The term bolt in plain English has many definitions as applied to the Engineering Principle. It can be the movable rod that slides into a socket to fasten a door. It is the portion of a lock that moves from and back to the case. It can be fastening rods, pins or screws, usually threaded to receive a nut.1 Other mechanical items also carry this name, but the examples here are likely the most common. The common factor between each of these definitions is that there is an object (often rod-like) that is inserted into something else, often for the purpose of some sort of fastening. Given this broad definition, the question might be asked, when is a bolt called a screw? Not so fast!
Much like bolt, screw has many definitions in plain English. As applied to the Engineering Principle, a screw can be a fastener with a tapered shank and helical thread. It is also a threaded cylindrical rod that engages a threaded hole, and used to fasten in some fashion.2 (Of course, there’s the famous Archimedes’ screw, which fastens nothing, but sure moves a lot of water uphill.) From the plain English definitions, one might say that ultimately all screws are bolts except for one strange caveat. Bolt, as a fastening rod, is made to receive a nut. From the definition for screw, it appears that screws are made to be driven into a threaded hole. But doesn’t a nut usually have a threaded hole? So where is the distinction?
As strange as it may seem, the distinction may simply be where the threaded fastener is torqued. It might be said a screw is normally torqued via its head, and a bolt is normally torqued via the applied nut. In this case, the decision whether to call the threaded fastener a bolt or a screw is based on how the fastener will normally be applied. This is the logical conclusion if one takes the plain English definitions at face value, and willfully ignores the fact that a having a nut does not magically turn a screw into a bolt, and not having a nut does not magically turn a bolt into a screw.
So, this leads into researching the topic further, having to go back to traditional applications within the Engineering Principle for these terms. The following is oft quoted:
Bolts are defined as headed fasteners having external threads that meet an exacting, uniform bolt thread specification (such as M, MJ, UN, UNR, and UNJ) such that they can accept a nontapered nut. Screws are defined as headed, externally-threaded fasteners that do not meet the above definition of bolts.
I will state that I’ve seen this quoted several times, but cannot find an attributable source. That aside, traditionally a bolt meets a particular uniform specification so that it can receive a nontapered standard nut. Screws are everything else (such as tapered screws that form their own thread during initial insertion). This would suggest that the terms bolt and screw are not interchangeable. In fact, one is not a subset of the other. It would also suggest that there is a major misuse of the term screw since almost everything labelled as a screw is really a bolt, according the above definition.
Looking for some formal definition might be of help here. Believe it or not, the U.S. Government has made an attempt at such in a document called What Every Member of the Trade Community Should Know About: Distinguishing Bolts from Screws.3 This document references ASME B18.2.1 1981 and Fastener Standards, 6th Edition as sources. I do not believe either of these standards are current, even though this government document is dated January 2008. The document authoritatively (note the sarcasm) goes on to define bolt and screw as if these standards provide a clear guidance regarding the matter of definitions.
Bolt – A bolt is an externally threaded fastener designed for insertion through the holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.
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.
You know what, those definitions do not seem all that unreasonable. Of course, the U.S. Government, being what it is, needs a 21 page document to make these two statements. (It makes comments on everything from the Internet to a plead for small businesses to rate agency responsiveness to small business needs.)
However, English is one of those funny languages where definition of words is not by decree, but rather by use. (I say this sarcastically since almost every language, except for a few like French and perhaps German, works in this way. Ironically, to the best of my limited knowledge about them, neither French nor German have separate words for bolt and screw, in this context.) How do many people use these terms? This is not a democracy. The majority has a say in this, but not exclusively. Definitions are added simply by many people using a word in a particular way (majority or not). So, the question points back to each of us. How have you used these terms? Is there a distinction, or are these synonyms?
In practice, when applied to threaded fasteners, my use of these terms may be simply this; a bolt is fastened with the use of a generic wrench; a screw is fastened with the use of some sort of dedicated driver, such as screw driver, hex head driver, Torx Plus driver, or torque driver. Ironically, even these basic definitions also have many exceptions, so even these are not universal. They certainly contradict the traditional definitions. They also do not provide any mechanically significant functional distinction. So, even though they may be commonly used, they do not provide any usefulness when classifying a threaded fastener.
As far as I can tell, there is no consensus on this issue. Whether a person calls a particular threaded fastener by the term bolt or screw seems to be fairly arbitrary these days. It is based more on personal preference, rather than any formal definition.
