What's the Difference Between a Bolt and a Screw?

A screw, by definition, is not a bolt. A bolt is designed such that a nut (or other turning device) is required for operation. A bolt is not designed to be turned. What most people refer to as a bolt is in fact a 'cap screw', which is designed to be turned (or screwed). Cap screws may, or may not be used with nuts. The distinction is subtle, but significant in the design of the fastener. If threaded all the way to the back of the head, it becomes a 'machine screw'

A screw, by definition, is not a bolt. A bolt is designed such that a nut (or other turning device) is required for operation. A bolt is not designed to be turned. What most people refer to as a bolt is in fact a 'cap screw', which is designed to be turned (or screwed). Cap screws may, or may not be used with nuts. The distinction is subtle, but significant in the design of the fastener. If threaded all the way to the back of the head, it becomes a 'machine screw

from http://warehousemro.blogspot.com

The deference is not always clear. The following is to clarifry or confuse.

1 A bolt is usually made with a parallel thread and always used with a nut. The nut must be turned. The length, pitch, direction of the thread may change according to requirements. the pitch on the nut must match the bolt.

2 A screw is usually tapered all the way and supposed to screw into some or other material for keeping 2 or more pieces together.

3 Some screws for example self tapping screws may have a tapered and parallel portions and may even use a metal plate "nut".

4 Nuts must stay away of screws.

5 If the same bolt as described in 1 is used in a tapped hole it becomes a screw.

6 if you cut the head of a bolt and thread the back it becomes a stud. A stud seems to be a combination bolt-screw. but the user may choose the sides.

7 A bolt, nut and expanding sleeve for fixing things to concrete should not be called a bolt, because the bolt has to be turned .

8 if space prohibit the correct placement of the bolt (bolt in front and nut at the back) and the bolt has to be turned it becomes a scrolt (screw-bolt - my own word)

9 bolts with alternative or dual ways of turning (slit, star, Allen etc) are screws.

Good list sir!!!!

It is an absolutely perfectdazzling list! ;)

Anyway, as a non-english speaker, and working abroad most of the time using english as technical language, I must say that is a good thing to have clear in mind what you need. After some bad experiences, every time I have the shadow of a doubt I use to attach a picture with the main dimensions detailed.

Thanks a lot for this moment!!

A machine screw or Set Screw are really a bolt with thread all the way to the head. A bolt as such only has 1 1/2" of thread (like say cylinder head bolts on a car). The term derives from locks, where a pin (called the bolt) locks (holds) the catch into place. It then became any locking pin and hence now it is a form of fastener. Gate bolts are the bolts which hold the hinge into the post the have a domed head with a square piece under the dome. ( not the gate locking bolt). With these the hole is drilled through the post and the square part is either hammered into the post or held by a locking plate. Sometimes the hinge has square holes in which case the domed end is on the hinge with nut and washer on the other end.

frankly continue sending a picture as everyone mixes the terms up and different english speaking countries have slightly different terms.

Actually, the fastener with a square piece under the dome part of the head is called a carriage bolt.

7 A bolt, nut and expanding sleeve for fixing things to concrete should not be called a bolt, because the bolt has to be turned .

Just to futher clarify nomenclature, #7 is an anchor.

Thanks I could not remember the anchor. English is not my mother tongue.

I actually call them anchor bolts. I would not know what to expect when someone talks about an "anchor screw"

Please refer to "confuse" in first line of my post.

The difference between bolts and screws are usually obvious and differences of words used can generally be ignored as long as the intention is clear.

I usually stick to the general names used in my area. Different languages create other problems and joys.

ChefdeChocolate (post #4) has the right idea.

"7 A bolt, nut and expanding sleeve for fixing things to concrete should not be called a bolt, because the bolt has to be turned."

Turning the bolt or nut makes no nevermind as to whether it is a bolt or not. I have made many assemblies held together with nuts and bolts. Many times the bolts and nuts are both turned by hand until finger tight. and in many cases the nut was not convenient to turn with a wrench, it would be held in position with a wrench and a ratchet wrench on the head of the bolt turned till the connection was tightened to the proper torque called for. You tighten what you can get to conveniently, be it the bolt or the nut.

