Licensed Professional Engineers

FORENSIC CLUES # 21 - "Wood Chippers" by John L. Ryan and L.D. Ryan


A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.

Issue 21: Vol. 18 August/September 2006

"Wood Chippers"

By John L. Ryan and L.D. Ryan

Wood Chipper Accidents

Wood chippers are used to turn tree waste into useable material and/or a more manageable form to aid in disposal. Chippers cut trees, limbs, and tree clippings into small pieces, usually ranging between one and two inches in size. These clippings can then be used for various purposes. The clippings can be used as mulch or ground cover, as well as to make paper and paper products in a paper mill.


Wood Chipper

How Chippers Work

Chippers consist of three basic components: a loading hopper where material is fed into, the chipping mechanism, and exit chute or collection means. Chippers are often hand-fed with limbs and other tree debris, and discharge into a truck bed, onto the ground, or other container. Some chippers have conveyor systems that feed material into the chipper. Self-powered chippers are normally powered by internal combustion engines. Tractor attachment chippers are powered through the PTO (power take-off unit).


Several basic chipper designs encompass most chippers produced today. Drum chippers were the original type of chippers that were placed in production many years ago. These chippers use a large drum that rotates and provides the pulling force and drives the wood material into the cutting mechanism. Common problems with drum chippers include noise production, material getting jammed in the chipper, and operators becoming pulled and entangled into the chipper. An accident with a drum chipper normally results in severe injury or death.

Disk chippers use separate mechanisms for the cutting and drive actions. Cutting is performed by a disk with blades on it, feeding is accomplished with hydraulic rollers that are usually reversible. These chippers can be easier to safeguard, because of the separate drive and cut mechanisms.

When the chipper drive mechanism grabs hold of the tree or limb, it pulls the limb into the machine. The chipper operator at this point releases the limb, allowing it to be processed by the chipper.

How People Get Injured

Wood chipper operators normally get killed when they get pulled into a wood chipper and are cut up by the machine. Operators get pulled into chippers when clothing or a body part becomes snagged on a tree limb that is being fed into the chipper. Operators can also get injured if they are hand feeding a limb, and do not release the limb before the drive mechanism pulls the limb into the chipper. Wood chippers involve an extreme amount of energy, making many chipper accidents fatal.

Branch feeding.jpg

Current Safeguarding Techniques

Some chippers have feed control bars situated on top of the loading hopper. Control bars are used to control the operation of the chipper by stopping the drive wheels, reversing the drive wheels, and engaging the drive wheels. While this is promoted as a safety device, these bars do little to protect many accident scenarios, since an operator who is entangled in a chipper will not have reaction time to engage the bar.

National Safety Council's Design Hierarchy

Engineering design principles, as well as the design priority method outlined by the American Society of Mechanical Engineers (ASME) and the National Safety Council, follow a methodical process to identify and remove any and all safety hazards. Once hazards have been identified, engineers and designers must neutralize these hazards. The first step in the design hierarchy, the most effective method of safeguarding hazards, is to remove the hazard in the design. This is the best case scenario, because a product that has had the hazard removed from it will keep product users safe.

Solutions to Wood Chipper Hazards

Like most hazardous products, solutions to solve product hazards are not extremely complicated, nor cost prohibitive. After identifying product hazards, the next step in the design hierarchy calls for eliminating the hazard through design. This means the best solution is to change a design so that the product is no longer hazardous. Products such as wood chippers have inherent hazards * hazards that are the result of the function of the product. Inherent hazards are difficult to remove through design. With current technology it is unlikely that a chipper could be designed that would cut wood but not flesh. Drum type wood chippers use a flywheel to drive the drum that both pulls material into the chipper and cuts the material. Unfortunately, this results in a chipper that cannot be stopped in an emergency. These chippers take up to 45 seconds for the drum to come to a stop. Disk chippers, or self-feeding chippers, use separate mechanisms to feed the material and process the material. This allows for very quick emergency shut-down. While this design change does not eliminate the hazard, it reduces the severity of the hazard, and allows for more effective safeguarding using guarding techniques. The hazardousness of wood chippers could arguably be reduced through design also by removing drive system, so operators could not be sucked into the chipper, although extra precautions (such as adding a manual feed system that removes the operator from the vicinity of the blades) would have to be made to prevent point of operation accidents without a drive system. When design solutions are impossible or not used for various reasons (usually increased cost or lack of knowledge of solution), the next step of the design hierarchy must be used, which is safeguarding the hazard with guarding solutions. While not as effective as a design solution, guarding solutions are much more effective than the next step in the design hierarchy, developing warnings. Many manufacturers chose to warn about a hazard and not guard the danger. This is not only unethical, OSHA regulations make it illegal. Failing to guard a hazard when guarding solutions existed at the time of manufacture spells certain litigious failure. One of the simplest methods used for safeguarding wood chipper hazards is guarding by distance. If a person is not physically able to get near to the hazard, that person cannot be injured by the hazard.

This is the concept behind wood chippers that use extended feed tables. Extended feed tables or chute systems normally require a minimum of 60 inches of length to help prevent operators from getting entangled in the drive system, or sucked into the machine.

b&w feeding.jpg

Chippers can be found on the market that have cables attached to the control bars of chippers, which are placed inside the feed hoppers of wood chippers. These are intended to allow the operator to stop the chipper in an accident. This safety feature depends on human behavior, and is therefore an unacceptable method of safeguarding. A more effective method of control safety would be to use a control bar that required constant pressure to activate the drive. If the operator removed his or her hand from the lever, the chipper would stop and/or reverse. This would ensure that an operator who gets sucked into a wood chipper would activate the emergency stop. The same effect is obtained by using a pressure-sensing mat where the operator stands. In order for the machine to work, the weight of the operator must be on the mat. Other solutions are available by applying guarding technology to the problem. Various designs have been studied, including having operators wear gloves with metal particles inside of them. Sensors on the hopper would detect the presence of the gloves, and would stop the feed wheels when the operator’s hand was sensed by the safety device. An inventor has developed SawStop, an ingenious device that brings a table saw blade to a stop in fractions of a second upon contact with human flesh. This same technology could be applied to wood chippers and other machines. Whenever human flesh touched the blades or even the part of the hopper nearest the blades, the chipper could be brought to a stop.

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Articulated Ladders

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