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Guide to Fasteners - Part I - Overview of Bolts and Screws

written by: johnsinit • edited by: Lamar Stonecypher • updated: 4/1/2009

Bolts and screws are commonly used fasteners to keep the machine parts together. There are many different types of bolts and screws – varying types of heads, threads and more. Different versions are used depending on the application requirement.

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    Many people know that without fasteners like screws, bolts and nuts, no industry can survive. These are the basic hardware parts required by any machines, whether simple household items to complex high-end machines in industries. It is many times essential to use high quality nuts, bolts and screws. From small furniture crafting to oil sector industries the most essential hardware items include these fasteners – bolted joint is the most trustworthy part connection – moreover, it usually gives an option of dismantling and reinstalling. Anyway, the generalization and systematization process and the most modern technological advancements have changed the field of fasteners for the best.

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    One of primary feature used by most of the machines is threaded joint as part of the design. The bolted joint can hold severe loads and the parts can be separated and reconnected repeatedly. Today, these fasteners are available in different materials like stainless steal, copper, brass and alloy steels. With the advancement of the technologies even porcelain and glasses are used as screws for many applications including canning jars and electrical insulators.

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    Bolt vs Screw

    Though many times confused by amateurs, bolts and screws are different types of fasteners. Bolts are headed fasteners with external threads suitable for non-tapered nut. They require a nut (or some other thread) as a counter - to lock the joint. The bolts are made with uniform threads satisfying the standard specifications such as UNJ, UNR, MJ etc. and are used in most industrial applications – secured with a spring-washer or a locktite. To sum it up, bolts are designed to use as a fastener placed through already drilled holes in the machine parts and these requires a nut to tighten it from the other end.

    Screws are headed externally threaded types of fasteners without meeting the standard specifications of the bolts. They are shafts with provision at one end to turn the screw and have a helical thread on its surface, which is capable of piercing strongly into the surface. They do not require a nut and are used mostly in home-based application – such a connecting a picture to the wall. It is actually the simplest example of converting torque into linear force.

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    Head types

    Depending up on the design of the heads the screws/bolts are named differently. Pan head is a low disc with chamfered outer edge. Hexagon head bolts are generally used for bigger loads. Socket head is a cylinder with (usually) Hex socket (Allen) opening – those bolts usually have the smallest head. Flat head or countersunk screws are conically shaped with tapering inner face and flat outer face. It will allow the screw to sink into the material – providing “flat surface". There are also many special types of bolts with different heads including oval or raised head, bugle head, cheese head. Even square head screws are present – as well as there are headless screws and bolts.

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    Bolt and Screw types