At one time there were "leaded cinch anchors" which consisted of a threaded metal insert, locked into the concrete by a lead sleeve compressed with a tool and hammer. Depending on size a Machine Screw or a Bolt was used to hold down the item to be anchored to the concrete.

For large machine tools and structural steel the anchor bolts are usually long 1" and larger steel with one end threaded, the other bent 90 deg. and cast in place in the concrete base. The item is set over the bolts and shimmed to height. Grout then fills the space under the machine or s.s.

A Bolt usually has a full sized body diameter under the head and the opposite threaded portion is threaded and is a bit under sized due the the thread specs, the diameter of the top of the threads being slightly smaller than the body.

Virtually any rule you can quote or conjure up can be countered by one or more exceptions.

See also Post #31 below.

Virtually any rule you can quote or conjure up can be countered by one or more exceptions.

How true, Stan. And to prove it, see below:

A Bolt usually has a full sized body diameter under the head and the opposite threaded portion is threaded and is a bit under sized due the the thread specs, the diameter of the top of the threads being slightly smaller than the body.

The exception being that cold-formed external threads must start with a body that is undersized, because forming the threads, instead of cutting them, displaces material outward, increasing the major diameter of the threaded portion.

One of our products uses a U-bolt (no one mentioned that one!) which is designed to use cut threads, however cold-formed threads are allowed as an option, decreasing the diameter of the stock used and therefore the cross-section of the un-threaded portion. This caused some problem for one of our customers who required the larger diameter for his application, so he had to change to a special order requiring the use of machined threads only, instead of our stock part which could be either.

One of our products uses a U-bolt

A "u-stud"? Does your wide say that?

"Does your wide say that?"

I will ASSume that is a typo, and not a commentary on the size of a portion of my wife's anatomy!

LOL. This was too short to spell check -- and even if I had, it still wouldn't have helped!

I like the Scrolt, If we all use this, It may end up in the dictionary!! Another one is TROBLEM: A problem that when solved ends up in trouble!!

Then what is a lag "bolt"?

Yah, that may be what I called them. See, common usage is not necessarily proper, even if that expression happens to be common usage which may actually have been a mistaken notion for me. :^)

"Then what is a lag "bolt"?"

The answer is obvious it is SLOW and therefore lags behind the other kinds.

From Post #20. "I like the Scrolt,"

Suggests: "Scrolts for Dolts who don't know (Nuts and) Bolts"

Change of "THREADS" Why when you open a can of mixed NUTS is the Brazil Nut almost always on top??

Duuno if a bolt always has to have a nut!

Cylinder head bolts on car engines don't!

I'd say if it has a hexagonal head or needs a spanner to turn it it is a bolt.

I call my dog 'blacksmith' 'cos if I kick him he makes a bolt for the door!

Other difference...we don't all love a good bolt. (sorry..but someone had to say it sooner or later!)

Del the cat,

U just couldn't resist, could ya? Well done mate, I liked it though. I had the imperrisson until now that the screw normaly refers to small size (<1/4") and bolt to the above but, after reading all the comments, it seems I was wrong.

It is good to clarify things sometime.

Hey Del,

1 stud + 2 nuts = 1 screw

A bolt is a mechanical part of a firearm that blocks the rear of the chamber while the powder burns.

In climbing, a bolt is a permanent anchor fixed into a hole drilled in the rock.

Cheers...

" Save the whales. Collect the whole set."

Or you could be like Rosalind Rolland, a senior researcher at the New England Aquarium in Boston, who answers to the siren songlike cry of "Brown Stain Ahead." She collects right whale POO! Also looks for the pretty bubble of flatulence.

Hi Stan,

Yeah, I've got a comment to that: What!

(I'm having an exitential meltdown)

-John

sorry Del-

a fastener used on your ic engine to secure the head to the cyl block is still called a hex head cap screw, unless it has an "allen head". then it's called an allen head cap screw.

