The job between the stump and the truck
Once a tree is down, the wood still has to travel from where it fell to a landing beside a road where trucks can reach it. This stage is called extraction, forwarding or skidding, depending on the method. It is often one of the costliest parts of a harvest, because it involves repeated trips over rough ground. The machine choice here also decides how much of the forest floor is disturbed.
This review looks at the main machine types. How trees are felled and processed in the first place is the subject of the companion article on timber felling machines.
Skidders
A skidder drags trees or logs behind it. Skidders are typically wheeled, four-wheel-drive machines with a frame that articulates in the middle for tight turning. They are used mostly in whole-tree or tree-length systems, where trees have been felled and bunched but not cut to final length.
There are two common ways of attaching the load:
- Cable skidders use a winch line with chokers, which are loops placed around logs by a person on the ground. They can pull from a distance with the winch, which helps on awkward spots, but they need a person to set chokers.
- Grapple skidders use a hydraulic grapple at the rear to grab the bunch directly. They save labor and are faster per cycle, and they are common where trees are already bunched.
One end of the load is lifted off the ground and the other end drags, which reduces friction while still disturbing the soil along the path. Skid trails therefore need planning.
Forwarders
A forwarder carries logs on its back. It has a loader crane and a bunk, which is a frame that holds a stack of cut logs, and it picks up the piles left by a harvester and drives them to the roadside. Because the load is carried instead of dragged, a forwarder typically disturbs less soil than a skidder and can work well on sensitive ground.
Forwarders belong to the cut-to-length system. The harvester processes the trees into logs of set lengths at the stump, and the forwarder collects them. Sorting by product can be done at the pile, which reduces handling later.
Most forwarders are articulated with six or eight wheels, and some use tracks or track bands on wheels to spread weight on soft ground. The crane and grapple are powered by hydraulics, so hoses and cylinders take the same punishment described for harvester heads. Choosing hoses and fittings suited to the job matters, which is why the principles in industrial hoses carry over to the forest.
Cable yarding and other systems
On steep slopes, wheeled and tracked machines may not be safe or may damage the ground. Cable systems drag or carry logs uphill or downhill using steel cables on a tower or a mobile yarder. Some operations use winch-assisted machines, where a cable from an anchor machine helps a harvester or forwarder hold position on a slope. In specific situations helicopters or horses are used, such as where road building would be too damaging. These methods are slower or costlier per unit in many cases, but they open ground that other machines cannot work.
Terrain and soil concerns
Terrain often decides the machine more than the preferences of the crew do. Factors include:
- Slope. Steep ground limits wheeled machines and raises rollover risk. Machines are designed for specific slope limits that appear in the manual.
- Soil strength and moisture. Wet, fine soils rut and compact easily. Many operations schedule work in dry or frozen seasons, or use designated trails and a mat of branches to carry machines.
- Ground pressure. Wider tires, tracks, bogie axles and lighter loads reduce pressure on the soil. Tire pressure can be set to suit the ground, and it is one of the reasons why tire pressure monitoring is useful on this equipment.
- Streams and sensitive areas. Rules typically require buffers and approved crossings.
- Distance to the landing. Longer distances favor machines that carry a load, because dragging over long distances is slow and hard on both the wood and the ground.
The kind of machine that tackles this work is closely related to rough terrain trucks and other off-road vehicles, which share the same traction and articulation ideas.
The landing and loading
At the landing, logs are sorted, piled and loaded onto trucks by a knuckleboom loader or a front-end loader with a grapple. Some landings include processors or chippers. Landing design, with enough room to turn and stack, affects the productivity of every machine in the chain. The grapples used here are close relatives of the load handling attachments seen in other heavy industries.
Maintenance and uptime
Extraction machines make many repetitive trips on poor surfaces. Tires, frames, articulation joints, winches, cranes and hoses all wear. Regular machinery maintenance, including checking tires for cuts and pressure, greasing pivots and examining hoses, keeps the crew productive. Because a crew is only as fast as its slowest machine, one broken forwarder can idle a harvester, which makes equipment uptime a practical matter instead of a theoretical one.
Safety
Log transport has its own hazards, including rolling or shifting loads, steep ground, winch cable failure, and ground crew working near moving machines. Machines have cab guarding to protect operators from intrusion, and good practice keeps people clear of cables under tension and of the swing area of cranes. Trained and certified operators, adherence to manufacturer limits and local regulations, and clear communication on site are the common themes. Fire is also a risk in dry conditions, and some machines have fire suppression systems fitted. This is general information, not engineering advice.