The Case of the Rock and the Hard Spot
Synopsis:
A stitch in time saves nine was a foreign concept to the operator of a gravel pit, and the resulting tear was costly.
Background:
It was claimed that a vandal threw a bear trap into a rock crusher at night and that the next morning an employee started the crusher without seeing the trap, resulting in destruction of the crushing components and damage to the machine structure.
Assignment:
TI was requested to determine the cause of and extent of damage.
Analysis:
This is known as a cone crusher where a cone of hard but very tough steel oscillates within an inverted bowl of similar steel, crushing rocks to progressively smaller size as they move from the top of the cone to the bottom where they are discharged onto a conveyor. The Insured stated that the remnants of the bear trap and chain were thrown into brush at the edge of the gravel pit but a thorough search of the area found no evidence of the trap, its chain or scrap metal of any kind. No police report was filed even though the initial service call itself amounted to $12.5k and there was major unrepaired damage to the machine itself beyond the replaceable wear components. TI took detailed measurements of the damaged cone and bowl as well as detailed photos of the damaged areas and then found new replacement parts at a regional dealer with which to compare them. The damaged cone and bowl were worn well beyond the point at which the manufacturer recommended replacement and the tear in the bowl was found to be the result of thinness due to wear.
The edges of the tear were well-rounded showing that the tear propagated slowly over time, allowing rock in the crusher time to round the edges. Had the tear occurred instantly due to tramp iron in the crusher the edges would have been sharp.
