In a message dated 97-10-07 02:53:03 EDT, dick@libelle.com (Dick Flanagan)
writes:
<< In essence the discussion was that not all 4130 was created equal, that
there was some manufacturing or hardening or =something= process that
altered the ultimate strength. The bottom line being you wanted to specify
that your 4130 be obtained on the stronger side of whatever that process
was or that you should at least be aware of the process and its affects on
the mast strength.
>>
Chrome moly seamless tubing is a generic term for many different alloys of
steel containing chrome and molybdimum. It's primary purpose is in the
process industries where inside diameter tolerances and straightness are
often the most desired characteristics. Most steel suppliers do not
understand our application as mast material.
The most common alloy is 4130 CDS (cold drawn steel). It's typical yield
strength (the point where it will flex and not return to its original shape)
is from 80,000 PSI to 120, 000 PSI, depending on the heat treating it
received at the mill. That's quite a variation in strength and you need to
specifiy 110,000 PSI or higher for big arrays.
For instance, a TH7 on top of a 10' piece of 1/4" wall carbon steel would
bend the mast, while the same TH7 could safely be installed 15' above the
tower on a piece of 3/8" wall chrome moly rated at 120,000 PSI.
Other alloys include 4140 and they can be normalized, annealed, quenched and
tempered. All these processes have to do with yield strength and they are
heat and cool processes. Each time a piece of material is heated and cooled,
it can become warped and has to undergo a straightening process before it is
useful as mast.
Hope this info helps. Remember, aluminium is great for antenna elements but
not for mast material!
Tom, K5RC
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