The effect on tether tension when using trees to redirect live machine tethers during forest harvesting on steep slopes

Highlights

• This study develops a new model that includes friction and cutting forces to estimate cable tension around a rub tree.

• Data was collected and used to estimate the friction coefficient and cutting deformation coefficient in the new model.

• The change in tension is independent of the depth the cable cut into the tree.

• Interaction between the surface toughness of the wood and the frictional surfaces of the cable requires further study.

Ground based forest harvesting machines are being used on steeper slopes with the aid of tethers to help maintain traction and stability. It is common to position a base machine containing the tether winch above the forest harvesting machine. This configuration is termed a live tether. The live tethers may be run around rub trees to reduce the time required to reposition the tether. This study developed a new model that includes both friction and cutting forces to estimate cable tension on either side of the rub tree. A dataset collected in the field was used to estimate the parameters in the model. The model appears to be promising; however, further work is required on the interaction of the cable cutting surfaces and the surface toughness of the wood. In general, it was found that the difference in the tension on either side of the rub tree increased with increasing change in angle, that the cable on average cut 3 mm deeper with each 10 m of the cable travel, and that the difference in tension was not correlated with the depth cut into the wood.

Introduction

Lindroos, La Hera, and Haggstrom (2017) suggest the main drivers of change in mechanized harvesting are the availability of new technology, the demand for new products, and the introduction of new regulations. Changes in regulations and the increased availability of new technology have been driving increased use of ground based mechanized harvesting systems on steep slopes. Harrill, Visser, and Ramond (2019) recount the initiation of the New Zealand Steep Land Harvesting Program that had the objectives of reducing harm, increasing productivity, and reducing costs through a heavy emphasis on mechanization. Tethering machines that are working on steep slopes is now a commonly accepted method to manage machine mobility and safety (Cavalli and Amishev, 2019, Holzfeind et al., 2019). Tether winches can be located on the harvesting machine in which case the tether is static, alternatively, the tether winch may be located on a base machine resulting in a live tether (i.e. the tether moves with respect to the ground).

The tether may be passed around trees or stumps in order to redirect it and to reduce the time required to reposition the base machine. Best practice documents are concerned about the management of tethers passed around trees and stumps (Naillon and Rappin, 2019). Their concerns include failure of the tree or stump, the line sliding off a stump, and for live tethers, inaccurate tension readings when the tension is measured at the base machine. The problem of a cable running around a circular post where the only interaction is Coulomb friction has been solved for many years (Sokolnikoff & Redheffer, 1958). Kimbell (1981) considered the problem of logging cables running around notched stumps as a V-belt running inside a notch, where friction is defined as a linear function of the normal force; however, these tests did not permit significant cutting of the cable into the tree. In addition, Kimbell (1981) used non-swaged cable, while most live tethers being used today are swaged. When using a rub tree to redirect a live tether that is supporting ground based harvesting equipment, the cable will quickly cut into the tree and the V-notch model may not be appropriate.

The objectives of this paper are to 1) to develop a new model for the tension in a cable running around a tree that includes a cutting force in addition to friction, and 2) to measure the percent increase in the tension for a limited sample of trees and to determine whether this is significantly affected by variables such as the depth the cable cuts into the tree and the change in direction of the cable as it passes around the tree.