ENERGY LOAD OF WORKERS EMPLOYED AT TIMBER HARVESTING

Witold Grzywiński

ABSTRACT

The performed investigations comprised 13 workplaces in the process of timber harvesting in premature stands managed in a motor-manual and mechanized technologies. Measurements of net energy expenditures were conducted with the help of the energy expenditure-measuring device MWE-1. Minute values of energy expenditure for individual activities (n=24) as well as energy costs for the working day (8h) were determined. The unit net energy expenditure ranged from 3.1 to 36.2 kJ/min. On the other hand, the recorded net energy cost of a workday ranged from 1.8 thousand to 11.5 thousand kJ. The following workplaces were classified as very heavy ones: fellers, feller’s assistants and skidders of short timber, while positions of operators of harvesters and forwarders were categorised as light.

Key words: energy expenditure, categories of work intensity, motor-manual harvesting, mechanized harvesting, intermediate cutting..

INTRODUCTION

Energy expenditure is defined as the amount of energy necessary to perform a definite physical work. Its value is used as an indicator of the degree of work arduousness [17]. Works carried out in forestry, especially those associated with timber harvesting are, in their majority, of manual nature and are characterised by a considerable dynamic effort and are also usually considered as very heavy [1, 8, 11]. Research data concerning the energy load in the process of timber harvesting available in literature, in their majority, go back to 1960s and 1970s [2, 3, 5-7, 12-14, 25, 27] and focus, primarily, on the work of the feller - operator of the chainsaw. Only few studies were concerned with the work energy costs in technologies employing multipurpose machines [28, 33].

The aim of this study was to present the analysis of the current energy load of forest workers employed in the harvesting of timber in premature stands. In spite of continuous technological advance and mechanisation of forestry works, the motor-manual technology employing the power chainsaw is still extremely common in the process of timber harvesting and the use of multipurpose machines is limited. The hard physical effort is still a significant element associated with the arduousness and harmfulness of workplaces in State Forests [24].

MATERIAL AND METHODS

The performed experiments comprised workplaces in the process of timber harvesting in pine premature stands employing motor-manual and mechanized work technology.

Motor-manual technology
The investigations were carried out in the Zamrzenica Forest Division (the Regional Direction of State Forests in Toruń) and involved the treatments of: early thinning in a pine stand 45-years old and late thinning in a 71-year old pine stand. Both stands were growing on the fresh coniferous site type. The short wood method was employed. In the case of the early thinning, 2.4 m long round timber assortment was worked, whereas in the late thinning - 6 m long logs were cut from the butt end part of the trunk and the rest of the stem was worked into 2.4 m long billets. The felling and other timber treatments were carried out by a feller using a chainsaw who, additionally, participated in the gathering of billets. The feller’s assistant helped in the toppling of felled trees, debarked stumps and gathered and stacked billets. The skidding of large-sized assortments was conducted single-handedly with the assistance of the LKT-81 tractor, while the skidding of 2.4 m long billets was also done by one person using a f arm tractor Ursus C-330 with a trailer (early thinning) and a horse with a cart (late thinning). Table 1 presents the characterisation of foresters working in the motor-manual technology.

Mechanized technology
The investigations were carried out in the Gidle Forest Division (the Regional Direction of State Forests in Katowice) in a 71-year old pine stand growing on the fresh coniferous site type. The timber was harvested with the help of the Timberjack 1270B harvester and the skidding – using the Timberjack 1010B forwarder. The investigations also included a variant of the mechanized technology, which employed the use of an additional field (midfield) on which the tree felling was carried out by the feller and his assistant, while the remaining operations were conducted mechanically. Table 2 presents the characterisation of forest workers working in the mechanized technology.

In order to determine the net energy expenditure of individual activities carried out on the examined workplaces, the method of pulmonary ventilation was employed. It is a simple method, which also guarantees a considerable accuracy of the obtained results [19, 22]. A Polish expenditure-measuring device MWE-1 was used to determine energy expenditure.

