Entwicklung eines Grenzneigungsmodells für selbstfahrende Arbeitsmaschinen in der Forstwirtschaft (English shop)

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Wood as a raw material is enjoying increasing popularity both for material use and as a substitute for fossil fuels. In order to satisfy the long-term rise in demand, hitherto unused and neglected raw timber potentials must be made accessible to the market. One important area in this respect are the slopes of the low mountain ranges, which, owing to the topographical difficulties involved in timber harvesting, hold partly unused timber reserves. In addition to the economic restrictions on management, there is frequently considerable uncertainty about the limits of highly mechanised timber harvesting on steeper slopes.

These limits arise, on the one hand, from the stability of the machines and, on the other, from the soil damage caused by trafficking. The latter occurs to an increased extent on slopes as a result of inappropriate trafficking with excessive slip. Agricultural investigations (SÖHNE 1952a) show that the risk of soil erosion increases with rising slip. For the present question concerning operational limits for highly mechanised timber harvesting, the slip limit that is acceptable in terms of soil ecology is considered to be 25 %. More pronounced “spinning” of the drive wheels leads to almost complete shearing off of the topsoil, so that the loosely lying soil particles are washed out during heavier precipitation. In order to prevent this additional strain on forest soils, objective limits for the use of ground-based timber harvesting technology are required.

On the basis of tractive force measurements on level ground, a limiting-gradient model for a forwarder was therefore developed. Considerations regarding the downhill slope force and thus the gradient resistance that the machine has to overcome reveal a direct relationship between the gradient and the ratio between the measured tractive force and the machine’s own mass. The so-called coefficient of traction can be determined on the basis of tractive force and slip measurements under level conditions and presented as a function of drive slip. With the aid of linear regression models, the traction capacity can be described in simplified form and compared with the downhill slope force. Besides the maximum tractive force, the coefficient of traction at 25 % slip is of particular importance, since it can be converted into the gradient of trafficking that is still acceptable for the soil.

The South Lower Saxony Uplands (Südniedersächsisches Bergland), with its typically loess-dominated sites, served as the study area. The tractive force measurements identify soil water content and the stone content of the topsoil as the most important soil parameters for the gradeability of the forwarder. Whereas variation of the tyres and the tyre inflation pressure had only minor effects, a marked improvement in traction capacity results from mounting traction aids on the forwarder. On the basis of these factors, a limiting-gradient prognosis for machine operation acceptable in terms of soil ecology as well as an absolute operational limit oriented towards resistance to slipping were calculated.

The soil-ecological operational limit was examined in the course of validation runs on slopes with gradients of up to 40 %. This yielded an agreement of over 80 % between the prior assessment (traffickable with max. 25 % slip) and the actual trafficking. Only one trial showed an underestimation of the slip level, while in four cases trafficking would have been omitted even though it would have been possible without major damage from slip. Verification of the absolute operational limits was not envisaged within the scope of the trials. Individual trafficking runs at steeper gradients showed, however, that the mobility of the forwarder ends well before this absolute limit is reached.

Both the soil-ecological limiting gradients specified and the driving trials at gradients of up to 40 % make it clear that purely wheel-based highly mechanised timber harvesting reaches its limits on slopes. In order to protect the soil against erosion damage and for the safety of the machine operators, early use of traction aids or (traction) assistance winches is therefore recommended. Provided that the soil has adequate bearing capacity, chains and tracks make it possible to traffic gradients of up to 35 % relatively free of damage even under moist conditions. Gradients above this can only be trafficked in a manner acceptable in terms of soil ecology under very good conditions, or they require the additional use of traction winch technology. The winch reduces slip during trafficking, and the traction aids ensure the machine’s resistance to slipping in the event of technical problems with the winch or the cable. The limits of this technology arise, on the one hand, from the soil-dependent absolute limiting gradient of the prognosis model and, on the other, from the technical possibilities of the individual machine to compensate for slope gradients for the operator and the loading equipment.