A skidder drags material from the woods to a landing or roadside.
Track or Wheel
Wheeled skidders are built on an articulated chassis with the cab and engine mounted on the forward articulation and either a cable drum and arch or grapple mounted on the rear articulation. Many modern skidders include both a cable drum and a grapple. Wheeled skidders typically have a small blade mounted on the front that can be used to push material out of the way and level small ground obstructions.
Tracked skidders may be modified construction crawlers or purpose built. They use either a grapple or cable to skid trees. Tracked skidders typically have a full dozer blade and can be used to create landings and roads. There are two styles of tracked skidder used, flex track and rigid track. Rigid tracked machines have a typical tracked undercarriage, which consist of a rigid frame supporting the tracks with no suspension. Flex tracked machines have bogey wheels that support the track. The bogey wheels are suspended independently. Due to this suspension, flex tracked machines maintain greater contact with the ground which provides greater traction on broken ground.
Tracked skidders are generally used where large loads are being pulled up steeper slopes or where the logger wants a machine that can help build roads and landings. Tracked machines have more pull per horsepower and lower ground pressure.
Grapple or Cable
Skidders come with either a grapple or a cable drum and sometimes both. A grapple is capable of picking up more than one stem at a time. A cable skidder has a skid line with chokers attached. The number of chokers used will depend on the size of trees being extracted. Cable skidders have a fixed arch over which the cable runs through a fairlead. The arch provides lift to the ends of the logs.
Single Arch, Double Arch, or Swing Boom
Grapples are attached to the skidder by means of an arch or boom. The arch provides lift and aids in positioning the grapple to grab a load of one or more logs.
Single arches have a one piece arch with a single set of cylinders controlling position. The motion they produce is an arc, moving the grapple both vertically and horizontally with one motion.
Double arches have a two piece arch with a set of cylinders controlling each section. One section is pinned to the skidder vertically and controls the grapple’s movement horizontally. The other section is pinned to the vertical section horizontally and controls the movement of the grapple vertically. This type of arch provides more control over the grapple.
A swing boom resembles the arm on a backhoe. It allows the grapple to be positioned vertically, horizontally, and laterally. This type of boom allows the skidder to pick up material next to the skidder and provides more options when dumping the load. Very large grapple skidders with boom mounted grapples are sometimes referred to as grapple forwarders. These resemble a clambunk skidder except that they mount the large grapple directly to the boom.
Slope: There are three factors that influence the slope on which equipment can operate:
- Safety - the safe operating slope at which a piece of equipment can operate without rolling or tipping.
- Gradeability - the slope on which a piece of equipment can mechanically operate. Over this slope limit fluids leak out of containers and the engine can no longer pull the load.
- Soil disturbance and erosion - the point at which either of these occurs is dependent on soil type and moisture levels/precipitation patterns.
The slope on which skidders may operate depends in part on the direction of skidding. Favorable slopes, where the load is being dragged downhill, are preferred to adverse slopes as this allows gravity to aid in the extraction. The slope guidelines given here need to be balanced with the type of soil over which the skidders are operating and the horsepower/drawbar pull of the equipment. These figures are based on gradeability and do not take into account safety or soil conditions.
- Favorable slope: up to 50%
- Adverse slopes: up to 15%
- Favorable slope: up to 45%
- Adverse slopes: up to 10%
The ability of a skidder to turn safely is reduced when slopes are steep. Operational layout should consider whether the skidder can safely turn around on the slope. Broken ground that provides level benches on which to turn around may decrease this danger. Swing boom grapples allow the skidder to grab wood lying on steeper ground while the skidder stays on safer slopes. Cable skidders are capable of extracting wood on steep slopes that are within the line length of the skidder from easier ground.
Soil Conditions: Wheeled skidders may be fitted with dual tires or high flotation tires for operation in wet soils. Cable skidders may be used to skid in areas of intermittent wet areas or loose soils. Logs can be pulled over these areas with the skid line while the skidder stays on firmer ground. Because skidders drag the logs, soil disturbance is greater than extraction methods that fully suspend the load.
Extraction Distance: Tracked skidders have slow ground speeds and should be limited to shorter extraction distances, typically less than 500 feet. Wheeled skidders are capable of faster speeds and can extract material up to 1000 feet. Very large grapple skidders, also called grapple forwarders, are available that can extract material greater distances. This is due to their ability to extract a larger payload per turn which offsets the extra time spent traveling in the yarding cycle.
