A lag bolt is a mechanical fastener with a threaded body that is driven through two adjacent surfaces for the purpose of adjoining them securely. Also known as a lag screw, this piece of hardware is most often used to link wooden elements with other wooden parts or concrete foundations. Lag bolts are generally large, heavy duty and often industrial bolts, though smaller variations are available for use in furniture and cabinetry industries.
Popular in construction, masonry, locomotive, mining and civil engineering industries lag bolts are used as fasteners at integral joints of such structures as bridges, railway trestles, decking and even barn construction. Wooden components such as posts and beams are commonly connected with these devices as they offer an extremely secure connection without causing significant damage to the woodwork. To ensure a continually high degree of clamping force when tightened, lag bolts are made of strong materials that are not likely to stretch, expand or crack over time.
The specific material depends on the intended use and environmental pressures to be inflicted upon the bolt. Zinc-plated steel and hot dipped galvanized steel are popular choices as they are low cost and still offer some resistance to rust. Stainless steel and silicone bronze, however, offer heightened protection against the elements as they are highly resistant to corrosion from moisture and chemicals that may be present in some industrial settings.
The manufacturing process used to create lag bolts is similar to that of other bolt types. A steel or bronze wire rod is first heated consistently for an extended period of time and all rust particles are removed. If needed, the metal is coated for added protection. Cold forging is then used to shape the rod at room temperature and the rod is forced through dyes at a high pressure to be shaped into long perfectly round rods. Once cut down to the desired length, a second dye shapes the head of the lag bolt which may be either square or hexagonal.
Cold forging using high pressure rollers is then used to press in the thread pattern. The finished course-pitched lag bolt is then ready for use in a variety of applications. A pilot hole is made in the first piece of wood or other material and the bolt is inserted. A wrench, spanner or pliers are then used to drive the screw into the second bit of material. The threading bites into the wood to hold the two components together.
During installation, caution must be used so as not to damage the head of the screw. The head provides compression while preventing the bolt from being driven completely through both materials. In some instances a washer is also used to protect softer lumbar, ensuring that the head will not sink into the wood. It is important to consider the materials to be fastened as well as the strength and corrosion resistance required of the bolt materials. Other considerations include the bolt length and diameter.