
Lag screws, also known as lag bolts, are heavy-duty wood screws with a hex head. They are used to fasten wood to wood or metal to wood, and can also be used to attach items to concrete and masonry when combined with lag expansion shields. Lag screws are available in a variety of materials, including steel, zinc-plated steel, hot-dip galvanised steel, and stainless steel. The size of a lag screw is determined by its diameter and length, with standard sizes ranging from 1/4 to 3/4 in diameter and lengths ranging from 1 to 16.
Characteristics | Values |
---|---|
Head Type | Hex |
Materials | Steel, Stainless Steel |
Steel Finishes | Zinc Plating, Hot-Dip Galvanizing |
Stainless Steel Finishes | Hot-Dip Galvanizing |
Common Sizes | 1/4", 3/8", 5/16", 1/2", 5/8", 3/4" |
Lengths | 1" to 16" |
Thread Length | Minimum: half the nominal screw length + 1/2" or 6" (whichever is shorter) |
Thread Type | Right-Hand |
Thread Count | 10 per inch (1/4" diameter) to 3 1/4 per inch (1 1/4" diameter) |
Point Type | Gimlet, Semi-Cone |
Hex Lag Screws
The diameter of Hex Lag Screws varies, with common sizes including 1/4", 5/16", and 3/8". The length of the screws can also vary to fit different applications, with lengths such as 1-1/2", 2", 3", and 6" available. It is important to select the appropriate size and material for the specific application to ensure optimal performance and durability.
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Steel and Stainless Steel
Stainless steel lag screws are known for their superior corrosion resistance. They are often used in marine environments and other harsh conditions. The austenitic alloys in stainless steel develop their strength through work hardening during the fastener manufacturing process. The only heat treatment normally available for these alloys is annealing, which is done at approximately 1900°F to a dead soft condition and is not usually thermally reversible.
Lag screws made from steel are typically made from AISI 1006-1022 or equivalent steel, while stainless steel lag screws are often made from 18-8 stainless steel. The tensile strength of stainless steel is 100,000-125,000 psi.
Lag screws are heavy-duty construction fasteners with sharp threading that only covers part of the bolt, allowing for adjustments as needed. They are generally used for wood-to-wood and metal-to-wood fastening, although they can also be used to attach objects to concrete and masonry when combined with lag expansion shields. They are available in a wide range of sizes, typically ranging from 1/4" to 3/4" in diameter, with lengths spanning from about 1" to 16".
When selecting a lag screw, it is important to consider the dimensions of the screw and the material it will be used with. The pilot hole for the body of the screw should match the screw size perfectly. For example, a 3/8" screw requires a 3/8" hole. The head of the lag screw should also have sufficient space for wrench clearance.
In terms of length, the total length of the lag screw should be at least twice the thickness of the material being attached. For example, if you are attaching a 2" thick piece of wood, the lag screw should be a minimum of 4" long.
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Length and Diameter
The length of a lag screw is measured from under its head to the threaded tip. Standard lengths range from about 1" to 16". The minimum length of the thread should be equal to half the nominal screw length plus 0.5" or 5", whichever is shorter. Short lengths are fully threaded, while longer lengths have a minimum thread length.
The standard diameter of a lag screw ranges from 1/4" to 3/4". The most prevalent materials for lag screws are steel and stainless steel. Steel screws are commonly zinc-plated or hot-dip galvanised, while hot-dip galvanised and stainless steel are recommended for use with pressure-treated wood.
Hex lag screws, which are large wood screws, are available in diameters from 1/2″ to 1-1/2″. These screws are designed to be installed in wood by rotating the six-sided head and typically have a gimlet point. However, non-standard hex lag screws may feature a semi-cone point instead.
Lag screws with a diameter of less than 1/2" typically require a pilot hole for installation. A stepped pilot hole is recommended, with the hole for the unthreaded body of the screw matching the screw size, and the hole for the threaded section based on screw size and wood density.
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Thread Length
The minimum thread length for a lag screw is typically defined by standards and good engineering practices. According to standard guidelines, the minimum thread length should be equal to half of the nominal screw length plus 0.5 inches, or 5.00 or 6.00 inches, whichever value is shorter. This specification ensures that the screw has sufficient engagement in the material it is being fastened to, optimising its holding power and preventing stripping.
In practice, lag screws with shorter lengths are often fully threaded, meaning the thread runs along the entire length of the screw. This design enhances the screw's holding power in shorter fasteners. For longer lag screws, the minimum thread length specification becomes more critical, ensuring that a substantial portion of the screw is threaded for secure fastening.
When selecting lag screws, it is essential to consider the specific application and material into which the screw will be fastened. The length of the threaded portion directly impacts the screw's ability to engage with the material and withstand the applied loads. Insufficient thread length can lead to reduced holding power, compromising the integrity of the joint or connection. Therefore, choosing lag screws with appropriate thread lengths is vital to ensure safe and effective fastening in any construction or assembly project.
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Pilot Holes
When using lag screws, it is important to understand the concept of pilot holes and how to drill them. A pilot hole is a small hole drilled into a piece of construction material, such as wood, concrete, or plastic, before driving a screw. Its purpose is to guide a larger drill, allow the insertion of another hole-making tool, or locate, guide, and provide clearance for a self-threading screw.
Drilling a pilot hole before using a screw helps to prevent the wood from splitting. It also makes it easier to install screws in dense hardwoods and ensures that the screw will be installed straight. Pilot holes are especially important when installing screws near the edge of a piece of wood or into a very thin piece of wood, as these tasks have a high risk of the wood splitting.
To drill a pilot hole, follow these steps:
- Select the right drill bit: Choose a drill bit that matches the inner diameter of the threaded part of the screw, not the diameter of the threads. The pilot hole should be slightly smaller than the diameter of the screw.
- Measure and mark the pilot hole location: Determine the intended location of the fastener and mark the center point for the pilot hole.
- Install the drill bit in the chuck: Place the drill bit fully into the chuck (the part of the drill that holds the bit in place) and tighten it completely.
- Drill the pilot hole: Set the drill direction to forward, grip the drill with your forefinger on the trigger, and align the drill perpendicular to the surface. Start slowly and then accelerate.
- Install the screw: Once the pilot hole is drilled, fit the drill with a screwdriver bit and position the screw's tip into the pilot hole. Drive the screw in, following the path of the hole.
By following these steps, you can effectively create pilot holes and ensure a smooth and secure installation of lag screws.
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Frequently asked questions
Lag screws, also known as lag bolts, are heavy-duty wood screws with a hex head. They are used for fastening wood to wood or metal to wood, and attaching objects to concrete or masonry when combined with lag expansion shields.
Lag screw sizes typically range from 1/4" to 3/4" in diameter, with lengths spanning from 1" to 16".
Lag screws are commonly made of steel or stainless steel.
There are two main types of lag screws: hex lag screws and square lag screws. Hex lag screws are the most common type and feature a six-sided head. Square lag screws are less common and have a four-sided head.
Lag screws can be purchased from various retailers, including online stores such as Fastener SuperStore, Fastener Mart, and AFT Fasteners.