Bolt or screw?

[File HandleParticipant @filehandle]

In my collection of many fittings and fixings is a small bag labeled by the supplier as "Hexagonal screws and nuts 10 UNF".
Because they have nuts, should they be called bolts? Or are they screws because they are fully threaded.
It makes sense that a bolt only needs to be threaded for the length that will have a nut on plus a bit more, as it will be a better fit in the holes of the 2 parts that it holds together. But the same could be said if two parts are joined by a screw going into the second part.
i.e. is it a bolt if it screws into a nut and a screw if it screws into something other than a nut? might be controversial. Or doesn't it matter which term is used?
Secondary why are small sized given a number rather than a dimension? Did it just follow on from this being the case with BA?

[File HandleParticipant @filehandle]

Definition?

[Martin ConnellyParticipant @martinconnelly55370]

Bolts with a metric thread, Whitworth, Unified, Ba etc. are screws, machine screws as opposed to wood screws, self tapping screws or other specified types of screw. So some screws are bolts, but not all of them

A coach bolt is a bolt that may, or may not, have a machine screw thread. So not all bolts are machine screws.

Sometime a bolt is considered to be a machine screw with a section of plain shank, but not always.

Are you still confused? It's the mongrel nature of English vocabulary that means such words are used with poor definition.

Wire gauges have increasing numbers for smaller sizes. I have read that this reflected the number of time the wire was drawn to reduce its size, might be true. If you then make small screws from wires with these sizes then it could be argued that that is why small screws follow a similar convention to wire gauges.

BA threads ore different though. The basic size 0BA is a metric thread of Ø6mm and pitch of 1mm. Each size smaller reduces the diameter by a factor of 0.881 and the pitch by a factor of 0.9. So if you want to calculate the size of any BA thread of size n then you can use 0.9^n for pitch and 6*0.881^n for diameter (rounded to 2 decimal places).

Martin C

Edited By Martin Connelly on 26/09/2023 18:32:27

[Bill Davies 2Participant @billdavies2]

Long ago I was taught bolts had a plain section, which importantly gave support to a hole, whereas a screw holds parts together. But with multiple anglophone countries and their definitions, anything goes.

Bill

[Chris Pearson 1Participant @chrispearson1]

There are many sorts of bolts, such as a projectile fired from a crossbow, part of a firearm, a door catch; but this is what OED says about our sort:

"A stout metal pin with a head, used for holding things fast together. It may be permanently fixed, secured by riveting or by a nut, as the bolts of a ship; or movable, passing through a hole, as the bolts of a shutter."

A screw has a helical thread, but they are not necessarily for holding things together. You probably have one on the front of your lathe.

[JasonBModerator @jasonb]

As Bill says I term a bolt as having some plain shank under the head, a screw is threaded for it's full length.

UN threads below 1/4" use the number system as the increments become smaller than fractional equivelents

[VicParticipant @vic]

Posted by JasonB on 26/09/2023 18:23:50:

A screw is threaded for it's full length.

Except when it isn’t

[Robert Atkinson 2Participant @robertatkinson2]

I'm firmly in the fully threaded = screw, plain shank = bolt. For load bearing applications screws should only be in tension. Bolts can be in shear, tension or both.

There are always exceptions e.g. cap head screws but even the plain shank versions of those are normally used in tension.

You can replace a screw with a bolt but should never replace a bolt with a screw.

Robert

[noel shelleyParticipant @noelshelley55608]

Bolts or setscrews ! You know the rest ! Noel.

[Chris Pearson 1Participant @chrispearson1]

Posted by Robert Atkinson 2 on 26/09/2023 19:30:16:

I'm firmly in the fully threaded = screw, plain shank = bolt. For load bearing applications screws should only be in tension. Bolts can be in shear, tension or both.

I agree with the first point, but as I said above, some bolts are screws and some screws are bolts.

My understanding is that if, for example, two plates are bolted together, the tension in the bolts clamps the plates such that the friction between them exceeds any shearing force. The shanks of the bolts themselves should not be subjected to a shearing force.

