Torque required to break screws - typical values anywhere?

I'd like to have some 'ball park' figures for the torque which typical wood screws can handle. Can anyone give me some typical figures or point me at a web site that has them? I would emphasise that I'm after the torque figure, i.e. the amount of twist needed to break them. I realise this doesn't directly relate to the strength of the screw holding things together which would simply be given by the tensile stregth of the screw.

I'd prefer the figures for metric screws (i.e. diameters in mm) and in Nm but I'm quite happy to convert if necessary. The actual screws I'm using are ScrewFix TurboGold and the stainless steel versions of the same. I would guess however that most good quality screws are going to be similar, I'm not after accurate figures, just an idea of what is likely to break them.

Reply to
usenet
Loading thread data ...

Actually, it pretty much is tensile strength. Steel (of any sort) pretty much has a constant stiffness. If you stretch it a given amount, it'll stretch a certain distance.

If you think about what's happening in the screw, if you draw lines along the shank of the screw, when you turn it, it gets twisted into a spiral. As each line is now longer, that means that it's under tension on the outside, and the inside is under compression. At some point the force just gets too high, and the screw either tears, snaps or bends.

IMO, this figure is utterly irrelevant in most cases, it's hard to get most types of screwdriver bit to exert enough force on the screw to get it to fail this way. And if you could, it's not relevant as by the time you've reached that number, in nearly all cases the screw has gone too deep.

Reply to
Ian Stirling

I managed to shear off the head of a dry-wall screw last week with a normal screwdriver! (just wouldnt go in that fraction more I wanted, ended up using a nail in that spot :o)

Reply to
a

Get a torque wrench and "break" a few. You will soon get what you want easier than asking such a rediculous question,

Reply to
Jim

Yup. I often use my mains drill for screwdriving and it's got much more torque than most cordless ones and no clutch. A screw breaking is a rare event - they usually 'cam out' first with Pozidriv

Reply to
Dave Plowman (News)

Well, you can get screws made of alloys with the tensile strength of extra-mature cheddar...

Reply to
Ian Stirling

I've broken quite a few Torx screws (SpaX) with my 18V combi, including some very large ones (150mm size 8).

Reply to
Grunff

Ooer - a competition...

I`ve sheared 12mm bolts before now - not the monkey metal kind :-}

Reply to
Colin Wilson
[snip]

Is there such a thing as a ridculous question? I thought it was only ridiculous answers?

Reply to
azerty

Yes I *did* realise there would be a fairly direct relation to tensile strength but that doesn't directly help me work out the 'torque strength'.

Anyway it is quite easy to break Pozidriv screws in my experience and I also use some hex head coach screws where you can apply effectively unlimited torque.

Reply to
usenet

snipped-for-privacy@isbd.co.uk wrote in news: snipped-for-privacy@uni-berlin.de:

Ask the buggers who fitted my double glazing

mike

Reply to
mike ring

Well I may need to do that ultimately, I do have a torque wrench and it would be easy enough to try it with one of the hex head coach screws. However I would have thought that at least some of the better screw manufacturers would publish such figures.

Reply to
usenet

Well, for "high tensile" bolts you can usually work out the values from the numbers stamped on the heads. But then, you might want to know the strength of the bolts if (say) you were using six of them to hold a 2 inch flange on a 100 psi system. But you are not usually doing stress analysis on wooden structures held together with screws. Why were you asking? Find a friendly metallurgy lab and you might get them to measure the hardness for a particular type of screw, then there are formulae. Usually what you want in carpentry is a joint that is as strong in shear as the wood. You don't need much area of the weakest steel to get enough normal force across the joint; friction and/or glue do the rest.

Reply to
OldScrawn

As I was trying to explain in my original question I'm not in the slightest bit interested (well, not for this question anyway) what the

*tensile* strength of screws is. So it's not about how well they hold things together.

What I'm after is how easy it is to break them, especially with a power screwdriver.

Reply to
usenet

formatting link
good?

Regards Capitol

>
Reply to
Capitol

BUT WHY????? If it is so you don't break any during 'installation' turn your torque ring down so the screw is proud and then increase it till it's flush! Nobody on here seems to know the answer to a question that seems irrelevant!!

