1. Introduction
Cable yarders are an important system of skidding and transporting timber and their use in the mountainous regions of Europe is widespread [
1,
2].
During the last ten years, the use of all-terrain mobile tower cable cranes has been increasing due to their adaptability in operating both in uphill as well as in downhill configurations. However, modern cable yarding technology fills this gap, and productivity models can assist users in their work, to maximize the performance of their machines [
3]. On difficult terrain, cable cranes represent a cost-effective alternative system compared to ground-based logging, and results in a much lower site impact too [
4,
5]. Over the past two decades, forest operations have been developing timber harvesting guidelines to mitigate the environmental impacts of tree felling and transport. Efficient forest management is essential for the resilience of these ecosystems. The use of this combined logging system helps reduce the effect of timber harvesting [
6]. Cable yarders are used mainly for timber harvest using parallel or fan-shaped schemes [
7], and new double-hitch carriages have been applied for full suspension. This different method requires longer to load up but permits a similar performance [
8].
Worldwide, the principal use of these machines is in coniferous stands and rarely in deciduous stands [
9]. Cable yarding in hardwood is widespread in the Appalachians in the Northeast of the US [
10,
11,
12,
13,
14], southern Germany [
1], southern Italy [
15,
16], Bulgaria [
9,
17], and Turkey [
18]. In Bulgaria the quota of deciduous tree species is 71%, while the quota of total assets growing is 55.5% [
19,
20]. Therefore, the use of these machines is important in the Bulgarian logging industry. In one example, a cable logging team of three workers tested a yarder for single-span layouts, while a different team of four members worked using both a yarder and skidder [
21,
22]. In the Northeast US, Huyler and LeDoux [
13] found that yarding delays were approximately 35% of the total cycle time for operational, mechanical, and non-productive time descriptions on steep slopes. The delays are separate from the delay-free times used to estimate the total cycle time [
13].
To raise the performance of cable yarding worked in European beech (
Fagus sylvatica L.) stands in the Ograzhden Mountains (southwest Bulgaria), Dimitrov [
17] showed that productive times for bunching (28%), extraction (21%), and carriage unloading (13%) should be decreased. In the mentioned paper, the performance of the yarder was defined as moderate (3.22 m
3/PMH). This productivity is comparable with other research carried out in a mixture of Oriental spruce (
Picea orientalis (L.) Link.), Normann fir (
Abies nordmanniana (Stev.) Mattf.), and Scots pine (
Pinus sylvestris (L.) forests in Turkey (6.6 m
3·h
−1, 5.5 m
3·h
−1, and 5.0 m
3·h
−1) [
23]. In Italy, Zimbalatti and Proto [
15] showed that three cable cranes extracted inferior volumes of timber compared to their load potential; in fact, the average extracted volume was inferior compared to the load capacity of the carriage. Melemez et al. [
18] showed that for a slope of terrain greater than 50%, the most efficient extraction occurs by the use of a skyline. The study carried out by Stoilov [
9] in a deciduous stand of European beech (90%) and European hornbeam (10%) in the Sredna Gora Mountains, Central Bulgaria, showed that the mean productivity of a truck-mounted tower yarder was close to the maximum for that type (9.12 m
3·PMH
−1 and 8.41 m
3·SMH
−1). The gross costs for yarding uphill a whole tree by the studied tower yarder were estimated at 146.52 € per PMH and 13.02 € per m
3. Fixed costs (26.84%) and variable costs (29.86%) were higher than labor costs (21.44%).
Most operations may be considered economically skillful if the costs of operations are determined carefully, taking into account all factors in a high-yield stand [
10]. Schweier [
1] showed that the harvesting costs of two different systems in flat terrains are similar, determining the cost of using a Koller K507 at 33 €·m
−3, while that of a Valentini V400 cable crane was 36 €·m
−3. Between the two systems, the Koller K507 was more cost-efficient in processing timber of large dimensions.
Combining a tower yarder with other extraction machines, although known for a long time, is not often implemented [
24]. However, such combinations have recently been observed, such as with a clambunk skidder.
