Investigation of Log Length Accuracy and Harvester Efficiency in Processing of Oak Trees

Harvester use in broadleaves has recently become more effective economically. However, difficulties
with delimbing have shown that not all harvesting heads are suitable and efficient for
broadleaved species. The typical obstacles are mainly large tree sizes, bends and forks in the
trunks and large branches. For these reasons, it is difficult to obtain specific log lengths according
to the settings in the harvester on-board computer. The objective of the research was
to determine: 1) the accuracy of the log lengths from the bottom, middle and top parts of oak
trees, and 2) harvester efficiency in the utilisation of the trunk for logs. The research was carried
out on 61-year-old oaks from which logs with an expected length of 250 cm were processed.
To achieve this length, a margin of error was set in the harvester computer with minimum
and maximum lengths of 252 and 257 cm. For thinning operations, a Ponsse Ergo harvester
with a H7 harvesting head was used. After harvesting, manual log measurements were carried
out on 280 logs: 69, 142 and 69, from bottom, middle and top parts of the trees, respectively.
The largest share of assortments satisfying the minimum requirement of 250–257 cm was
obtained from the middle part of the trees (93%), followed by bottom logs (91%) and top logs
(88%). The highest frequency of logs, which were too short, were found to be the top logs (9%),
while bottom logs were most often too long (6%); therefore, different length settings should be
applied to limit such inaccuracies. Analysis of the last log from the highest part of the tree
indicated a strong goodness of fit between the top diameter and the DBH; the mean value of
the top diameter was 13.3 cm over bark.