August 7, 2014 - No Comments
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 mechanised 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.
by Jerry Locker – Global Harvester Technical & Training Manager, Blount International Inc.
The typical chain shot projectiles are shown in Image 1.
Saw chain pieces usually travel in the cutting plane of the guide bar (Image 2), but can deviate to either side (Image 3). Although the shot cone (an Oregon® term) reflects the most likely chain shot path, deflection can occur; substantially expanding where 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 (image 4).
If the saw chain is not contained by the saw box or a chain shot guard, the broken saw chain’s free end can speed up rapidly, carrying immense dynamic energy (image 5).
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 (image 6).
Can chain shot be eliminated?
In simple terms ‘No!’ 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 significant 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.
The mechanical timber harvesting industry recommends that bystanders stay at least 70 metres from the harvester to reduce the risk of injury. However, chain shot parts can ricochet off objects in their path and come out of the plane of the bar. Just because a bystander is out of the plane of the bar does not eliminate the risk of a chain shot injury; however, it does reduce it.
Although chain shot cannot be eliminated, the risk can be reduced.
Reducing the risk of ‘chain shot’
Equipment should be designed with appropriate guards, shields, window enclosures and cases to minimise the exposure of the machine operator, ground personnel and bystanders to the cutting plane of the cutting system and shot cone.
Ground personnel and bystanders should stay at least 70 metres 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.
Windshields and windows must be made of appropriate material as test results from SMP Svenska Mankinproving AB found that:
- 12mm material was penetrated by chain shot.
- 19mm polycarbonate – LEXGARD® MP750 laminate: projectiles penetrated and caused a 5mm deformation on the rear surface of the window.
- 19mm polycarbonate/Acrylic – LEXGARD® MP750 laminate: projectiles penetrated the outer polycarbonate layer, but were contained although the acrylic layer and the rear polycarbonate layer were partially delaminated.
- 32mm polycarbonate – LEXGARD® SP-1250: projectiles penetrated to a max. depth of 18mm.
Guards and shield
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 recom–mendations for chain speed. For Oregon® harvester chains, the recommended chain speeds are Oregon® .404-pitch harvester saw chain – 40 m/s (8 000ft./min) or less, and Oregon® ¾-pitch harvester saw chain – 30m/s (6 000ft./min).
- Inspection of the cutting system (saw chain, guide bar and drive sprocket)
- 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.
- Discard a saw chain that has broken twice.
Guide bars must be cleaned and dressed on a regular basis in agreement with manufacturer 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.
- .404-pitch cutting systems.
- Should use approximately 7.6 litres (2.0 gallons) of guide bar/saw chain lubricant or more per 8-hour shift in harvesting operations, more when used in processing operations.
- ¾-pitch cutting systems:
- Should use approximately 9.5 litres (2.5 gallons) or more per 8-hour shift in a harvesting application, more when used in processing operations.
Saw chain tension
- Maintain proper chain tension.
- Automatic tensioning systems:
- For the Oregon® .404-pitch chain we recommend 490N (110lbs) force on the bar to tension chain.
- For the Oregon® ¾-pitch chain we recommend 668N (150lbs) force on the bar to tension chain.
Manual tensioning systems
- 3 mm (1/8 inch) gap at the centre of the guide bar when grasped by hand and pulled away from the guide bar rails.
- 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; and
- Always use new parts when assembling and repairing the saw chain.
- Maintain the saw chain in agreement with your manufacturer’s recommendations;
- Never force a dull saw chain to cut. A sharp chain places less wear and tear on the cutting system;
- The 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; and
- 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 equalise wear on a daily basis; and
- At shutdown/during extended breaks or equipment maintenance, 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; and
- Reduce saw chain speed.
Oregon® offers a Mechanical Timber Harvesting Service School, which focuses on current industry concerns, harvester products, recommended maintenance practices, and product trouble shooting. A similar school is available for chainsaw-based cutting systems. Additional details are available at www.oregonchain.com/pro/service/service_schools.htm.
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 a serious injury or death to the machine operator, ground personnel, or 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.
*Additional details are available at: www.oregonchain.com/harvester.
**Published in April 2014