On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
wrote:


"Kristian Ukkonen" wrote in message
...
On 2/22/2012 8:29, RogerN wrote:
My most immediate use will be to tension the guy wires on the
antenna tower
I'm preparing to put up and later hopefully to do tests on
construction of a
homemade crane. I'd like to verify/test my force calculations to
keep the
design factor correct.

Isn't the normal way to measure tension of wires by attaching a
gauge to measure the elastic deformation (increase of length by unit
length) of the wire by the force? Basically a long caliper attached
to the wire say 30cm apart.. Same is used to measure tension of band
saw blades too.

The elongation is two parts per thousand at the (arbitrarily) defined
yield point of steel.
http://www.keytometals.com/Article53.htm
Right above "Ductility".

This small amount is difficult to measure accurately in the field,
like on a steel cable guy line, being only 1mm per meter at a 2x
safety factor.

jsw


That .2% is the amount of non-elastic stretch used to define the yield
point; it's not the amount of elastic deformation before yield. The
suggestion was to measure *elastic* strain (delta length/length) in
order to determine tension. The change in length before yield will be
approx = stress/Young's modulus x length, stress equal to
tension/area. Still a small number.

The bigger problem is that I'm quite sure the stress/strain behavior
of a spiral wound cable won't be nice and linear like a solid wire's.
The sailboat tuning devices someone else mentioned work by measuring
the force required to deflect the wire thru a given angle, and don't
rely on the elastic properties of the wire.

--
Ned Simmons