Coach bolts or coach screws?

[dg]

Apart from reasons of practicality, is there any reason for not using coach screws instead of coach bolts for fixing large timber sections - ie is the strength and longetivity of the fixing the same?

dg

[The Natural Philosopher]

Personally I prefer bolts,, but screws are pretty damned good. Screws CAN pull out..rare but possible...Bolts OTOH..

[Andy Dingley]

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

Bolts are 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.

Screws are 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: bolts in particular can apply tensile forces, although screws are 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 to screws than metal would be.

[dg]

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

[robgraham]

For a start lets do the pedant thing and at least get them called by their right names - they are "coach bolts" and "carriage screws".

The argument on sheer and tensile is cobblers as diameter for diameter the sheer strength is going to be the same. On the other hand the tensile strength of a bolted joint with a suitable load spreading plate is clearly always going to be greater than a screw into the wood. The choice is related to whether you can get at both sides of the joint - if you can then its a bolt - if you can't it's a screw. Can't really be simpler can it?

Rob

[normanwisdom]

Basically screws are for fixing a thing TO a thing, bolts for fixing things TOGETHER.

cheers Jacob

[Andy Dingley]

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.

[Roger Mills]

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.

[Andy Dingley]

I'll take Machinery's Handbook as authoritative here, over someone who confuses "shear" and "sheer".

That's a question as to whether you're using a loose nut or not, not what type of fastener you've used.

A bolt (albeit not a coach bolt) can use a fixed nutplate or a tapped hole and be inserted by rotating the bolt from one side. Conversely a parallel-threaded screw can have a nut applied to it.

[Andy Dingley]

Not good design though. It's very dificult (i.e. impossibly impractical) to maintain compressive forces and thus friction in a design of bolted timber. Moisture or thermal movement, then compressive yield in the timber, cause such a joint to go loose and fail after a few seasons.

[Roger Mills]

True, but you can mitigate against that by using a 'spikey plate'[1] between the 2 bits of wood. That will keep the shear force away from the bolt even if the full compressive force isn't maintained.

[1] Not sure of the technical term!

[The Medway Handyman]

Too true Roger - eveybody forgets that bit.

[The Medway Handyman]

And you can't easily use a bolt to make a tee joint.

[The Medway Handyman]

I use loads of the Screwfix Turbo Coach Screws

or 3 of those in 6 x 2 timber and it aint not going anywhere. Incredibly strong joint.

[Roger Mills]

Yes, they're good.

Pity that even Screwfix doesn't know the difference between a *coach* bolt and a *carriage* screw!

[Andy Dingley]

You'll see me using "spikey plates" after they pry my twybill from my cold, dead fingers....

[dg]

Timber connectors

dg

[dg]

I thought they were just different words for the same thing - after all isn't a carriage a coach?

Tomatoes, tomartoes?

dg

[Lurch]

On 29 May 2007 16:08:41 -0700, dg mused:

You see, one of those is a made up word and one isn't, that's a different thing altogether.

[Owain]

Those made up words are a bummer aren't they; much better refer to the long-distance-talkie-by-wire-machine.

Owain