A blower used to exhaust air from an industrial process stopped functioning when the blower wheel drive shaft fractured. The process, and thereby most of the plant, had to operate at a reduced volume until the blower wheel could be replaced. The blower wheel had been installed during a shutdown a week before the incident. The blower wheel was a spare installed when the existing blower wheel was sent for scheduled remanufacturing.
The blower wheel shaft fractured as a result of fatigue cracking. The crack initiated at a machined step in the shaft. The blower wheel input shaft diameter decreased from 18 inches at the wheel end to 4 inches at the shaft end in three machined steps with sharp edges.
The existing blower wheel that had been replaced was a different design with an input shaft that had one smoothly radiused decrease in diameter rather than sharp steps. The steps in the spare blower wheel input shaft created stress risers at the sharp machined edges of the steps, areas where stresses are concentrated rather than distributed evenly through a material.
The installation of a blower includes connection of the blower wheel drive shaft to the blower motor drive shaft. The mechanical coupling between the two shafts has some limited flexibility to reduce bending stresses in the shafts from slight misalignments. Careful alignment of the motor to the blower is necessary to reduce the bending forces acting on the shafts as they rotate. These bending forces cause wear to the bearings in both the motor and the blower in addition to fluctuating bending stress in the drive shafts. In this case, the blower wheel was replaced by skilled millwrights who used a laser alignment tool to align the motor and blower to standard industry tolerances.
Investigation revealed that the spare blower wheel was an earlier design that was previously used without problem in the blower. It was replaced by the blower wheel with the smoothly radiused decrease in shaft diameter as part of routine maintenance during an earlier shutdown. The manufacturer had improved the blower wheel shaft design because of fatigue failures in other shafts. The improved shafts with the smoothly radiused decrease in shaft diameter required additional machining but did not have the stress risers caused by the machined steps with sharp edges.
Evaluation of the fractured shaft determined that the design with the machined steps violated the engineering principle of eliminating stress risers whenever practicable, a basic principle taught in sophomore level machine design courses. The original design of the shaft could work if the shaft happened to be almost perfectly aligned to the motor drive shaft but was not robust enough to avoid fatigue fractures when the alignment was not as precise, but still within industry standards. The original design also was not as robust in its ability to withstand the imbalances in the blower wheel that increase between maintenance cycles. The blower wheel drive shaft design was determined to be the root cause of the incident.
John Phillips, senior consulting engineer at Warren, has more than 30 years of crane and heavy equipment experience and more than 20 years of experience in forensic engineering. A licensed professional engineer in South Carolina, North Carolina, Georgia, Louisiana and Ohio, he’s NCEES registered both as a model engineer and with The United States Council for International Engineering Practice, USCIEP. John has designed crane systems, supervised installation, tested and certified lifting equipment even serving as a project engineer for maintenance and certification of nuclear weapon lifting and handling systems. He is a certified fire and explosion investigator and fire and explosion investigator instructor by the National Association of Fire Investigators. John is a member of the American Society of Materials and American Society of Testing and Materials, as well as a voting member of ASTM Ships & Marine Forensic Sciences, Forensic Engineering, and Performance of Buildings committees.