[BazyleParticipant @bazyle]

As well as not trusting things written on the internet as definitive one should be especially beware of adverts which were probably prepared by graphic artists not engineers.

[Mark RandParticipant @markrand96270]

Posted by noel shelley on 26/09/2023 19:31:41:

Bolts or setscrews ! You know the rest ! Noel.

Note that many colonials incorrectly refer to grub screws as set screws…

But yes, it it's got a (parallel) shank that's the same or larger size than the thread, for shear loading and location, it's a bolt. Including woodspoiling coach bolts.

Edited By Mark Rand on 26/09/2023 20:51:46

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bazyle on 26/09/2023 20:31:26:

As well as not trusting things written on the internet as definitive one should be especially beware of adverts which were probably prepared by graphic artists not engineers.

.

However … I think it’s true that cap-head screw is a very widely accepted term for such items.

Either that or socket head cap screw

https://nationalstocknumber.org/nsn/5305-01-295-8163

MichaelG.

Edited By Michael Gilligan on 26/09/2023 21:30:43

[not done it yetParticipant @notdoneityet]

The difference? You tighten the nut onto a bolt. You tighten the screw with a screw. Simple enough?

[Neil WyattModerator @neilwyatt]

There's no 'official' definition, just accepted usage, and as such it's always open to interpretation. You can always find a source that agrees with your personal definition and another that disagrees with it.

I used to feel it was that bolts hade a plain section, but nowadays I tend towards screw being simple fixings whilst bolts have a secondary role such as providing a location or closing an eye.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

RagBolts are an interesting variation on the theme

MichaelG.

[mark costello 1Participant @markcostello1]

I was taught a bolt is 1/4" od. or bigger. A screw is anything smaller.

[peak4Participant @peak4]

Here you go, have a try at downloading the ISO standard;
P29 on seems to define the terminology

https://regbar.com/wp-content/uploads/2019/09/ISO-1891-2009-ÖNEMLİ.pdf

For hex heads, fully threaded seems to be a screw, whereas partially threaded is a bolt.
Various other forms are subsequently defined.

See also ISO 898-1, who's bibliography pointed me towards the one above
https://pppars.com/wp-content/uploads/2021/07/ISO-898-1.pdf

ISO888 defines the various thread lengths of metric bolts.
https://regbar.com/wp-content/uploads/2019/09/ISO-888-2012.pdf

Bill

Edited By peak4 on 26/09/2023 22:00:03

[Barry ThurgoodParticipant @barrythurgood45522]

As one who prepared bolt schedules back in the day in construction industry steel fabrication. Bolts have a plain shank for a length as near as you can get to the thickness of the parts being connected unless the parts are very thick in which case 1.0 – 1.5 times bolt diameter into the part on the thread side. If threaded full length they are screws irrespective of diameter. Bolts can act in shear, tension or a combination of shear and tension. Nowadays for most “simple” structural steelwork screws are used. There have been many reports in the journal of The Institution of Structural Engineers on the use of screws and shear “slippage” as the threads crush when the shear load is applied, bear in mind tolerances for bolt/screw holes <=20mm – bolt dia plus 2mm, >20mm bolt dia plus 3mm! Personally, for systems resisting lateral loads such as bracing or sway frames I specified Friction Grip bolts but opens up a whole new can of worms.

[Howard LewisParticipant @howardlewis46836]

My understanding has always been that a fastener with the thread extending right up to the head, is a Setscrew.

If there is a plain section between the underside of the head and the end of the thread, it is a Bolt.

If the fastener has a parallel thread, it can accept a nut, and in smaller sizes can be described as a Machine Screrw..

Grubscrews have no head, and the thread is continuous

Otherwise, to me a screw, implies a wood screw with a tapered thread.

A fastener intended for tapping its own thread is in my definition of a Self Tapping Screw, colloquially a Self Tapperer.

HTH

Howard

.

