Improperly Machined Crankshaft/Pitman Arm Cause of Chemical Plant Accident?

Chemical plants produce a multitude of end products.  They use a variety of chemical processes to change raw materials into useful products or chemical compounds that will be used by other chemical plants or manufacturers to produce valuable products.  For a variety of reasons, these chemical plant processes can become uncontrolled.  The result is often a fire, explosion or a "runaway" chemical process that causes equipment failures and/or environmental release of process chemicals.

Metallurgical failure analysis and/or root cause analysis is often required to provide the answer to chemical plant fires, explosions and plant and equipment accidents. 

A crankshaft/pitman arm had repeatedly failed in a concrete plant.  Fracture occurred at a transition from a smooth 2 7/8 inch shaft to the threaded shaft end where the pitman arm was connected.  The fracture surface exhibited classic two way bending metal fatigue with multiple fatigue crack origins around the failed shaft circumference.  A measurement of the radius transition from the smooth shaft to the threaded end was achieved with a machined 1/32 inch radius.  This very small transition  radius was much less than the 1/8 inch radius specified on the engineering drawings.

Photograph A: Metal fatigue fracture initiating
at sharp radius transition.

Photograph B: Two way bending metal fatigue
 failure initiating at improperly machined corner.

Failure of the subject shaft was the result of manufacturing defect.

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