Volume 19, numéro 2, fall 2008

EN :

Acoustic technology has been successfully used as a non-destructive technique for assessing the mechanical quality of various wood products and species based on stiffness. Many mechanical harvester/processor manufacturers have implemented mechanical sensors to measure tree diameter and length as well as optimal bucking algorithms on their equipment. There is a growing interest in incorporating technologies for measuring internal stem features into a harvester head. The objectives of this study, therefore, were to i) determine and investigate the factors arising from incorporating acoustic instruments on a mechanized harvester head that might influence resonance-based acoustic signal and velocity readings and quality in Douglas-fir, and ii) investigate the issues and considerations associated with suggested working strategies in regard to harvest productivity impacts and processing decisions.

After taking into account some feasibility considerations, it was determined that the hold of the machine grapple would not compromise the accuracy of resonance-based acoustic velocity readings. There were three working procedures suggested for measuring resonance-based acoustic velocity: 1) after the stem is delimbed and run through the measuring equipment, 2) once a portion of the stem is measured and the length of its unmeasured portion is forecast, and 3) after the tree is felled by the harvester but before any further processing is done.

Regardless of the working procedure, it was determined that logs produced from lower sections of the tree are stiffer than those from upper portions. If the processor head traverses the stem partially or completely, the removal of bark and branches and their effect on acoustic velocity readings should be taken into account. Forecasting routines could be developed to account for imperfect and even non-existing information about tree length with the second or third working procedure. Results yielded by the two methods used for stem height (and consequently acoustic velocity) prediction in this study (linear regression model and a k-nearest-neighbor) were considered rather promising. Testing feasibility concerns with the resonance-based acoustic technique were observed if the entire stem was intact to the very top offshoot bud.

EN :

The goal of this cross-sectional study was to investigate the impact of different physical work station designs, expressed in two different brands of forest vehicles, on the muscle activity patterns in the neck and upper extremities among the vehicle operators.

Surface electromyography (sEMG) was continuously recorded bilaterally on the trapezius (TM) and the extensor digitorum muscles (EDM) during one working day among operators driving Timberjack and Valmet vehicles, either as harvesters (n = 7 and 6, respectively) or forwarders (n = 9 and 9, respectively).

Both the construction of the crane in relation to the chassis and the design of the control levers vary between the Timberjack and Valmet vehicles, which demand different ergo-nomic performance by the operators. The operators mostly handle control levers in the harvesters or forwarders, the latter with a more varied work load, in a fixed, seated working posture in the cabin for long hours with little rest. The sustained low-level muscle activity was quantified by periods with muscle activity above 0.5 percent EMGmax into 10 predetermined duration intervals from 1.6 to 5 s up to above 20 min (SULMA periods). These SULMA periods were analyzed both for number in the different intervals and cumulated periods above the predefined values. Amplitude and frequency parameters were analyzed and the number and total duration of muscle rest periods were calculated.

The operators driving Valmet harvesters had a significantly higher number of long cumulated SULMA periods above 10 min in the left TM, and showed a higher level of static muscle activity and less total duration of muscle rest in TM bilaterally. The operators driving Timberjack forwarders had a significantly higher number of SULMA periods between 10 and 20 min in the right TM. No difference was found between the operators in the EDM activity pattern.

The results of our study showed that operators driving Valmet harvesters had more sustained low-level activity in the neck than those driving Timberjack, including a higher number of long cumulated SULMA periods, higher static level, and less muscle rest. Despite a small sample, the results in muscle activity pattern raise the question of needs for improvements of the forest vehicle workstation design.

EN :

It is only possible to pivot (horizontally rotate) conventional harvester cranes at the crane pillar. A new type of harvester crane has an extra pivoting point on the outer boom, which makes it possible to reach behind residual trees and thus probably ease thinning work. This paper quantifies differences in harvester time consumption in thinning between a conventional crane and a pivoting outer boom (POB) crane by the use of a simulation study and a field study. Simulations were made in two mapped stands. A harvester equipped with a POB crane was used in the field study. The work of a conventional crane was performed with the same machine by not using the pivoting function. Six blocks were created with densities ranging from 1,230 to 3,100 trees per hectare and tree choice was restricted.

The POB crane required 4 to 8 percent less time compared to the conventional crane in the simulation study and 7 to 15 percent less time in the field study. In the field study, the mean time consumption of the POB crane was significantly lower for the work elements machine movement backwards and crane out. The number of machine movements backwards was significantly lower for the POB crane, and 17 percent more trees could be cut per machine position by the POB crane. The pivoting function was used on 29 percent of the cut trees. Based on the consistent results from the simulation and the field study, it was concluded that the pivoting function significantly increased productivity in thinning.

EN :

Redwood (Sequoia sempervirens (D. Don) Endl.) responds favorably to sprout clump thinning in clearcut environments, but appropriate silvicultural techniques for this practice have not been developed. Four power tools for this unique application were evaluated: standard mid-sized chainsaw (long saw), small arborist chainsaw (short saw), power hedge trimmer, and a customized power brushcutter. Linear mixed effects model analysis was conducted to determine tool differences in clump processing speed, transit time, cycle time, and worker difficulty (peak heart rate). Machine costs and unit production costs were calculated separately for each tool, and perceived ease-of-use by workers was noted. Clump difficulty (sprout clump size) was a strong determinant of processing time and affected the performance of each tool differently. The long saw performed well across the range of clump difficulties but forced the highest heart rates in operators and was the most costly. The short saw was especially proficient on easy to moderate clumps, was among the least difficult, and was the least costly; it was the clear favorite for easy clumps. The hedge trimmer was among the fastest and least difficult, but its capability was constrained by sprout caliper. The brushcutter’s lackluster performance and several inadequacies prevent its recommendation. Regardless of tool selection, the condition of the sprout clump is a factor that must be taken into consideration since it strongly affects average production rates (clumps processed per productive machine hour) and area costs. Sprout age and clump size should guide tool selection, but should also be considered by operators in selectively choosing a subset of crop stumps for sprout clump thinning.

EN :

The productivity of harvesting stump and root wood was studied in Norway spruce (Picea abies) stands. The objective was to create productivity models (m³/E0h) for stump wood extraction, stump wood forwarding, and site preparation, in addition to identifying work phases and improvement opportunities in the extraction and forwarding chain. Productivity models were based on time studies with professional operators. The independent variables in stump wood extraction were stump diameter (cm) and the number of stumps per hectare. For forwarding, the independent variables were volume of stump wood removed (m³/ha) and forwarding distance (m). When removing 350 stumps per ha with an average diameter of 40 cm, productivity was estimated at 7.9 m³/E0h. Increasing the number of stumps removed from 350 to 800 stumps per ha, increased productivity to 10.8 m³/E0h. Forwarding productivity was 7.8 m³/E0hwithaforwardingdistanceof250mandaload size of 7.0 m³ when removing 60 m³ of stumps per ha.