高空作業設備旋轉盤鎖固螺栓斷裂原因分析

Failure Analysis of Slewing Ring Bolts in an Aerial Work Platform

自國外進口之中古高空作業設備操作時，因旋轉盤上的8顆緊固螺栓全數斷裂，導致銜接的伸臂倒塌，連同高空作業中勞工墜落罹災。關於此件職業災害事故，本文將從材料破損分析的觀點，探究螺栓斷裂之主要肇因。分析方法涵蓋事故背景調查、螺栓斷裂面巨觀及微觀觀察、金相組織分析、硬度量測，以及螺栓受力模式討論等。分析結果顯示：8顆螺栓斷裂形貌呈現疲勞破壞與過負載破壞兩種形態，其中疲勞破壞主要集中於負載較高之螺栓，顯示在長期反覆應力作用下，疲勞裂紋容易從螺牙牙谷處開始形成，並逐步成長擴展終至完全破斷。由於部分螺栓因疲勞斷裂後，原先的負載轉嫁至其餘螺栓承受，但是，其承受應力已超過螺栓材料之抗拉強度，進而相繼發生過負載斷裂。此結果顯示雇主在實施自動檢查時，對於螺栓等關鍵零組件之定期檢測與更換至為重要。

An occupational accident happened to an imported and used aerial work platform for its eight bolts securing the slewing ring bearing failed. This failure caused the collapse of connected boom arm of elevated platform and resulted in the worker's death by falling. This study investigated the primary cause(s) of the bolt fracture through a material failure analysis, which included collecting mechanical design and maintenance records, macroscopic and microscopic examination of fracture surface, metallographic analysis, hardness measurements, and evaluation of the bolt loading conditions. The results revealed two distinct types of fracture mechanisms among the eight bolts, i.e., fatigue fracture and overload fracture. Fatigue crack predominantly occurred at certain bolts subjected to higher cyclic loading, initiating at the thread roots due to prolonged repetitive stress and gradually propagating until final failure. Once a part of bolts failed by fatigue, the originally distributed load was abruptly transferred to the remaining bolts, which consequently experienced stresses exceeding their tensile strength, ultimately resulting in immediate overload fracture. These findings underscore the importance of a regular inspection and perhaps replacement of essential components, particularly for those used aerial work platforms imported from abroad, to assure operational safety and prevent catastrophic failure.