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Effects of radial force and tip path overlap on the ring debarking efficiency of frozen balsam fir logs.

Abstract

This study investigated the effects of the static radial force applied by ring debarker tips to frozen balsam fir logs, and tip path overlap on debarking quality. The radial force is applied by a hydraulic cylinder acting on a lever that forces a tip attached to a tool arm to remove the bark from the log. Each pressure system is set into a rotating ring that peels the log while it is fed through the debarker. The experiment consisted of debarking balsam fir logs using three levels of static radial force, each with two values of overlap between the tip paths. Debarking quality was evaluated by three parameters: the wood fiber loss volume, the percentage of wood in bark residues, and the percentage of bark remaining on log surfaces. Thirty logs of 150-mm diameter at the small end were debarked under frozen wood temperatures (winter) for each cutting condition. The results showed a significant effect of the static radial force on all quality indicators and a significant effect of tip path overlap on the bark remaining on logs. The wood loss volume and the amount of wood in bark residues decreased as radial forces and tip overlap decreased, but more bark remained on the log surfaces. Finally, soaking the logs in warm water for 20 minutes at 8[degrees]C was sufficient to substantially improve debarking performance. These results give useful information to estimate the changes in debarking quality that can occur within the range of radial pressures and overlaps covered by the study.

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In Canada, sawmill chips are an important raw material for the pulp industry. In the Province of Quebec, the most recent statistical data show that 64 percent of the wood raw material to this industry is supplied as chips by sawmills (Parent 2000). These chips typically come from logs that have been debarked by ring debarkers. Wood chips that do not meet pulp manufacturers' specifications in terms of maximum acceptable amount of bark are rejected. Additional amounts of bleaching products are necessary to remove bark components that affect paper quality (Parham 1983, Koch 1985). Sawmills are usually able to meet chip quality requirements in summer when the logs are not frozen. However, debarking performance decreases when logs become frozen in winter, and the bark allowance in chips can be exceeded. Given that lumber recovery has significantly increased in recent years and the volume of chips produced per log has consequently decreased, levels of bark remaining on logs that used to be acceptable in wood chips have now become critical. On the other hand, loss of wood fiber during debarking affects lumber recovery by reducing log diameters. For example, several mills located in northern Quebec and Ontario have reported that their roundwood consumption increased by 0.5 [m.sup.3]/thousand board feet (MBF) in winter as a result of poorer debarking performance (Laganiere 2003). Therefore, debarking systems need to be more efficient to satisfy pulp mill chip quality requirements while keeping wood fiber loss to a minimum. Winter debarking problems have been attributed to an increase in bark/wood bond strength (BWBS) as the log temperature decreases under frozen conditions (Perem 1958, Berlyn 1965, Calvert and Garlicki 1974). Cambial activity also affects BWBS, which decreases during spring, reaches a minimum in June to July, and increases again in August (Wilcox et al. 1954, Harder et al. 1983); BWBS therefore peaks in winter. The loss of moisture content also increases BWBS (Perem 1958, Berlyn 1965, Calvert and Garlicki 1974, Miller 1975, Einspahr et al. 1984, Hatton 1987, Wingate-Hill et al. 1989, Belli 1996, Duchesne and Nylinder 1996).

Several treatments to decrease BWBS have been studied. Slitting bark in the longitudinal axis improved bark removal, but too much wood was torn out in the process (Berlyn 1970). Chemical (Hale 1944), enzymatic (Ratto et al. 1993), fungal and bacteriological (Kubler 1990) treatments were able to reduce BWBS, but such methods are not currently used commercially. BWBS also decreases when logs are heated with saturated steam (Miller 1975). Some Quebec sawmills processing balsam fir logs use warm water ponds to heat frozen bark in order to improve debarking efficiency. Balsam fir logs are generally recognized as the most difficult to debark during the winter months within the spruce-pine-fir group usually processed in Eastern Canadian sawmills. This could be explained by the fact that the basic densities of balsam fir's inner bark and sapwood are relatively close (324 kg/[m.sup.3] and 299 kg/[m.sup.3], respectively) (Lamb and Marden 1968).

