So What's the Difference Between a Bolt & a Screw?

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cribbj

"Supra" Moderator
Staff member
Well, I finally found a concise answer to this burning question that comes up every now & again and keeps us all awake at night:

From ASME standard B18.2.1 (1996):

"A bolt is designed to be inserted through a hole and secured with a nut, while a screw is designed to be used in a threaded hole—sometimes along with a nut."
 
So I guess the HOLE determine if the "Insert" be a Bolt or a Screw. An example would be the STARTER SCREW because the block is threaded to house the SCREW. I cant think of too many BOLT in our engine.

Great find John.
 
And I always thought one of the distinguishing characteristics of a screw was that it had to be fully threaded, while a bolt "could be" partially or fully threaded.

I also thought, as Bushwhacker did, that a screw cut its own threads, but that theory went out the window when I discovered things like "machine screws" that looked just like "bolts".

Now we know the difference by definition, and I'm sure we'll all sleep better tonite. Except for the next burning question: How can you TELL a screw from a bolt? :D
 
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When I go to Home Depot, if the bag is labeled as screw, then that's screw. If the bag is labeled as bolt, then that's bolt. LOL. Actually, I think manufacturers may even make mistakes labeling them as well.

To me, bolt and screw both have threads. The difference is bolt has a flat end where it can go in a nuts. Screw has a sharp pointy end that can't go in a nut; however, it can cut its own thread into the material.

As John mentioned, machine screw can work like bolt as well. I found on Wikipedia something about screw & bolt:

" There is no universally accepted distinction between a screw and a bolt. The 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.)"

http://en.wikipedia.org/wiki/Screw
 
The determination is not the male portion rather the female portion. Interesting.

FEMALE HOLE THREADED = SCREW
FEMALE HOLE UNTHREADED = BOLT
 
I always thought of it as.
A screw has an internal driven head, e.g. philips, flat, torx, spline drive.
and a bolt has an externally driven head, e.g. hex head.
But then Allen drives don't fit, because I would consider them a bolt.
 
Ah... shall we delve deeper?

What we know as a "screw" and a "bolt" are actually complex objects utilizing a "simple" machine.

Starting with an incline plane (very simple) both wrap or spiral this inclined plane around a round "rod" to create "threads" .

So, what we commonly call "threads" are in fact an inclined plane wrapped around a "round" object that can transmit... our favorite force... torque.

A screw is actually a round rod with "threads" designed to utilize its inclined plane as a means to secure itself in place by means of "cutting" a complementing inclined plane in a maleable material. It may or may not be used to secure more than one object to another. If it holds two or more objects together it becomes a "screw bolt".

A "bolt" (as we commonly refer) is a cylinder utilizing a spiraled incline plane designed to interact with a complementing inclined plane (bolt or other previously threaded material). It is not designed to "cut" its receiving "threads" but rather utilize previously machined threads in another object. As rotating force (torque) is applied it is designed to secure itself into said complementing inclined plane creating clamping force. This clamping force is what is known as "bolting".

So a viable definition is a screw creates its own receiving "threads" while a bolt utilizes previously created receiving threads.

A bolt HOLDS two or more objects together utilizing a preformed receiving matching set of threads while a screw SECURES ITSELF into "virgin" material cutting its own threads in the process.
 
Ah... shall we delve deeper?

What we know as a "screw" and a "bolt" are actually complex objects utilizing a "simple" machine.

Starting with an incline plane (very simple) both wrap or spiral this inclined plane around a round "rod" to create "threads" .

So, what we commonly call "threads" are in fact an inclined plane wrapped around a "round" object that can transmit... our favorite force... torque.

A screw is actually a round rod with "threads" designed to utilize its inclined plane as a means to secure itself in place by means of "cutting" a complementing inclined plane in a maleable material. It may or may not be used to secure more than one object to another. If it holds two or more objects together it becomes a "screw bolt".

A "bolt" (as we commonly refer) is a cylinder utilizing a spiraled incline plane designed to interact with a complementing inclined plane (bolt or other previously threaded material). It is not designed to "cut" its receiving "threads" but rather utilize previously machined threads in another object. As rotating force (torque) is applied it is designed to secure itself into said complementing inclined plane creating clamping force. This clamping force is what is known as "bolting".

So a viable definition is a screw creates its own receiving "threads" while a bolt utilizes previously created receiving threads.

A bolt HOLDS two or more objects together utilizing a preformed receiving matching set of threads while a screw SECURES ITSELF into "virgin" material cutting its own threads in the process.

both ansi and iso organizations should adopt your definition. It's easily the best I've seen. Thank you!
 
and let me heap another load on the fire
source- http://euler9.tripod.com/bolt-database/boltdef.html

The correct definition of bolt and screw is as follows.

