Date of incident: April 2021
Notice of incident number: 2021192870005
Employer: Dry land sort company

Incident summary

At a salvage log collection site, a logging truck operator (worker) was unloading a log trailer from a self-loading logging truck. While the self-loader was lifting the log trailer from the logging truck, the boom of the self-loader broke apart from the logging truck. The worker, who was seated at the self-loader controls attached to the boom, was ejected from the seat, and landed on the ground. The worker sustained serious injuries.

Investigation conclusions

Cause

• Slewing ring bolts broke sequentially. The incident occurred when the bolts securing the turret and upper assembly (the boom and jib) of the self-loader to the slewing ring broke in a sequential manner. The first bolts to fail were on the tension side of the slewing ring. When they failed, the load was transferred to the other bolts in the slewing ring. The other bolts aligned with the boom suffered partial fatigue followed by an overload of the remaining cross-section, which caused them to fail. Consequently, the remaining bolts, those adjacent to the self-loader boom on the boom side, were unable to maintain support of the turret and upper assembly. The turret separated from the slewing ring, ejecting the worker from the operator’s platform.

Contributing factors

• Inadequate torque value applied to slewing ring bolts. The slewing ring bolts were not torqued to values specified by the manufacturer. Due to the bolt shank length and washer thickness, the slewing ring bolts were threaded to the end, resulting in the inability to achieve the specified torque. The bottoming out of the threads may have indicated an adequate torque; however, this would have been a false reading, which resulted in inadequate clamping force between the turret and slewing ring.
• Inadequate equipment specifications. The manufacturer specified that the bolts connecting the turret to the slewing ring bearing were to be ¾-inch-10 bolts (diameter and thread pitch). However, the required length of bolt was not specified in the operator’s service manual. The type and thickness of the slewing ring bolt washers were also not identified. In the absence of specific bolt criteria, bolts of an incompatible shank length and washers of inadequate thickness were used. These factors prevented the necessary torque values from being obtained before the thread run-in bottomed out. In addition, the proximity of the bolts’ thread run-ins to the mating threads also caused larger stress concentrations than if the bolts had more pitches of threads above the mating thread.
• Inadequate equipment inspections. The employer had not performed inspections of the slewing ring bolt torques at the intervals required by the manufacturer. Although the length of bolt shank and washer thickness of the slewing ring bolts prevented achieving the required torques, the required inspections might have identified anomalies or bolts that had been broken or damaged prior to the sequential failure of the bolts at the time of the incident.
• Insufficient standards for the type of equipment. Because the self-loader is a piece of specialized equipment designed and used only for logging operations, it is exempt from the requirements of the Occupational Health and Safety Regulation that apply to other similar but non-logging cranes. The self-loader does not fit in an appropriate category of the CSA Standard Z150-1998 or ANSI Standard B30.22-2005 and as such is not required to undergo the stringent inspection and recertification criteria that apply to similar, non-logging cranes.