HK Timbers Technical Information
What to look for when buying storage boxes
Potato boxes are supplied and labelled in accordance with British Standard 7611 which determines the strength performance required for different stacking heights of box. Our box designs have been tested both in house and at external laboratories. Boxes for other purposes or other crops are manufactured and stamped to the same standard providing that the overall sizes and design fall within the size and performance requirements of the Standard.
MATERIALS USED
The types of timber used in the manufacture of boxes is critical to their longevity. Below is an outline of the HK selection process.
The majority of components being the sides, ends and bases of the boxes are predominantly manufactured from European Redwood or Whitewood. Selection is made from Scandinavian, Russian or European supplies with particular emphasis on using reputable mills with a proven record of supplying consistently well graded goods. No timbers are used from the Lower Gulf areas of Scandinavia as these are found to be too fast grown. (i.e. weaker).
The principal qualities required of these timbers are that they:
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Are out of slowly grown material, giving the required strengths in respect of bowing stiffness and density.
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Are kiln dried to a moisture content of between 12% and 18% which in turn fully seasons the timber, kills any insect infestation and gives an increase in strength (see section explaining relationship between strength and moisture.)
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Are purchased on the basis that they are TOTALLY CHEMICAL FREE to prevent any possible contamination to produce stored in them.
There are currently NO “food safe” timber treatments available.
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Are supplied from sustainable sources only.
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Are full to size i.e. no less than 25mm in thickness (see section explaining effect of thickness on bowing / bend strengths) This point is critical when considering planed boxes which can often lead to boards only 22mm or 23mm thick.
To summarize, the selection of raw material is based on:
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Knot content
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Speed of growth
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Square-ness
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Accuracy of measure
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Moisture content
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Nail-holding properties
OTHER DETERMINING FACTORS
CONNECTOR PLATES:
Two vertical steel connector plates are pressed one into each long side of the box to prevent racking or lozenging forces encountered during handling. These are hidden from the crop as they are sandwiched between the side boards and the vertical battens. Plates are classified as M20 x nominal 1.0mm thick galvanized plate with 8mm teeth. They comply with BS EN 10147. Teeth not pressed into timber (i.e. in gapped boxes) are crushed to remove sharp points.
END BRACES:
Into each of the two short end panels of a box, a diagonal brace is fitted to limit racking or lozenging forces encountered during handling. The profile of these is critical to facilitate the emptying of boxes and to prevent the accidental trapping of crop giving rise to unwanted debris left in the box. We produce a brace with a 10mm radius on the top edge and more importantly a 25 degree chamfer on the underside.Internal checks on head size and shank serration are carried out.Regular batch testing of nails is carried out by an external laboratory to determine/confirm bend strengths using the Robiment test.
Effect of thickness on strength and stiffness of 25mm timber boards:
Effect on strength (likelihood of breaking)
25mm start point
24mm 92.2% of original strength
23mm 84.6% of original strength
22mm 77.4% of original strength
21mm 70.6% of original strength
Effect on stiffness (outwards bow)
25mm start point
24mm 88.5% of original stiffness
23mm 77.9% of original stiffness
22mm 68.1% of original stiffness
21mm 59.1% of original stiffness
Results are applicable to all species, all lengths, e.g. a 10mm bow on sawn timber would give a 12.8mm bow with 23mm (10 x 100/77.9).
Examples of comparative breaking strengths of timber species, at different moisture contents, tested to BS 373 / ISO 3133
A major factor in what makes a strong box or pallet is the strength of wood species. It is not adequate to simply specify softwood, even if geographically defined, as for example Baltic softwood, because species usually has a greater effect on strength than the area where the tree is grown. The list below shows the difference in the mean strength (modulus of rupture or MOR) of species at different moisture contents. These figures are given as mean breaking strengths, and the higher the figure the greater the breaking strength.
Species MOR @ 12% mc MOR @ 20% mc MOR@Green*
European Redwood 83 57 44
European Whitewood 72 51 39
UK Sitka Spruce 67 45 35
* Green = Timber freshly felled before air drying or kiln drying.