Bolt or screw?

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?

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

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

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.

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

Except when it isn’t

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

Bolts or setscrews ! You know the rest ! Noel.

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.

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.

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

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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

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

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

RagBolts are an interesting variation on the theme

MichaelG.

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

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

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.

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

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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