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A carriage bolt is a type of threaded fastener designed to be used with wood, although specialized versions can be used to fasten metal components. Also known as plow bolts or coach 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, though other materials can be used to make them as well.
The design of a carriage bolt includes a large round, domed head with an externally threaded bolt projecting from it. The head is typically at least twice the diameter of the bolt shaft, and usually does not have slots for a screwdriver or other driving device. Underneath the head, there is a square fitting that grips the wood as the bolt is placed and fastened, keeping it from twisting or turning and otherwise maintaining a snug fit. In a variation on the basic carriage bolt, the square fitting under the rounded head is replaced with ribbing that holds the bolt in place when it is 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 tightened by hand for a reasonably snug fit, or tightened with the assistance of a hand tool that will lock the carriage bolt in place, ensuring it cannot move. The thread size of the bolt must match the thread size of the nut in order to be screwed on properly; thread sizes can vary significantly according to application. Specialized versions of the bolt and nut combination may be made from metals such as galvanized steel that resist corrosion so they can be used in wet environments or places in which 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 in by its round, smooth head. The square fitting that holds the bolt in place means that the nut can be tightened firmly, however, creating a solid fit. Often, the shaft of the bolt is smooth immediately under the fitting, as this area is completely within the material being joined, and therefore does not need to be threaded. The wide head — sometimes called a cup or mushroom head — is ideal for use with timber, where its large size prevents it from being pulled deeply into the wood without the need of a washer, although they can be used.
Pieces of timber are often secured together using one or more carriage bolts during construction of homes, outdoor decks, and pilings. These bolts are commonly used in chain link fences, patio and ready-to-assemble furniture, machinery assembly, and even in automobiles. Since the square fitting keeps the bolt from turning, carriage bolts are a good choice for situations in which the head of the bolt cannot be easily accessed.
When using carriage bolts, it is important to select a bolt of the right width and length for an application. A bolt that is too narrow may lead to the fastening failing under strain, causing an object to break. If the carriage bolt is too big, it can stress the surrounding wood or metal, causing cracks. Length is also important, as the bolt must be long enough to go through the material being fastened, but not so long that the protruding end becomes a nuisance.