Mechanical Felling

There are three basic types of mechanical felling machines. Single-function machines are only capable of directionally felling a tree, while dual-function machines (called feller-bunchers) are able to fell a tree and move the cut stem to a pile. There are also multifunction machines (called harvesters) that can fell, delimb, buck, and place the processed pieces. Mechanized felling is faster and safer than manual felling. For example, a shear feller-buncher can fell a tree in 3 to 6 seconds compared to 30 and 90 seconds for a person with a chainsaw. Feller-bunchers are capable of felling and bunching up to 200 trees per hour in favorable conditions. Manual felling cannot maintain steady levels of productivity comparable to mechanized felling (Conway 1976).

Mechanical Felling

Three basic types of cutting mechanisms are used for mechanical felling: shears, disk saws, and chainsaws. Shears are like a large pair of scissors that use hydraulic power to snip through trees. Disk saws are large, circular saw blades (typically several inches thick) that rotate at high speed. The third option uses a slightly larger version of the manual chainsaw to cut through the stem. In all of these designs, the mechanical felling machine will grasp the tree to be cut, power the cutting mechanism through the stem, and apply force to the tree to control the direction of fall.

Shear felling machines are the least expensive to purchase and maintain. However, the shears may cause some splitting damage to the logs along the grain of the wood. These splits may extend up to 2 to 4 feet into the butt log, depending on temperature, species, wood density, blade thickness, blade configuration, and anvil width. Because of this damage, shears are typically used in small diameter or low-value materials. Shears have an advantage over saw blades when working in rocky conditions where saw blades would be more exposed to damage. Disk saws are generally very fast and need less frequent maintenance than chainsaw-type cutting heads. However, disk saws remove a wider cut than chainsaws, creating more waste.

Any of the three cutting mechanisms can be attached to one of two basic types of carriers: drive-to-tree or swing-to-tree machines. Drive-to-tree machines have to maneuver to approach each cut tree and then drive away to either drop the stem in a bunch or cut an additional tree. Swing-to-tree machines, on the other hand, have the cutting device attached to the end of a boom. The machine generally stays in one position while cutting, swinging and placing the stems in a bunch. Any of these combinations can be designed for a wheeled or tracked carrier. Drive-to-tree machines tend to be less expensive, but may result in greater site disturbance and suffer from stability limitations on slopes.

These machines not only improve productivity by harvesting stems faster, they also reduce the overall harvest costs by saving time in the limbing and skidding functions. When stems are bunched together, the limbing process requires less time moving from stem to stem. Bunching also reduces the amount of time required to gather a full load for skidding. Even the directional felling of the single-function mechanical feller reduces the time required for skidding by orienting the butts of the logs. Another advantage of mechanical felling is lower stump heights and greater fiber recovery than manual felling.

Mechanized harvesters have become more prevalent in the Appalachians. Bolding and Lanford (2002) describe the operation of a wheeled harvester capable of handling 28-inch-diameter trees on slopes up to 46 percent. Self-leveling tracked machines can operate on slopes up to 50 percent and special adaptations such as walking excavator-based harvesters have been reported to operate on slopes up to 70 percent. Steep terrain harvesters working in conjunction with cable yarding systems can reduce lateral yarding distances by prebunching cut-to-length timber closer to the cable corridor (Baker and others 2001).

One logging company in West Virginia has replaced its 3-machine system (grapple skidder, loader with delimber, and feller-buncher) with a tracked harvester and a tracked forwarder.  These tracked machines are capable of operating on very steep slopes and can handle stems up to 35 inches in diameter. Since the conversion, production has increased up to 100 tons per week, two in-woods employees have been eliminated, and safety of the operation has been improved. The mechanized operation is able to work an extra 15 days per year because it is less sensitive to weather delays. These machines also improve the utilization of the tree by consistently topping the stems at 4 inches. The processing of the logs at the stump leaves the limbs and tops in the woods for better nutrient cycling and reduced soil disturbance (Beach 1999).

The main advantages of mechanized felling are:

• Higher production rate.
• Improved safety.
• Reduced residual stand damage due to controlled felling and proper placement of felled stems.
• Higher skidding productivity.
• Sorting and merchandizing is possible.
• Mechanical cutting can be completely separated from skidding since there is no need for assistance with hung trees.
• Feller-bunchers can build optimum load-sized bunches for grapple skidding.
• Higher wood utilization due to lower stump height.
• Mechanical cutting can be safely used in weather and lighting conditions that are unacceptable for manual cutters, increasing operational hours.

Disadvantages include:

• Distinct limits of stem size capability.
• Higher initial capital investment.
• Potential wood damage with shear-type cutters.
• Soil disturbance associated with in-woods machine traffic.
• Stability limitations on steep slopes.
• Operational areas limited by rough terrain, boulders, dense residual stand spacing.