Harvester

A harvester is a type of heavy vehicle employed in cut-to-length logging operations for felling, delimbing and bucking trees. A harvester is typically employed together with a forwarder that hauls the logs to a roadside landing.

Harvesters are employed effectively in level to moderately steep terrain for clearcutting areas of forest. For very steep hills or for removing individual trees, humans working with chain saws are still preferred in some countries. In northern Europe small and manoeuvrable harvesters are used for thinning operations, manual felling is typically only used in extreme conditions, where tree size exceeds the capacity of the harvester head or by small woodlot owners.

The leading manufacturers of harvesters are Timberjack (owned by John Deere), Valmet (owned by Komatsu) and Ponsse.

Harvesters are built on a robust all terrain vehicle, either wheeled or tracked. The vehicle may be articulated to provide tight turning capability around obstacles. A diesel engine provides power for both the vehicle and the harvesting mechanism through hydraulic drive. An extensible, articulated boom, similar to that on an excavator, reaches out from the vehicle to carry the harvester head. Some harvesters are adaptations of excavators with a new harvester head, while others are purpose-built vehicles.

A typical harvester head may consist of (from bottom to top, with head in vertical position)

· a chain saw to cut the tree at its base, and also cut it to length. The saw is hydraulically powered, rather than using the 2-stroke engine of a portable version. It has a more robust chain, and a higher power output than any saw that can be carried by a human.

· two or more curved delimbing knives which reach around the trunk to remove branches.

· two feed rollers to grasp the tree. The wheels pivot apart to allow the tree to be embraced by the harvester head, and pivot together to hug the tree tightly. The wheels are driven in rotation to force the cut tree stem through the delimbing knives.

· diameter sensors to calculate the volume of timber harvested in conjunction with a measuring wheel which measures the length of the stem as it is fed through the head.

All of this can be controlled by one operator sitting in the cab of the vehicle. A control computer can simplify mechanical movements and can keep records of the length and diameter of trees cut. Length is computed by either counting the rotations of the gripping wheels or, more commonly, utilising the measuring wheel. Diameter is computed from the pivot angle of the gripping wheels or delimbing knives when hugging the tree. Length measurement also can be used for automated cutting of the tree into predefined lengths. Computer software can predict the volume of each stem based on analysing stems harvested previously. This information when used in conjunction with price lists for each specific log specification enables the optimisation of log recovery from the stem.

Harvesters are routinely available for cutting trees up to 900 mm in diameter, built on vehicles weighing up to 20 t, with a boom reaching up to 10 m radius. Larger, heavier vehicles do more damage to the forest floor, but a longer reach helps by allowing more trees to be harvested with fewer vehicle movements.

The approximate equivalent type of vehicle in full-tree logging systems are feller-bunchers.

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