The force applied to the log through the debarking tips can remove as much wood as bark, which explains the importance of knowing the force required to break the bark without tearing out wood fiber. The main parameters involved in enhancing debarking quality are the radial or normal force applied by the tips to the log, the feed speed, the rake angle, the cutting edge radius, and the sharpening angle (Calvert and Garlicki 1974, Lapointe 1976).

Warm water soak treatments have been used successfully to thaw out log surfaces and facilitate debarking, but in Quebec and a number of other jurisdictions, used pond water cannot be rejected to the environment without treatment (QLMA 2001). The associated costs have led many sawmills to stop using water ponds. Most of Quebec sawmills still using water ponds process balsam fir, as the debarking of frozen balsam fir would incur excessive wood loss without thawing the log surface. Alternative thawing techniques based on microwave (Gilbert and Turcotte 1994) and infrared light (Bedard and Poulin 2000) have been proposed, but they are not yet commercially applied.

The main objective of this work was therefore to provide specific processing information for improved debarking of balsam fir logs under frozen conditions. The specific purpose of this investigation was to quantify and compare debarking quality resulting from two tip path overlap factors (i.e., feed speeds) and three different radial force levels. Warm water immersion treatment efficiency to thaw out log surfaces prior to debarking was also evaluated.

Materials and methods

A total of 209 balsam fir (Abies balsamea [L.] Mill.) logs were chosen from the wood yard of a sawmill at Degelis, in the Temiscouata area, Quebec. The trees had been harvested a few weeks before the test and were bucked into 3.05-m logs. The logs had a mean small-end inside bark diameter of 150 mm and a mean taper of 11 mm/m (Table 1). They were without crook or visible decay, had straight grain and a minimum of knots. The experiment consisted of debarking balsam fir logs using three levels of static radial force, each one with two values of overlap between tip paths. This experiment was carried out during winter with about 30 logs for each debarking condition (conditions 1 to 6, Table 1). Another group of logs was evaluated after soaking in warm water at 8[degrees]C for 20 minutes to evaluate the effect of this treatment on debarking. Before processing, inside bark log diameters were measured at distances of 0 mm, 150 mm, 500 mm, 1000 mm, 1500 mm, 2000 mm, and 3000 mm from small ends and at large ends. Bark was removed on each side of the log with a 35-mm diameter wood drill for every measurement. The exposed wood was painted to identify this position after debarking. Log diameters were measured to the nearest millimeter on one axis with a forest calliper. After diameter measurement, the logs were sorted into seven homogeneous groups with similar diameters and tapers (Table 1).

Debarking experiments

The logs were always fed small end first into the debarker. All bark residues were collected immediately under the debarker and placed in plastic bags for later analysis. The debarking operation was performed with a 460-mm diameter Carbotech ring debarker (Cambio type) equipped with five tool arms and tips of 46.5-mm width. A 3-mm corner radius was used according to the recommendation given by Lapointe (1976), who suggested radii between 1.6 mm and 4.8 mm to debark frozen spruce and balsam fir logs. A hydraulic circuit acting on a hydraulic cylinder coupled to a rubber band allowed the tool arm to apply a radial force to the log by lever effect. The stiffness of each rubber band was verified for the five tool arms to ensure they applied similar forces. The static radial force applied in the normal plane was verified to the nearest 5 N with a hydraulic cylinder that was used to lift the tip edge up to its normal position when debarking a 150-mm-diameter log.

We selected a radial force of 35.6 N/mm (1656 N total force), which is commonly used in sawmills for a 150-mm log diameter as the highest force used in this study. Preliminary observations in different sawmills showed that this radial force level was too high. It was then reduced by 11 percent (31.7 N/mm, 1472 N total force) and 22 percent (27.7 N/mm, 1288 N total force) (Table 1). Each radial force level was studied at 19 and 35 percent of overlap between consecutive tip paths. These overlaps were selected in accordance with values currently used in sawmills. An overlap factor over 10 percent had previously been suggested to ensure complete bark coverage by the tips (Lapointe 1976). Given that the rotation speed of the debarker was 323 rpm and the tip width was 46.5 mm, these overlaps corresponded to feed speeds of 61 m/min. and 49 m/min., respectively. The overlap was calculated as follows:

O = (1 - [[1000f]/[RNw]]) X 100

where:

O = overlap in percent

f = feed speed in m/min.

