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Time study and statistical analysis – Studija vremena i statistička obrada podataka
The repetitive time method, which is also know as snapback method is one of the most common time study methods in forestry, (Eker and Acar 2014; Gulci et al. 2017a; Melemez et al. 2014; Proto et al. 2018; Spinelli and Magagnotti 2011) and was used in this study for analysis of system productivity. Time measurements were made during 30 trips for both uphill winching and skidding on skid trail. The main work stages for both operations are listed in Table 2. The time spent on resting, injuries, and other work-related delays were separated from other work stages (Björheden et al. 1995). Tree diameter and tree length data measured in the field were used to compute tree volume, and then productivity of each trip was calculated based on timber volume and total cycle time.
One-Way ANOVA, Pearson correlation test, and linear regression analysis were used for the analysis of the data obtained from time measurements. The differences between the averages in the study with equal number of samples were evaluated by the Tukey multiple comparison test (R Core Team 2018; SPSS 2017). For both operations (phases), the effects of tree volume on productivity was investigated by using ANOVA at the 0.05 significance level. Tree volume extracted in each trip was divided in to three classes including low (<0.40 m3), medium (0.41 – 0.80 m3), and high (>0.80 m3). Pearson correlation test was applied to determine the relationship between tree diameter (X1), length (X2), volume (X3), and skidding distance (X4) and total time (Y). Finally, Linear Regression Analysis was used to determine the mathematical models of total cycle time for whole-tree extraction methods.
Productivity of the farm tractor was determined during whole tree uphill winching and skidding backward by using time study method. The same trees extracted from the harvesting unit were transported during both working semi-phases. The average tree volume transported in each trip was 0.64 m3. The average winching distance and skidding distance were to be 30 meters and 91 meters respectively. The ground slope at uphill winching area and on a skid trail was 49% and 7%, respectively.
Productivity analysis – Analiza proizvodnosti
The average time study data for each work stage in uphill winching is indicated in Table 3. The results indicate that winching trees to the prebunching area was the most time consuming work stage (49%), followed by pulling steel rope to the fallen tree (23%) and moving to the winching station (22%). In a previous study, it was reported that the most time consuming work stage during winching whole-trees by farm tractor was again winching trees to roadside landing (40%) (Gulci 2014).