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.

Because various organizations have thoroughly confused everyone regarding this simple definition, the remainder of this article dispels several myths and discusses the full rationale explaining the above, correct definition.

(1) Several dictionaries, Machinery's Handbook, part specification sheets, military specifications, bolt manufacturers, and vendor catalogs are wrong and have botched the above, unambiguous definition. Their definitions of bolt versus screw are arbitrary, random, ambiguous, nondefinitive, and do not align with common sense. Having "credible" sources such as Machinery's Handbook, ASME, ISO, and military specification sheets misusing and arbitrarily misdefining the words throws the whole world off. Then eventually most dictionary authors follow suit and copy some arbitrary version of the incorrect definitions floating about. All of these incorrect definitions and misuses, no matter how credible the source may otherwise seem, should be completely ignored. It should be apparent to you that technical definitions that make no sense are not credible.

(2) Another major confusion factor is the fact that bolt threads are sometimes generically referred to as "screw" threads in specifications, even though they are actually bolt threads, in an attempt to generically refer to the threads themselves, whether internal or external and regardless of which part they exist in. Words can have more than one definition, and this particular usage of the word "screw" is an attempt to describe helical threads simply in reference to the act of screwing. Using the word "screw" when "screwing" is meant does not cause a bolt, whose threads meet the specification of bolt threads, to be suddenly metamorphosed into a screw. If the threads meet the specification of bolt threads, such that they can accept a nut (regardless of whether you install a nut or not), then the threads themselves would more aptly be called "bolt threads" in the specifications, instead of "screw threads," to dispel confusion, as the nut is no more a screw, by the strict definition, than a bolt is a screw.

(3) Let us get the record corrected with the previously-stated, correct definition, which stands unaffected by several other factors. And let us now, by stating the correct facts, dispel several myths that have no affect upon the above definition.

The extent to which the shank of a fastener is threaded, whether fully or partially, does not affect the above definition in any way. Bolts can come fully or partially threaded, as do screws.

Whether or not a nut is installed does not affect the above definition in any way. Common sense tells you a bolt is not suddenly metamorphosed into a screw each time you extract the nut (if you chose to install a nut instead of using the bolt in a threaded hole, insert, or nutplate); nor is it suddenly transformed into a screw each time you choose to not use the bolt but just let it sit there on a table unused.

Bolts are untapered. Screws are often tapered but can also be untapered.

Screws always cut their own internal threads when initially installed, as there is generally no tool meeting the arbitrary specification of their threads to tap out the internal threads beforehand. Conversely, however, it is possible for a bolt to be self-tapping. The only criterion in regard to the bolt versus screw definition is whether or not the self-tapping fastener, non-cutting threads meet the strict specification of bolt threads, meaning they can be correctly mated with a nut.

The type or size of head on a fastener does not affect the above definition in any way. Bolts come with almost every imaginable head; screws do also, including hexagonal. Likewise, the configuration of the driving (or holding) tool surfaces in the head, whether internal or external surfaces, does not affect the above definition in any way.

The fastener nominal diameter does not affect the above definition in any way. Bolts do not suddenly and mysteriously no longer accept nuts just because they become small, miniature, or micro.

The term "machine screw" is a misnomer. A bolt, clearly having bolt threads, is not suddenly metamorphosed into a screw just because someone arbitrarily misnamed it in a specification, book, organization, or industry.

Whether or not specifications incorrectly or loosely refer to bolt and nut threads as "screw threads," or even erroneously refer to bolts as "screws," does not suddenly transform the bolt into a screw. The inability of the technician who drafted the specification to master or understand language, grammar, measurement systems, coherent, unambiguous, internationally-standard units of measure, correct mathematical expressions, etc., does not mean the coherent engineers are suddenly thrown into an abyss without coherent definitions. Intelligent engineers must be able to sort out the technical facts from among the fiction and typos. Whenever someone has botched definitions, world class organizations such as ISO and IEEE need to step up to the plate and redirect the incoherent, aimless, arbitrary, lower-level entities. All of these incorrect definitions and misuses of the word "screw" should be completely ignored, as there is no need to continue to copy past mistakes and propagate confusion. The previously-stated, correct definition should be used in new specifications, standards, and publications.

Return to Structural Analysis Reference Library. © 2002 Garrett D. Euler
 
LOL, I will never look at a bolt or screw in the same way again :D

Given that theory, would a bolt that is cross threaded suddenly be transformed into a screw, as it is cutting its own thread?
 
Bolt, threads, screw. So where does a stud fit in here ??? Lol..
I always thought small bolts as screws.. Under 10mm ??
With out a nut... Usually due to me dropping and losing them !!!
 


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