Carriage bolt is a kind of hexagon bolt, specially used for carpentry, especially for fastening metal to wood. The carriage bolt is designed as a domed head to prevent loosening from one side, and an enlarged head shape also prevents the bolt from being pulled through the wooden structure. The presence of a domed head means that carriage bolts cannot be installed by means of pushing the bolts axially during installation, such as a spanner drill adapter.
The square head part of the carriage bolt allows the use of a spanner to be installed to increase the lever. When tightened into a soft material (such as wood), the square section also provides a certain degree of locking capability. An unthreaded section under the square section of the carriage bolt increases strength and resistance to shear.
The carriage bolt also provides several secondary benefits, such as the aesthetic appeal of its round head, which can give a finished appearance to many applications. Although used for wood, carriage bolts have machine threads and are equally suitable for metal applications. However, carriage bolts may be more difficult to install because they are not compatible with drill adapters.
It should be noted that this definition varies from manufacturer to country-for other users, what is considered a carriage bolt may be a coach bolt or a lag bolt, so you should always exercise caution to ensure that you buy the correct bolt.
What is a coach bolt?
Coach bolts, usually called coach screws, are self-tapping fasteners with hexagonal head bolts. Coach bolts are usually used to fix wood to metal and wood to wood. The unthreaded section of coach bolts increases the strength of its structure and makes it more resistant to shearing forces compared to full-threaded variants (such as self-tapping hexagon bolts).
Due to their suitability for heavier applications and their compatibility with large tooling, coach bolts are usually larger in size than other self tapping screws.
What is the difference between coach bolts and carriage bolts?
Although coach bolts and carriage bolts are different types of screws, they have similarities in general head shapes, and they are suitable for use with wood. The most significant difference is that the coach bolt has a self-tapping thread, which allows it to create its own thread in the wood. In contrast, the carriage bolt has a machine thread, so an appropriately sized pilot hole is always required.
The difference in the application of these two screw types is not simply attributed to the difference between the carriage and the coach. Confusingly, carriage and coach are almost synonymous, and carriage bolts and coach bolts can be found in many designs of both types of vehicles.
However, there is little concrete evidence regarding the origin of the term "carriage bolt". There is a theory that it is derived from the old French "cariage", which does not refer to the carriages in a vehicular sense, but is closer to the English word "carry". It may be because this type of bolt is used to carry applications, rather than specifically designed for use in the manufacture of carriages.
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