We were asked to perform the structural integrity and fatigue life of an original and modified longwall mining relay bar.
Longwall mining is used to more efficiently remove underground coal deposits in large rectangular blocks. After a “pass” of coal has been mined, the equipment assembly including the armored face conveyor and shields must be moved forward to repeat the process. The shield, which stabilizes the immediate roof strata where the coal has been removed, is slid forward on a relay bar by a hydraulic cylinder.
Before the shield is removed, it is lowered and the base-lift cylinder is engaged, which picks the front of the shield off the mine floor to more easily slide it forward. Due to the weight of the shield, force exerted by the base lift cylinder, and the height of the front of the relay bar above the ground, the relay bar deflects, causing significant stresses.
- The original design indicate a high stress concentration at the end of the doubler plate as the base-lift cylinder moves over and past this region on the relay bar. This concentration greatly reduced the estimated life of the relay bar.
- Surface fatigue cracks were found at the end of the doubler plate on the top and bottom surfaces.
- In the region of the surface cracks was a reduction in plate thickness of approximately 0.050 inches, which was caused by the repeated base-lift cylinder sliding across and wearing down the relay bar
- A significant gap of approximately 0.1400 inch was found between the upper plate and vertical stiffener in the region of the end of the doubler plate.
The relay bar is constructed of S890Q, which has a minimum yield strength of 129 ksi and minimum ultimate strength of 136 to 159 ksi.
The modified design, which included extending the doubler closer to the end of relay bar, resulted in a movement of stress concentration to a new location. Although located in a region of lower total deflection, the primary membrane stress still exists and causes high stresses at the end of the extended doubler plates.
Finite element analysis results determined the maximum stress and larger section of high stress is located on the bottom plate instead of the upper plate as seen in the original design. To reduce the stress in this location, the center vertical stiffener was extended to the solid section at the end of the relay bar. The analysis of this configuration showed a substantial reduction in stress at this location.
O’Donnell Consulting Performs Structural and Fatigue Analysis on Equipment in Industries including Mining, Energy and Oil / Gas.
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