Go on admit it you're not really sorry are you?

Right...... hmmmm....so....

when I was teaching my daughter some car mechanics on her manky old Cavaliar...I should have said.

'You can torque down those cylinder head hex head cap screws now!' ?

I think not old chap, I feel one would be severely mocked by those ruffians at the garage with that sort of talk!

If we are discussing semantics..then definitions change with useage, so I think I'll stick with cylinder head bolts..as I am sure this is the common usage.

Yeh, and which end of your boiled egg do you lop off?

Cheers

Del

PS (my Daughter stiil does her own maintenance even tho' that Cavalier has long since gone!)

yes, common usage does prevail in automotive world. if you go to the auto parts store you say "head bolt", if you are talking to the engineer that designs those blasted things you say HHCS.

when i was assembling a set of massive wood dryers for a local lumber mill, the parts list didn't have a single bolt listed. for attaching the buildings to the foundation, i had SS anchors. for attaching part "A" to part "B", i had SS-HHCS with size notations in both english and metric.....why they mixed the fasteners i have no idea. forced me to have to carry almost twice the number of tools on the job.

Del,

No problem. Should you be severely mocked by those ruffians at the garage, just hit 'em with your purse.

Just kidding, of course, but I couldn't resist.

Regards,

Bluestone

Lol!

This side of the pond we call it a handbag sweetie!

Del

Can one be bolted by the IRS?

No but you can be blown up by the IRA!

Screws are threaded throughout the entire length of the shaft. Bolts have an unthreaded portion located directly under the bolt head. A bolt can be used without a nut if the hole it's to fit into has a matching thread for it.

The definition:

Bolt: Horse Bolt out of the barn door - or (Bolts are heavy duty)

Screw: It is a man thing. Or When someone mess up things or (Light weight bolt)

I don't know, I've never been BOLTED.

But you sure must be one NUT is you always SCREW around.

Seriously, it's just a matter of naming convention or protocol. As long as you got what it means thats it. There are also others asking the difference between pipes and tubes.

Remember, there are no STUPID QUESTIONS, just STUPID PEOPLE.

CR4 is supposed to be a discussion forum of learned technical people, not mediocre individuals whose minds are inversely proportional to just about anything.

I rest my case.

A screw has a thread and a pointed tip. A bolt has a thread and a nut. Both are made of metal. I GUESS SO!

Bolt: 1)A roll of fabric.

2) To run quickly

Screw: 1) To have intimate relations with.

2) To get even with. As to Your Screwed!

A bolt is in defining the cylindrical part of a fastener. The bolt part of a fastener does not need to be round. Like bolt of a door lock or the square under the head of a carriage bolt. A screw is in defining the spiral shape of the threads. So if a faster is thread at all it can be considered just a screw.

If any portion of the fastener is nonthreaded it can also be considered a bolt.

A bolt is a type of screw but a screw is not necessarily a bolt. Screw implies a fastening cylinder encircled by an attached inclined plane. A bolt is a subset of screw.

Another common language expression is nut and bolt. It's improper usage to call a screw with a sharp point for self penetration a bolt. Yet we do have the expression "lag bolt" for a thick screw with a hex head!

All this time I just felt a bolt was a fastener and a screw, well.... I feel much more enlightened.

if you use a screw driver than it is "screw" ( mostly for fixing light weight applications)

if you use a spanner than it is "bolt" ( heavy applications)

So if I have a 2" socket head capscrew, and I use it (or 12 of them, rather)to fasten a 4000 lb coupler to a huge 7000 hp electric motor, does that make it a bolt, or does it make your screw theory incorrect? Can I tighten that with a screwdriver? Many answers here were correct, i just dont understand the $.02 from people who obviously dont know what they are talking about. Must be the people I work with who just like the sound of their own voice.

Common usage with the trades I work with:

Bolt: Any fastener 1/4" diameter or larger with machine thread

Screw: Any other threaded fastener

T bolts are enjoyed on a cold day while hanger bolting around ones stove bolt.