The clambunk skidder lends to replacing or being used together with skidders, feller–bunchers, and processors [
25]. Stokes and Schilling [
26] have demonstrated that clambunk skidders mitigate the adverse effects of operations on conventional and challenging sites, while also enhancing the efficacy of the harvesting system. The use of this machine is more advantageous as the extraction distance and number of passes over the site increase, reducing road construction. On a clambunk skidder, the load is positioned on an integrally mounted loader and it is held by top-opening jaws [
27]. In South Africa, Oberholz [
28] has obtained a productivity of 5.7 m
3/PMH to 50 m
3/PMH in pine stands using a clambunk skidder with a capacity of 18 t, with uphill slopes 0%–25%, downhill slopes 0%–40%, and skidding distances of 50–1000 m. The variable, fixed, and administration costs were the most significant for cost analysis. Clambunk production costs differed with skidding distance because this variable involved travelling times. In Brazil, Dos Santos et al. [
29] have studied the average harvesting cost of a clambunk skidder in a stand of eucalyptus. It was equal to 1.7 €∙m
−3, for an average distance of extraction of 241.23 m with a volume average of 0.39 m³·tree
−1. This cost ranged from R
$3.14 to R
$6.93 m
−3 (around 1.10 to 2.40 €∙m
−3) for extraction distances of 106 m and 366 m, respectively [
29]. In a study, Ghaffariyan [
30] reported an average productivity varying from 9.3 m
3/PMH to 78.0 m
3/PMH in coniferous forests and plantations. The type of machine, the volume of logs, the slope of the terrain, and the skidding distance affect the performance of the skidder.
The aim of this present study was to examine the viability and to improve the use and operational efficiency of the two-stage extraction system, consisting of a tractor-mounted tower yarder and clambunk skidder in deciduous stands, and to analyze the time consumption, harvesting productivity, and costs for both machines. The adoption of these machines is useful for improving the economic and environmental performance of wood harvesting in deciduous forests, also involved in the Natura 2000 network.
3. Results and Discussion
The summary of the experimental data from 120 yarding and 30 clambunk skidder cycles for each of the selected variables used in the cycle time and production equations is shown in
Table 4.
3.1. Duration of Work Cycle Elements—Tower Yarder Unit
The work phases of the tower yarder (
Figure 4) were subdivided into the following times (including delays): lateral outhaul and hook (LOH) occupied 26% of the time, followed by carriage inhaul (CI) 6%, carriage outhaul (CO) 6%, unhook (U) 3%, and lateral inhaul (LI) 3%, respectively. The percentages of times excluding delays were as follows: LOH occupied 60% of the time, CI 14%, CO 13%, U 7%, and LI 6%, respectively. The predominant part of the cycle time (
Figure 1), excluding and including delays, was dedicated to the lateral outhaul and hooking (LOH) of the tree (60% and 26%, respectively), followed by carriage inhaul (14% and 6%, respectively) and carriage outhaul (13% and 6%). The other phases had approximately similar proportions: unhook (7% and 3%, respectively) and lateral inhaul (6% and 3%, respectively), excluding and including delays. Delays accounted for 56% of the total phase time and could be divided into operational and mechanical delays, respectively, for 52% and 4% of the total phase time of the studied cable yarder machine. Operational delays, 92% of all delays of the tower yarder, were predominantly due to the clambunk work phase (travel loaded to landing, unloading, sorting and piling, and travel unloaded). Within this time, the tower yarder can only make one phase (turn), waiting for the clambunk skidder to winch the extracted stems, due to the danger of them sliding back to the cutting area. A total of 66% of the work phase time, without delays, involved operations of lateral yarding (29% including delays). These operations include the lateral pull of the main line and the extraction of the load. In these conditions, the machine works at a moderate and low level of carriage payload capacity usage, with an average yarding distance of 123.83 m from nominal length, an average lateral yarding distance of 12.84 m, and an average slope of 17.25°. To estimate the yarding phase time (with and without delays), a regression analysis on time data was performed to create a prediction equation, using the independent factors in
Table 4. The delay-free cycle time (Equation (1)) and the cycle time with delays regression models (Equation (3)) obtained for cable extraction with significant factors are displayed in
Table 5. In Equation (1), the minimum values of T
net,TY may reach for this eventuality lower rates of yarding distance L
TY and terrain slope angle
i. The total cycle time of the tower yarder (Equation (3)) will decrease as the yarding distance L
TY is decreased and the lateral yarding distance
l and the number of load stems n
TY are increased.
