Sawn timber sorting

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Wood is a natural product with different growth conditions and genetic makeup, so that there is a wide range of properties. The spread of the properties is increased by the processing into building material, which is why it makes sense to sort cut goods into classes of the same strength in order to guarantee optimal use and the best quality. Sorting is done according to dimension, purpose or appearance. The most important is the grading according to the strength, whereby the grading characteristics are related to the cross-section and compared, both visually and mechanically.

Visual sorting

With visual sorting, externally recognizable features are recorded with the naked eye. Depending on the strength rating, it can be divided into different grading classes. Until well into the 20th century, sawn timber was only sorted visually. But the visual sorting is time-consuming, imprecise and requires a lot of staff. Visual sorting is currently still an important part of the sawing industry. Most sawmills are small businesses where the purchase of a machine sorting system is not worthwhile.

Sorting parameters

The following parameters are taken into account when sorting visually:

Knottiness
The knottiness is a very important parameter when sorting. It has a great influence on the strength of the wood. Depending on the cut goods, different measurements and ratings are made.
Fiber slope
With the fiber inclination, the deviation of the fiber direction from the longitudinal axis is measured. It affects the strength in smaller amounts.
Pith
Present or not. Due to its different structure, it affects the strength in small amounts
Tree ring width
Wood that has grown quickly has wider annual rings and thus a lower bulk density or strength.
Cracks
Depending on the type of crack, a reduction in strength can be determined. Small cracks are mostly ignored. Lightning cracks / ring peeling are not permitted; Shrinkage cracks sometimes have to be measured and evaluated depending on their size.
Discoloration
Change in the natural wood color, e.g. B. Blue stain (does not have to be considered), brown or white rot (not permitted)

Other parameters such as curvature, twist, reaction wood, insect damage and tree edge also have an influence on the strength and are included in the evaluation of the sawn timber.

Sorting classes

Every European country has its own sorting regulations and sorting classes. Your own sorting class can then be assigned to a Europe-wide uniform strength class.

In Germany, the DIN 4074 standard regulates the sorting classes:

Squared timbers and boards and planks subject to bending on edge
S7 / S7K, S10 / S10K, S13 / S13K
Slats
S10, S13
Boards and planks
S7, S10, S13

This sorting class is for sorting softwood. When sorting hardwood, there is an L in front of the S.

Machine sorting

Due to the increasing demand for wood, the processes in further processing must also run faster and more precisely. At the beginning of 1960, the first sorting machines were designed and used in Australia and the USA, which could assess the sawn timber by means of mechanical flexural strength. In 1970 another three sorting machines were developed in England and Scandinavia with improved machine and measuring technology. At the beginning of 1980 two more sorting machines were developed which, using the latest computer technology, were able to record several measured values ​​in parallel, which together led to a common classification. The so-called Finnograder and ISO-GreComat. Experiments have shown that the modulus of elasticity is the best individual parameter and can be further improved in connection with another parameter such as knotiness. In 1993 the first sorting machine (EuroGreComat) came onto the market, which recorded the modulus of elasticity, bulk density and knotiness of the wood and was able to compare them with one another. The advantages are clear: higher sorting speeds and a high degree of accuracy in strength grading, as well as better exploitation of the use and raw material wood.

Sorting parameters

With these three parameters, wood can be sorted very precisely. In some cases, additional sorting parameters are also determined and taken into account.

Sorting classes

In the case of machine sorting, in contrast to visual sorting, the classification follows directly into the strength classes. Automatic sorting is regulated by EN 14081.

  • Europe-wide strength classes for softwood: C14, C16, C18, C22, C24, C27, C30, C35, C40
  • Europe-wide strength classes for hardwood: D30, D35, D40, D60, D70

Each strength class has specified material parameters for the sorting parameters. In visual sorting, sorting class S13 corresponds to strength class C30, S10 corresponds to C24 and S7 corresponds to C18.

Perspectives

The mechanical sorting of sawn timber has many advantages. On the one hand, wood can be saved, on the other hand, it can also be used more economically thanks to the more precise sorting in strength classes. That would make wood much more competitive with steel and concrete. In spite of this, machine sorting has not yet established itself over visual sorting. On the one hand, the purchase of the machines is expensive and also involves some changes in the operation. Further costs arise from the strict Europe-wide standards. In order for a sorting machine to be approved, a connection between the sorting parameters and the strength must be established. Each machine must therefore pass a test series of 450 samples if a new type of wood is to be sorted, as individual approval is required depending on the origin and type. This is due to the different soil and environmental conditions, which means that properties such as knottiness or bulk density can have great differences, although it is the same species. And since many sawmills process wood of different origins in parallel, machine sorting is still too cumbersome and can only be found in larger companies in practice. However, researchers found that a classification by country would make little sense, as the effort for the minimal property differences would not be worthwhile. It would make more sense to divide Europe into 3 sections (Northern, Central and Eastern Europe). In the meantime, some European countries are already being combined to form common growth areas. This makes machine sorting easier and more attractive for companies in order to increase the yield of the wood. For example, it can reduce the cross-sections of trusses if the beams have reliable guaranteed properties. In the future, more and more attention will be paid to using the wood optimally and thus enabling savings of up to over 20%.

Marking of the sawn timber

Lumber must be clearly marked. If this is not possible for optical reasons, a delivery note must be enclosed. This makes it possible to trace the wood back. Sawn timber is usually identified by an embossing stamp or an inkjet printer, but it can also be identified by a pressed or glued-on badge. The exact regulations are in DIN 4074 and DIN 14081. The sawn timber is labeled according to the CE marking, with which various information must be given. For example, the manufacturer, the code number for identification and the strength or sorting classes are on the stamp.

Individual evidence

  1. ^ "Press Service Wissenschaft, TU Munich, March 11, 2011" Retrieved on December 20, 2018
  2. ↑ "International Wood Construction Forum ". Accessed on December 20, 2018
  3. ↑ "Merkblatt 4074" Retrieved on December 20, 2018
  4. ↑ "Merkblatt 4074" Retrieved on December 20, 2018
  5. ↑ "LWF aktuell 77/2010" Retrieved on December 20, 2018
  6. ↑ "LWF aktuell 77/2010" Retrieved on December 20, 2018
  7. ↑ "LWF aktuell 77/2010" Retrieved on December 20, 2018
  8. ^ "Press Service Wissenschaft, TU Munich, March 11, 2011" Retrieved on December 20, 2018
  9. ↑ "LWF aktuell 77/2010" Retrieved on December 20, 2018
  10. ↑ "Merkblatt 4074" Retrieved on December 20, 2018