Stripper bolts are used by entertaining dancers to get as many eye bolts as possible.

Sex bolts precede finishing screws and then U bolt.

Sorry, but I had to set a screwy pitch to this thread.

"I want to no about Differentiation between bolt and screw"

After schlepping through the definitions, erroneous definitions, totally wrong, and pseudo-definitions post to this thread thus far I would hesitate to even attempt to straiten out the confusion.

"Machinery's Handbook" has 65 pages devoted to Screw Thread Systems.

There are 52 pages of Bolts, Nuts and Machine Details.

In regard to this thread I will be As Cleopatra is alleged to have said to Mark Anthony, "I'm not prone to argue."

There has been a lot of information posted on this thread by a lot of nuts! All well-meaning of course, but the problem is there is no one all-encompassing differentiation. It boils down to common, and sometimes industry-specific, usage. However, some of the information above is not quite accurate.

One poster described the differences between "hex head cap screws" and "Allen head caps screws", while both are used to describe a type of "socket head cap screws". "Allen" comes from the original manufacturer of this type of screw, now known as a hexagon "Socket Head Cap Screw". Allen, as well as others, also make the hexagonal driving tool, commonly called an "Allen key", "hex key", or "Allen Wrench" here in the US. There are also spline "Socket Head Caps Screws" which use a slightly different drive than a plain hexagon. Many people forget to use the word "socket", and use "Hex Head Caps Screw", for these same fasteners, while a true "Hex Cap Screw" is something entirely different, basically a Hex Bolt with finished threads all the way to the Cap or Head.

Also, it is not true that all screws have threads all the way (or nearly so) to the top, as many longer SHCS do not! Anything over 2 inches long will only have only 1.5 inches of thread.

The Machinery's Handbook, usually the Bible of the mechanical trades and of mechanical engineers, has this to say:

"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 which 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.)"

"An externally threaded fastener which must be assembled with a nut to perform its intended service is a bolt (Example: heavy hex structural bolt.)"

"An externally threaded fastener which must be torqued by its head into a tapped or other preformed hole to perform its intended service is a screw (Example: square head set screw.)"

The confusion on sizes of the fastener determining whether it is a bolt or screw is probably because most machines screws are available in very small size, generally only up to 3/8 in, while hex bolt size generally start at about 1/4 inch and go up from there. However, Socket Head Cap Screws can be obtained in diameters up to 2in.

Socket Head Shoulder Screws have a larger smooth diameter for some length between thread and head (unlike most machine screws where the unfinished portion is generally the same diameter or less than the thread major diameter). Yet these same fasteners are often referred to as Shoulder Bolts, possibly because they may be used with a nut in some applications.

However there are some screws never referred to as bolts, and vice versa. Have you ever heard of a Set Bolt? Or a "T" Screw? Or a Sheet Metal Bolt (even when used with "panel nuts")? But I have seen both Eye Bolts (flat end, use with a nut) and Eye Screws (with a point, makes its own threads). Lag bolts are also known as Lag Screws.

Cornfuzin', ain't it?

Check this link. Their definitions concur with the common engineering understanding that a bolt is used with a nut, a screw is not.

Colloquially, there as many definitions are there are people, as you can see from this thread.

The difference between a bolt and a screw...

A bolt seems less pleasing to the senses, ie a crossbow bolt or a mad dash.

A screw is generally an activity performed with the fairer (and more beautiful) sex.

Lets use this in a sentence:

Or maybe the morning after a new screw one might suddenly bolt for the bedroom door in alarm and fear!! Especially, after a night of copious amounts of imbibed beer!!

Sorry I could not resist either

Or the opposite:

"The lady was very happy with the beautiful bolt of cloth she had just bought, until she saw found out the price she paid was a real screw job!

Or:

"He felt pleasing bolts of electricity dancing up and down his spine with anticipation as he got into bed with his lover, who simply responded, "Go screw yourself".

