Wood chippers: a necessary technology for forestry economy

In logging and in areas where clearcutting is carried out it is now necessary to leave a clean area behind after this work has been performed. The most convenient method is to chip or reduce the logs felled. The chips produced can be used as fuel, for the production of chipboard panels or compost. Manufacturing industries now market a wide range of wood chippers suitable for the requirements of various work operations

The forest environment in Italy is very different compared to those in other parts of Europe. Most of the forested areas from which it would be possible to produce great quantities of biomass are, in fact, located in zones with rugged topographic features which are difficult to access. Moreover, the strong fragmentation of ownership of the wooded land heritage makes it very difficult to plan forestry interventions in vastly extended areas. These are the most important factors acting as obstacles in Italy to the large scale use of operations with self-propelled machines especially designed for work in forested environments which require big investments and great space but are often not suited for the steep slopes on which most of Italy's wooded areas are found. This is the reason that most felling operations in the country are carried out manually with a chainsaw whereas in the forestry-wood-energy production chain the Whole-Tree harvesting system is used with extraction of the entire tree by a winch for short distances or an overhead cable for longer distances followed by chipping the log on the worksite.

 

This is the procedure used in part because it has become increasingly necessary to leave the felling worksite clear of branches and to ensure this the most convenient method is to reduce the woody materials felled. Chipping waste materials as well as logs and large stem sections can be done for the purpose of producing chips to burn for the generation of energy as well as for making chipboard panels or compost.

Chips are turned out by mechanical cutting for transforming the woody materials into biomass with various characteristics in relatively uniform fragments generally 2 to 5 cm in length, width of no more than 2 cm and a few millimeters thick. Once the chips are reduced to a fairly uniform size the product is easy to transport and dry for quicker burning because of their larger surfaces exposed to the air compared to more commonly used firewood.

On the side of practical considerations, the major advantages of chipping are that, first of all, the process makes waste materials into something of value, the operation on branches reduces their initial volumes by two-thirds and chips can be used for feeding plants and automating combustion because of their small sizes. On the other hand, the disadvantages are the great energy requirement for the chipping process, the problem of conserving chips, especially with humidity content over 35-40%, and a rather low mass volume in that the volume of chips is, at equivalent weight, double that of processed or bucked logs. Chipping is done by chippers or chipping machines with various operational functions and manufactured in a wide variety of models. For their transport, chippers used in silviculture can be mounted or towed by tractors which provide power for the machine operations off their PTO, they can be towed by an off-road vehicle or a pickup truck and equipped with their own engines, usually a diesel power plant, or more powerful models usually rated at over 300 kW can be mounted on a truck bed. In this latter case, chipping is done using the machines' own engine and the truck powers the crane used for the chipper. Chipping machines can be divided into three general categories according to cutting utensils: drum, disk and screw auger. In the first type, cutting is done by two to four blades, depending on the model, mounted in a rotor which rotates on a longitudinal axis. The length of the chips depends on the length of the knives pointing inwards inside the rotor which can be adjusted. The disk chipper employs a steel disk with a flywheel function and knives mounted on the surface, again variable in number. And then there is a flap which acts as an anvil to block the advancing wood in place. In this case, the size of the chips depends the extension of the anvil and the knives. The screw auger chipper mechanism is a screw with a hard cutting edge rotating on its own axis in a housing to produce chips in sizes varying from 50 to 80 cm. To facilitate feeding, the wood for chipping is fed into the hopper in a pyramid shape usually by hand or, for heavier models, logs are fed in by a crane. There are also on the market models equipped with conveyer belts which carry materials to the feeding hopper. These materials then reach the cutting edges by simply falling on the screw or are driven in through a pair of teethed rotors with at least one of them providing traction. There are no utensils driving the wood in the screw chipper because the screw itself handles this function.

A fan located behind the cutting utensils drives the chips out for discharge through a gooseneck chute which can be oriented. A limitation of these types of chippers is that

they produce fairly rough-hewn chips and provide little possibility for regulating their size. The power required by chippers varies according to differing parameters and, more precisely, in relation to the diameters of the materials to chip, the coefficient of density which, in turn, depends on the tree species, and humidity content. There are small chippers with power at up to 50 kW which can handle diameters of up to 20 cm; average models at 50 to 200 kW for work on diameters up to 30 cm and big machines rated at more than 100 kW for operations on larger diameters.

Safety

Safety is a fundamental factor for the end-users of these machines because, due to the type of operation they perform, they are extremely dangerous. This means that full respect for safety norms is essential. All chippers must be equipped with a button in an easily accessible location for quickly stopping the machine and one or more controls in strategic places for stopping and reversing the feeding rotors. These devises are required because, especially with small chippers with manual feeding, one of the greatest dangers is the possibility of the operator getting dragged into the machine. On these models, the feeding hopper floor has been lengthened and grips or handles have been added to prevent accidents of this type. To avoid kickback of materials discharged from the gooseneck chute the operator should maintain a safe distance from the machine whereas an operator of a crane is protected by the cab. In all cases, personnel anywhere near a chipper must reduce the possibility of an accident also by wearing appropriate protective clothing along with safety glasses and a hard hat. Another consideration is the level of noise produced by these machines which should be checked by wearing anti-noise headphones.