On 29 May, 16:47, Andy Dingley wrote:
On 29 May, 15:41, dg wrote:

Apart from reasons of practicality, is there any reason for not using
coachscrewsinstead ofcoachboltsfor fixing large timber sections -

They're entirely different devices, for different purposes. One's a
bolt, one's a screw.

Boltsare plain-shanked dowels used to resist shearing (sideways)
forces. For convenience in installation compared to a plain dowel, the
ends are threaded so that a locking nut can be attached.

Screwsare compression fasteners intended to compress two pieces
together. They shouldn't be loaded sideways in shear, as they're
relatively thin and poorly attached, thus unable to resist this well.

Obviously there's some crossover:boltsin particular can apply
tensile forces, althoughscrewsare much less happy in shear. In
principle though, use each for its intended purpose and don't confuse
them. This applies equally to design in wood or metal, although wood
is usually a little less fussy about applying shear toscrewsthan
metal would be.

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?

I can see how a coach bolt with washers would resist being pulled out,
and coach screw could potentially be weaker in this respect
(limitations of the timber not the screw), but for lateral shear, both
would seem equally adequate.

dg