Harvester Safety Information

What is Chain Shot?

Chain shot is the high velocity separation and ejection of a piece or pieces of cutting chain from the end of a broken chain in saw chain based mechanized harvesting systems. Chain shot potentially exposes both machine operators and bystanders to a risk of serious injury or death. Chain shot typically occurs near the drive end of the cutting system but can also come from the bar tip area. 

How Chain Shot Happens

First, the saw chain breaks.

Second, after a saw chain break, the “free” end of the saw chain begins to whip away from the break.

Third, 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.

Finally, at the peak of the whip, saw chain pieces may break loose and be ejected at high speed.

Anatomy of a Harvester Cutting System

a.) Drive sprocket
b.) Chain catcher
c.) Chain shot guard
d.) Outside mount pad
e.) Pivot arm mount pad
f.) Direction of chain travel
g.) Bar groove
h.) Guide bar
i.) Inside mount pad

Chain Operator and Bystander Safety

There is risk of a potential serious injury or death to the machine operator, ground personnel, and bystanders from chain shot. A Chain Shot Event (CSE) occurs when a piece or pieces of cutting chain from the end of a broken saw chain in mechanized timber harvesting or processing is ejected at a high velocity. Chain shot typically originates near the drive end of the cutting system but can also originate from the guide bar tip area. Saw chain pieces usually travel in the cutting plane of the guide bar, but can deviate to either side. Although the "Shot Cone Zone" reflects the most likely chain shot path, deflections can occur, substantially expanding where chain pieces may travel. To minimize risk, operators should keep out of the Shot Cone Zone. Ground personnel and bystanders should be at least 230 feet away from cutting operations and out of the Shot Cone Zone.

Do not use saw chain that:

• Has broken twice.
• Is severely damaged.
• Has excessive saw chain stretch.
• Has broken or cracked components.
• Has loose rivet joints—if you can rotate the rivets with your fingers, they are too loose.

 

Operational Recommendations

• Never engage in a cut with yourself, ground personnel, or bystanders in the Shot Cone Zone. 
• Always cut as close to the ground as possible to reduce the distance that any potential ejected pieces could travel.
• Your cutting systems operates in challenging environments and depend on sufficient lubrication to minimize wear and extend the life of operation.
• Check the tension of your saw chain regularly. The recommended amount of force applied by the guide bar to tension saw chain.

 Note: Following these best practices consistently will increase your safety and the life of your cutting system.

 

Chain Speed, Feed Force, Service Life & Safety

It is well recognized that higher chain speeds and/or applied feed forces (with attendant power input) generally equates to faster cutting speeds and shorter service life of the saw chain, guide bar, and drive sprocket. In general, higher chain speeds result in increased potential for chain breakage and potential injury. Never operate chain at speeds higher than recommended.

Minimizing the Risk of a Chain Shot Event

For maximum protection, machines should be equipped with the appropriate safety attachments. Please follow the recommendations below and reference your equipment’s operator manual.

 

Guards & Shields

Because of the high speeds, high stress, heavy loads, wear factors, and varying levels of repair and maintenance given to saw chain-based mechanized wood harvesters/processors, there is a possibility that saw chain or saw chain pieces can be thrown from the machine at high speed and velocity. Machine operators, ground personnel, and bystanders are exposed to a risk of serious injury or death. Equipment should be equipped with appropriate guards, shields, and window enclosures to minimize the exposure of the operator, ground personnel, and bystanders to the cutting plane and Shot Cone Zone of the cutting system. Bystanders and ground personnel should be kept at least 230 feet way.

 

Windows 

• The glazing of the operator's enclosure window should comply with local codes for impact resistance. The glazing should be replaced if any scratches (or other damage) obstructs the operator's effective viewing of the cutting operation.
• It should be noted that the UV portion of the light spectrum degrades the properties of polycarbonates (i.e. through time your protection decreases). Consult your equipment manufacturer or replacement glazing supplier for recommended replacement interval.
• Before upgrading, always check with your equipment manufacturer as it may change the operational integrity of the operator enclosure. 

 

Chain Catcher

A chain catcher can assist in containing thrown saw chain, and is a complement to guards and shields. Replace worn out chain catchers. Please follow the recommendations in your equipment manufacturer’s operators manual.

 

Chain Shot Guard

A chain shot guard is a piece of material mounted behind the drive sprocket. This guard performs two functions:

• Absorbs the energy of a broken saw chain coming in contact with the saw box, and is designed to prevent saw chain parts from breaking off and being ejected.
• Acts as an extension of the saw box, designed to reduce the opportunity for thrown saw chain or saw chain parts to escape the saw box.