Fastener withdrawal characteristic
The fastener withdrawal characteristic (FWC, written Cw in the Uniform Standard) is a multiplier that rates how much a nail's threads improve its resistance to pulling out. A plain-shank nail scores 1.0; the Uniform Standard sets minimums of 1.9 for reusable pallets and 1.5 for single-use pallets.
Also called FWC, Cw, withdrawal characteristic
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How FWC is calculated
The Uniform Standard (2012, Annex F) gives a public formula based on the nail's geometry:
Cw = 1 + K x (thread crest diameter - wire diameter) x (H / thread length)
K is 22 for helical nails and 60 for annular nails, and H depends on the thread count and angle. For the 2-1/4 in x 0.112 in helical nail on the PDS GMA example (0.124 in thread diameter), the result is 2.58, matching the spec sheet.
Typical values
| Nail | FWC |
|---|---|
| Plain shank | 1.00 |
| Helical and annular pallet nails (PDS sheets) | 2.58 to 3.69 |
Minimums in the Uniform Standard (Table 4)
| Reusable | Single-use | |
|---|---|---|
| Minimum Cw | 1.9 | 1.5 |
| Minimum head-to-shank ratio | 2.25 | 2.00 |
Why it matters
Withdrawal strength decides whether deckboards stay tight after repeated handling. FWC multiplies the basic withdrawal equation, so a nail with FWC 2.58 resists pull-out about 2.6 times better than a plain nail of the same wire size in the same wood. See pallet nails and MIBANT.
Questions
- What is a good FWC for pallet nails?
- At least 1.9 for reusable pallets under the Uniform Standard. Quality threaded pallet nails on real spec sheets run about 2.6 to 3.7.
Related
- MIBANTMIBANT (Morgan Impact Bend Angle Nail Tester) rates pallet nail stiffness as a bend angle under ASTM F680. A lower angle means a stiffer, better nail.
- Helical nailA helical nail has a spiral thread on its shank that turns as it is driven, resisting pull-out. It is the standard pallet nail for medium and dense hardwoods.
- Annular nailAn annular (ring-shank) nail has rings around its shank that lock into the wood fibers. It is recommended for softwood and low-density hardwood pallets.
- Pallet nails: types, sizes and how manyPallet nails are usually 2 to 3 in helical or annular threaded nails, 11 to 12-1/2 gauge. A GMA 48 x 40 uses 84 nails: 2 or 3 per joint by board width.
- Uniform Standard for Wood PalletsThe Uniform Standard for Wood Pallets (Woodpack Global, formerly NWPCA) sets US lumber grades, tolerances, nail counts and repair rules for wood pallets.
- FlushA flush pallet has its deckboard ends even with the outer stringers or blocks, with no wing or overhang. Most GMA 48x40 and block pallets are flush.
Sources
- Uniform Standard for Wood Pallets (NWPCA, now Woodpack Global), 2012 edition
- Guide to Understanding the PDS Specification and Analysis (stringer, inches)