The measurement of the pulmonary ventilation was carried out in accordance with the methodology recommended in relevant literature [20, 21]. Prior to the initiation of measurements, the worker worked in a half-mask for a few minutes in order to get accustomed to new working conditions and achieve the state of functional equilibrium. This was followed by a 5-minute period of measurement of pulmonary ventilation and energy expenditure. After the measurement period, a break lasting several minutes was applied, which was followed again by a measuring phase. At least three measuring cycles were carried out for each working activity, which allowed to obtain the mean minute energy expenditure for individual activities.

A continuous timekeeping of the workday was done for each workplace. All measurements were referred to the 8h workday bearing in mind considerable differences in the real time work on individual workplaces. Values of the unit effective work expenditure (kJ/min) were multiplied by the time necessary to carry out individual activities as evidenced from the timekeeping and, after adding, the energy expenditure of a given shift (8h) was obtained.

The work on individual workplaces was classified into an appropriate degree of intensity of manual work on the basis of values of the shift net work expenditure [18].

RESULTS

Energy expenditure of individual activities
The unit net energy expenditure of individual activities associated with timber harvesting ranged widely from 3.1 to 36.2 kJ/min (Table 3). Energy expenditure below 10 kJ/min was determined for assisting activities and for rest, while the main activities of the effective work time were characterised by considerable higher levels of energy loads. Cutting as well as treatment operations carried out with the assistance of a chainsaw required energy expenditure of the order of 20.8 to 22.6 kJ/min. A similar level of energy expenditure was recorded during walks with the chainsaw. Activities performed during the skidding of long timber with the help of an LKT tractor operated by one person (securing of the load) were characterised by the energy expenditure below 20 kJ/min.

The lowest values of minute energy expenditures were observed on operators’ workplaces in the mechanized technology of timber harvesting. The minute energy expenditures of operators of harvesters and forwarders ranged from 3.1 to 5.6 kJ/min. The lowest value of the minute energy expenditure of all the activities occurring on the examined workplaces was recorded for the operator of the harvester.

The highest minute energy expenditures were recorded in the case of manual work connected with lifting and carrying loads: manual loading of billets 2.4 m long onto trailers (36.2 kJ) and unloading and stacking round timber assortments (29.3 kJ).

The highest variability of the spent energy was recorded in the case of dynamic activities in which it was the worker himself who decided about the work rate as well as during walks. The variability coefficient of the value of the energy expenditure in the case of activities connected with timber harvesting reached up to 25%, although for the majority of activities, it did not exceed 10-15% (Table 3).

Energy expenditure of a work shift
Figure 1 shows the energy cost of a workday in the process of timber harvesting in the course of intermediate cuttings in the analysed work technologies. The lowest net energy expenditure calculated per one work shift was observed in the case of workplaces of operators of multipurpose machines and it ranged from 1.8 thousand to 2.6 thousand kJ. A single-person skidding of long timber with the help of the LKT tractor resulted in the energy consumption of the order of 6.9 thousand kJ.

The energy cost of a feller’s shift averaged 8.6 thousand kJ, while the work expenditure of the feller’s assistant ranged from 7.3 to 9.6 thousand kJ and depended on the proportion of the assisting work time. As in the case of the minute expenditure, the highest values of the work energy costs in the course of the work shift were recorded during the skidding of short timber: the carter – 11.5 thousand kJ, skidder-farm tractor driver – 10.6 thousand kJ (Fig. 1).

Evaluation of the work intensity
Seven of all the analysed workplaces (53.8%) were classified as very heavy work and further 2 – as heavy work (Table 4). Only four of the examined positions, those of the operators of the harvester and forwarder were identified as light (net energy expenditure below 3.3 thousand kJ/8h).

