Hexagon Head v. Allen/Cap Head screws/bolts

Over the years I've accumulated a number of different types of screws/bolts (let's say screws even though it may be the wrong word) with a variety of heads. I have, for example, a large quantity of round head, cheese head, panel head & countersunk screws in the BA range - all with plain screwdriver slots. Plus a few hex head bolts. I also have a large quantity of M6 panel head & counter sunk headed screws, again all slotted, but am gradually building a stock of hex head screws, and a tiny number of cap head screws. Similarly with 8mm. Smaller metric screws are a mixture of whatever I can recover from scrapped items - slotted head, Philips, Pozidrive, hex head, whatever.

In all, then a rather eclectic mix of heads, and as a result I've tended to use whatever seemed satisfactory at the time.

Recently though, I've used a plate and fastened it with two 8mm hex head bolts and two 8mm cap head bolts for no other reason than that is what I had available in the desired lengths, and that set me thinking. What advantage, if any, does the cap head give over the hex head other than that of accessibility?

I can see that a cap head countersunk can be tightened more than, and more safely as well, than a slotted countersunk, and possibly a Pozidrive countersunk, but can't see any real advantage over hex head as long as there is suitable access.

So, thoughts anyone?

Peter G. Shaw

Strength.

Chris Gunn

I suspect that the choice of head is down to ease of assembly, especially in a production environment. Cap head screws are usually high tensile but most head types can be sourced in high strength materials if required.

My 2c,

Rod

Space, heads are smaller and can be accessed from the top little side space is required around the head. All screwed fixings are available in various strength values, nuts are usually weaker than the male threaded fixing.

Cap heads and Csk are ideal for counterboring so you dont see side on.

Bolts not fully threaded a screw is.

Used to be a case all screws were high tensile minimum Grade 8.8 with hardest at 12.9 quality ones will be marked accordingly. Since theres only one producer outside of china anything can happen.
Stainless screws are soft and only come in A2 or A4 nothing more than corrosion resistance, heads chew up very easily. Stopped using them a few years back unless specially required.

Hex heads you will get more torque on them due to a greater surface area.

High tensile cap heads I use for tooling but for all my steam models I use hexagon nuts and bolts/screws simply because they were what was used when the full sized versions were built - they just look right!

Rik

Horses for courses.

Where there's plenty of headroom for a key but little sideroom for a spanner, the socket-head is a practical winner. You can also use it on grubscrews where the slot gives insufficient torque, and the hex head is effectively impossible to get at.

Where there's sideroom but little headroom, the hex head is favoured.

On my lathe, most of the screws are socket-head. Hex heads are there on the compound slide, for example.

Back when I was a jig & tool designer, I'd say the whole department used socket-heads in more than 80% of instances.

Edited By Mick B1 on 25/04/2018 21:16:17

Time is money in a production environment. Cap heads are easier and safer to fasten with windy tools and being round don't need a washer to protect the surface finish of the component being fastened from a socket.

Brian

Well... spanners round off hex screw heads. Cap head screws wreck allen keys.

I suspect you can put more torque through a hex head than a cap head, though.

In UK usage 'Screw' is correct, a bolt has a plain shank that fits a hole.

In the USA a screw screws into something, a bolt is used with a nut.

Clearly the USAnian definition is ambiguous, which causes them much grief as it's used to decide the rate of tax that applies or something like that...

Neil

In engineering, isn't a 'screw' short for an 'engineering screw', meaning a threaded piece that needs a nut or similar thread in a metal piece to secure, as opposed to a screw that chippys use to attach thing together in wood?

(Ignoring air screws and screw propellors etc etc!)

Chris

I’ve always thought of all Cap head socket screws as screws whether they’re threaded all the way or not. If you do a google search many others agree. just one random page:

**LINK**

The term Set screw is also an interesting one.

There used to be some really nice pictorial charts of fasteners showing the convention but I believe even these were often incomplete.

