What’s a Carriage Bolt?

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Carriage bolts are threaded fasteners used with wood or metal components, with a large domed head and square fitting to grip the wood. They are commonly used in construction, furniture, and automobiles. It is important to select the correct size and length for the application.

A carriage bolt is a type of threaded fastener designed for use with wood, although specialized versions can be used to secure metal components. Also known as plow bolts or anchor bolts, carriage bolts can be found in many hardware stores in an assortment of lengths, diameters, and thread pitches. They are usually made from various types of steel for strength and durability, although other materials can also be used to make them.

The design of a carriage bolt includes a large domed carriage bolt with an externally threaded bolt protruding from it. The head is typically at least twice the diameter of the bolt shaft and usually has no slots for a screwdriver or other driving device. Below the head, is a square fitting that grips the wood as the bolt is positioned and secured, keeping it from twisting or turning and still maintaining a snug fit. In a variation on the basic carriage bolt, the square under the carriage bolt is replaced with a rib that holds the bolt in place when used with other metal components. The length of the bolt itself will vary according to its intended use, as will the diameter of the bolt and bolt head, regardless of the type of materials being fastened.

Like other bolts, the carriage bolt is designed to be used with a nut that helps secure it in place. The nut can be hand tightened to a reasonably snug fit or tightened with the aid of a hand tool which will lock the carriage bolt in place, ensuring it does not move. The thread size of the bolt must match the thread size of the nut to be screwed in correctly; Thread sizes can vary greatly depending on the application. Special versions of the bolt and nut combination can be made from metals such as zinc plated steel that resist corrosion so they can be used in damp environments or in locations where the bolt is likely to be exposed to corrosive chemicals.

Typically, carriage bolts are used in pre-drilled holes; this is necessary, in most cases, because the bolt cannot be driven by its smooth, round head. The square fitting that holds the bolt in place means the nut can be tightened tightly, however, creating a solid fit. Often, the shaft of the bolt is smooth immediately below the fitting, as this area is completely within the material to be joined and therefore does not require threading. The large head – sometimes called a cup or mushroom head – is ideal for use with wood, where its large size prevents it from being pulled deep into the wood without the need for a washer, although they can be used.

it is used
Lumber pieces are often fastened together using one or more carriage bolts when building houses, outdoor decks, and pilings. These bolts are commonly used in chain link fences, patio and ready-to-assemble furniture, machine assembly, and even in automobiles. Because the square fillet prevents the bolt from turning, carriage bolts are a good choice for situations where the bolt head cannot be accessed easily.

When using carriage bolts, it is important to select a bolt of the correct width and length for an application. A bolt that is too tight can cause the fastener to fail under stress, causing an object to break. If the carriage bolt is too large, it can put stress on the surrounding wood or metal, causing cracks. Length is also important, as the bolt needs to be long enough to pass through the material being fastened, but not so long that the protruding end is annoying.

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