Woodchipper

Woodchipper

[wood-chip-er]

A tree chipper or wood chipper is a machine used for reducing wood (generally tree limbs or trunks) into smaller parts, such as wood chips or sawdust. They are often portable, being mounted on wheels on frames suitable for towing behind a truck or van. Power is generally provided by an internal combustion engine from 3 to 1,000 horsepower.

Tree chippers are typically made of a hopper with a collar, the chipper mechanism itself, and an optional collection bin for the chips. A tree limb is inserted into the hopper (the collar serving as a partial safety mechanism to keep human body parts away from the chipping blades) and started into the chipping mechanism. The chips exit through a chute and can be directed into a truck-mounted container or onto the ground. Typical output is chips on the order of one to two inches (3-5 cm) across in size. The resulting wood chips have various uses such as being spread as a ground cover or being fed into a digester during papermaking.

Most woodchippers rely on energy stored in a heavy flywheel to do their work (although some use drums). The chipping blades are mounted on the face of the flywheel, and the flywheel is accelerated by an electric motor or internal combustion engine.

Large woodchippers frequently are equipped with grooved rollers in the throat of their feed funnels. Once a branch has been gripped by the rollers, the rollers transport the branch to the chipping blades at a steady rate. These rollers are a safety feature and are generally reversible for situations where a branch gets caught on clothing.

Styles

High-Torque Roller

Shredders that make use of high-torque low-speed grinding rollers are growing in popularity for residential use. These shredders are driven with an electric motor and are very quiet, dust free, and self feeding. Some of these machines are equipped with an anti-jamming feature.

Drum

The first commercially marketed chippers were of a design that was drum-based. They are still produced and sold today. The chipping mechanism in a drum-style chipper is a large steel drum powered by the motor, usually by a belt. It is mounted parallel to the hopper and spins towards the output chute. The drum also serves as the feed mechanism, drawing the material through as it chips it. This caused it to be colloquially known as a "chuck-and-duck" chipper, because material would start moving through the chipper very quickly as soon as it made contact with the drum.

These chippers have many downsides. The drum-style chipper is not as safe as newer designs. If an operator becomes snagged on material being fed into the machine, injury or death is almost certain. These chippers are also very loud. The chips produced can be very large, and if thin material is inserted, it may be cut into slivers rather than chips. Finally, since the drum cannot be disengaged from the engine, if too large or too long material is fed through the machine, it will stall, usually with the material stuck firmly in the drum.

Newer models have overcome many of these disadvantages with digitally-controlled reversible hydraulic feed wheels and muffling systems. The reversible feed system allows the newer style drum chippers to handle larger diameter materials.

Modern drum-style chippers usually have a material capacity of 6 to 19 inches (15-48 cm).

Disk

A newer chipper design employs a steel disk with knives mounted upon it as the chipping mechanism. In this design, (usually) reversible, hydraulically powered wheels draw the material from the hopper towards the disk, which is mounted perpendicularly to the incoming material. As the disk spins, the knives cut the material into chips. These are thrown out the chute by flanges on the drum. This design is not as energy-efficient as the drum-style design, but produces chips of more uniform shape and size. Most chippers currently used by commercial tree care companies are disk-type.

Consumer grade disk-style chippers usually have a material diameter capacity of 6 to 24 inches (15-60 cm). Industrial grade chippers are available with discs as large as 160 inches in diameter, requiring 4000-5000 horsepower.

Other

Much larger machines for wood processing exist. "Whole tree chippers" and "Recyclers," which can typically handle material diameters of two to six feet (60-180 cm), may employ drums, disks, or a combination of both. The largest machines used in wood processing, often called "Tub Grinders," may handle a material diameter of eight feet (250 cm) or greater, and use carbide tipped flail hammers to pulverize wood rather than cut it. These machines usually have 200 to 1,000 horsepower. Some are so heavy that they must be moved by a semi-trailer truck. Smaller models can be pulled with a medium duty truck.

Knives

Although chippers vary greatly in size, style, and capacity, the knives they use are similar. They are rectangular in shape and are usually four to six inches (10-15 cm) across by six to twelve inches (15-30 cm) long. They vary in thickness from about one-half to two inches (1-5 cm). Chipper knives are made from high grade steel and usually contain a minimum of 8% chromium for hardness.

Safety

Thirty-one people were killed in wood-chipper accidents between 1992 and 2002 in the US, according to a 2005 report by the Journal of the American Medical Association.

References

US Patent: 7070132, Low-Speed High-Torque Chipper-Shredder Machine

External links

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