The Case of the Broken Office Chair Bolt

2014029 aThe Problem:

While in continuous use in a clean workplace, the bolt of an office chair fractured.  When the bolt failed, the occupant fell and was injured. The chair was “highly adjustable” and the manufacturer’s instructions described easy-to-make adjustments.  There were no “warnings” about unsafe conditions inherent in the chair.   Examinations of the chair mechanisms did not reveal wear.  The chair mechanisms and fractured bolt were not contaminated with grease, dirt or substances that could conceal cracks in the bolt surfaces.

2014029 bMy Assessment:  

A workplace office chair should occasionally be examined for damage, missing bolts or ineffective operational features.   Maintenance personnel should make adjustments or repairs to an office chair that becomes structurally deficient and functions improperly.  Laboratory examinations of the bolt and chair mechanisms did not reveal evidence of additional cracks or pending fractures.  The “link end” and attachment mechanisms apparently failed in shear as a result of over-tightening stressed and deformed the threads, elongated the bolt and provided initiation sites and fractured at the root of the threads.  

2014029 cThe Consequences:  

During the investigation, it was determined that the manufacturer did modify the end link connection with a hard thermoplastic material integral with the seat link and a replacement bolt. The change was the topic of a recall. The upgrade was incorporated in new chairs.  A plastic spacer was added to protect the bolts.  

The Lessons Learned:

Visual examinations of workplace chairs can reveal evidence of wear, elongation, cracks, bends or pending physical damage. Workplace office chairs with damaged or broken parts should be discarded.  Ineffective parts should be replaced with proper factory replacement parts. Loose bolts should only be tightened to the proper torque by qualified personnel. 

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