Wood screw design: influence of thread parameters on the withdrawal capacity

Abstract

Self-tapping wood screws are important fasteners in timber construction. A characteristic of these screws is their high axial load-bearing capacity, which also depends on their withdrawal capacity. This is used for structural design and is thus an important optimization parameter for wood screws. To increase the withdrawal capacity, knowledge of the influence of the thread parameters, such as outer diameter, pitch or flank angle, is required. The influences of pitch and flank angle on the withdrawal capacity have not yet been sufficiently subjected to research and are therefore investigated in this study. A total of ten specially developed screw prototypes with pitches in the range of 3.04 mm and 5.9 mm and flank angles in the range of 35° and 45° are used. The screw prototypes are flat ribbed bars with uncoiled screw thread. The effect on the withdrawal capacity is measured using an experimental setup based on test standard EN 1382:2016. The pitch showed a significant influence (p = 0.000, f = 0.37) whereas the influence of flank angle was not significant (p = 0.283). A smaller pitch leads to a higher withdrawal capacity, irrespective of the flank angle. The experimental results are explained based on the theoretical models of bonding mechanisms and conical stress distribution. To optimize the withdrawal capacity of a ø 8 mm screw, a smaller pitch is preferable. The determined influence of the pitch can also be used to improve the accuracies of calculation models for the withdrawal capacity.