interesting question. No one really addressed how to come up with the length of
splice.
If this was a normal bolted joint, you might say something like 8-10x bolt
diameter for spacing and end spacing.
But this is not a normal joint. The through bolts really don't do anything
except keep things from sliding apart.
If it's a riveted joint, then it's different..it would be more like a
compressed
joint.
I think what happens, no matter how closely spaced, for short joint overlap, is
that the tubing deforms a little under load, so that you get a lever effect,
where the fulcrum is the bottom area near the lip on the big tube, and one end
of the lever is the top end of the smaller tube (pressing against the inside of
the big tube)
I've noticed that the aluminum corrosion seems to match these areas, when
taking
tubes apart. (in through-bolted joints)
So, if you have a 6" overlap, and an overall half element that goes out 20-33'
feet, you can imagine there's quite a leverage multiplier between that fulcrum
and the other lever point.
So you can imagine how a small overlap, like 3" could end up with distorted
tubing.
I think with enough overlap, this lever effect gets spread over more area,
which
is better.
Now bigger tubing that takes bigger loads, is also wider in diameter and
thicker. So what's interesting, is that for all our joints, the length need may
be similar.
I think emprical data is probably your best bet..looking at what similar
antennas do for overlap, at similar cases.
The looser the joint, the worse it is. But you will have some looseness. the
outer and inner diameters don't match.
Actually you can probably tell what's good by putting the tubes together at a
splice length, and holding it out horizontally and rocking it up and down. The
overlap is "about right" when it doesn't feel like it rocks up and down so
much.
Shouldn't need more than 12", shouldn't be less than 4" (you start getting not
enough material around the bolt holes then.)
Even with loose fitting joints, the rocking decreases with increased splice
length. So that's the model I would use: minimize rocking/oscillation to some
amount.
-kevin
ad6z
On 9/5/2011 5:26 PM, Robert Chudek - K0RC wrote:
> I am rebuilding an old Telrex 40m dipole that was damaged in a recent
> windstorm. One side of the dipole buckled slightly from excess force. It
> didn't break, rather it crumpled on the leeward side and the element was
> about 20 degrees out of straight.
>
> Is there a formula for how far a smaller diameter tube (not pipe) should
> be inserted into a larger tube and still retain the full strength of the
> smaller tube?
>
> In my particular case, I have a 2.0" OD 6061-T6 tube, 0.058" wall
> thickness that will slip into a 2.5" OD tube. The larger tube has the
> proper wall thickness so the smaller tube is a "nice" slip fit.
>
> I am wondering if 4" is adequate, but that's just a SWAG.
>
> 73 de Bob - KØRC in MN
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|