DISCUSSION

The majority of the analysed workplaces in the process of timber harvesting in pine premature stands were characterised by high- and very high energy expenditures, which reached the net maximum of 11.5 thousand kJ per work shift. Higher work energy expenditures in modern forestry were observed only in some silviculture work [10] and, outside forestry, in agriculture [9] and some segments of heavy industry [15]. Out of the 13 analysed workplaces, the limit of 8380 kJ (2000 kcal) considered as the upper acceptable boundary of energy expenditure for the work carried out by a male on a permanent basis was exceeded in 7 cases. It is true that pertinent current regulations do not specify clearly acceptable standards of energy expenditure for men, nevertheless, it is assumed that work characterised by the work expenditure exceeding 2500 kcal (10475 kJ) is maximally hard and may lead to exhaustion [17]. In the case of workplaces analysed in the described investigations, the energy expenditure excee ding 10475 kcal during one work shift occurred only during the skidding of billets (in the case of the carter and skidder-driver of the farm tractor).

The net minute work expenditure recorded in the course of felling amounted to 21.6 kJ (5.15 kcal). This value corresponds to the medium interval of the energy expenditure for the single-person chainsaw cutting, which can be found in domestic literature (4.5-5.97 kcal/min) [6, 13, 30, 31]. Similar values of energy consumption can also be found in foreign literature on the subject from 1950s and 1960s [after 6].

Similar values to those found in literature were obtained in this study for the operations of the timber bucking, and slightly lower – for debranching. It appears then that the significant decrease in the weight of chainsaws witnessed during the past 40 years failed to influence substantially the decrease of energy expenditures during the felling and other tending operations. This can, most probably, be attributed to the increased rate of work since the reduced weight of the chainsaw allowed to perform work activities more effectively. This explains the observed small differences in the energy expenditure during the tree debranching operations, which, as dynamic activities, associated with holding and carrying the chainsaw from place to place, should be characterised by a reduction of energy expenditure values when compared with data from earlier years. During the cutting and timber bucking, the chainsaw weight is partially transferred on to the treated tree, hence the effect of its weight on the worker’s load is smaller. In addition, the above-mentioned activities are carried out in a static position (in particular, the timber bucking), which also affects the reduction and a smaller degree of variability of the energy expenditure in the course of performing these operations. The hypothesis appears to be corroborated by results of Brazilian researches who found a considerably higher energy cost for all the tending operations carried out using a chainsaw [4].

The increased work rate of the chainsaw is the result of the shortening of the cutting time caused by increased rotational speed of the engine and, consequently, of the speed of timber machining. The rotational speed of the chainsaw engine, in comparison with 1960s, tripled, while the speed of wood machining – almost doubled [32].

The recorded energy expenditure for the activities of walking with the chainsaw was by about 20% lower than that found in literature [6, 13], which undoubtedly was the result of the smaller weight of chainsaws used in forest works now in comparison with chainsaws used in 1960s and 1970s. This is in keeping with conclusions of Soundberg and Linholm who reported that the weight of the mechanical chainsaw affects, primarily, the effort of the worker as he carries it about and transfers it from place to place rather than the actual course of sawing [after 23].

The performed investigations revealed that the energy expenditure of the feller’s workday, in the case of the motor-manual technology, averaged 8735 kJ (2085 kcal). The few available literature data referring to the energy expenditure when harvesting timber in stands of young age classes report similar values [3, 5]. On the other hand, the effective energy expenditure of the feller in final cuttings found in literature obtained using measurement methods ranges from 1167 to 2400 kcal [4, 6, 13, 14, 27]. The above-mentioned wide range of values of the energy expenditure obtained for the same workplace could be attributed to varying stand conditions, terrain conditions, season of the year when the harvesting was carried out and – first and foremost – to differences in the rate of work.