R = rotation speed in rpm

N = number of tool arms or tips

w = tip width in mm

A seventh group of logs was immersed in water at 8[degrees]C for 20 minutes prior to debarking. This group was then debarked under a 27.7 N/mm static radial force with 19 percent overlap (Table 1).

The tips were new and freshly ground before the experiment to minimize the effect of tool wear on debarking performance. The rake angle of the tip, defined as the angle between the inside face of the tip and a tangent to the log surface at the contact point, decreases as log diameters increase. In our case, the rake angle recommended by the equipment manufacturer varied from 60 degrees for the small end to 59 degrees for the larger end of the log. The sharpening angle of the tip was 54 degrees while its edge had a radius of 0.6 mm. The temperature outside the sawmill was -11[degrees]C during the test. The logs were brought into the sawmill in small groups to prevent thawing before processing.

Laboratory measurements

Log diameters after debarking were measured with a forest calliper at the same positions marked prior to debarking. Log volumes were estimated with the frustum of cone formula for each log section where diameters were measured. The loss in wood volume of individual logs was calculated as the difference between the volume of wood fiber available before debarking and the volume remaining after debarking divided by the volume before debarking (in %). This difference was considered a parameter to evaluate debarking performance. The loss in wood volume associated with chunks torn off the logs during debarking was taken into account and estimated according to their geometry.

Bark residues were air-dried indoors for a few days to facilitate separation. A bark sample of about 1 kg for each log was taken using a Domtar particle separator. Given their variable dimensions, the bark particles were then screened with a classifier in three classes: large particles (material retained in a 25.4-mm by 25.4-mm square hole screen), intermediate particles (material passing through a 25.4-mm square hole screen and retained by a 12.7-mm square hole screen), and small particles (material passing through a 12.7-mm square hole screen). All large and intermediate particles were manually separated into their wood and bark components. For the small particles, the wood/bark components were evaluated from a specially selected sample of about 75 g. After separation, the samples were conditioned to equilibrium at 20[degrees]C and 65 percent relative humidity. The respective masses of wood and bark were measured to the nearest 0.01 g. The amount of wood in bark was expressed as a weighted percentage of the total debarking residue mass.

The percentage of bark left on the log surface was the third debarking quality parameter. Remaining bark was traced on a transparent plastic film with a permanent marker. The area of each trace was then measured with a planimeter. The remaining bark was then calculated as a percentage of total log area without bark. The total log surface area was estimated from the diameters measured before debarking.

Results and discussion

Univariate analyses of variance (ANOVA) (SAS Institute 1990) were performed to examine the effects of radial force and overlap on the proportions of wood volume loss, wood in bark and bark remaining on logs after debarking (Table 2). The analyses were performed on data transformed to a log 10 basis to ensure variance homogeneity. The analysis showed that radial force affected all three debarking parameters, while the effect of tip path overlap was only significant for the proportion of bark remaining on logs.

The F-values also indicate that radial force affects debarking quality more than overlap. The interaction force X overlap was not significant, which means that the effects of these parameters vary independently from each other. Comparisons were drawn to assess the differences among radial forces and between overlap levels for the three parameters characterizing debarking quality. The results of these comparisons are given in Table 3.

Effect of radial force

For a given overlap, wood volume loss and wood in bark increased, while remaining bark decreased, as the radial force increased (Table 3 and Fig. 1). The tool tip tended to penetrate deeper into the bark as the radial force increased, which may have increased wood tearout. For wood in bark, there was a significant difference among the three radial force levels for the two overlaps tested. The variation of the radial force also had a significant effect on wood volume loss. For the two overlaps tested, differences in wood loss were statistically significant between 27.7 N/mm and 35.6 N/mm, while there was no significant difference between 27.7 N/mm and 31.7 N/mm of radial force. The 35.6-N/mm radial force had a much more negative impact on wood loss during debarking than the other radial force levels studied.

