Design Modification Creates Hazard
Case Synopsis: An engineering firm designed and built a process system with an elevated operator platform that also served as a loading platform. Sliding gates along one edge of the platform were normally opened while loading materials at the start of the process, but were then closed and latched during production. The sliding gates were intended to provide the same level of fall protection as the guardrails along the other edges of the platform. The sliding gates were closed and latched when an operator went on the platform to obtain a sample for quality control testing. At one point the operator leaned against a sliding gate, which unexpectedly separated from the platform, and caused him to fall to the plant floor.
Expert Analysis: Approved design drawings showed sliding gate rollers mounted on axles welded to supports. The welded axles were intended to secure the gates and prevent them from moving when subjected to the same types of forces as the guardrails. Unfortunately, the sliding gates were not fabricated and installed in accordance with approved drawings and specifications. The sliding gates were fabricated with threaded axle bolts instead of welded axles. The threaded axle bolts were not secured with locking fasteners, pins, or any mechanism to prevent them from loosening. Consequently, a threaded axle bolt gradually loosened and backed out of its mounting hole. As a result, when the operator leaned against the sliding gate, the roller disengaged from the axle and the gate swung away from the platform. An engineering analysis determined that the accident occurred as the result of an unapproved, undetected design modification.
Result: The case was resolved.