Have been thinking a lot about the findings of the thermo cam session. I found out that the hysteris effect (= loss of power when a slingshot band is kept stretched) may be caused by the bands cooling off. The obvious question: What if you heat the bands in stretched condition - wouldn't that bring back the power?
In order to test this, I made a little device that lets me keep a length of band stretched with the fish scales attached to one end. It has to have a hook for the removal of the scales as they switch off after a while, and have to be recalibrated after that.
It works! Tested this over a few days now.
- Bands are 30mm x 20mm x 150mm (active length)
- Stretched to 770mm (factor 5,1)
- Have about 4,35 kg initial draw
- Draw goes down quickly to3,25 kg
- Draw stays 3,25 kg for days (same room temperature)
- When heat is applied (with heat gun), the draw can be increased to 4,35 easily
- Bands tear at 4,6 to 5 kg
So it works, the proof is there. The cooling of the bands is the reason for the hysteris, which can be turned around if the bands are re-heated.
The goal: To build a slingshot crossbow that can be kept cocked for days without loosing power.
Bought the heat films today (9 of them for good measure):
These films work on 12V DC, and are listed at 36 W. I plan on making an encapsulated slingshot crossbow (the top can be folded away for cocking the weapon) with several of these films as a heating. A motorcycle battery will provide the power. The front of the slingshot will be closed with brushes so the heated air can not escape.
A switch and a thermometer will be installed as well, of course. Gotta know when it gets too hot.
Interesting tests ahead of us! Stay tuned.