Annular Ring / Helical / Plain
Annular ring, helical and plain nails represent some 99% of the volume of fasteners used in wood packaging and pallets in the Western world. The annular ring nail dominates in Europe and the helical nail in North America. The plain nail is well used in the packing case trade and where clinching is necessary in the pallet industry such as board to stringer. A minimum bending strength is necessary if nails are to avoid crippling (bending under axial load) when being driven into difficult timber. This can happen whether being applied by nailing machine, pneumatic tool, or by hand. Crippling always occurs at the weakest position in the nail and is a particular problem with annular ring nails, low grade steel, long nails and dense woods.
Ring nails after testing by MIBANT (top)
and ISO static bending method (bottom)
A minimum bending strength is also necessary if a high performance is expected of wood packaging and pallets, or a manufacturer is building to a design which defines nail bending strength. Suppliers or manufacturers can test for this, the methods produce results quickly.
The MIBANT impact hammer-drop test method was developed in the USA to measure bending strength of helical or twisted nails and many years later an ISO standard was developed defining a static test method. The MIBANT tests strength in the upper part of the nail and the ISO static method near the point. The ISO static method offers a better system for use with annular nails which are far weaker nearer their point where the ring root diameter reduces the effective diameter resisting crippling. This is not the case with helical or twist nails which have a more consistent strength top to bottom. The ring nails in the illustration have been tested by the MIBANT method (top) and ISO static bending method (bottom). There are also other non-standard test methods such as those developed for the Europallet to test nails when made up into complete pallet joints.
The helical nail is less likely to cripple in dense woods than the annular ring so has long been used in North America where hardwoods and denser softwoods are common. The annular ring nail has a greater resistance to pull-out when fully driven in and for this reason is more popular with the lower density pines and spruces of Europe. The net result is a very similar joint strength each side of the Atlantic.
Although critical to the safety of some wood pallet and packaging products such as full perimeter base pallets and bulk storage potato boxes, minimum bending strength is sometimes ignored by nail specifiers. Nail makers outside of the EPAL Europallet nail quality scheme, seem disinclined to mark nail bending strength on packaging. To simplify matters and offer a pallet or packaging nail buyer a simple way to order nails to a minimum bending strength a worldwide standard has been produced defining 3 levels of bending strength. This is given in EN ISO 15629: Pallets for materials handling - Quality of fasteners for assembly of new and repair of used wooden flat pallets. Nail quality in this standard is rated High, Medium or Low bending strength. ‘Machine quality’ is sometimes mistakenly assumed to mean high bending strength, but machine quality is not a term found in European or ISO standards and generally in the industry relates to consistency in type, length and head size to avoid hopper fed nail machines jamming.
The system of stating bending strength completely avoids manufacturers ‘jargon’ since it encompasses diameter, ring depth and final steel quality after wire drawing. Stating nail bending strength is more common in the USA, Canada and Korea, but outside of the EPAL Europallet nail quality scheme, is rarely seen in Europe. Although bending strength is described as the ‘bottom line’ of performance specifications it must still be coupled with the number of nails and the nailing pattern in a pallet or case joint. Those things together give a defined joint strength.
The North American style of expressing nail strength as MIBANT bend angle is a measure of expressing bending strength as is using the EN ISO method. The MIBANT angle is expressed in degrees of bend and is the single performance unit, but the angle of bend using the EN ISO method as shown above has zero significance. The EN ISO static bend method unit of measurement is the Nm (Newton.metre). There is some correlation between the two methods.
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52b A guide to minimum pallet nail sizes (PDF 33kb)52i The effect on joint strength of deep rings in annular nails (PDF 28kb)52j International strength quality levels for pallet nails (PDF 27kb)52m Mibant, Rockhi, Verus - Wood packaging nail test methods (PDF 56kb)59a INDEX OF EUROPEAN NAILING standards and related documents (PDF 24kb)