Ian Stirling wrote:
wrote:
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.

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.

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.

--
Chris Green