Tree Size: Skidders should be matched to the trees they will be dragging. Factors to consider are the power required to pull larger trees as well as larger grapples capable of grasping enough stems to make it productive. Tracked skidders are well suited to skidding large trees due to their high traction and pulling force.
Grapple skidders permit the operator to stay in the cab during the yarding cycle. This provides an extra level of safety for the skidder operator. Someone must work outside the protection of the cab to pull line and set chokers. The cables on cable skidders may break if not properly maintained or if the logs hit a stump or snag while being dragged. This poses a potential danger to the operator and those working in the area.
Skidders work very well in clearcuts where damage to residual stems is not a concern. In partial cuts damage to standing trees can be minimized through proper skid trail planning and felling techniques. There are few constraints in the use of skidders to extract material other than resource damage and slope limitations.
Grapple skidders work well with feller bunchers. Feller bunchers are capable of building and placing full bunches in a manner that facilitates the productivity of the skidder. Cable skidders work well with manual felling where the trees are scattered in the unit.
Products and Markets
Skidders are very versatile and can be used to skid whole tree, tree length, or log length material.
The following is a selection of representative research studies and reports done on harvest systems that include skidders. These reports may be used to get an idea of productivity and impacts of different systems and uses of skidders as well as some of their limitations. When reading these reports, keep in mind that they describe specific systems and stand treatments. Trying to apply the lessons learned from these reports to systems and treatments outside of the studies’ scope may have unintended or unforeseen consequences.
This is not a complete listing of research on skidders. Additional information can be found at the USDA Forest Service Treesearch website. This site provides reports on research performed by Forest Service Research and Development scientists and their collaborators.
Title: Effect of tire size on skidder productivity
Author: Brinkcr, Richard W.; Klepac, John F.; Stokes, Bryce J.; Roberson, Joe D.
Description: During the Spring of 1996 a collaborative effort among Mead Coated Board, the Auburn University School of Forestry, and the Southern Research Station was initiated to evaluate skidder production performance as a function of tire size and soil condition (i.e. wet and dry season)_ The objective of the study was to determine production and cost differences among 28L-26. 30.5L-32. 67x34.00-25. and 66x43.00-25 tires. The first portion of the study was completed for dry sites and there proved to be no significant differences in productivities among the tires tested. Wider tires do not hinder productivity under dry conditions. Additional data for the same tires will be collected under wet conditions in order to make comparisons among the tires and between the seasons. The goal is to develop a strategy for optimal tire management.
Title: Productivity of rubber-tired skidders in southern pine forests
Author: Kluender, R.; Lortz, D.; McCoy, W.; Stokes, B.; Klepac, J.
Description: Sixteen stands were harvested at intensities (proportion of basal area removed) ranging from 0.27 to 1.00. Logging contractors used one or two rubber-tired cable and/or grapple skidders. Harvested sites were similar in slope, tree size, and stand composition. Thirteen of the stands had even-aged structures while the other three were uneven-aged. Skidding time per cycle was directly related to skidder type, distance, and number of stems hauled, and inversely related to harvest intensity. Skidding productivity (grapple skidder, per productive hour) was sensitive to distance, stem size, number of stems in a load, and harvest intensity. Productivity was more sensitive to tree size than harvesting intensity.
Title: Development and Analysis of SRIC Harvesting Systems
Author: Stokes, Bryce J.; Hartsough, Bruce R.
Source: In: Proceedings, First Biomass Conference of the Americas: Energy, environment, agriculture, and industry; 1993 August 30-September 2; Burlington, VT. Golden, CO: National Renewable Energy Laboratory: 302-308. Volume 1.
Description: This paper reviews several machine combinations for harvesting short-rotation, intensive-culture (SRIC) plantations. Productivity and cost information for individual machines was obtained from published sources. Three felling and skidding systems were analyzed for two stands, a 7.6-cm (3-in) average d.b.h. sycamore and a 15.2-cm (6-in) average d.b.h. eucalyptus. The analyses assumed that whole trees were chipped at roadside.
Costs and production were summarized for each system. The systems were: (1) Continuous-travel feller-buncher, skidder, and chipper; (2) 3-wheel feller-buncher, skidder, and chipper; (3) chainsaw, skidder, and chipper. In the 7.6-cm stand, system productivities were 9.9, 7.3, and 7.5 BDLT/SMH, and costs were $20.9, $20.8, and $18.0 per BDLT for the three systems, respectively. System production rates for the 15.2-cm stand were 24.3, 10.2, and 12.5 BDLT/SMH, and costs were $8.7, $10.9, and $13.2 for systems 1, 2, and 3, respectively.