN20 sizing was retained to permit a direct correlation between the results of both tests.

The fatigue life of D-section stainless steel staples was measured for two test pressures; 250 Bar and 510 Bar. The median fatigue life is approximately 76,000 cycles over this pressure range. A 0.14% probability of crack initiation exists after approximately 14,600 cycles.

The measured fatigue life of the tested D-section stainless steel staples did not vary appreciably with the magnitude of the cyclic pressure. This trend was also observed in the previous testing conducted on the square section spring steel staples and is largely attributed to the yielding of the staple legs at very low pressures (approximately 120 Bar).

A favourable “knee” was observed in the results plot and is hypothesised to be caused by minor variations in the materials, fits and the manner in which the staple yields, conforming to its physical surroundings. It is likely that these variations lead to a changed loading mechanism for some staples.

Because bending stresses in DN20 square section spring steel staples exceed yield, the test results are not transferable to other staple configurations; testing of other staple configurations will be required to establish their performance characteristics.