[bernard towersParticipant @bernardtowers37738]

Yes I'm with the setscrew def. and was always under the illusion that screws were for wood or if engineering type threads they had heads with slots in

[File HandleParticipant @filehandle]

I had always use the partial plain shank for a bolt, and fully threaded for a screw, until a few years ago someone on the web said I was wrong and that a bolt screws into a nut and if it screws into something else it is a screw. When i thought about it this idea did have some merit, but if you are selling them the same thing might have its name determined by how you intend to use it, confusing.
No definition really works.
Partially threaded capscrews have already been mentioned.
Coach bolts might be partially or fully threaded, if you ask for a coach screw you will get one that screws directly into wood (a hefty wood screw with a square or now mainly hex head) not a fully threaded coach bolt.
For some jobs I use what I call gutter bolts, but more often roofing bolts. round slotted head and square nuts. I was given several boxes of then decades ago when an agricultural building constructor had a clear out from his van. These are fully threaded, but calling them screws would cause confusion as roofing screws are used to go directly into wood or steel, being self drilling.
If you ask the general public I suspect the most will call anything with a parallel thread and head a bolt.
But its clear that no definition really works. Perhaps like the Eskimos have hundreds of names for snow, we need more names for bolts / screws to avoid confusion. Probably why no descriptor is complete without a photo.
i really wrote the initial post to see what the language police thought.
Thanks for all of the contributions.

[kyriParticipant @kyri]

I’d like to chime in, as this topic has haunted me for close to a decade now – let me explain:
I’m a Nordic engineer with a degree in mechanical engineering and product development, which in most of the EU and especially here in Finland, adhere to the generally-accepted-as-bar-none German engineering practices and standards. Therefore the DIN standard, later superseded by the ISO standards, have been my in-grained ruleset for mechanical elements, fasteners and materials, along with the terminology, definition and manufacturing practices of derived mechanical components and assemblies.
I traverse the English speaking internet for pretty much everything STEM-related, so as to have a larger set of sources and a broader peer group for information. Unfortunately this got me in a less than favorable situation, as after years of influences from colloquial discussions with, and technical documentation from, ANSI & ASME users, I had started picking up some habits from them.

The screw vs. bolt discussion has been ingrained in my mental shame repository, as I was practically laughed out a project meeting for suggesting a bolt flange construction for ease of on-site assembly, instead of pre-fabricated welded subassemblies. The albeit snarky comment from my older colleague: “there are no bolts in engineering” was made even more eviscerating when in my youthful defensiveness, I went on to parrot the definition I’d come to see online: “screws attach directly to the base material, bolts use through holes and secure the connection with an adjacent female threaded fastener, such as a nut”. This was promptly followed up by the retort “if you had two ISO 4762 hex socket screws, put a nut on one and thread the other into a tapped hole, one would spontaneously transform into a bolt?”.
The roast was a chastening experience, as I came to appreciate how easily erroneous and ill-defined terminology seeps from passed-down colloquial speech into technical topics, due to the conversate nature of applied science.

Ever since I have gotten an uncomfortable shudder every time I come across this topic, as it reminds me of that situation where I ultimately had to concede the logical fallacy and accept I was wrong and they were right (tl:dr):

a mechanical component can’t and shouldn’t be defined by its arbitrary end use or application, but rather by its definitive standardized properties such as geometry, so as to rid any notion of ambiguousness.
Therefore at least in the engineering that adheres to the metric standards: There are multitude of mechanical element that utilize machine screw threads, with varying shape, thread geometry and layout – some partially threaded, some with precision shoulders, some with just a shaft and threads on both ends. But there are no bolts.

Of course this is not the case for the engineering world still adhering to the imperial system, ANSI & ASME, but that is such a small and diminishing subsect on the mechanical engineering world stage, that even though English is the predominant global engineering language, it would serve them well to start to wean off the dated standards during the transition from predominantly pre- to post-internet and global co-operation engineering workforces.

[Harry WilkesParticipant @harrywilkes58467]

When I worked in the metal finishing industry when work came in from the like of GKN, Atlas bolts and others based in the Darlaston area if the fasten had a plain shank under the head it would be labelled as a bolt .

H

[JAParticipant @ja]

They are the same. Big full stop.

This is an arcane debate: Very important to a very few, utterly irrelevant to most. Rather like “How many angels can sit on a pinhead?”.

JA

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