Lag screws, like wood screws, require prebored holes of the
proper size (Fig. 7–6). The lead hole for the shank should be
the same diameter as the shank. The diameter of the lead
hole for the threaded part varies with the density of the wood:
For low-density softwoods, such as the cedars and white
pines, 40% to 70% of the shank diameter; for Douglas-fir and
Southern Pine, 60% to 75%; and for dense hardwoods, such
as oaks, 65% to 85%. The smaller percentage in each range
applies to lag screws of the smaller diameters and the larger
percentage to lag screws of larger diameters. Soap or similar
lubricants should be used on the screw to facilitate turning,
and lead holes slightly larger than those recommended for
maximum efficiency should be used with long screws.
In determining the withdrawal resistance, the allowable
tensile strength of the lag screw at the net (root) section
should not be exceeded. Penetration of the threaded part to a
distance about seven times the shank diameter in the denser
species (specific gravity greater than 0.61) and 10 to 12 times
the shank diameter in the less dense species (specific gravity
less than 0.42) will develop approximately the ultimate
tensile strength of the lag screw. Penetrations at intermediate
densities may be found by straight-line interpolation.
The resistance to withdrawal of a lag screw from the endgrain
surface of a piece of wood is about three-fourths as great
as its resistance to withdrawal from the side-grain surface of
the same piece.
hope that helps
Alan
