On the Influence of End Shrinkage Cracks on the Quality of Lumber
Abstract Generated abstract
This paper examines how end drying checks affect the quality and grading of sawn timber, especially in the context of kiln drying and automatic process control. The authors propose an economic criterion for permissible end checking, comparing the value of a full-length lower-grade board with that of a shortened higher-grade board after trimming, and derive a formula for the maximum allowable end trim length. Using price-list data and grading limits for coniferous timber, they calculate permissible check lengths for several grade transitions and compare them with standard requirements. The analysis suggests that end checks have relatively little effect when upgrading first and second grade timber, but become more significant for third and fourth grades, where small excesses over standard limits can increase trimming waste.
Full Text
UDC 674.047.3.004.12
ON THE INFLUENCE OF END DRYING CHECKS
ON THE QUALITY OF SAWN TIMBER
G. I. BANDAEVSKII, A. A. GURCHENOK
(Submitted by the seminar of the Department of Automation of Heat-and-Power Processes of Industrial Enterprises)
Carrying out kiln drying of sawn timber, especially hardwood timber, without cracking under industrial conditions is very difficult, almost impossible. Despite the fact that, in developing standard schedules for kiln drying of wood, the permissible value of the cracking criterion is taken as the basis [1], in practice a reduction in the grade of sawn timber because of drying checks is a very widespread phenomenon. The heterogeneity of the chemical and mechanical properties of a stack of sawn timber placed for drying in a kiln, the nonuniformity of the fields of temperature, humidity, and velocity of circulation of the drying agent, and a number of other causes do not make it possible to obtain strictly deterministic dependences of the cracking of sawn timber on the assigned drying schedules.
In developing systems for automatic monitoring and control of the process of drying sawn timber with allowance for quality characteristics, the question arises of the economy of applying one drying schedule or another at minimum drying cost while preserving quality. Optimization of the processes of kiln drying of sawn timber, in our view, should not assume the complete elimination of drying checks, but only the limitation of their dimensions. In the particular case, the problem reduces to limiting the checking of the sawn timber being dried to its permissible value, which represents a “set point” for an automatic-control device. Solving this question, in view of the statistical character of the dependences being studied, is very complex and requires a very large number of experiments.
A certain optimally permissible amount of checking can be determined not only with allowance for the cost of drying the sawn timber, but also with allowance for the purpose of the various grades of sawn timber.
Coniferous sawn timber of selected and first grades is used in whole form and for cutting mainly into large blanks. Lower-grade sawn timber, in addition, is also used for cutting into blanks of higher quality and smaller dimensions.
The choice of the permissible amount of end checking of sawn timber, taking into account the requirements of GOST and the economic expediency of cutting into blanks of higher quality, does not completely solve the question posed, but it makes it possible to outline certain const-
constructive ways of solving it, i.e., to assess the influence of drying checks on the quality of sawn timber.
The need for further research in this direction is also determined by the fact that at present there is no consensus in assessing the influence of end checking in dried sawn timber on its quality. Some authors [2] believe that, with proper end trimming of sawn timber, the influence of purely end checks on the quality of the sawn timber is eliminated. Other authors [3], investigating this question, have arrived at the opposite opinion and recommend taking measures to eliminate end checking of sawn timber during the drying process [4]. In a quantitative assessment of the influence of end drying checks on the quality of sawn timber, we must proceed from the fact that the value of the boards shortened after trimming must be no less than the value of the long boards before they are shortened [5]:
\[ \frac{N'}{N} > 1, \tag{1} \]
where \(N'\) is the value of the board after shortening, in wholesale prices \((\text{rub.}/\text{m}^3)\);
\(N\) is the value of the board before shortening \((\text{rub.}/\text{m}^3)\).
When shortened, each board is divided by value into two parts
\[ N' = N_1 + N_2, \tag{2} \]
where \(N_1\) is the value of the part of the board with the higher value;
\(N_2\) is the value of the part of the board in which there is a grade-determining defect.
To avoid overstating the calculated indices and to simplify the calculation, let us solve the problem for \(N_2 = 0\), when the end offcut with checks will have no value.
For condition (1) to be fulfilled, the inequality \(N' > N\) must hold. Let us express the values through the overall dimensions (Fig. 1) and the prices of the boards:
Fig. 1
\[ ab(l - 2x)C_1 > ablC. \tag{3} \]
Here
\(a, b, l\) are, respectively, the thickness, width, and length of the board, m;
\(C\) is the price of \(1\ \text{m}^3\) of a lower-grade board (before shortening), rub.;
\(C_1\) is the price of \(1\ \text{m}^3\) of a higher-grade board (after shortening), rub.;
\(x\) is the length of the end offcut from one end of the board, m.
We shall assume that drying checks at both ends have the same length.
