A 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.
Wood 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 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. As large branches are consumed by the machine, the inertia of the flywheel causes it to gradually slow down; when the branch is consumed, the engine causes it to speed up again. This is what produces the rising and falling siren-like howl of these machines. 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 operator lets go of it and 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.
Shredders that make use of high-torque low-speed grinding rollers are growing in populartiry for residential use. These shredders are driven with an electric motor and are very quite, dust free, and self feeding. Some of these machines are equipped with an anti-jamming feature.
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 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 reversible hydraulic feed wheels automatically controlled with the digital Auto-Feed Plus® 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.
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 on a perpendicular angle to the incoming material. As the disk spins, the knives cut the material into chips. They 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 of this type.
Disk-style chippers usually have a material diameter capacity of 6 to 24 inches.
Much larger machines for wood processing exist. "Whole tree chippers" and "Recyclers," which can typically handle material diameters of two to six feet, 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 or greater, and used 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.
Although chippers vary greatly in size, style, and capacity, the knives they use are all very similar. They are rectangular in shape and are usually four to six inches across by six to twelve inches long. They vary in thickness from about one-half to two inches. Chipper knives are made from high grade steel, and usually contain a minimum of 8% chromium for hardness.
Chippers were controlled by various means since their inception. However the harsh application and environment caused an incredible lack of reliability and performance. In 1999, Kurt Snider developed a proportional start/stop auto feed control that was tested by Morbark. Although this was the first digital feed control for this type of machinery, the net result was sawdust. The following summer, Kurt produced the first digital, programmable, reversing control with a LED display, Auto-Feed Plus®. This control eliminated the self feeding of material into the feed wheels when the engine bogged down. In addition, this technology, by enabling the machine to stop and reverse the feed wheels after the engine RPM's drop to a pre-determined set point, saves on knife wear, clutch wear, adds to the overall longeivity of the machine. This technology has so reveloutionized the industry, virtually every North American manufacturer of tree chippers and most world wide, now use some variation of reversing auto feed.
Chippers are by nature dangerous machines, designed to turn whatever is put into them into small chips. While most modern chippers have safety features built in, it's important to remember that some don't, and even those that do need to be used with caution and under the guidance of an experienced operator.
Users of chippers should wear comfortable clothing that is not too loose, and preferably of strong fabric that won't get punctured by branches. Goggles should always be worn, and when using non-self-feeding types a brush mask is advisable. Ear protection is a must, especially for larger chippers.
Chippers should always be fed butt end first, so piles need to be stacked before the chipping starts. In order to prevent branches from becoming entangled, piles are stacked in a fan-like pattern, in a third of a circle around the chipper (leave room to each side for branches that get caught up and need to be thrown aside). For very large brush piles, these fans can be in two rows (one in front of the other). The back row should not sit on any part of the front row, to avoid the need for pulling branches free while the chipper is running. There should always be at least ten feet of work space between the piles and the chipper chute, to prevent tripping.
Depending on the type of wood, the angles of the branches, and the capacity of the chipper, some branches may need to be cut to prevent jamming. Branches that are close in length to the feeding capacity width should never be thrown in, since they can often jam between the feed wheel and the disk.
Roots and raked materials should never be chipped, since they often contain rocks that can damage the blades. The chipping of thorned materials should be avoided when possible.
When chipping diseases materials, the load of chips should be marked as such, and not used for mulching near any plant that is vulnerable to the same pathogen.
Before starting the chipper, check to make sure the discharge chute is pointing in the right direction, and is secured. Make sure everyone is ready before starting the engine. Hand signals for "wait", "put it in", "don't put that in", and "turn it off" should be gone over with new crew members. All safety equipment should be put on before starting the engine, shoes tied, and any foreign materials gotten out of the way. If chipping in a residential area, make sure that no children, dogs, or other "civilians" are in the area.
Once the chipper is started and spun up, workers can begin adding materials into the infeed shute. If more than one worker is using the chipper, the crew members need to keep a careful eye on one another, and make eye contact before adding a new armful. If small material doesn't go in right away, it's often easier to simply let it be pushed in by the next load, rather than blocking the way.
If the infeed chute is too full with small branches and jams, use a long branch to try to pull the small branches through, or if necessary use a plastic Grain Scoop or similar tool that is too large to fit into the actual chipper, and is plastic or wood just in case it does touch the blades or rollers.
http://autofeedplus.com/ Auto Feed Plus® web site with videos
US Patent: 7070132, Low-Speed High-Torque Chipper-Shredder Machine