We observed changes in the productivity of log-loading work using a grapple loader and determined how an operator can gain proficiency in operating forest machines by evaluating the subsequent machine work of the operators who were inexperienced in forest machine operations. Consequently, we found that the work time tended to decrease with an increase in the work experience of the operator. This could be because of the improvement in machine operation techniques such as simultaneous operation of actuators or effective uses of machine abilities and the acquisition of work knowledge regarding log treatment or efficient trajectory of the work machine. This consistent decrease in work time and improvement in productivity can be expressed using a log-linear learning model, with a determination coefficient of >0.90 in regression analysis. This showed that an operator’s long-term improvement in forest machine operation can be predicted by observing the early progress of the operator. In addition, we revealed that observation of the work should be performed at least five times to complete an exact model.

In this study, we investigated the log-loading work performed by operators exhibiting different proficiencies to clarify the relationship between the number of logs grasped and moved to the forwarder carrier using a grapple and the time required to complete one work cycle. The work was performed on logs of 2.35 m³ and 0.138 m in volume and average diameter, respectively, using a grapple with a cross-sectional area of 0.16 m². The work was classified into four groups: fast work (<20 min), moderately fast work (20–30 min), moderately slow work (30–40 min), and slow work (>40 min). Operators in the fast or moderately fast work groups who grasped more logs during each cycle were more productive; however, this was not the case for the operators in moderately slow or slow work groups, and the effective number of logs grasped per cycle was estimated to be 9.0 or 5.7. The decrease in productivity due to the operator grasping an excessive number of logs could be because of 1) the logs collapsing during transportation to the forwarder carrier, thus requiring the logs to be reloaded and 2) the logs being scattered on the forwarder carrier during release, thus requiring additional time to align the logs. These results indicate that operators involved in the loading of logs work more efficiently when loading the optimal number of logs per cycle according to the operation technique.