The process behind the delimbing of trees

Abstract: In modern forestry, two types of machines are generally used for thinning, harvesters and forwarders. These machines are large and weigh several tens of tons, this means that they damage the ground when they drive around in the woods. AirForestry is a startup company that focuses on manufacturing lightweight and electric forest machines that will do much less damage due to the lower weight. As their ambition is to develop an all-electric harvester, high efficiency in the work process is required to be competitive. The purpose of this thesis was to find which factors affect the debranching process and to find what approximate effect the harvester head's drive motors would need to have to achieve a stable debranching process. If the results of the research were successful, a new knife design would be developed for thinning. The work carried out began with a planning report which included introduction, literature study and method was written. The method chosen was to first develop a test rig to be able to measure what amount of work and what maximum impact force a single branch would need to be cut. Afterwards a study of branches and their distribution on trees in the thinning phase were conducted. The branch distribution and the data from the test rig, would make it possible to estimate the energy required to cut down a tree. Impact forces along the tree provide an opportunity to not only estimate the life cycle of the knives but also an analysis of how the remaining units hold. The branching process was assumed to be affected by both the design of the knives and which branches are present and how these are placed. The literature study showed that frozen wood gives higher forces when sheared, so it was investigated as well. The test rig was designed as a guillotine to make it easy to manufacture and due to potential energy being the only source of energy to power the rig. The work of cutting a branch was measured with a high-speed camera and the maximum impact force was measured with an accelerometer. Different knife designs were tested together with different angles and branch types. All variables tested in the rig were performed on several different branch diameters. The study of branch distribution along the tree was done on trees that were considered to be in the thinning stage and all branches with a diameter greater than 5 mm were noted together with distance and whether it was dead or not. The test rig's test data were later used in a multiple linear regression to determine if a variable effect of not. The regression was then used to estimate the power required to debranch a pine to be 3.1 kW and 6.6 kW for a spruce. Only the diameter of the branches and the tree sort could be proven to have a statistically significant effect on both the work required and the maximum impact force.