Origin and Cause
Power Shovel Fire
One of the largest, completely mobile, earth moving machines in the world was employed at a mine for the purpose of removing overburden to provide acess to the underlying coal seam. The scoop of this power shovel would easily accommodate a Greyhound bus. The excavator was powered by two 3,000 horsepower diesel engines that provided hydraulic power for movement and digging, and electrical power for lighting and battery recharging.
The excavator was examined to determine the cause of a fire. Two fractured connecting rods that penetrated the walls of the cylinder block were observed on one of the engines. The resulting fire consumed practically all of the combustible material located within the rotating equipment section of the excavator.
Often the cause of connecting rod fracture is traceable to destruction of the connecting rod bearing. Commonly, the cause of a connecting rod bearing failure is the spinning of a crankshaft main bearing. The bearing inserts spin when lubrication of the journals is inadequate and the resulting friction heat results in bonding the inserts to the journals. The oil supply to the rod bearings is generally fed through the main bearings of large engines. If the oil flow is disrupted by a seized and spun bearing, the adjacent connecting rod bearing will seize and the rod will be broken.
Disassembly of the burned engine revealed that two main bearings had spun which resulted in the fracture of the connecting rods. An unusual pattern was observed on the mating surfaces of the other main bearings and crankshaft journals. This pattern is commonly described as being similar to a crow’s foot. The cause of such a pattern is electrical current being conducted between the crankshaft and the main bearings through the oil film. The electromagnetic shaft current is induced by rotation of the crankshaft or inherent electric charge. The current flow results in bearing erosion and finally in fusion of rotating and stationary components.
The high temperature generated by friction in the bearing resulted in the subsequent fire when the interior of the engine block was opened to air after the connecting rods penetrated the walls.
The root cause of the fire was determined to be the lack of sufficient electrical grounding between the crank shaft and the frame of the machine.
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