Wood screws is fasteners that are used to join pieces of wood together, and the strength of a wood screw depend on many different factors. A person must consider the densities of the wood, the gauge of the wood screw, and the depth of the wood screw to ensure a secure connection. If you select a wood screw that is too small or too short, the wood screw may fail, and a failed wood screw can cause a structure to become unstable.
Pullout strength are the amount of force required to pull a wood screw straight out of the wood. Pullout strength increases when you increase the thread engagement, and pullout strength also increases when you use denser wood. Dense woods like oak provide more pullout strength because dense woods compress less around the threads of the wood screw.
How to Choose the Right Wood Screw
Softwoods like cedar provides less pullout strength because softwoods compress more easily around the threads of the wood screw. Additionally, increasing the length of the wood screw increases the pullout strength because every extra inch of penetration increase the holding power of the wood screw. Shear strength is the resistance of a wood screw to forces that try slide boards past one another.
Structural wood screws are good for shear strength because manufacturers design structural wood screws to bend rather than snap when they face sideways pressure. Many people may attempt to use drywall screw for structural purposes, but drywall screws are brittle and may shatter when shear force is subjected to them. Coarse threads often provide better performance in softwoods because coarse threads bite deeper into the fiber of the softwood, and coarse threads help to prevent the threads from stripping.
The species of wood affect how a wood screw performs in the wood. Hardwoods like white oak have high pullout values, but softwoods like cedar have much lower pullout values. You must choose a wood screw size that match the wood species, because using a wood screw that is too large for a specific wood can cause the wood to split.
Plywood and manufactured wood products like MDF also behave differently than solid wood. The edges of MDF or particleboard are weak, so the edges of MDF or particleboard may crumble if you use a standard wood screw. The gauge of a wood screw refers to the thickness of the wood screw.
A larger gauge mean the wood screw has a thicker shank, and a thicker shank provides more metal to resist bending. For example, a #8 wood screw is suitable for everyday tasks, but a #12 wood screw is more better for heavy loads. You should pre-drill pilot hole when you use thick wood screws in thin wood or in the end grain, because pre-drilling pilot holes prevents the wood from splitting.
The head type of the wood screw also affect how the wood screw holds the wood together. Flat countersink heads provide maximum pullout strength, but pan heads are better for spreading shear loads. Failure of a wood screw can happen in several ways, and failure often occurs because of mistakes made during installation.
A wood screw may strip if the driver slip, and a wood screw may snap if the shear strength is exceeded. You must also consider corrosion when you use wood screws in exterior project. Exterior projects require galvanized or stainless steel wood screws because galvanized or stainless steel wood screws resist rust.
If you use wood screws that are not resistant to rust, the rust will decrease the strength of the wood screw over time. When you design a project, you should of use a safety factor to ensure the wood screw is strong enough. A common practice is to design for a margin of four times the expected load, and you should adjust this margin if you are using wet wood.
The strongest wood screw joint is a joint where the gauge of the wood screw match the thickness of the wood, the thread of the wood screw match the species of the wood, and the coating of the wood screw match the exposure to moisture.