[FIGURE 1 OMITTED]

As regards bark remaining on the logs, statistically significant differences existed among the three force levels for both overlap values. Bark was removed more efficiently as radial force increased. The results clearly indicate that a radial force of 27.7 N/mm was insufficient to remove bark. The importance of remaining bark for chip production was estimated by the OPTITEK sawmill simulation software (Grondin and Drouin 1998), considering a lumber recovery factor of 4 [m.sup.3]/MBF. The results indicated that, at the 27.7 N/mm level, the amount of bark in wood chips could reach 2.9 percent for 19 percent overlap and 1.6 percent for 35 percent overlap, which exceed the allowable limit of 1 percent currently used by Canadian pulp mills. At the 31.7-N/mm level, the percentage of bark in chips could reach 1.0 percent for 19 percent overlap or 0.6 percent for 35 percent overlap. These values would be even lower with a radial force of 35.6 N/mm. Conversely, the loss of wood volume and the wood in bark were lowest for both overlaps at the 27.7-N/mm radial force level. As wood loss was not significantly different between the 27.7-N/mm and 31.7-N/mm levels, but was significantly different between the 31.7-N/mm and 35.6-N/mm levels, a radial force of 31.7 N/mm would appear to be a suitable compromise with respect to wood fiber recovery and chip quality requirements when debarking frozen balsam fir logs.

A previous study (Calvert and Garlicki 1974) reported that a suitable rake angle of the tip would vary between 75 and 80 degrees for balsam fir logs. The rake angle used in this study was 60 degrees. The effect of a smaller angle had to be compensated for by the application of a higher radial force in order to obtain the same debarking quality. A greater rake angle will reduce the force required because tips are more aggressive. However, the rake angle should not exceed 90 degrees in order to prevent the tip from digging into the wood. Furthermore, the aggressiveness of the rake angle (and penetration into wood) will be reduced if the sharpening angle remains the same.

The tip edge radius of 0.6 mm used in this study was within the range suggested by other authors (Calvert and Garlicki 1974, Pokryshkin 1975, Lapointe 1976). If the edge radius is too sharp, the tip will tend to penetrate deeper into the log, causing further fiber loss. On the other hand, too large an edge radius is less aggressive, and the radial force has to be increased to ensure complete bark removal from the log. A higher radial force can also damage tool arms and mechanical parts due to high stress. In fact, a preliminary study has shown that radial forces were higher when the tip edge radius increased (Laganiere 2003).

Effect of overlap

As shown in Tables 2 and 3, as overlap increased from 19 to 35 percent, the loss in wood volume and the percentage of wood in bark increased, but less bark remained on the logs. However, the ANOVA showed that these effects were not statistically significant for wood loss and wood in bark, even though in one case the probability level was close to 0.05 (p = 0.08). The results suggest that the number of replicates was probably not sufficient to detect significant effects for these two parameters. This is supported by the fact that a statistically significant difference in remaining bark existed between the two overlap levels studied. Overlap between tool tip paths should ensure that all bark is removed from the log. Tool tips pass twice on some areas already debarked or partially debarked. In our experiment, 38 percent (19% overlap) and 70 percent (35% overlap) of the total log surface area was debarked with two passes of the tool tips. As overlap increases, the possibility of removing all bark increases. However, increasing overlap causes tool tips to exert additional shearing action on bark free log surfaces, which may increase wood fiber tear out. The results of this study confirmed this hypothesis.

A higher tip overlap left less bark on log surfaces. However, the overlap factor was found to be significant only at the 27.7-N/mm radial force level, where 35 percent overlap produced a 50 percent decrease in bark remaining on the log as compared to 19 percent. The same trend was observed with the other radial force levels, but differences between overlap levels were not statistically significant.

Tip path overlap was set by modifying the feed speed of the debarker while the rotation speed of the ring remained constant. As feed speed increased, the overlap decreased. Given that a higher feed speed induces more stress in the mechanical parts of the equipment, the logs were expected to be more stressed during debarking, with increased wood loss and wood in bark. In the tests, however, the opposite happened, with lower wood loss and wood in bark as feed speed increased, indicating that the machine was sufficiently strong to keep the logs stable at higher feed speeds.

