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 mechanized timber harvesting. Chain shot 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.
Industry research indicates an average of 1 in 50 broken chains had parts missing that may have been the result of a chain shot event.
How can operators reduce the risk of chain shot ?
1. Operators and bystanders must never be in the plane of the bar when the chain is in motion on the bar.
2. Appropriate windshield material must be installed.
3. Chain speed must be 40 m/s (8000 ft/min) or less for .404 pitch Harvester chain and 35 m/s (7000 ft/min) or less for 3/4 pitch Harvester chain.
4. A chain shot guard should be installed near the drive sprocket.
5. Bystanders must be at least 70 meters (230 ft.) away from the harvester.
6. Chains should be inspected frequently and damaged or cracked chains removed from service.
7. Always use new parts when repairing chains.
8. Industry groups recommend that chains should be discarded after the second break.
9. Remove all dull and/or worn chains from service.
10. Always sharpen chains to factory specifications.
11. Maintain proper bar and chain lubrication.
12. Maintain proper chain tension.
13. Replace the drive sprocket when it has visible signs of wear.
How does chain shot occur ?
A chain shot consists of two breaks in a chain as demonstrated in the computer simulation. First, the loop of chain breaks and forms two ends. One end moves past the drive sprocket or bar nose and is rapidly accelerated due to a whip-like motion of the chain end. The "whip action" causes the second break releasing small parts at super sonic speed.
Chain shot can cause chain parts to be thrown in many directions, especially those along the plane of the saw bar.
How do I know what size bar I have?
Your bar's cutting length (or "called length") is different from its overall length. The cutting length is the distance from the front of the saw to the tip of the farthest cutter rounded to the nearest inch.
Should I do anything special with a new chain?
Yes. Taking these few simple steps prior to using it can extend the life of your new chain:
1. If possible, soak the chain in oil to allow oil to penetrate all chain components.
2. Never run any chain on an over-worn drive sprocket, especially a new chain. Replace drive sprocket system after every two chains, or sooner.
3. Adjust chain (see "How tight should I tension my chain?)
4. Run new chain at half throttle for several minutes before doing any cutting; this allows oil to reach all parts of the bar and chain. Let sprocket, bar and chain warm up fully.
5. Stop the motor, let the chain cool, then check and adjust tension.
6. Keep the first several cuts light. Keep extra oil on the bar and chain during these first cuts, and do not apply heavy pressure. Check chain tension often during the first half hour of use.
Why do my chain pull or cut to one side?
Several things can make a chain cut crooked or pull to one side. The following are the causes and the remedies:
Uneven top plates can cause a chain to cut crooked. It's important to keep all top plates equal length.
Dull cutters damaged by rocks can cause a chain to cut crooked. It's important to remove all damage from cutters prior to cutting. Cutting with a dull chain can accelerate wear to the bar and chain.
Different depth gauge settings from left to right. Keep all depth gauge settings equal from left to right.
Different top-plate angles from left to right. If you sharpen your chain at a 25º setting on your left hand, your right hand cutters should match.
A worn guide bar. A badly worn bar cannot be repaired. If your system is cutting crooked and you have narrowed it down to the bar, more than likely the rails or the inside of the bar are damaged beyond repair.
What is chain gauge?
Chain gauge is the drive link's thickness where it fits into the guide-bar groove. The gauge of the chain and the gauge of the guide bar must match. Several gauges - such as .050", .063", etc.
What is chain pitch?
Chain pitch is the distance between any three consecutive rivets divided by two. Pitch defines the size of the chain. Chain is made in several pitches - 1/4" is the smallest, 3/8" is the most popular. Other parts of the cutting elements are pitch-related. The drive sprocket must be the same pitch as the chain, and so must the nose sprocket in sprocket-nose bars.
How do I measure the length of my chain?
The length of your chain is determined by counting the number of drive links. Common examples are 70 for "D" or "72" chain and 56 for "S" or "91" chain.
How tight should I tension my chain?
For a sprocket nose bar, turn your saw's tension-adjustment screw until the bottoms of the lowest tie straps and cutters come up and contact the bottom of the bar rails, then turn your tension-adjustment screw an additional 1/4 turn. Also, on sprocket nose bars, the snap test should be performed. Grasp the chain along the bottom of the bar, pull down, and let go. The chain should snap back to its original position, solidly contacting the bottom of the bar rail.
NEVER TENSION YOUR CHAIN WHEN IT IS HOT!
How often and what type of lubrication should I use?
Keep your saw's chain-oiling system filled with clean bar-and-chain oil. Never put used oil or old motor oil in your saw or on your chain. Be sure your chain, bar, and sprocket are always receiving oil from the saw during operation. Fill your oil reservoir each time you fill your gas tank.
How long the drive sprocket an my saw last?
It's important not to run a new chain on a badly worn drive sprocket. Replace drive sprocket systems after every two chains, or sooner.
How do I know that my chain is dull and I have to sharpen it?
Keep in mind that a sharp chain will cut large-size chips. A chain that is dull or has abrasive damage will create sawdust. It's time to sharpen when you're having to push on the saw or the saw is no longer self-feeding.
How do I sharpen my chain?
Always inspect your chain prior to sharpening. Check for the following:
- Bent or burred drive links
- Broken cutters or tie-straps
- Loose rivets or broken rivet heads
If broken parts are detected, take it to a servicing dealer for replacement of parts or replace the entire chain. The following steps will help you correctly sharpen (with a round file) an saw chain:
1. Be sure to have the correct size file and file guide.
2. When hand filing it's important that 1/5, or 20 percent, of the file's diameter is always held above the cutter's top plate. Using the correct file guide is the easiest way to hold the file in this position
3. Keep the correct top-plate filing angle line on your file guide parallel with the chain. Many cutters have a guide mark stamped near the rear edge of the top plate that can also be used as a guide for filing angle.
4. Sharpen cutters on one side of the chain first. File from the inside of each cutter to the outside. Then turn your saw around and repeat the process for cutters on the other side of the chain.
5. If damage is present on the chrome surface of top plates or side plates, file back until such damage is removed.
6. Keep all cutters equal. Start with the cutter with the most damage and hand file all cutters back equally.
Note: Do not file or alter the tops of kickback-reducing bumper tie straps or bumper-drive links, except on 33SL, 34SL and 35SL chains. Only on 33SL, 34SL, and 35SL will the bumper tie straps be filed down while the cutter depth gauges are filed.