I think everyone would agree that a stud is not a bolt; either fully threaded or threaded on either end with a shank between threads.

Then.....how come I can take a stud, bend a nice 180 deg. radius in the center of it and I end up with a U bolt?

"Then.....how come I can take a stud, bend a nice 180 deg. radius in the center of it and I end up with a U bolt?"

By your logic, how can you take a bolt, cut the head off it and machine a thread on the un-threaded end and you end up with a straight stud?

Hmmmmmm....

Bolt is a fastener used for fastening or assembling two or more parts by Torquing a Nut where as a screw is a fastener used for fastening or assembling two or more parts by Torquing the head of the fastener

"Bolt is a fastener used for fastening or assembling two or more parts by Torquing a Nut where as a screw is a fastener used for fastening or assembling two or more parts by Torquing the head of the fastener"

NOT SO (fast.) As mentioned in an earlier post I have assembled a lot of parts where a ratchet wrench was applied to the head of the bolt and the nut held stationary and similarly assembled many parts with machine screws where a ratchet wrench was easier to use on the nut.

'It''s not what you torque but how you torque it' or 'one size doesn't fit all!'

Definitions are usually a bit less accurate in Spanish than in English... but in English are certain words that can make you really freak out cause they have a lot of different meanings.

Nut and bolt are good examples... but what about the word "Yield"?

In Spanish we call crew the male threaded part. It is sub defined specifying the type of "head" ie: Hex, Allen, etc. On the other hand, a Nut is a female threaded part. It can be Hex, winged, etc.

The spelling.

But kidding (or seriousness) aside, the distinction is largely, if not totally, a matter of habits and folkways. In essence, one knows a bolt when one sees, or hears, the term, even though, the term "screw" can always be substituted for the term bolt...whereas the term, bolt, cannot always be substituted for the term, screw.

Starting with basics, screw is one of the basic machines, a specialized kind of inclined plane. In this sense, any fastener with helical or spiral thread is properly referred to as a screw. On the other hand, a bolt (by that name) can also have (screw) threads, but it does not, of necessity, have to have threads in order to be called a bolt...nor does it necessarily have to be rod or cone shaped...nor does it necessarily have to be turned in order to be engaged for fastening purposes. So it might be said that, apart from language customs, a bolt derives its name, not so much from any particular property (in the sense the word as a noun) but, rather, from its use (in the sense of the word as a verb). So we could say, that when fastening two things together, we are bolting them together...and if that fastener used for the action of bolting the two items together happened to be a screw (machine), then it follows that we can call the screw (machine) a bolt, and everyone will intuitively understand what we mean. Same as they would understand a bolt (action, or verb) as applied to a rod or bar which accomplished its purpose of fastening by, say, being pushed (not screwed) into a receptacle (as in, the bolt on a lockset or dead bolt lock, or rifle), or engaged by lowering or rotation (as in, bolting the barn door with a rod or beam).

So you see, the question is probably best answered by saying, that it is a question that really needs not to be asked. That said, though, the trend, for sake of uniformity (and logistical simplification), in recent years, has been towards increasingly exclusive use of the term, screw, and less use of the term, bolt--the former being more uniform and less subject to misinterpretation...and also less problematic when it comes to parts listings and specifications. As an example, the uniform standard practice in U.S. federal and military/navy parts nomenclature is to use the term, screw, exclusively...at least this is the case with all provisioning and catalog-ing documentation. What gov't/military/navy folks might call these items when tangibly held in their hand would of course be a matter of personal habit (and peer pressure or ridicule)...except during supervised training...when one goes by the manual or else..and the manual says, screw!

I am relying on an explanation described by Machinery's Handbook, and my memory of what was explained there. A screw does not require an external device (such as a nut) for securement, whereas a bolt is an ASSEMBLY of a screw and an external fastening device such as a nut.

Hence, a screw assembled with a nut becomes a bolt.

But we all know that many of us (even me) describe anything smaller than 1/2" diameter as a screw, and anything larger as a bolt. Why? Becuase of the people with who we associated early in our careers.