As already mentioned, a radial force of 27.7 N/mm proved too low, and an important amount of bark remained on the logs with both overlap factors. At the 31.7-N/mm and 35.6-N/mm radial force levels, there were no significant differences between the two overlap levels in terms of bark remaining on the logs. The 35.6-N/mm radial force/35 percent overlap combination yielded the lowest level of remaining bark but it also produced the highest level of wood loss of all combinations studied. The 31.7-N/mm radial force/35 percent overlap combination therefore appears to provide the best debarking conditions, with acceptable levels of wood loss and remaining bark.

In addition to increased feed speed, tip path overlap can be reduced by decreasing ring rotation speed and/or tip width. A lower ring speed reduces the centrifugal force of tool arms, while increasing their reaction time. The tips apply a more consistent force to the log during geometry changes. Experience also shows, however, that tip width must be kept as narrow as possible to remove more bark when log taper is too large. Narrow tips also remove bark more efficiently around knots and in depressions.

Effect of bark thawing on debarking quality

The immersion of frozen logs in warm water for 20 minutes before debarking significantly improved debarking quality. The treatment was studied only with the 27.7-N/mm radial force/19 percent overlap combination (Table 4). Under these conditions, wood fiber loss was reduced from 2.7 to 0.6 percent, the percentage of wood in bark decreased from 15.8 to 6.8 percent, and the percentage of bark remaining on the logs was reduced from 24.5 to 16.1 percent (Table 4). As stated earlier, the 27.7-N/mm radial force used in combination with the thawing treatment had been too low to efficiently remove frozen bark from the logs. Under these conditions, the amount of bark expected in pulp chips was estimated to be 2.2 percent, which would not even meet Canadian pulp mill requirements. Nevertheless, the bark level decreased by 0.7 percent compared to the frozen condition. On the basis of the results obtained with frozen logs, it can be assumed that the use of a 31.7-N/mm force/35 percent overlap combination in the thawing test would have yielded superior debarking quality.

Conclusions

The results of this study on the ring debarking efficiency of frozen balsam fir logs show that the radial force applied by tips to the logs has a significant effect on the loss of wood volume, the percentage of wood in bark, and the percentage of bark remaining on the logs. Tip path overlap also affected debarking quality, but its effect was lower within the range of operating parameters covered in the study. The loss of wood volume and the amount of wood in bark decreased as the radial force and overlap factor decreased, but more bark remained on the log surface. Under the conditions of this study, a radial force of 31.7 N/mm combined with an overlap factor of 35 percent appeared to be the best setting for debarking frozen balsam fir logs. A superficial thawing pretreatment improved the three parameters used in the study to characterize debarking quality. Even though only one group of logs was tested, the thawing treatment demonstrated potential to improve debarking quality. Further study of the interactions among thawing treatment, radial force, and overlap is required to optimize debarking conditions for frozen balsam fir logs.
Table 1. -- Description of log groups used in the experiments.

Mean parameters and Log group
cutting conditions 1 2 3 4

Mean parameters
 Small end
 diameter (mm) 150 (2) (b) 150 (2) 149 (2) 150 (2)
 Large end
 diameter (mm) 180 (3) 182 (4) 184 (3) 186 (4)
 Taper (mm/m) 10 (0.6) 11 (0.8) 12 (0.6) 12 (0.8)
 Log area under
 bark
 (d[m.sup.2]) 162.2 (2.2) 161.4 (2.4) 162.8 (2.6) 164.2 (2.5)
 Number of logs 30 28 30 30
Cutting conditions
 Static radial
 force (N/mm) 35.6 35.6 31.7 31.7
 Overlap (%) 35 19 35 19

Mean parameters and Log group
cutting conditions 5 6 7 (a)

Mean parameters
 Small end
 diameter (mm) 150 (2) 149 (2) 149 (2)
 Large end
 diameter (mm) 182 (3) 180 (3) 182 (3)
 Taper (mm/m) 11 (0.4) 11 (0.8) 12 (0.8)
 Log area under
 bark
 (d[m.sup.2]) 163.4 (2.4) 162.1 (2.3) 161.7 (1.9)
 Number of logs 28 30 32
Cutting conditions
 Static radial
 force (N/mm) 27.7 27.7 27.7
 Overlap (%) 35 19 19

(a) Logs immersed in water at 8[degrees]C for 20 minutes before
debarking.
(b) Numbers in parentheses are standard errors of the mean.

