Lateral and Withdrawal Resistance of Tapping Screws in Three Densities of Wood

The ability of type-A tapping screws to withstand withdrawal and lateral loads in red oak, Douglas-fir, and redwood was investigated. Lateral load varied linearly with the wood’s specific gravity and as the square of the screw root diameter. Tapping screws had 6 to 15 percent higher maximum withdrawal. loads than common wood screws of the same root diameter. The lead-hole size to obtain optimum withdrawal load was 60 percent of the screw’s root diameter. End-grain withdrawal strength was 86 to 71 percent of the values for side-grain penetration. The withdrawal strength of tapping screws varied nearly linearly with depth of penetration. No practical difference in withdrawal strength was indicated between radial and tangential faces.

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