by A. H. Lloyd, Imperial Forestry Institute, Oxford, England
Mechanization of the extraction of timber from difficult sites by the use of the Wyssen cable system or skyline crane has aroused much interest during recent years. It is a Swiss invention originally designed for logging from Alpine forests but it is now used over a wide range of forest types and conditions, from softwood forests in Canada to tropical hardwoods in the Sudan and other parts of Africa, and also, recently, in mangrove swamps in Asia. During the present year it has been installed for the first time in New Zealand and in Scotland for the extraction of thinnings from softwood plantations.
It is the purpose of this article to examine briefly the capabilities and limitations of the skyline crane from the forestry point of view, particularly in regard to the terrain and the type of forest.
The skyline crane is a cable system but differs from all other cableways in that it is a travelling crane and can pick up loads directly from the ground at any point along the cableway. It can also haul in logs from a limited distance on either side of the cable. It therefore combines the uses of the well-know American skidder with transport by gravity cableway. The operation is simple, as can be seen by reference to the diagram.
The Wyssen system consists essentially of a single fixed main cable or skyline along which runs a specially designed carriage or travelling crane. This carriage is controlled by a light operating cable connected to a yarder, which is a portable winch with a small engine assembled as a single unit on skids. The yarder is anchored near the upper terminus of the cableway, as shown. When the carriage reaches the point along the cable at which it is desired to collect a load, a special skyline stop, controlled by a light trip-line from the ground, halts the carriage and, on contact, automatically releases the choker-hook which falls slowly to the ground, attached to the end of the operating cable.
The choker-hook is then carried by hand and attached to the load which may be a single log or a number of small logs bundled ready with choker-cables. At a given signal to the winch operator, the load is hauled upwards to the carriage crane on the main cable, as shown in the third illustration, where a single two-ton log is being hoisted up to the carriage coupled to the skyline stop. As soon as the special choker-hook strikes the base of the carriage, an automatic release frees the "stop" from the carriage which, with the suspended load, then slides down the cable by gravity to the off-loading point, the speed being controlled through the operating cable by the brakes on the winch-drum. In the diagram the carriage is shown approaching the off-loading point after being released from the skyline stop seen halfway up the cable.
The diagram shows an ideal topographic profile for a skyline operation. The forest is very open and has suitable tall trees to form natural supports for the cable and also to provide firm anchorages. In dense even-aged forests, particularly in young plantations, the leek of suitable spar trees or natural anchorages adds considerably to the labor involved in installation, but a log buried horizontally a few feet in the ground can provide a good alternative anchorage. Skidding timber from either side of the cable is also difficult in very dense young plantations and short feeder tracks may have to be cleared and the timber ground skidded by man or horse to these tracks. The current practice in Scotland to bring out softwood thinnings in their full original lengths entails the use of very high supports anti adds considerably to installation difficulties as the bundled poles are carried vertically.
In open high forest practically no harm or damage to the remaining stand is caused by skyline logging, and it has indeed often been proved that much less damage is caused than with ground skidding by man or horse, especially on hill slopes where soil erosion can also frequently be caused by tractors anti dragging paths.
The skyline can extract timber either uphill, downhill or on level ground. The steepest safe gradient for the cable is 40 degrees (or 100 percent) between supports: where a mountain side is too precipitous the lower anchorage may be sited at some distance from the foot of the slope in order to reduce the cable gradient. The minimum slope for operating by gravity is about 8 degrees and, if less than this, a second winch-drum may be needed to haul the operating cable.
The maximum distance between cable supports depends on the weight of the load and the gradient; the smaller the slope the deeper is the catenary curve and therefore the closer together the supports must be to keep the load off the ground. This is one of the disadvantages in using the skyline on level or almost level terrain and can only be countered by increasing the normal tension on the main cable.
Logging uphill from a deep v alley or ravine offers less difficulty than from level ground. The arrangement of cables is the same but the loads are hauled up the main cable by the yarder.
The average normal load is limited to 1½ tons although occasional logs of 2 tons may safely be extracted. A heavier model to take 10-ton loads is now being designed, but this will entail very heavy cables and reduce the portability and simplicity of the equipment. The present model is designed for rapid installation by ordinary workers without any previous mechanical knowledge, but several weeks' special training under experienced operators is essential. Five experienced men can install a skyline cableway in from 3 to 14 days, depending on the length of cable and the availability of natural supports. The ordinary operating distance is Up to 1½ miles (2.5 km.) long.
Choker-hook a bundle of logs being hauled in to tile main skyline.
The skyline crane promotes intensive utilization of the forest as, when installed for logging, additional small material such as firewood can often be extracted at a profit. Forests badly damaged by fire, insects or wind can be clearer! quickly, although windfelled areas create special difficulties in operation. The output of timber per man-day is high and operations can continue in all weather except deep snow.
Costs of extraction of timber by skyline crane are difficult to compare with other means of transport as the skyline is generally used in forests inaccessible by other methods or where costs would in any case be much higher than the average, particularly if hillside roads have to be constructed and maintained.
The equipment is costly as it has to be light in weight but strong enough to withstand rough usage in the forests. Breakdowns in an overhead crane would he both dangerous and difficult to repair, away from workshops. The controlling yarder has to be sensitive to sudden changes of stresses and speeds due to logging operations which are out of sight and often at the end of a mile of cable. This necessitates expensive multiple gears and special braking equipment, and the initial cost of a complete skyline crane is therefore high. The costs of operation and maintenance are, however very small in comparison with tractor operations, and the equipment has a much longer life.
A two-ton log being hoisted up to the skyline crane carriage.
Actual costs of extraction per cubic meter of timber vary too greatly with local conditions to be usefully quoted here, but figures are obtainable for forests in several European countries and in Canada.