Table 2. -- Univariate ANOVA of wood volume loss, wood in bark, and bark
remaining on log.

Source of Wood volume loss Wood in bark
variation MS F-value P > F MS F-value P > F

Force 0.5326 10.89 0.0001 0.9896 36.28 0.0001
Overlap 0.1508 3.08 0.0810 0.0431 1.58 0.2104
Force X Overlap 0.0286 0.59 0.5579 0.0039 0.14 0.8670

Source of Bark on log, area
variation MS F-value P > F

Force 5.4594 40.91 0.0001
Overlap 1.2001 8.99 0.0031
Force X Overlap 0.0690 0.52 0.5971

Table 3. -- Means (%), standard error of the mean (%), and multiple
comparison tests between means for wood volume loss, wood in bark, and
bark remaining on log. (a)

 Wood volume loss Wood in bark
Radial force 19% 35% 19% 35%
(N/mm) (%)

27.7 2.7 (0.5) 3.0 (0.5) Aa 15.8 (1.6) Aa 17.0 (1.6) Aa
 A (b) a (c)
31.7 3.3 (0.5) ABa 3.7 (0.5) Aa 19.4 (1.6) Ba 22.0 (1.6) Ba
35.6 4.5 (0.5) Ba 5.9 (0.5) Ba 27.4 (1.7) Ca 31.1 (1.6) Ca

 Bark on log, area
Radial force 19% 35%
(N/mm) (%)

27.7 24.5 (1.9) Aa 12.2 (2.0) Ab
31.7 7.9 (1.9) Ba 4.8 (1.9) Ba
35.6 4.7 (2.0) Ca 3.0 (2.0) Ca

(a) Numbers in parentheses are standard errors of the mean.
(b) Means within a column (radial force comparison) followed by the same
uppercase letter are not significantly different at the 5 percent
probability level. Comparisons apply to individual debarking parameters.
(c) Means within a row (overlap comparison) followed by the same
lowercase letter are not significantly different at the 5 percent
probability level. Comparisons apply to individual debarking parameters.

Table 4. -- Means (%), standard error of the mean (%), and multiple
comparison tests between means for wood volume loss, wood in bark, and
bark remaining on frozen and thawed logs of balsam fir. (a)

 Wood volume loss Wood in bark
Frozen Thawed Frozen Thawed
 (%)

2.7 (0.3) A (b) 0.6 (0.3) B 15.8 (1.1) A 6.8 (1.0) B

 Bark on log, area
Frozen Thawed
 %

24.5 (3.9) A 16.1 (3.6) B

(a) Numbers in parentheses are standard errors of the mean.
(b) Means followed by the same capital letter are not significantly
different at the 5 percent probability level. Comparisons apply to
individual debarking parameters.


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Benoit Laganiere

Roger E. Hernandez*

The authors are, respectively, Research Scientist, Forintek Canada Corp., 319 rue Franquet, Quebec, QC, Canada G1P 4R4 (formerly a Graduate Student at Laval Univ.); and Professor, Dept. of Wood and Forest Sciences, Laval Univ., Quebec City, QC, Canada G1K 7P4. The authors wish to thank Luc Bedard, Odile Fleury, Yves Giroux, Alan Kostiuk, Ahmed Koubaa, Yves Levesque, Richard Moreau, Anne Richard, and Ghislain Veilleux for their assistance with this project. They gratefully acknowledge Bowater Forest Products Inc. in Degelis, QC, Canada for supplying equipment and mill facilities for this study. The mention of trade names does not constitute endorsement by the authors or their institutions. This paper was received for publication in August 2002. Article No. 9536.

*Forest Products Society Member.
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Date:Mar 1, 2005
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