From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: atm{at}shore.net
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


On Tue, 19 Aug 2003 18:45:19 -0700 (PDT),  Ross Sackett
 wrote:

>
>--- Emery Greg  wrote:

>> If so, are the bolts not in double shear?  It has been a long
>> time since Mechanics 1 and Strength of Materials - I
>> never thought I would use that stuff again :)

{aside] You and me both, Greg !!

>The connector bolts as I imagine them are in line with
>the tubes (on axis in fact) and loaded in tension-compression.

You can think of it either way:-

You're either going to butt the tubes end - end, or have an overlapped
joint of some sort. If they're butted together over the full cross
sectional area, the fastener needs to provide some clamping force, and
provided this isn't exceeded, the joint will be as stiff as the tube.

If there is an overlap, it's difficult (but not impossible) to make a joint
without any change in cross sectional area, so there may be a minor effect
on stiffness (+ or -). The function of the fastener (I would argue) is
still to clamp one part of the joint against the other. It is therefore the
friction between the two parts that matters, rather than the fastener
strength or stiffness.

The commonly seen arrangement of a sleeve (internal or external) to join
two tubes is actually a good idea, but it is usually spoilt by being a poor
fit on/in the tubes, and/or being secured by sticking a bolt across the
diameter and tightening it down.

At *best* this will clamp the sleeve on one diameter, and turn it oval,
spoiling whatever fit there ever was and allowing wobble and slop. If the
sleeve were precisely fitted and could be clamped uniformly to the tube,
then this would be a good joint.

There'd a very readable book about building race-cars called "Engineer
to Win" by Caroll Smith which has a whole chapter on this sort of
thing, and has sound advice from cover to cover (mostly applicable to
building *anything*)

Regards, all.

Andy

Andy Gray, Dyserth, N. Wales, UK.

P.S. - The query about using Young's Modulus for hollow tubes : It works
the same. You just need to use the cross sectional area of the metal/etc
that there actually is in the tube.

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