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Home > News > What is the screw

What is the screw

2015-10-23
product-c4-da-4f-51188da7d2237528ba250feb61A screw, or bolt, is a type of fastener, typically made of metal, and characterized by a helical ridge, known as a male thread (external thread) or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as a female thread (internal thread), often in the form of a nut or an object that has the internal thread formed into it. 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.

A screw will almost always have a head on one end which contains a specially formed shape that allows it to be turned, or driven, with a tool. Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. There are exceptions; for instance, carriage Bolts have a domed head that is not designed to be driven; set screws often have a head smaller than the outer diameter of the screw; J-bolts have a J-shaped head which is not designed to be driven, but rather is usually sunk into concrete allowing it to be used as an anchor bolt. The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded.[1] The distance between each thread is called the "pitch".

The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread; a common mnemonic device for remembering this when working with screws or bolts is "righty-tighty, lefty-loosey." Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to counterclockwise torque (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. The left side pedal of a bicycle has a left-hand thread.

More generally, screw may mean any helical device, such as a clamp, a micrometer, a ship's propeller or an Archimedes' screw water pump.


Differentiation between bolt and screw
A carriage bolt with square nut
A structural bolt with a hex nut and washer.

There is no universally accepted distinction between a screw and a bolt. A simple distinction that is often true, although not always, is that a bolt passes through a substrate and takes a nut on the other side, whereas a screw takes no nut because it threads directly into the substrate. Machinery's Handbook describes the distinction as follows:

A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut. A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head. An externally threaded fastener which is prevented from being turned during assembly and which can be tightened or released only by torquing a nut is a bolt. (Example: Round Head Bolts, Track Bolts , plow bolts.) An externally threaded fastener that has thread form which prohibits assembly with a nut having a straight thread of multiple pitch length is a screw. (Example: Wood Screws , Tapping Screws .)[2]

This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw[3][4] and bolt.[5][6][7]

The issue of what is a screw and what is a bolt is not completely resolved with Machinery's Handbook distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction, and usage variations.[8][not in citation given] Some of these issues are discussed below:
Machine Screws

ASME standards specify a variety of "Machine Screws"[9] in diameters ranging up to 0.75 in (19.05 mm). These fasteners are often used with Nuts as well as driven into tapped holes. They might be considered a screw or a bolt based on the Machinery's Handbook distinction. In practice, they tend to be mostly available in smaller sizes and the smaller sizes are referred to as screws or less ambiguously as machine screws, although some kinds of machine screw can be referred to as stove bolts.
Hex cap screws

ASME standard B18.2.1-1996 specifies Hex Cap Screws that range in size from 0.25–3 in (6.35–76.20 mm) 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".[10] 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.[11] In 1991 responding to an influx of counterfeit fasteners Congress passed PL 101-592[12] "Fastener Quality Act" This resulted in the rewriting of specifications by the ASME B18 committee. B18.2.1[13] was re-written and as a result they eliminated the "Finished Hex Bolts" and renamed them the "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.
Lug bolts and head bolts

These terms refer to fasteners that are designed to be threaded into a tapped hole that is in part of the assembly and so based on the Machinery's Handbook distinction they would be screws. Here common terms are at variance with Machinery's Handbook distinction.[14][15]
Lag screw
Lag screws, also called lag bolts
A side view

Lag screws (also called lag bolts, although this is a misnomer) are basically large wood screws. Square-headed lag screws and hex-headed lag screws are covered by ASME B18.2.1 standards, and the head is typically an external hex. A typical lag bolt can range in diameter from 1⁄4 in (6.35 mm) to 1 1⁄4 in (31.75 mm), and lengths from 1⁄4 to 6 in (6.35 to 152.40 mm) or longer, with the coarse threads of a wood-screw or sheet-metal-screw threadform (but larger).

The materials are usually carbon steel substrate with a coating of zinc galvanization (for corrosion resistance). The zinc coating may be bright (electroplated), yellow (electroplated), or dull gray hot-dip galvanized. Lag bolts are used to lag together lumber framing, to lag machinery feet to wood floors, and for other heavy carpentry applications. The adjective lag came from an early principal use of such fasteners: the fastening of lags such as barrel staves and other similar parts.[16]

These fasteners are "screws" according to the Machinery's Handbook criteria, and the obsolescent term "lag bolt" has been replaced by "lag screw" in the Handbook.[17] However, in the minds of many tradesmen, they are "bolts", simply because they are large, with hex or square heads. In the United Kingdom and Australia, lag screws are known as coach screws.
US government standards

The US government made an effort to formalize the difference between a bolt and a screw because different tariffs apply to each.[18] The document seems to have no significant effect on common usage and does not eliminate the ambiguous nature of the distinction between screws and bolts for some threaded fasteners. The document also reflects (although it probably did not originate) significant confusion of terminology usage that differs between the legal/statutory/regulatory community and the fastener industry. The legal/statutory/regulatory wording uses the terms "coarse" and "fine" to refer to the tightness of the tolerance range, referring basically to "high-quality" or "low-quality", but this is a poor choice of terms, because those terms in the fastener industry have a different meaning (referring to the steepness of the helix's lead).
Historical issue

Old USS and SAE standards defined cap screws as fasteners with shanks that were threaded to the head and bolts as fasteners with shanks that were partially unthreaded.[19] The relationship of this rule to the idea that a bolt by definition takes a nut is clear (because the unthreaded section of the shank, which is called the grip, was expected to pass through the substrate without threading into it). This is now an obsolete distinction.
Controlled vocabulary versus natural language

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 themselves 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.

The word bolt is also a very old word, and it was used for centuries to refer to metal rods that passed through the substrate to be fastened on the other side, often via nonthreaded means (clinching, forge welding, pinning, wedging, etc.). The connection of this sense to the sense of a door bolt or the crossbow bolt is apparent. In the 19th century, bolts fastened via screw threads were often called screw bolts in contradistinction to clench bolts.

In common usage, the distinction (not rigorous) is often that screws are smaller than bolts, and that screws are generally tapered while bolts are not. For example, cylinder head bolts are called "bolts" (at least in North American usage) despite the fact that by some definitions they ought to be called "screws". Their size and their similarity to a bolt that would take a nut seem linguistically to overrule any other factors in this natural word choice proclivity.
Other distinctions

Bolts have been defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as ISO metric screw thread M, MJ, Unified Thread Standard UN, UNR, and UNJ) such that they can accept a non-tapered nut. Screws are then defined as headed, externally threaded fasteners that do not meet the above definition of bolts.[citation needed] These definitions of screw and bolt eliminate the ambiguity of the Machinery's handbook distinction. And it is for that reason, perhaps, that some people favor them. However, they are neither compliant with common usage of the two words nor are they compliant with formal specifications.

A possible distinction is that a screw is designed to cut its own thread; it has no need for access from or exposure to the opposite side of the component being fastened to. This definition of screw is further reinforced by the consideration of the developments of fasteners such as Tek Screws for roof cladding, self-drilling and self-tapping screws for various metal fastening applications, roof batten screws to reinforce the connection between the roof batten and the rafter, decking screws etc. On the other hand, a bolt is the male part of a fastener system designed to be accepted by a pre-equipped socket (or nut) of exactly the same thread design.

https://en.wikipedia.org/wiki/Screw

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