On Fri, 24 Feb 2012 20:24:25 +0200, Kristian Ukkonen
wrote:

On 2/24/2012 0:03, Ned Simmons wrote:
On Thu, 23 Feb 2012 16:29:41 -0500, "Jim Wilkins"
"Kristian 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.

Here are two patents describing such devices:

http://www.google.com/patents/US5333...page&q&f=false

http://www.docstoc.com/docs/32588662...Patent-4562743

For bandsaw blades:

http://www.toolcenter.com/62126.html
http://bbs.homeshopmachinist.net/showthread.php?t=21171

Although I have to admit that it makes sense to fix two wheels against
the cable and deflect the cable at the middlepoint between wheels
a known amount, and measure force needed for deflection. Elegant.

http://www.checkline.com/cable_tensiometer/CTM

BUT it again requires for force measurement.. The ones above only
require measurement of elongation with dial indicator, which is cheap
compared to load cell and electronics..

In practice, I think that most people who use the deflection-type
gages use them more for relative measures. From experience, you gain a
good idea of what values work for you, and at what point you start
breaking blades.

--
Ed Huntress