[TowerTalk] mast for two big antennas

[Jim Lux]

On 12/27/12 10:19 AM, Mike Ryan wrote:

> This is 4130 chrome molly steel tubing. The word tubing is the primary
> descriptor I think, in that there is no seam such as you might find when
> using some kind of pipe. I bought a half dozen or so 24 ft lengths from
> the Dillsburg Aeroplane Works up in Pennsylvania back in the late '80s
> when I was putting together a station in northern Virginia.  I had them
> galvanized before using them. These were 125,000 psi tubes very similar
> to the materials used to build racing car safety cages as I recall.

I'm not a metallurgist, but wonder if the heat from the galvanizing (I 
assume it was done hot-dip into molten zinc) affects the heat treatment. 
  While that might not change the alloy composition, it might change the 
yield strength.

My little handbook says 4130 cold-drawn is tensile strength 85-110kpsi, 
yield of 70-85kpsi.

My understanding is that non-heat treated CrMo is fairly soft (hence the 
low yield strength) and not a whole lot "stronger" than 1XXX type 
steels. However, if heat treated it's a whole lot stronger (and more 
stiff/brittle, as well) so it gets used for suspension components, 
axles, and the like.

I suppose that we're constrained by the OD to fit in clamps, bushings, 
bearings and the like, but I wonder if a larger diameter tube of cheaper 
thinner steel might not be an easier way to go (lifting a 20 foot stick 
of 2" tubing with 1/4" wall is going to be a chore).  That r^4 term 
really helps a lot..