The net energy expenditure of the feller’s assistant was similar to that of the feller himself and ranged from 7357 to 9627 kJ (1756-2298 kcal). Its value was affected by the proportion of activities of the assisting time, in particular, of the technological break, which is characterised by low minute energy expenditure. This was particularly evident in the early thinning where, because of a considerable proportion of technological breaks, the lowest energy expenditure for the feller’s assistant was recorded. The collection of the round timber assortment exerted a significant influence on the value of the energy expenditure of the feller’s assistant. The minute energy cost of this activity amounted to 29.3 kJ (9.99 kcal). The same values quoted in domestic literature are considerably lower – 3.85-6.54 kcal (16.1-27.4 kJ) [6, 7], but they refer most probably to 1 m long billets, whereas in the discussed experiments, 2.4 m long assortments were worked.

The mean energy expenditure determined for the loading of the round timber assortments was 36.2 kJ/min (8.64 kcal/min), while for the unloading of the same timber – 23.8 kJ/min (5.68 kcal/min). Values quoted in relevant literature are by 44% lower in the case of loading and by almost 21% lower for the unloading [14]. The author of the quoted investigations does not specify the length of the investigated assortments but it can be assumed that the data refer to billets 1 m long. Only Botwin [2] gives values of the obtained work expenditure for the loading of 1 m long billets (8 kcal/min) similar to those obtained in this study.

The gathering and stacking as well as loading and unloading of 2.4 m long billets involves the risk of exceeding the weight unit value (50 kg) accepted for manual lifting and transfer of objects by a single worker employed on a temporary basis [26]. In the case of permanent workers, the allowable weight of relocated objects is smaller and cannot exceed 30 kg. Bearing in mind the fact that a pine billet 2.4 m long and of 20 cm diameter weighs approximately 60 kg, the above-mentioned activities should be performed by two persons, especially when harvesting timber during late thinnings.

In the course of the discussed investigations, timber skidding was performed by one person using a skidder (LKT). The net work energy expenditure of the driver-skidder was 6.9 thousand kJ, while the minute energy expenditure during driving reached 16.9 kJ. The obtained values of the unit energy expenditure fall within the middle interval of values found in relevant literature referring to skidding by two persons (driver and helper) [16, 29].

The lowest arduousness of work was recorded in the course of the machine technology of timber harvesting. The minute energy expenditure of workers operating the multipurpose machines ranged from 3.1 to 5.6 kJ, while the net shift energy load did not exceed 2.7 thousand kJ. Czech researchers [28, 33] reported similar levels of energy expenditure for operators of the harvester and processor concerning both the minute and shift loads.

The observed values of energy expenditures in mechanized technology were several times lower in comparison with the traditional motor-manual technology of timber harvesting. Minute values of energy expenditure of the main work operations were 4 times lower during timber skidding using the forwarder in comparison with the traditional skidding (a horse, farm tractor, skidder) and 7 times lower when timber was harvested using the harvester when compared with harvesting with the assistance of a chainsaw. On the other hand, the energy expenditure of the shift in the mechanized technology was, on average, 4 times lower.

CONCLUSIONS

1. The energy expenditure of the majority of workplaces during timber harvesting was high and very high. According to the energy expenditure criterion, nearly all the workplaces in the motor-manual technology of work were classified as very heavy (over 8380 kJ/8h). Only workplaces of operators of multipurpose machines were classified as light.

2. The current level of energy load in the process of timber harvesting in the motor-manual technology does not differ significantly from the levels from 1960s and 1970s. Values of minute energy expenditures similar to those from 30-40 years ago were recorded for cutting, bucking and debranching, while those for walking with a chainsaw were by about 20% lower.

3. The application of the mechanized technology for timber harvesting allowed to reduce the minute energy expenditure of major work operations by 4 to 7 times and to decrease the energy cost of the work shift, on average, by 4 times.

4. The application of the mechanized technology of timber harvesting with a midfield on which a feller and his assistant cut trees is not justified ergonomically.

5. Gathering, stacking as well as loading and unloading of 2.4 m long billets by one person exceeds the acceptable values of weight of objects that can be carried by one person, especially at large diameters of harvested timber assortments.

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*The research work was carried out within the framework of the project ordered by the General Direction of State Forests No. 32/99.

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