Chain Shot Primer
— Jerry Locker
Originally appeared in Canadian Forest Industries, Sept/Oct 2012 (reprinted with permission)
‘Chain shot’ is not a new term to the forestry industry. However, events within the last two years have brought this term to the forefront of our industry. Chain shot is the high-velocity separation and ejection of a piece or pieces of saw chain from the end of a broken loop of saw chain in mechanized timber harvesting. Chain shot typically originates near the drive end of the cutting system, but can also originate from the guide bar tip area. In either case, it poses the same risk of serious injury or death to the machine operator, ground personnel and bystanders. Saw chain pieces usually travel in the cutting plane of the guide bar, but can deviate to either side. Although the shot cone (a term used by Oregon) reflects the most likely chain shot path, deflection can occur, substantially expanding the distance saw chain pieces may travel.
How Chain Shot Happens
After a saw chain break, the free end of the saw chain begins to whip away from the break.
If the saw chain is not contained by the saw box or by a chain shot guard, the broken saw chain’s free end can speed up rapidly, carrying immense dynamic energy.
At the peak of the whip, saw chain parts may break loose and be ejected at high speed, especially if the free end of the saw chain strikes the saw box.
Can Chain Shot Be Eliminated?
In simple terms, no, it can’t. Properly designed chain shot guards and shields reduce the danger of a chain shot from the drive sprocket area. However, there is currently no known way to place similar guards in the bar tip area without signifi cant disruption to the cutting operation. Because no guarding is currently possible in the bar tip area, chain shots can be generated and pose the same risk of injury and death as those generated at the drive sprocket area.
To reduce the risk, your equipment should be designed with appropriate guards, shields, and window enclosures and care should be taken to minimize the exposure of the machine operator, ground personnel and bystanders to the cutting plane of the cutting system and shot cone. The mechanical timber harvesting industry advises ground personnel and bystanders to stay at least 70 metres (230 feet) away and outside the shot cone of a working harvester. Chain shot projectiles travelling at the speed of a bullet can travel far beyond the recommended setback distance. The setback distance will help reduce the risk of a chain shot injury, but not eliminate it.
It is important that windshields and windows be made of appropriate material. Test results from SMP Svenska Mankinproving AB found that 12 mm material is being penetrated by chain shot. For 19 mm Polycarbonate – LEXGARD MP750 Laminate, projectiles penetrated and caused a 5 mm deformation on the rear surface of the window. The test on 19 mm Polycarbonate/Acrylic – LEXGARD MP750 laminate found that projectiles penetrated the outer polycarbonate layer, but were contained by the acrylic layer and the rear polycarbonate layer partially delaminated.
As for 32 mm Polycarbonate – LEXGARD SP-1250, projectiles penetrated to a maximum depth of 18 mm.
Chain shot guards and chain catchers should be in place and in good working condition. Refer to your equipment manufacturer for details. Do not exceed the manufacturer’s recommendations for chain speed. Cutting systems must be inspected frequently. Cutting systems must be maintained in agreement with manufacturer’s recommendations. Dull, damaged saw chains should be immediately removed from service for inspection, repair or replacement. Saw chain loops, which have broken twice, must be removed from service. Saw chains must be maintained in agreement with the manufacturer’s recommendations. Guide bars must be cleaned and dressed on a regular basis in agreement with manufacturer’s recommendations. Drive sprockets must be replaced when observed wear exceeds manufacturer’s recommendations. Drive sprockets must be aligned with the groove of the guide bar. Ensure the saw chain/guide bar lubrication system is functioning properly: 0.404-pitch cutting systems should use approximately 7.6 litres (2.0 gallons) of guide bar/saw chain lubricant or more per eight-hour shift in harvesting operations – more when used in processing operations. Three-quarter-pitch cutting systems should use approximately 9.5 litres (2.5 gallons) or more per eight-hour shift in a harvesting application – more when used in processing operations.
• Never engage in a cut with the machine operator, ground personnel or bystanders in the shot cone zone.
• Always engage in a cut as close to the ground as possible.
• Always use new parts when assembling and repairing saw chain.
• Maintain saw chain in agreement with your manufacturer’s recommendations.
• Never force a dull saw chain to cut. Sharp chain places less wear and tear on the cutting system.
•Saw chain should be sharpened or replaced with a sharp chain at least once per operational shift, or more if damaged.
• Depth gauges (rakers) must be maintained through the life of a saw chain.
• Never exceed your saw chain manufacturer’s operation recommendations.
In cold cutting conditions:
• Use a lighter-weight lubricant, if possible, doubling the flow rate.
• Periodically cycle the guide bar without cutting (air cuts) to increase lubricant present on the cutting system.
• Reduce bar feed force.
Conduct proper guide bar maintenance:
• Clean the guide bar groove from bar tip to bar tail, and keep the oil hole open.
• Turn the guide bar over to equalize wear on a daily basis.
• Cycle the guide bar several times to remove moisture from the guide bar tip.
• Ensure you maintain proper chain tension, checking it often.
• At breaks and at the end of each shift, relieve saw chain tension to prevent damage to the guide bar tip, saw motor and/or the saw chain as the saw chain cools and contracts.
• Reduce saw chain speed.
In simple terms, your harvester head, when operational, should be treated as if it is two loaded guns (the bar tip and the bar tail) that pose a risk of serious injury or death to the machine operator, ground personnel and bystanders.
The risk of a chain shot event cannot be eliminated, but the risks can be reduced by following the recommendations provided by your equipment manufacturer, your cutting system manufacturer, and the operational recommendations presented here.
Jerry Locker is the OEM harvester manager – North America for Oregon. Additional details regarding Harvester products, technical and safety information and Mechanical Timber Harvesting Service School, and links to websites offering additional information on chain shot are available at www.oregonchain.com/ harvester.