Chain shot injures operator behind cab window
In 2004, the operator of a mechanical harvester was trimming the butt end of a 12-inch log. The saw bar and chain were directly in line with the front window of the operator's cab. The saw chain had been repaired with used connecting links. While the saw chain was operating at high speed, the chain broke. The whipping action of the broken chain end then caused a drive link to separate from the chain. It penetrated the ½-inch polycarbonate windshield of the cab. This "chain shot" struck the operator, causing severe internal injuries.Conclusions
Findings as to causes
- Saw chain failure: The saw chain from the mechanical harvester saw broke as the butt end of a 12-inch fir log was being cut. A drive link (with two side links attached) broke free and was hurled towards the cab of the forest machine, penetrating the cab window and striking the operator.
- Saw bar orientation: The mechanical harvester operator oriented the saw bar in line with the forest machine cab while attempting to square the butt of the log.
- Inadequate windshield protection: The windshield of the forest machine cab made of ½-inch thick polycarbonate was insufficient to protect the operator from chain shot involving ¾ pitch saw chain.
Findings as to underlying factors
- Saw chain repair: Chain repairs had been made with used saw chain parts. Some rivet heads were inadequate in both volume and shape, leading to a weakness in the chain strength.
- Saw chain speed: The saw chain speed exceeded design limits. Higher speed increases the likelihood of breakage.
- Saw chain tension: The saw chain was tensioned by hand. Recommended tension was 150 pounds. It is very difficult for an operator to gauge the force on the lever used to tension the chain.
- Excessive sprocket wear: The wear on the sprocket spurs was excessive as the sprocket had never been replaced despite a number of chain replacements. This contributed to saw chain damage and increased the likelihood of breakage.
- Severe side loading of chain: The "drifting" of the cut indicates that the saw bar and chain were severely side-loaded; it is not known if the cause was bending or twisting. In addition, the depth gauges were half the recommended depth. This caused extra loading forces on the saw bar during the downstroke, contributing to side loading.
- Education and training: The education and training for the harvester operator with regard to avoiding chain shot was insufficient. The employer did not have a copy of the manufacturer's recommendations for saw chain repair and maintenance.