Understanding Bolts and Screws
Is it a bolt or is it a screw? A simple question you might think, but one that generates substantial debate. There's actually a very small distinction between the two, so much so that sometimes a screw is called a bolt and vice versa.
In researching this controversial subject, we discovered a number of interesting viewpoints regarding the bolt/screw difference…
The American Heritage® Dictionary of the English Language, Fourth Edition
BOLT – A fastener consisting of a threaded pin or rod with a head at one end, designed to be inserted through holes in assembled parts and secured by a mated nut that is tightened by applying torque.
SCREW – A metal pin with incised threads and a broad slotted head that can be driven as a fastener by turning with a screwdriver, especially: a. A tapered and pointed wood screw. b. A cylindrical and flat-tipped machine screw.
Merriam-Webster Online Dictionary
BOLT – A metal rod or pin for fastening objects together that usually has a head at one end and a screw thread at the other and is secured by a nut.
SCREW – A nail-shaped or rod-shaped piece with a spiral groove and a slotted or recessed head designed to be inserted into material by rotating (as with a screwdriver ) and used for fastening pieces of solid material together.
Webster's Revised Unabridged Dictionary
BOLT – A strong pin, of iron or other material, used to fasten or hold something in place, often having a head at one end and screw thread cut upon the other end.
SCREW – Specifically, a kind of nail with a spiral thread and a head with a nick to receive the end of the screwdriver. Screws are much used to hold together pieces of wood or to fasten something. Webster's Revised Unabridged Dictionary
BOLT – A screw that screws into a nut to form a fastener.
SCREW – A fastener with a tapered threaded shank and a slotted head.
World Book Online Reference Center. 2004. World Book, Inc. 2 Oct. 2004. Bianchina, Paul. "Bolt." Lewis, W. David. "Screw."
BOLT is a type of fastener formed of a metal rod that has an enlarged head at one end and a screw thread at the other. Bolts may be screwed directly into a threaded hole in the part they are to hold, or they may be held in place by nuts. A nut is a block of metal with a hole in the center. The hole is threaded to match the threads on the bolt.
SCREW is an inclined plane wrapped in a spiral around a shaft. The screw is one of the six simple machines developed in ancient times. The other five are the lever, the wheel and axle, the pulley, the inclined plane, and the wedge.
A screw consists of two main parts—the body and the thread. The body of a screw may be a cone or a cylinder. The center line of the body is called the axis. The thread is the inclined plane that sticks out from the body. The distance between two adjacent (neighboring) crests of the thread is known as the pitch.
Uses. Screws have many practical applications, especially as fasteners. The most common are the wood screw and machine screw. When rotated, such screws can be made to move into, or out of, an object. As the screw rotates one full turn, it travels a distance equal to its pitch. Most wood screws and machine screws have a slotted or recessed head into which a screwdriver is placed in order to turn the screw. Screws come in various sizes and shapes. They are made of steel, copper, aluminum, and other metals that are easy to form.
AccessScience@McGraw-Hill. Charles Birnstiel, "Bolt", last modified: September 19, 2001. Frank H. Rockett, "Screw fastener", last modified: July 15, 2002.
BOLT – A cylindrical fastener with an integral head on one end and an external screw thread on the other end designed to be inserted through holes in assembled parts and to mate with an internally threaded block, called a nut, which is turned to tighten or loosen the bolt. Tensioning the fastener by turning the nut differentiates a bolt from a screw, which is tightened by turning its head.
SCREW FASTENER – A threaded machine part used to join parts of a machine or structure. Screw fasteners are used when a connection that can be disassembled and reconnected and that must resist tension and shear is required. A nut and bolt is a common screw fastener.
Some very specific guidance comes from three respected sources, which are in agreement…
Millwrights and Mechanics Guide 4th Edition, Carl A. Nelson, © 1989 Macmillan Publishing Company
The bolt is described as an externally threaded fastener designed for insertion through holes in assembled parts. It is normally tightened and released by turning a mating nut. A screw differs from a bolt in that it is supposed to mate with a[n] internal thread into which it is tightened or released by turning its head.
Machinery's Handbook 26th Edition, © 2000 by Industrial Press Inc.
Differentiation between Bolt and Screw. – 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 a thread form which prohibits assembly with a nut having a straight thread of multiple pitch length is a screw. (Example: wood screws, tapping screws.)
Machinery's Handbook 26th Edition, © 2000 by Industrial Press Inc.
2.1 Bolt. 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.
2.2 Screw. 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.
3. Explanatory Data. A bolt is designed for assembly with a nut. A screw has features in its design which makes it capable of being used in a tapped or other preformed hole in the work. Because of basic design, it is possible to use certain types of screws in combination with a nut. Any externally threaded fastener which has a majority of the design characteristics which assist its proper use in a tapped or other preformed hole is a screw, regardless of how it is used in its service application.
The U.S. Customs and Border Protection issued An Informed Compliance Publication, March 2006, What Every Member of the Trade Community Should Know About: Distinguishing Bolts from Screws, which states…
U.S. Customs and Border Protection uses fastener industry standards to distinguish bolts from screws. When a fastener is described in a fastener industry dimensional standard as either a screw or a bolt, we follow that standard. When we have no dimensional standard, we go to "Specification for Identification of Bolts and Screws," ANSI - ASME B18.2.1 1981 (the "specification"). In Rocknel Fastener, Inc. v. United States, Slip Op. 00-112 (Ct. Int'l Trade, decided August 29, 2000), the court sanctioned ANSI/ASME Standard B18.2.1 as "provid[ing] a well-recognized, comprehensive basis for the common and commercial meaning of bolt and screw as understood by the fastener industry in the United States."
Since the United States Court of International Trade recognizes ANSI - ASME B18.2.1 1981 as "provid[ing] a well-recognized, comprehensive basis for the common and commercial meaning of bolt and screw as understood by the fastener industry in the United States," we can describe bolts and screws thusly…
A bolt is a fastener that has external threads (a nut, for example, has internal threads), is inserted into an unthreaded hole of one or more parts, and is tightened or loosened by turning its nut. Carriage bolts, elevator bolts and plow bolts, for example, are bolts because you can't turn their heads to tighten or loosen them.
A screw has external threads just like a bolt. But a screw is inserted into a threaded or unthreaded hole, and is tightened or loosened by turning its head. It may or may not be used with a nut. Wood screws and sheet metal screws make their own threads in the hole, so they are not used with a nut. Machine screws and cap screws, for instance, thread into threaded holes or are used with a nut. In either case, though, you tighten or loosen the screw by turning its head.
With regard to Hex Bolts and Hex Cap Screws it is commonly accepted that it's a Hex Cap Screw if it has a washer face bearing surface and if it doesn't it's a Hex Bolt. That belief is supported by ASME Standard B18.2.1.
Naturally there are exceptions to every rule as the Millwrights and Mechanics Guide 4th Edition observes: "These definitions obviously do not always apply, since bolts can be screwed into threaded holes and screws can be used with a nut." And several other examples come to mind: A toggle bolt consists of toggle wings, which is a special spring-loaded collapsible nut, and a screw—you always tighten the screw, but the assembly is called a toggle bolt. Stove bolts are machine screws supplied with machine screw hex nuts. Lag screws are sometimes called lag bolts. And fully-threaded cap screws are also called tap bolts.
Most of the fasteners at Fastener Mart follow the bolt/screw rule. But even if they don't, we've worked hard to make it easy for you to find what you're looking for.
And of course now that the United States Court of International Trade has ruled, we suppose the question is: Has the bolt/screw debate finally been put to rest?