John

Reply to
John

Nearly, it tells one everything one could possible want to know about the screws except how easy (or otherwise) it is to break them when screwing them in. It doesn't even seem to tell you how tightly you ought to screw them in to get the figures for fixing strength they give.

Reply to
usenet

Well it's really to find out how much torque is needed to screw them in successfully and how much will break them. I'm looking for a new cordless driver and want to know what sort of torque figure is useful and how much is simply too much. The cordless driver specifications give their maximum torquer but nowhere can I find any figures for how much torque is actually needed to screw screws in.

Reply to
usenet

I think that it's going to be a tough one to work out.

If it was threaded components going into metal, then it's rather easier because you are trying to avoid distorting something, stripping threads or shearing the stud/bolt and the materials are reasonably predictable.

For screwing into wood, there are a whole bunch of variables:

- The species and to some extent moisture content of the wood.

- The type of screw - e.g. standard, double threaded, serrated edge etc.

- Size and depth of pilot hole if any.

- The type of screwdriver

- Whether lubrication is used

For example, at one end of the spectrum, I have a Senco collated screwdriver which takes screws on a plastic band. Typically they are

3 or 4mm types with a square slot head. It's really fast for panelling and drywalling work and will comfortably drive the screws through ply or plasterboard into constructional softwood.

At the other end, I was rehanging an oak door into an oak frame today because I needed to reverse the opening side of the door. Since oak contains tannins which eventually corrode steel as well as for aesthetic reasons I wanted to use brass screws. However, brass will not stand up to high torque driving (it will shear or the screwdriver cams out) and oak is tough material, so to achieve this, I drilled carefully sized pilot holes and put in normal Spax screws to cut a thread. Then I removed the steel screws and put in the brass ones easily.

I used stainless steel screws in aspects of my cabin construction and found that these had a tendency to shear unless there was a reasonable pilot hole.

Some of the Spax and Turbogold etc. screws are claimed to be able to be driven into wood without a pilot hole. I've found this to be moderately true into softwood as long as it's away from an edge or end and one isn't looking for a particularly good finish. Trying this with hardwood, doesn't seem to work. Either the screw shears or cams out, and there is a mess.

For applications using the larger sizes of Spax screw, I tend to pick the pilot hole drill carefully and then using a combined drill countersink, drill to the correct depth. Occasionally with larger ones, a touch of some lubricant (e.g. candlewax) on the threads helps.

So in terms of the torque needed to drive the screw, I think it would be pretty hard to determine that because there are too many variables with wood. If the concern is over having too much torque and shearing the screw, then I would say that if it happens, the pilot hole arrangement was not right or if shearing happens when the screw is fully home, the clutch setting needs to be reduced.

Reply to
Andy Hall

I see where you're coming from, but I'm not entirely sure that it has any relevance other than purely academic or theoretical. It's an interesting question, though, and I'm surprised that ability to withstand up to X amount of torque isn't somewhere within screw specifications. Perhaps your best bet is to hunt down and contact the manufacturers of the particular screws that you're using - I don't really see that you can work out shear strength for a particular size of screw in general because they're all going to be made from slightly different materials and to slightly different specifications, which could have a disproportionately large effect on the shear strength.

All the manufacturers of Industrial-rated tools seem to give two figures for maximum torque - one for "soft" joints and one for "hard" joints - anyone have a clue what the difference is here, and whether there is actually a standardised test for measuring these?

As for buying a drill/driver, I think that the thing is to go for as high a torque as you can find for a particular voltage & price point - it will indicate better cells and motor. 20 or so step torque control is pretty much essential. Then just play about until you get the ideal maximum torque setting for a particular screw in whatever it's going into. Even if you expect to be driving only no.8 screws in normal use, you can always use the torque control to reduce the torque, but you can't turn up a less powerful drill if you have the need to drive in occasional long no.12s for instance.

Incidentally, you ought not to get close to shearing a screw in an ideal situation - received wisdom in the woodworking community appears to be that maximum holding strength from a screw is achieved when a pilot hole of the _correct size_ is drilled first. It's pretty much essential when using brass screws, but an often skipped step with stronger screws that can be driven directly into the material with a modern driver.

Reply to
RichardS

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.