Is it a Bolt or a Screw? (Nut jobs welcome)

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.

9 thoughts on “Is it a Bolt or a Screw? (Nut jobs welcome)

  1. Interesting, we were just having this conversation yesterday around the office here at Wayne. I knew the differences and some did not but you can argue that pan head screw can be used with a nut to hold together two pieces of plastic. Is it a screw or a bolt? I tend to lean more towards the mechanism driving the screw/bolt as well as the whether or not is uses a nut. BTWE, what is a Cap Head Screw?

    Steve

  2. Steve, funny how that works. 🙂 So, according to the traditional definition, a SHCS is a bolt, not a screw. But by what you and I learned, it is a screw. I guess it makes sense in a classical sense that a screw works differently than a bolt, as a screw tends to move material (as in the aformentioned Archimedes’ screw). In that sense, a fundamental function of a screw is to displace or move material through its turning action. This would also apply to lead screws. That said, maybe the common usage of the term screw is technically wrong.

  3. Nice article, I’ve come across this same topic several times…using smart part numbering systems (i know, i’ve read the rant http://www.fcsuper.com/swblog/?p=143) . I haven’t done any research into it but one of the instances this brings to mind is the difference between a shim and a washer. At one company we had part classes for both. Is it only in the use of the part, shims are between two parts used to fill a void whereas washers are used in conjunction with fasteners to distribute load or provide a locking affect? Is there a physical difference? I now I’ve used both as each other.

    Jason

  4. i tried viewing your blog through my mobile but the site layout was messed up.is your site not optimized for mobile or is my mobile the problem.i am using sony ericson xperia.

  5. Simply said
    Bolt is not fully Threaded, for it slots down a shaft and do not need full thread.
    Tqrg strengths on a bolts is stronger as it have to support strength on a smell section of the nuts treads. both nut and bolt is reliable in the small area it use to tighten the bolt

    Screw is Fully threaded up to the head. depending on the application used full length of thread is required for torque over the full length of thread. and not a short peace
    If you use a bolt to joint materials the bolts torque is to short and to strong and when torque is applied the internal hole tread will not be able to resit on such short tread and it will strip as we call it.

    Use the correct tool for the right job,

    Use the correct Application for the job.

  6. Johnny, thank you for your comments, but I do not think those distinctions are 100% factual nor are supported by other sources. For example, many bolts are fully threaded. Length determines whether a bolt is fully or partially threaded.

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