And with that, I am bolting to the next topic of interest.

Regards,

Ing. Robert Forbus

An inmate of a lunatic asylum escaped after sexually assaulting a nurse.

The headlines in the local newspaper (edited by a retired engineer) was

"NUT SCREWS & BOLTS"

Excellent venkat! Ha, ha, ha (LOL)

John

Yet more on this subject:

I was looking for the translation from metric 8.8 grade to SAE grade (5, it turns out) and came across this definition coincidentally. This is the ISO definition, apparently, and Euler makes a good case for sticking with it. However, per this definition, a set screw would be in no-man's land, because it has no head; however it more closely meets the definition for bolt than screw. Set bolt?

Hi Ken,

That link's a pretty good writeup. Euler sounds very adamant in his bolt/screw verbosity. He says: "Bolts come with almost every imaginable head".

I guess "set bolt" would indeed be the proper term as Euler sees it, since he does expound quite profusely that this or that "does not affect the above definition [his] in any way".

Maybe a "set bolt" is just like any other headed bolt but the head is simply the same size as the shank (works for me), but I'm sure the next time I need to purchase one I'll just ask for a set screw like everyone else. Can you imagine trying to explain to the salesperson that you need a set bolt? "Well, lets see, I think you go over there to aisle 12 and look in that drawer that has all the dust on it. By the way, whatcha go do with that thing anyway?

I thought the Brits always called "bolts" machine screws. Kris, where are you when we need you?

Well ! I can tell you what I know or was told, When going to school to be a mechanic, that question was brought up. A bolt had treads on one end and had a squared head, like that goes in the old wood burning stove. As what you call a bolt it really is a cap srew, but when a nut is applied on the other end it is a bolt, General a screw to my way of thinking is anything smaller than 1/4, now anything smaller than that is measure by wire gage 12-24, 10-24, 10-32, and so on.And this is for general info, cap screws are bolt as you would call them are measured from the bottom of the head, that give you the length and then the diameter, and tread size. I am 67 and been doing Automatic transmission for 45 years and other things pertaining auto are other mechanical things, like hydraulic's and a good bit of electorons, from bumber to bumber and all in between. Hope this helps.

Anymore...bolt is what I do when hearing or seeing nuts attempting to clarify what screw means.

A bolt passes through the joining materials and uses a mating threaded device (typically a nut) to provide a clamp force between the joining materials (using washers to distribute the load and even the clamping area). A screw fastens directly in to one of the joining materials by way of a threaded hole or by self tapping such as a wood screw. There are some grey areas such as when a nutplate or weld nut is used. Technically the fastener would be called a screw because the weld nut or nutplate is seen as the "threaded hole" but I can see it perfectly acceptable to call the fastener a bolt in this case as well because technically the fastener is designed to be used as a bolt by the original definition. Hope that clarifies things.

Bolts need not be screw threaded, nor do they need to interface with threaded counterparts such as nuts (or threaded receivers of any kind). As a matter of convenience and consistency is has become standard to refer to (colloquially-called) bolts with threads as screws...and nothing is lost (other than confusion and ambiguity) in such usage because, when you think about it, a threaded "bolt" essentially works and performs the same function as any threaded screw (with or without and engaging counterpart). So if one wants to think about "bolt" in colloquial terms, perhaps its best to consider a (threaded) bolt as a special case of bolts (retaining rods/bars/...members) in general; such that a bolt with threads, apart from being rightly & properly called a screw, could also be referred to as a "screw bolt" or "screw-type bolt."

A major thrust of the above-stated being....

...there is a point at which discussion of bolts versus screws becomes pointless. That point has arrived more than once in CR4.

1. A SCREW HAS FULL THREADS ALWAYS, WHERE AS A BOLT DONT HAVE FULL THREADS.

2. A BOLT USUALLY HOLD THE MATERIALS AND COMPRESS IT, WHERE AS THE SCREW IS HOLDED BY THE MATERIAL ITSELF.

give exact answer