Tests of a Portable Wood Chipper

A $2,700 portable wood chipper consisting of a two-wheel trailer, power unit, and chipper unit is easily hauled by pickup truck to chipping locations in the woods. An eight-knife version of the machine operating at 1,500 rpm, and a 12-knife modification operating at 2,000 rpm were tested on three different sizes of raw material zero to 1 inch diameter, 1 to 4 inches diameter, 4 to 6 inches diameter. Production in cubic feet of wood chips increased as diameter class increased except that the 12-knife machine was easily stalled by 4 inch and larger wood, so that production in this diameter class decreased with the 12-knife machine. The production in the smaller size diameter classes was considerably increased, however. A number of factors influenced the rate of production: straight lengths of brush increased production up to four times that of crooked material. Production in cubic feet was greater when the chipped material consisted of considerable green foliage, but the increase was due to the low density of this additional material and contributed very little to the ovendry weight. The size of the chips can be varied by changing the amount of projection of the knives from the cylinder and varying the clearance. This would, of course, affect the production. Chips from the 12-knife chipper were larger but less uniform in size than from the 8-knife chipper. Tests of the aspen (Populus spp.) chips show that they could be used for pulping and papermaking. However, chips produced from rough wood by portable chippers of this type are not generally used by the pulp industry, mainly because of small size and lack of uniformity of chips, high proportion of bark and leaves, mixed species, and uncertain supply. Dry weights of chips of 15 lbs./cu.ft. for oak (Quercus spp.); eleven for aspen (Populus spp.); and ten for pine (Pinus banksiana). The cost per hour of operation, including depreciation of the machine over 5 years, the labor of two men, and gasoline consumption of nearly 3 gallons per hour totaled $4.53. While some pulp mills may accept chips produced by this machine, some other uses for chips as in agriculture or as fuel might limit the practicality of this machine. Where burning is permitted, chipping is more expensive than burning, but less expensive than hauling.

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