Cal,
As John Barker has said, someone has strained the wire in your torque
meter beyond the linear range of the stress/strain curve for steel.
Most likely this was done by winding rubber too big for the torque meter.
I would refer you to the following web sites:
http://www.modelflight.com/torque.html
or
http://www.mindspring.com/~thayer5/ffpages/tools/torque/torquetech.html
If you use the formula on these web sites, your torque meter, with
4.92 inches of .015" diameter wire, would take about 1.17 in-oz torque
to make one 360 degree turn. That's probably adequate to reach the
breaking point of a single loop of 1mm (.040") or 2mm (.080") rubber.
If you are going to wind larger rubber than that to the breaking
point, you need a larger diameter wire in the torque meter.
If you put a .020" diameter wire in your torque meter, you would reach
a little over 3.5 in-oz in a 360 degree turn (one turn) of the torque
meter. That should be good enough to take a single loop of 1/8"
rubber to its breaking point.
After building a few torque meters I have come to believe that the
number of degrees your torque meter uses should never exceed 240 to
300 degrees. If you exceed this it, is likely you will deform the
wire out of the linear range of the stress/strain curve.
If you are conservative, as I am, and only use 300 degrees then your
torque meter, with the .020" wire, would reach a little over 3.0 in-oz
in that 300 degrees. Still plenty good for a single loop of up to
1/8" rubber. Being conservative I use "G" in the formula as a
constant of 11,500,000 regardless of the diameter of the wire.
Remember though, the torque meter will never show if someone has wound
a 1/4 inch loop to breakage and over stressed the wire. The torque
meter doesn't stop at 300 degrees. If someone keeps winding, it will
keep turning, but then you are back where you started from.
Tom Juell
--- In Indoor_Construction_at_yahoogroups.com, "John Barker"
<john.barker783_at_...> wrote:
>
> Cal
> If my memory is not playing tricks after all these years then the
equation
> you require is:
>
> Stress = modulus of rigidity x angle of twist x radius of wire /
length of
> wire.
>
> modulus is usually about 11.5 x 10^6 lb/sq.in
> angle of twist = 2 turns = 4pi radians
> radius of wire = 0.0075 in.
> length of wire = 125 mm = 4.92 in.
>
> I make that to be a stress of near 220,000 lb/sq.in = 98 tons/sq.in.
>
> To my mind that pushing it even though you colonials always claim to
have
> better steels than us oldies.
>
>
>
>
> [Non-text portions of this message have been removed]
>
Received on Mon Oct 15 2007 - 20:31:36 CEST