Generally speaking, metric steel cap head screws are supplied with a tensile strength of 1200kPa. Hexagon headed screws are only 8000kPa. (The marking "12.9" on cap heads - the "12" is the code for 1200kPa, and the first "8" of the "8.8" on hex heads is for 8000kPa). Hex grub screws are usually grade 14, but countersunk hex socket and button head screws are only rated at 10.9 as standard. No idea why! Unbrako do a special "Durlok" hex head screw with integral serrated washer - rated at 12.9 but I have never used them.

Obviously with stainless fasteners it's different.

Your typical cheese heads in steel are rated at 4. Some I've had from dodgy suppliers must have been rated about minus six...

I always understood that bolts were only partially threaded, whereas screws were fully threaded, but I suspect it's now just pedantic to distinguish them in this way.

.

Ah, but ... Allen keys are made of better stuff than most allen keys

MichaelG.

.

P.S. Allen's patent [Manufacture of Screws] is an interesting read:

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=960244A&KC=A&FT=D&ND=3&date=19100607&DB=EPODOC&locale=en_EP

Edited By Michael Gilligan on 26/04/2018 10:30:49

I think that less then M4 capheads are a right royal pain as the key slips or jams before you get anything like tight. Anyone recommend a decent brand of Allen key which won't break the bank

Nigel is nearly correct on grades, the UTS is the figure before the decimal point in 100MPa, the figure after the point is the ratio of yield to UTS, thus 8.8 has UTS 800MPa, yield 640MPa. 1200kPa is only 0.087 tons/sq.in.

I think strength has to be a major factor in choosing cap head screws, they are used almost exclusively on press tools. Cap heads are ideal when the head needs to be out of the way as they can be counterbored to give a flush surface. As they need no space around them to fit a spanner they can be used in situations where a spanner would be impossible to use. Motor vehicles now use many fasteners like Torx and the related types, ease of assembly and again accessibly are factors in this choice. If you look round a modern car you will see that many fasteners would make life difficult or impossible if hex heads were used. As much assembly is done by automated spindles they can be made slimmer if they don't have a hex socket on the end.

Mike

Bondhus or unbrako are my preferred brands of key. Many cap head screws are not that well made I find, stick to a brand like Unbrako and you will see a world of difference.

Mike

.

Duncan,

Your profile shows Cheshire, so I presume you know Macclesfield.

Visit the small engineering supplies shop on the Leek road ...

They sell single keys of excellent quality

... I simply bought two of everything they had in [Imperial and Metric] sizes under 3mm

MichaelG.

.

http://www.richarddelaney.co.uk

Edited By Michael Gilligan on 26/04/2018 10:39:32

Yes agreed, never had any problems with Bondhus. They supply tools to NASA so they must be pretty good.

I had a chat with a guy at one of the places I used to buy socket screws and he said that they had to buy in a lot of their stainless stuff from the Far East to remain competitive. He said they met minimum standards but weren’t as good as European fasteners. You get what you pay for it seems?

For a close fitting set I can recommend Wera... **LINK**

The extra length and ball ends are pretty handy IMHO

Cap heads are grade 12.9 (1200 N/mm^2 ultimate, 1080 N/mm^2 0.2% Proof). Hex head screws/bolts are usually grade 8.8 (800 N/mm^2 ultimate, 640 N/mm^2 0.2% proof) or weaker. If not marked, hex heads are grade 4.6, (400 N/mm^2 ultimate, 240 N/mm^2 proof), same with studding. Nuts are grade 8 by default, and are at least as strong as a grade 8.8 bolt/screw.

The grade 12.9 material is less shock tolerant. So, for strength use cap heads, for shock tolerance, use hex heads.

The head of a grade 10.9 button head is weaker than the thread, and under tension a button head will have its head pop off before the thread snaps.

Regards,

Richard.

Edited By richardandtracy on 26/04/2018 12:20:25