3.2. Duration of Work Cycle Elements—Clambunk Skidder
The clambunk skidder work cycles (
Figure 5) were subdivided into the following (including delays): winching (W) occupied 36% of the time, followed by travel loaded (TL) 22%, travel unloaded (TU) 11%, loading (LOAD) 4%, unloading (UNLOAD) 3%, sorting and piling (SP) 3%, and maneuvering (M) 1%, respectively. The percentages of time excluding delays were as follows: W occupied 45% of time, TL 27%, TU 14%, LOAD 5%, UNLOAD 4%, SP 4%, and M 1%, respectively. The delays accounted for 19% of the total cycle time and are divided into operational and mechanical delays, respectively, for 16% and 3% of the total cycle time of the clambunk skidder. The operational efficiency obtained during the study of the Timberjack 1710 clambunk skidder extracting whole eucalyptus trees for chipping was 78.30%, while mechanical availability was 87.38% [
29]. From
Table 5, the delay-free clambunk skidder cycle time T
net,CS regression model (2) shows that a reduction can be achieved by reducing the number of stems n
CS in the payload. The same conclusion applies to the total cycle time of the clambunk skidder T
CS given by Equation (4). Within the subdivision of operations of the clambunk skidder (excluding delays), winching (36%), and movement (34%) took up the most time, followed by loading and unloading (7%) and sorting and piling at landing (3%). The mean speed of the studied clambunk skidder was 4.25 km·h
−1. The clambunk skidder with a load had an average speed of 3.23 km·h
−1, and an average of 6.28 km·h
−1 without a load. The difference is 3.05 km·h
−1—almost double the unloaded weight since the skidding resistance forces of the rear parts of the stems do not act when moving without a load.
Orlovský et al. [
38], for an LKT 81 ILT cable skidder with knuckle-boom, stated an average speed of 3.75 km·h
−1 with load, and an average speed of 4.45 km·h
−1 without load, i.e., lower than the values we found. However, Spinelli and Magagnotti [
39] reported an average speed of 8.1 km·h
−1 without load and an average speed of 7.3 km·h
−1 loaded travel, for a 96 kW agricultural tractor, significantly higher than this study. Also, for a John Deere 548H, the average speed was 8.60 km·h
−1 and 6.00 km·h
−1, respectively, for travel loaded and unloaded [
38]. To reduce movement time, the clambunk skidder should increase its travel speed. Unfortunately, the terrain conditions do not afford a significant increase in speed.
3.3. Productivity Analysis
The productivity of the studied machines was described by the regression equations (
Table 6). The tower yarder productivity (without delays), described by Equation (5) in
Table 6, can be increased by reducing distance and skyline slope and increasing load volume. The same can be said for tower yarder productivity including delays (Equation (6)) by reducing lateral yarding distance. Another option is a truck-based yarder with a hydraulic crane with a grapple to place the yarded stems on the road. If the hydraulic crane is equipped with a processor head, crosscutting and subsequent forwarding of assortments (logs) is possible. In these two cases, one will move from a serial to a parallel production system.
From Equation (7), to increase the clambunk skidder delay-free productivity, the load volume per phase V
CS should be increased, whereas the skidding distance L
CS in it should be decreased. The clambunk skidder productivity including delays is calculated by Equation (8) in
Table 6, and the opportunities for increase are the same as for delay-free productivity.
Consequently, the productivity models indicate skidding distance and load volume extracted per phase (turn) as the most important factors affecting the skidding productivity of the studied clambunk skidder in the conditions of thinning in European beech forests. Of course, further studies could evaluate the influence of other variables to compare productivity in several stands.
The average productivity, obtained at an average skidding distance of 693 m, average load volume per cycle of 5.02 m3, and mean 6.6 stems per phase (turn), is 6.23 m3·PMH−1 and 4.93 m3·SMH−1 respectively.
These productivity rates are close to those reported by Orlovský et al. [
38] for the LKT 81 ILT with knuckle-boom loader, which, at an average skidding distance of 316 m and mean load volume per phase of 8.00 m
3, are 6.31 m
3·PMH
−1 and 4.20 m
3·SMH
−1, respectively. Borz et al. [
40] determined a higher value of productivity in terms of PMH and SMH (4.40 m
3·h
−1 and 3.10 m
3·h
−1, respectively) testing a TAF 690 skidder covering an average distance of 1700 m. The mean productivity for a TAF 690 PE in similar terrain conditions and a shelterwood system obtained by Stoilov et al. [
24] was 16.0 m
3·PMH
−1 and 14.00 m
3·SMH
−1, respectively, for an average distance of 264 m (2.6 shorter than that in this present study), mean load volume of 3.94 m
3 (1.3 lower than that in this present study), and mean 2.2 logs per travel.
