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The Case of the Fatally Failed Hinge Pin

Figure 1.  Exemplar Oven with DoorThe Problem:

The subject oven door hinge failed.  The incident chronology indicated a single-event explosion inside the high-temperature and internally pressurized oven while in operation.  The blast occurred on a tray of a mid-level shelf.  At the time of the explosion, the technician had prepared to open the oven door when the hinge pin failed and the door rapidly opened with great force.  The door disengaged from the hinges and knocked the technician across the room, resulting in his death.

Figure 2.  Proper Pin in Door HingeMy Assessment: 

WJMG was provided the ‘subject oven’ and door with ancillary items after the explosion.  WJMG also received an ‘exemplar oven’ which was identical to the ‘subject oven’.  The ‘exemplar oven’ was cleaned and dissembled for visual examinations, measurements and nondestructive testing of hinge parts and attachment locations.  Portions of the ‘exemplar oven’ hinges were compared and tested directly with the ‘subject’ oven steel hinge part that failed.

Figure 3. Improper Bolt in Door Hinge

The Consequences:  

It was reported that during maintenance events over past years, a steel hinge pin was lost and subsequently replaced with a threaded bolt (Figure 3.) until a new steel pin could be obtained.  The bolt that was substituted for the steel pin in the hinge was never replaced and failed as a result of fatigue and non-uniform loadings.  The threaded bolt fractured at the root of the coarse threads.  After repeated loadings, the bolt failed while in use as an interim replacement for the proper steel pin. Figure 2.

The Lessons Learned:

 Industry standards address the improper use of a ‘temporary fixes’ of high-temperature and pressurized ovens.  The maintenance staff was responsible for placing the correct steel pin into the hinge.  Do not substitute steel hinge pins with threaded bolts of unknown shear strength as a temporary fix on a high-temperature and internally pressurized oven.

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Welding Failures I Have Known, Interesting cautionary tales from Metals & Welding  expert, Dr Jesse A Grantham, PE