Screw joints are a common way of joining elements of furniture construction. They can be found in skeletons of chairs and tables, in frames of armchairs and sofas and in bodies of case furniture. As a general rule, screws are inserted into previously drilled holes, whose diameter must be adapted to the appropriate, characteristic part of the screw. In the part without thread, the hole diameter should be equal to the screw diameter d2, and in the threaded part, it amounts to d1 = 0.7d2 (Fig. 6.87). And the depth of insert should amount to a minimum of 4d2.
In some load states of furniture, screw connectors can work on pulling and pressure (Fig. 6.88). For screws with a diameter of d2 < 8 mm, the same calculation methods as for the nails apply, and in the case of screws with a diameter of d2 > 8 mm, we proceed in accordance with the rules that apply when calculating bolts.
For axial loads of screws in wooden constructions, in which connectors were placed perpendicular to the wooden fibres, the constructor should set load-bearing capacities. In this case, we can use the formula given in the norm PN-B-03150:2000:
Fig. 6.87 Example of a screw joint
Fig. 6.88 Scheme of work of connectors in a joint: a pulling and pressure, b bending and pressure: 1 connector, 2 veneer
Rd — f3,d(lef — d2), (6.229)
f3d — Wv; (6.230)
f3;k —(1.5 + 0.6d2)pqk, (6.231)
d2 diameter of the smooth part of the core,
lef length of the screwed part of the core from the sharp side,
pk density of wood,
kmod partial modification coefficient (Table 6.5), yM partial safety coefficient (Table 6.6).