A wood chipper or woodchipper is a machine used to reduce woodchips (usually tree branches or trunks) into small woodchips. They are usually portable, mounted on wheels on frames suitable for towing in the back of a truck or van. Power is usually supplied by an internal combustion engine between 2 and 700 kilowatts (3 to 1,000 horsepower). There are also models of high-powered shredders mounted on trucks and driven by a separate engine. These models usually have a hydraulic winch.
Wood chippers are typically made from a hopper with a collar, the shredder mechanism itself, and an optional collection bin for chips. A tree branch is placed in the chamber (the collar acts as a partial safety mechanism to keep parts of the human body away from the chipping blades) and starts the chipping mechanism.
The chips come out of a channel and can be guided into a truck-mounted container or floor. Typical output are chips of the order of 2.5–5 cm (1–2 inches) in size. The resulting wood chips have a variety of uses, such as spreading as a ground cover during papermaking or feeding into a digester.
Most woodchippers rely on energy stored in a heavy flywheel to do their job (although some use a drum). The chip removal blades are mounted on the face of the flywheel and the flywheel is accelerated by an electric motor or internal combustion engine.
Large wood chips are usually equipped with grooved cylinders in the throat of hoppers. When a branch is gripped by the rollers, the rollers move the branch at a constant speed to the chip removal blades. These cylinders are a safety feature and are often reversible for situations where a branch is caught on clothing.