Tethered Logging Systems
Tethered logging systems utilize cable winch systems on harvesters, feller bunchers, forwarders, loaders, and skidders to stabilize and assist equipment operations on steep slopes. The cable system allows the equipment to operate on slopes that would normally be considered unsafe for equipment or damaging to soils. The use of tethered assistance enables enhanced equipment stability on steep or adverse soil conditions, while also enhancing traction on more gentle grades.
Tethered systems consist of a cable winch mounted on a piece of equipment. The winch system either mounts to the working equipment or it is mounted to another piece of equipment, like a dozer, that also acts as the anchor. When mounted to the working equipment, the winch line is anchored to an anchor point, such as a stump or the base of a standing tree, somewhere on the slope. The mechanical influence of the winch is used for enhanced traction and mobility on steep slopes (often called “traction assist”) or for safety on steep slopes (preventing machine sliding and overturning).
Operators should adhere to the maximum slope of the equipment as specified by the manufacturer. Oregon OSHA restricts operation of ground-based timber harvesting machines on slopes in excess of the following limits unless specified by the manufacturer of the equipment.
- Rubber-tired skidders – 30 percent.
- Crawler tractors, tracked feller bunchers, tracked excavators and loaders – 40 percent.
- Other forestry equipment designed for steep slopes – 50 percent.
Soil type and moisture will affect the level of ground disturbance realized or required tensions for appropriate operation on site soils. Fine-grained soils are more prone to compaction when moderately wet (i.e. “malleable, but not sticky” when squeezed in your hand) and sinkage when saturated (i.e. “sticky” when squeezed in your hand). Coarser-grained soils are more prone to slip when dry or saturated, but less so when moist. Use of a tether with appropriate tension may decrease the relative disturbance levels for these soil conditions by (1) relying less on soil shear strength for traction, (2) decreasing ground pressures, and (3) reducing eccentricity in the equipment’s balance on steep slopes. In general, the weaker the soil conditions, the higher tether tension is required to reduce the level of soil disturbance.
Studies show that use of the tether may more evenly distribute ground pressures along the track length of equipment, especially on steeper terrain. Specifically, the tension imparted by the tether provides forces that oppose machine imbalance (eccentricity), which reduces peak ground pressure and improves track engagement, therefore resulting in reducing soil disturbance. Soil compaction is a function of soil density, moisture and ground pressures – by reducing the pressures imparted on the soil surface, soil porosity loss may be reduced during loading. Sinkage and rutting is a function of soil shear strength, moisture and ground pressures – by decreasing ground pressures, the permanent deformation and lateral shear of soils may be decreased, primarily by avoiding exceedance of the bearing pressure of surface soils. Soil displacement is a function of soil shear strength, moisture and equipment tractive effort – by decreasing ground pressures and reducing tractive effort required for mobility with a tensioned tether, displacement of topsoil may be reduced significantly as full shearing of the soil is not necessary for traction and a larger proportion of equipment track length may be in contact with the ground. Both ground pressures and track slip may be reduced through increased tether tension.
Cutting and Extraction Distance
The length of cable and ground profile affect the distance the equipment operates on the slope. A typical drum size of the currently available winch systems ranges from 1,000 to 2,000 ft. Tethered systems work by lowering the equipment into the stand, so it may not be possible to use mechanical equipment in lower parts of a unit. Units may require hand falling in the lower sections where the tethered equipment cannot reach.
Tethered systems allow the use of mechanical equipment on slopes that normally require hand falling. The system keeps personnel in a cab with its associated safety features, minimizing exposure to unsafe conditions encountered when hand falling. Furthermore, the ability of cutting machines to cut and bunch trees near designated skyline corridors enables grapple yarding that does not require choker setting during yarding operations. Similarly, tethered cut-to-length systems (i.e. harvesters and forwarders) decrease exposure in hazardous yarding conditions through reduced need to set chokers or yard logs to a landing.
Many operations still require hand falling of trees in areas that mechanized equipment cannot reach. In this case, hand falling following mechanical harvesting can increase the hazards for the hand faller. Performing hand falling prior to mechanical falling reduces the hazards. This requires identifying areas of the stand that cannot be harvested with the mechanical equipment.
Type of treatments individual tethered systems can undertake vary and mainly depend on the tethered working equipment (feller buncher, harvester, forwarder, loader, skidder, etc.).
Find more videos and images of tethered logging equipment by performing a internet search on the terms ‘tethered logging’.