Some types of plastic, such as nylon or polytetrafluoroethylene (PTFE), can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. The cylindrical portion of the screw from the underside of the head to the tip is called the shank; it may be fully or partially threaded. [1] The distance between each thread is called the pitch. [2] The hand tool used to drive in most screws is called a screwdriver. A power tool that does the same job is a power screwdriver; power drills may also be used with screw-driving attachments. Where the holding power of the screwed joint is critical, torque-measuring and torque-limiting screwdrivers are used to ensure sufficient but not excessive force is developed by the screw. The hand tool for driving hex head threaded fasteners is a spanner (UK usage) or wrench (US usage), while a nut setter is used with a power screw driver.

Bear in mind that these are just examples and the width across flats is different for structural bolts, flanged bolts, and also varies by standards organization. A screw and a bolt (see Differentiation between bolt and screw below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a male thread (external thread). A screw is a combination of simple machines: it is, in essence, an inclined plane wrapped around a central shaft, but the inclined plane (thread) also comes to a sharp edge around the outside, which acts as a wedge as it pushes into the fastened material, and the shaft and helix also form a wedge at the point. Some screw threads are designed to mate with a complementary thread, called a female thread (internal thread), often in the form of a nut object with an internal thread. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.

Recommendations

A fastener with a built in washer is called a SEM or SEMS, short for pre-as SEMbled. [26] [27] It can be fitted on either a tapered or non-tapered shank. Screws are often self-threading (also known as self-tapping) where the thread cuts into the material when the screw is turned, creating an internal thread that helps pull fastened materials together and prevents pull-out. There are many screws for a variety of materials; materials commonly fastened by screws include wood, sheet metal, and plastic. This article is about the fastener. For the screw as a mechanism, see Screw (simple machine). For other uses, see Screw (disambiguation). An assortment of screws, and a US quarter for size comparison A screw in macro view A bolt (with a nut) and a screw Screw head shapes [ edit ] (a) pan, (b) dome (button), (c) round, (d) truss (mushroom), (e) flat (countersunk), (f) oval (raised head) Combination flanged-hex/Phillips-head screw used in computers Pan head A low disc with a rounded, high outer edge with large surface area. Button or dome head Cylindrical with a rounded top. Round head A dome-shaped head used for decoration. [46] Truss head Lower-profile dome designed to prevent tampering. Flat head Conical, with flat outer face and tapering inner face allowing it to be countersunk into the material. The angle of the screw is measured as the aperture of the cone. Oval or raised head A decorative screw head with a countersunk bottom and rounded top. [46] Also known as "raised countersunk" or "instrument head" in the UK. [ citation needed] Bugle head Similar to countersunk, but there is a smooth progression from the shank to the angle of the head, similar to the bell of a bugle. Cheese head Cylindrical. Fillister head Cylindrical, but with a slightly convex top surface. Flanged head A flanged head can be any of the above head styles (except the countersunk styles) with the addition of an integrated flange at the base of the head. This eliminates the need for a flat washer. Hex head Hex shaped, similar to the head of a hex bolt. Sometimes flanged.

This section possibly contains original research. Please improve it by verifying the claims made and adding inline citations. Statements consisting only of original research should be removed. ( June 2022) ( Learn how and when to remove this template message) The distinctions above are enforced in the controlled vocabulary of standards organizations. Nevertheless, there are sometimes differences between the controlled vocabulary and the natural language use of the words by machinists, auto mechanics and others. These differences reflect linguistic evolution shaped by the changing of technology over centuries. The words bolt and screw have both existed since before today's modern mix of fastener types existed, and the natural usage of those words has evolved retronymously in response to the technological change. (That is, the use of words as names for objects changes as the objects change.) Non-threaded fasteners predominated until the advent of practical, inexpensive screw-cutting in the early 19th century. The basic meaning of the word screw has long involved the idea of a helical screw thread, but the Archimedes screw and the screw gimlet (like a corkscrew) preceded the fastener. Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips in the US; hex, Robertson, and Torx are also common in some applications, and Pozidriv has almost completely replaced Phillips in Europe. [ citation needed] Some types of drive are intended for automatic assembly in mass-production of such items as automobiles. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the home repair person.Find sources: "Screw"– news · newspapers · books · scholar · JSTOR ( October 2020) ( Learn how and when to remove this template message) ASME standard B18.2.1-1996 specifies hex cap screws whose size range is 0.25–3in (6.35–76.20mm) in diameter. These fasteners are very similar to hex bolts. They differ mostly in that they are manufactured to tighter tolerances than the corresponding bolts. Machinery's Handbook refers parenthetically to these fasteners as finished hex bolts. [19] Reasonably, these fasteners might be referred to as bolts, but based on the US government document Distinguishing Bolts from Screws, the US government might classify them as screws because of the tighter tolerance. [20] In 1991, responding to an influx of counterfeit fasteners, Congress passed PL 101-592, [21] the "Fastener Quality Act". As a result, the ASME B18 committee re-wrote B18.2.1, [22] renaming finished hex bolts to hex cap screw– a term that had existed in common usage long before, but was now also being codified as an official name for the ASME B18 standard. These abbreviations are not universally standardized across corporations; each corporation can coin their own. The more obscure ones may not be listed here.