Re: tensioning method for secondary?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "Laurence Davis by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <meknar-at-hotmail-dot-com>
> 
> i just wrapped another secondary.  22awg on PVC/sch40 4" pipe.  i don't have
> a coiling jig. its more like the pipe rests on my ankles and i spin it for
> about 3hrs.  BUT when i place the secondary in the basement
> to let it 'acclimate' to the temp before i poly it, it loosens.  therefore i
> assume the temp exp coeff of pvc vs copper is that the pvc contracts faster
> than the copper per degree.  instead of this being a problem, i decided that
> i would use it to my advantage.
> 
> cool the coil in a walk-in freezer, while retensioning. when coil is
> retensioned in the cold air, when moved to the warm air... voila nice tight
> secondary.  i'd like to compare the temp coeff to see if i can calculate how
> much temp change the copper can stand till the stress causes the copper to
> either stretch or break. but i'm not sure how to convert the "coeff of
> linear thermal expansion" to a cylinder expansion.
> 
> retensioning before poly is a pain, perhaps i just need to make a jig to
> wrap unattended -and- tensioned.


I suspect that the copper is stronger than the PVC, and that the PVC will
deform to match the copper.  Copper wire is pretty soft, but PVC probably
has an even lower modulus.  Even if the copper is much thinner than the
PVC.  In any case, to analyze it, consider it as a thin slice, 1 turn long. 

The diameter of a cylinder that is unconstrained (yours isn't) will change
exactly as the coefficient of linear expansion (that is, if the CTE says
you'll get a 1ppm change in length, then the diameter will change 1ppm).
You could use this to get the "resting" size of the cylinder, and then use
Hooke's law (load proportional to displacement) to figure out the stress.

What you've essentially got is two springs, with different spring
constants, acting against each other, with specified displacements on each
end.  A simple 2 equations and 2 unknowns kind of problem.


An interesting mechanical engineering problem..