Oregon Engineer
ArboristSite Operative
I viewed a video of a hot saw that broke a chain, which resulted in a 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 hot saw or harvester chain. The chain shot parts travel at supersonic speeds.)
The chain shot part was recovered after it blew through three sheets of steel, five boxes full of cardboard, and came to rest when it hit the side of a chain saw, in a box on a nearby shelf.
Close examination of the video showed the chain broke before hitting the wood. The saw handling technique of the operator caused a large increase in the chain tension before contacting the log. The increased chain tension spread the nose so the chain jammed in the nose and then the chain broke. The high operating speed of the hot saw chain then resulted in a chain shot as the end of the broken chain came back to the drive sprocket whipping the end pieces off the chain.
The operator made a dangerous but very common mistake that I often see on videos posted to this website by other hot saw users.
Simply put:
THE OPERATOR ROTATED THE BAR TIP DOWNWARD WITH THE MOTOR AT FULL SPEED.
I bet that does not sound so dangerous to you! That’s why it is a very common mistake!
There is a dynamic phenomenon that shows up very clearly in mechanized tree harvesters and hot saws. It’s called Coriolis Acceleration and results in a very strong outward force on the chain when the chain is rotating around the bar, AND the operator is rotating the chain saw and bar. The force is so strong it will pull the drive links out of the bar groove along the entire top and bottom of the bar, when the bar tip is rotated downward. The chain will only be contacting at the drive sprocket and bar nose. This results in an enormous increase in chain tension. The faster the chain speed and the faster the bar rotation, the STRONGER the outward force!
The problem technique: Before making a cut, many hot saw operators hold the bar horizontal when the saw is at idle. As the motor is accelerated to full speed the operator cannot prevent the bar tip from rotating up 10 to 30 degrees to the horizontal. Now the operator must force the bar tip downward so the bar is again horizontal before cutting. This is when the Coriolis acceleration pulls the chain totally out of the bar grooves. The chain tension skyrockets, and the chain has no support when it hits the log!
Alternative technique: Determine how many degrees your bar tip rotates up when the saw goes from idle to full speed. Then rotate your bar tip downward that amount while the saw is at idle before cutting. As the motor accelerates to full speed the bar tip will come to a horizontal position just prior to cutting. Then you can lower the bar and chain onto the log without rotating the bar.
If the chain on your hot saw runs in the opposite direction of standard chain saws then you must be careful not to rotate the bar tip up.
Get out your video cameras and have a friend film your cutting technique!!! Look very closely (most digital systems let you play back frame by frame) for how much the bar rotates as the motor comes to speed. If you have old videos then review them watching closely for rotation of the bar tip down and then the chain as it comes out of the groove. I’ll bet you find many cuts where the chain was out of the bar groove as the chain entered the log, not good.
Even a log stand that is TOO HIGH can force you into rotating the bar tip up, to get over the log, and then force you to rotate it downward instead of just lowering it.
Hot saw operators and their spectators are at a high risk of chain shot because the chain is operated far faster (2 or more times faster) and with many times more driving power than standard chain saws. The hot saw operator must pay close attention to many details that the average chain saw operator knows nothing about. Downward bar tip rotation, when the chain is at full speed, is a major technique problem that needs to be eliminated during hot saw cutting.
After you look at your old videos, please tell me what you see.
(Chain shot is the high velocity separation and ejection of a piece or pieces of cutting chain from the end of a broken hot saw or harvester chain. The chain shot parts travel at supersonic speeds.)
The chain shot part was recovered after it blew through three sheets of steel, five boxes full of cardboard, and came to rest when it hit the side of a chain saw, in a box on a nearby shelf.
Close examination of the video showed the chain broke before hitting the wood. The saw handling technique of the operator caused a large increase in the chain tension before contacting the log. The increased chain tension spread the nose so the chain jammed in the nose and then the chain broke. The high operating speed of the hot saw chain then resulted in a chain shot as the end of the broken chain came back to the drive sprocket whipping the end pieces off the chain.
The operator made a dangerous but very common mistake that I often see on videos posted to this website by other hot saw users.
Simply put:
THE OPERATOR ROTATED THE BAR TIP DOWNWARD WITH THE MOTOR AT FULL SPEED.
I bet that does not sound so dangerous to you! That’s why it is a very common mistake!
There is a dynamic phenomenon that shows up very clearly in mechanized tree harvesters and hot saws. It’s called Coriolis Acceleration and results in a very strong outward force on the chain when the chain is rotating around the bar, AND the operator is rotating the chain saw and bar. The force is so strong it will pull the drive links out of the bar groove along the entire top and bottom of the bar, when the bar tip is rotated downward. The chain will only be contacting at the drive sprocket and bar nose. This results in an enormous increase in chain tension. The faster the chain speed and the faster the bar rotation, the STRONGER the outward force!
The problem technique: Before making a cut, many hot saw operators hold the bar horizontal when the saw is at idle. As the motor is accelerated to full speed the operator cannot prevent the bar tip from rotating up 10 to 30 degrees to the horizontal. Now the operator must force the bar tip downward so the bar is again horizontal before cutting. This is when the Coriolis acceleration pulls the chain totally out of the bar grooves. The chain tension skyrockets, and the chain has no support when it hits the log!
Alternative technique: Determine how many degrees your bar tip rotates up when the saw goes from idle to full speed. Then rotate your bar tip downward that amount while the saw is at idle before cutting. As the motor accelerates to full speed the bar tip will come to a horizontal position just prior to cutting. Then you can lower the bar and chain onto the log without rotating the bar.
If the chain on your hot saw runs in the opposite direction of standard chain saws then you must be careful not to rotate the bar tip up.
Get out your video cameras and have a friend film your cutting technique!!! Look very closely (most digital systems let you play back frame by frame) for how much the bar rotates as the motor comes to speed. If you have old videos then review them watching closely for rotation of the bar tip down and then the chain as it comes out of the groove. I’ll bet you find many cuts where the chain was out of the bar groove as the chain entered the log, not good.
Even a log stand that is TOO HIGH can force you into rotating the bar tip up, to get over the log, and then force you to rotate it downward instead of just lowering it.
Hot saw operators and their spectators are at a high risk of chain shot because the chain is operated far faster (2 or more times faster) and with many times more driving power than standard chain saws. The hot saw operator must pay close attention to many details that the average chain saw operator knows nothing about. Downward bar tip rotation, when the chain is at full speed, is a major technique problem that needs to be eliminated during hot saw cutting.
After you look at your old videos, please tell me what you see.
