In an earlier contribution to this discussion,
Andy Dingley wrote:

On 29 May, 17:10, dg wrote:

I would have thought that say a 100mm long 10mm dia coach screw would
have an almost equal shear strength to a similar sized coach bolt?

Perhaps, but a screw in a hole has much less _overall_ shear strength
than a similar bolt. It's not about the screw being weaker, or even
the core of the screw being smaller than the overall diameter, it's
more about the relative strength against shear of screwthreads vs.
plain shanked dowels.

For wood it's less of a problem as wood isn't especially strong
against any shear force applied to a narrow steel dowel. For metal
it's significantly different -- no competent design shear-loads a
screwthread when it ought to be using a plain-shanked bolt, often in a
reamed and tightly fitting hole.

It's all a bit academic anyway, because if you hold two bits of wood tightly
together - be it with a bolt or screw - any shear force if provided by the
friction between the mating surfaces rather than by the bolt or screw.
--
Cheers,
Roger
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