Torque required to twist and cut loose Scots pine stumps

Abstract

Stump wood is a possible source of renewable energy, but before its potential as a fuel can be utilised to a high degree, new harvesting techniques should be developed to reduce the environmental impact (notably ground disturbance) of harvesting stumps. The forces required to lift and drag stumps out of the soil are known. In this study, two unknown and important parameters were addressed: the torque required to uproot stumps by twisting them and the torque required to cut lateral roots around stumps. A new, improved stump-twisting rig was designed and used in trials with 28 Scots pine (Pinus sylvestris) trees (breast-height diameter over bark, 153–427 mm). The measured torque requirements ranged from 10 to 50 kNm. Twisting stumps required more torque than cutting lateral roots around stumps and the required torque increased with increases in stump size. The results indicate that a wrist on a big feller-buncher, but not a conventional rotator used on forest machines, should be able to generate sufficient torque to cut the roots around stumps such as those used in this study.

Keywords:

• Bioenergy
• force
• cutting roots
• stump harvesting
• uprooting

Acknowledgements

This study was financed by the Swedish Energy Agency, the Swedish District Heating Association, the companies Jämtkraft AB and Skelleftekraft AB and the research program Efficient Forest Fuel Supply Systems. Hans Arvidsson at the Svensk Maskinprovning is acknowledged for providing and helping out with the measurement equipment and Kalvis Kons for assisting in field work and producing drawings. The personnel at Ätnarova Experimental Forests with Svartberget Research Station are gratefully acknowledged for their great support during the field work.