After transforming inequality (3), we obtain
\[ 2x < l\left(1 - \frac{C}{C_1}\right). \tag{4} \]
From formula (4) it is easy to obtain a formula for determining the length of the end cut from one end of the board
\[ x < \frac{l(1-K)}{2}, \tag{5} \]
where
\[ K=\frac{C}{C_1}. \]
The value of \(K\) can be calculated from the existing price list for sawn timber. Formula (5) makes it possible to determine the maximum length of end checks \(x\) as a grade-forming defect, according to the value of full-length and shortened boards for each belt and each type of sawn timber, taking into account the possibility of raising the grade of dried sawn timber by subsequent end trimming.
Table 1 gives the values of \(K\) for the first belt of coniferous sawn timber (price list No. 07-03).
Table 1
| In changing grades | Value of \(K\) for pine, trimmed, length 4–6.5 m | Value of \(K\) for pine, untrimmed, length 4–6.5 m, and trimmed, 2–3.75 m |
|---|---|---|
| First to select | 0.69 | 0.69 |
| Second to first | 0.71 | 0.71 |
| Third to second | 0.84 | 0.84 |
| Fourth to third | 0.81 | 0.81 |
| Fifth to fourth | 0.46 | 0.52 |
Substituting \(K\) into formula (5), we determine the maximum permissible lengths \(x_{\max}\) as a grade-forming defect.
The results of the calculations are summarized in Table 2.
Table 2
| Change of grades | \(x_{\max}\), [m], length 4 m | \(x_{\max}\), [m], length 6.5 m |
|---|---|---|
| First to select | 0.6 | 1.05 |
| Second to first | 0.56 | 0.9 |
| Third to second | 0.32 | 0.55 |
| Fourth to third | 0.38 | 0.6 |
According to GOST 8486-66 [6], for coniferous sawn timber, end checks of various lengths are permitted depending on the grade of the sawn...
material. In this case, the total lengths of both face and edge checks extending to the end, \(x_{\text{pk}}\), as well as the lengths of through end checks, \(x_{\text{sk}}\), are regulated. If the differences between these permissible length values in passing from grade to grade for each type of end check are greater than the end trim, then corrections must be made to the data of Table 2. For the most complete assessment of the influence of end drying checks on the quality of sawn timber, let us compile summary Table 3.
Table 3
| Grade transition | \multicolumn{3}{c}{Length 4 m} | \multicolumn{3}{c}{Length 6.5 m} |
|---|---:|---:|---:|---:|---:|---:|
| | \(x_{\max}\), [m] | \(\Delta x_{\text{pk}}\), [m] | \(\Delta x_{\text{sk}}\), [m] | \(x_{\max}\), [m] | \(\Delta x_{\text{pk}}\), [m] | \(\Delta x_{\text{sk}}\), [m] |
| First to select | 0.6 | 0.17 | 0.025 | 1.05 | 0.26 | 0.025 |
| Second to first | 0.56 | 0.16 | 0.025 | 0.9 | 0.18 | 0.025 |
| Third to second | 0.32 | 0.34 | 0.13 | 0.55 | 0.55 | 0.4 |
| Fourth to third | 0.38 | >0.38 | 0.17 | >0.6 | 0.6 | 0.25 |
It is clear from Table 3 that the differences between the regulated GOST values for the lengths of through end checks in all cases are smaller than the differences in the lengths of face and edge checks extending to the end, which simplifies the numerical determination of the permissible value of the lengths of end checks. Analysis of the data in Table 3 shows that an increase in the lengths of face and edge checks extending into the ends above their maximum permissible GOST values, and in passing from grade to grade, affects the quality of sawn timber in different ways.
When passing from the second grade to the first and from the first to the select grade, increasing the length of an end check in the initial material by the required amount does not affect the quality of the final product. When passing from the fourth grade to the third and from the third to the second, the lengths of end trims differ little from the differences in the lengths of checks permitted by GOST; therefore even a slight excess of the lengths of end checks, compared with their permissible GOST values in the initial sawn timber, affects the additional trimming operation in the direction of increasing waste in the form of cracked end offcuts.
In other words, when additional trimming is carried out in sawn timber of the fourth and third grades, end checks have a greater relative weight among other defects than in sawn timber of the second and first grades.
Conclusions
When designing an installation for automatic inspection of end cracking in sawn timber, it is advisable to use as “set points” the permissible lengths of end checks, which should be selected with allowance for the intended use and grade of the sawn timber.
References
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A. V. Lykov. Theory of Drying. Moscow, “Energiya,” 1968.
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G. N. Kharitonov, L. A. Andreev. Kiln Drying of Export Sawn Timber. Moscow, “Lesnaya promyshlennost,” 1965.
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S. I. Akishenkov. Improvement of kiln drying. LTA, Scientific Works, No. 133. Leningrad, 1969.
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S. I. Akishenkov. Investigation of End Checking of Sawn Timber and Methods for Its Elimination during Kiln Drying. Dissertation. Leningrad, 1971.
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V. V. Kislyi. On Improving the Quality of Sawn Timber by Means of Additional End Trimming. Moscow, MLTI Publishing House, 1968.
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GOST 8486-66. Softwood Sawn Timber. Moscow, Standards Publishing House, 1967.