Underground » Health and Safety
A common hydraulic fitting used in longwall systems is the Staple Loc type, available in many sizes from numerous manufacturers. A typical longwall installation might use in excess of 10,000 staples, with every one potentially subject to fatigue failure. Prior to the ACARP project, no test data was available to enable statistically highly confident fatigue life prediction of staples.
ACARP Project C19011 aimed to accurately determine the fatigue life of just one staple configuration. An intention of the test work was to establish meaningful testing protocols for use with additional staple configurations. Specifically, there was an intention to establish a pressure-life (P - N) curve to allow fatigue life prediction at any pressure and load spectrum. It was anticipated that this project would be the first of possibly three or four aiming to provide a method to calculate the statistical life expectancy of a wide range of staple types and sizes, and permit more informed assessment of staple replacement strategies.
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 test program. This proved both helpful and obstructive. DN20 square section spring steel staples do indeed have poor fatigue performance; with roughly 6,500 cycles mean life rather than the often specified 50,000 cycles. The low fatigue life reduced the duration of the fatigue test program. However, the tested fatigue life is only 6,500 cycles because DN20 square section spring steel staples yield at less than their rated working pressure. This was an unexpected finding, but one which explains the high failure rate with this staple configuration. The P - N data established for the staples tested is not transferable to other staples. Additional testing would be required for other staple configurations, processing and steel chemistry.
Notwithstanding the above, the testing protocols established for this project should be directly applicable to future staple tests, and the fatigue testing of "staple-less" fitting alternatives.