One crude way to tell if a torque meter has been overloaded is to
observe if the meter returns to a zero torque measurement when
unloaded. Once the wire has been loaded with a stress greater than
the yield point of the material, the wire will have a permanent
twist. This is how I tell if Science Olympiad students have
overloaded their torque meters.
A simple rule of thumb not to wind more than a certain angular amount
does not always work. If the length of the torque wire is increased,
let's say doubled, the wire can be twisted to twice the angle with
the same torque load (stress) or twice has much before the wire
permanently twists. A simple solution to prevent gross damage on
torque meters is to put a pointer stop on the dial so that the meter
cannot be twisted over a certain angular amount. Once the pointer
hits the stop, any increase in torque is countered by the stop rather
than the wire. One can use the various formulas and web sites posted
in other replies to work out the angle, wire diameter and length such
that the maximum permitted twist angle gives the desired maximum
torque reading without overstressing the wire.
The crude zero return method given above and the pointer stop do not
give a complete safeguard against torque meter degradation. Because
a sensitive torque meter with a reasonable length is desired, the
outer surface of wire will often be stressed near the yield point of
the material. Over time, the wire material will start to fatigue and
develop micro-cracks, usually near the surface where the stain is the
greatest. When this happens, the wire will not resist twist as well
even though it may still return to zero when unloaded. Thus, the
torque meter will read higher than actual torque as the wire
fatigues. This is why a torque meter needs to be tested for
calibration and accuracy every so often. When the Science Olympiad
students build their torque meters, I have them make sure that the
pointer is balanced just like a propeller. This way, the meter reads
zero when unloaded no matter what the orientation of the meter.
Then, to test the meter for accuracy, the students need only put a
known weight on the end of the pointer, orient the meter so that the
pointer is horizontal, and read the torque. The measured torque
should equal the length of the pointer times the weight (in ounce-
inches if English units are used).
Of course, all of this also applies when building and using torque
meters for F1D and HLS planes as well as Science Olympiad planes.
Leo, Bloomington IN
--- In Indoor_Construction_at_yahoogroups.com, "Tom Juell"
<vinfiz1911_at_...> wrote:
>
> 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.htm
l
>
> 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 - 22:24:37 CEST