For the John Deere 548H skidder, Proto et al. [
37] registered a harvesting performance of 30.5 m
3 per PMH and 25 m
3 per SMH in conditions of an average seven logs extracted per turn, at a mean extraction distance of 275 m, average and maximum winching distances, respectively, of 35 and 65 m, and a mean volume skidded per cycle of 3.90 m
3.
The tower yarder and clambunk skidder present a serial production system. In the serial system the production processes are performed individually in sequence [
41], since after the stems are extracted by the tower yarder, the skidding process starts.
The use of the serial production system for timber extraction allows both machines to be used more rationally and thoroughly, and, consequently, increases their productivity, improves the technical maintenance of equipment and current repairs, ensures the possible full compliance of forestry equipment with natural production conditions, and, ultimately, increases the efficiency of logging production. For a serial production system of machines to be effective in specific terrain and production conditions, it must be designed in such a way that their productivity must be equal or multiple and efficient for a given volume of production. This study ensures the full rate of utilization of equipment, reduces production costs, and allows for reducing manual labor in the forest stand to the minimum possible, i.e., only until felling, delimbing, pulling, and hooking the stems to the yarder mainline. The majority of operations are carried out at convenient workplaces near forest roads and landings, which reduces the risk of occupational injuries. In addition, the use of the forwarder crane supports the bucking, sorting and piling of the received assortments on the landing.
Therefore, by equating the delay-free productivity models of the two machines, parameter values can be determined. The effect of the yarding distance L
TY on the skidding distance L
CS for a skyline slope of 17° and carriage payload of 1 m
3 is shown in
Figure 6a.
An increase in yarding distance and skidder payload results in an increase in skidding distance where delay-free productivity is equal. An increase in carriage payload has a similar effect on the skidding distance at constant values of skyline slope of 17° and skidder payload of 5 m
3 (
Figure 6b). The influence of skyline slope with other parameters constant is shown in
Figure 6c. Increasing the skyline slope, other parameters being constant, leads to a decrease in delay-free yarding productivity, but an increase in the corresponding skidding distance.
3.4. Costs
The extraction net costs of the studied production system for stem yarding and skidding in a beech stand were calculated at 120.87 € PMH and the harvesting costs at 14.93 €/m
3 (
Table 7). The gross extraction costs of a Koller K501 for cutting trees in a predominantly beech stand were calculated at 13 €/m
3 [
9], slightly lower than that of the tower yarder and clambunk skidder studied here. The net extraction costs of a Timberjack 1010D combi–forwarder in similar production and terrain conditions were estimated at 4.1 €/m
3 [
20], which is three times lower.
In comparison, Proto et al. have determined that the costs of skidding for a John Deere 548H skidder in chestnut stands in South Italy were between 3.6 and 5.8 €·m
−3 (for an average distance reaching between 266 and 276 m, an average load between 3.88 and 4.01 m
3, and an average slope of 27%) [
37]. In Kardzhali Province (Bulgaria), Stoilov et al. [
42] reported an extraction cost of an HSM 805 ZL double-drum cable skidder equal to 14.24 €·m
−3 in a stand of conifers (for an average distance of 69 m, an average load 4.06 m
3, and an average slope of 34%).
In general, these costs are slightly above the extraction costs reported with other machines in this region. This is probably due to the lower performance of the studied extraction system, due to longer delays.
4. Conclusions
The studied extraction system has many advantages. Very often, on steep terrains, in the absence of sufficient landing space, logs which were obtained in a cutting area under difficult working conditions are yarded. The yarder’s unloading area must be cleared after each turn for new carriage loads. In the case studied, the yarded stems are pulled with the winch of the skidder. Otherwise, they will inevitably fall back into the clearing. It is also possible to pull the stems with an additional winch or tractor, even with animal power.
The studied system allows for the extraction of stems or stem sections, which reduces operations in clearing, felling, and delimbing, and the remaining operations of crosscutting, sorting, and piling are performed after skidding on the landing under more convenient working conditions.
A disadvantage of the studied extraction system is the long delays due to the skidder waiting for a sufficient load to be collected, and subsequently, during skidding, the yarder waits with the extracted load on the carriage for the skidder to return to pull the load onto the skid road.