Previous columns have discussed elements of basic chain saw safety, such as required PPE (personal protective equipment), safety features of chain saws and body positioning in relation to the saw when cutting, all of which are important for safe and efficient chain saw use. However, a full and complete understanding of the inherent reactive forces of chain saws will not only help climbing arborists operate saws more safely, but also provide them with a variety of progressive cutting techniques that can assist in getting jobs done more efficiently and under safer cutting conditions.
The push force is experienced by both chain saw and operator when cutting with the top of the bar. The cutter teeth in the chain on top of the bar are moving at a high rate of speed forward, toward the tip of the bar, and as they slow slightly upon contact with the wood, removing material, the reaction of the saw is to push backward into the operator. Operators in a strong, stable body position with both hands on the saw in the appropriate locations will absorb this push readily, although it can be quite powerful with larger chain saws. Chain saw users not prepared to absorb the energy of this push force, or not holding the saw correctly, will quickly find themselves in a dangerous situation.
The pull force is experienced by both chain saw and operator when cutting with the bottom of the bar. The cutter teeth on the bottom of the bar are moving at a high rate of speed backward, toward the body of the saw and around the drive sprocket. As they slow slightly upon contact with the wood, removing material, the reaction of the saw is to pull away from the operator toward the piece being cut. This pull force is also readily dealt with by operators in a strong, stable body position with appropriate hand location, but users unprepared for this pull will also quickly find themselves in a dangerous situation. Both the pull and push reactive forces can be quite dangerous for unprepared chain saw users in climbing operations, as they may pull or push a climber with a poor work position and stability into an extremely hazardous spot in the cutting plan or rigging system.
Kickback or “no” corner
Chain saw operators are often taught the partial truth that kickback, the rapid, violent movement of the saw upward and backward toward the user, occurs at the tip of the bar, and use of or contact with the tip of the bar should be avoided at all costs. Not only does total avoidance of use of the tip limit a chain saw’s effective cutting length, it also precludes the use of a variety of safe and effective progressive cutting techniques. In reality, though, kickback does occur at the tip of the bar, it is only possible at the upper quadrant of the tip, commonly called the kickback or “no” corner. A close examination of a cutter tooth’s actions as it rounds the bar nose sprocket downward illustrates what causes kickback to occur. The depth gauge, also called a drag or raker in some areas, determines how large a “bite” of wood the tooth will take. When the tooth turns downward over the sprocket at the upper quadrant of the tip of the bar, the depth gauge goes down first, allowing the tooth to take far too large a bite. Much as a drill bit stopped momentarily by a knot in a piece of wood will attempt to spin the drill in the user’s hands, this momentary stoppage of the cutter tooth by too large a bite of wood will attempt to spin the chain saw within the chain, causing it to rear violently and suddenly back toward the operator. This is described as a rotational kickback, in which the chain saw rotates upward and backward toward the user. Operators should also be aware of the possibility of a pinch kickback, in which the piece being cut pinches in on the bar, creating a pushing reactive force, which then can be converted to a rotational kickback as the upper tip contacts wood as the saw is pushed violently backward out of the piece and toward the user.
Starting or “go” corner
Use of the lower quadrant of the tip of the bar not only holds no potential for creating rotational kickback, but is quite advantageous in cutting techniques such as plunge/bore cutting or key notches. Examination of the cutter tooth’s actions as it rounds the bottom of the bar nose sprocket and heads back to the body of the saw and drive sprocket shows why no kickback is possible. As the tooth turns underneath the sprocket at the lower quadrant of the tip of the bar, the depth gauge goes down first, and is actually much higher than the cutting point of the tooth, preventing any possibility of too large a bite. For this reason, along with its use in beginning bore or plunge cuts, the lower quadrant of the tip of the bar is commonly called the starting or “go” corner.
Chain saws are excellent and effective tools that most climbing arborists use many times throughout the course of their workday. They should not be feared, but respected for their power and function; and this basic knowledge of the reactive forces they generate should help all chain saw users employ these powerful tools safely, efficiently and with respect.
Michael (House) Tain is a contract climber, splicer, educator and writer currently located in Lancaster, Ky.