Industry:
Electric Power Company
Vehicle Maintenance Department
Problem:
Bolts broke on the lower connecting arm of a boom lift truck. An
employee was close to 30 feet (9.1m) in the air, working
in the lift bucket. The boom crashed to the ground.
Injuries:
The employee sustained a compound fractured arm, multiple cuts and
bruises.
Damages:
Several thousand dollars in damages to the truck, boom, and lift
bucket. There were additional losses to the company in
man-hours by the employee and crew.
Observations:
Initial visual examination of the failed fasteners indicated a size,
grade, and type normal to this application. The identification
marks of the manufacturers were consistent with those
believed to provide reputable products. The remaining
portions of the threads were noticeably elongated, and
the fracture surface was smeared, typical of a shear.
No corrosion was apparent.
Investigation:
Independent lab tests showed the fasteners normal as to chemical,
physical, and dimensions (except for the fracture area).
However, interviews with the maintenance foreman and
his people, coupled with a review of their latest preventative
maintenance manual, revealed a startling practice. It
read, "In order to assure fasteners are secured properly,
tighten the bolt head or nut one quarter (1/4) turn at
each vehicle safety servicing, every three months."
Conclusion:
If a fastener was found loose at the scheduled maintenance interval,
tightening it one quarter (1/4) may or may not make it
properly tight. In the more probable situation of finding
a tight fastener, over tightening it every three months
will elongate and weaken the threads, thus setting up
this opportunity for failure by overload.
Recommendations:
1. We developed a fastener maintenance program addressing each nut
and bolt by specific strength, diameter, thread pitch,
and finish. Each joint connection was then charted, linking
proper tools with the tightening requirements, thus facilitating
the process.
2. Fastener training classes were then held for all maintenance
personnel.
As appeared in Fastener Technology
International magazine. |
