Thursday, January 19, 2012

Triumph of the mind (over hysteris)?

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.


  1. That is probably THE most important new information that has been proved. I am looking forward to the results of your heated bands. Well done, Joerg! Very important results!

    Battery powered heating elements add a certain level of nerdy technology that appeals to me. I can almost see sling-shot rifles covered in photo cells charging an internal battery with a butane attachment for an extra quick burst of energy in the not too distant future. :) ( I think you have already made something like this.)

    I'm also curious if the bands will be effected by different levels of humidity.

    All the best to you and thank you again for this breakthrough.

  2. Where did you get the heat films?

  3. I wonder if tubing would react differently in this and previous tests?

  4. Tubing is untapered (at least, the type you buy by the meter or foot) - but other than that, no, the reaction is entirely the same.

  5. I bought the films at an online electronics store,, 10 Euros a pop.

  6. It would be really cool to have streachable heating bands...
    So the heating is integrated in the bands. Don't know if OLEDs would work, but it would be awesome^^

  7. Well done Jörg! I wonder what happens if you stretch the bands in cold weather and outside and then take them inside and warm them... Will the draw weight increase?

    a fan from Finland :)

  8. Yes, the draw weight increases as soon as the bands warm up.

    1. Yes of course. But I was meaning that if you stretch the bands for example 10 N and then warm them, can you get draw weight for example 12 N? I mean increasing the draw weight just with heat?

      a fan from Finland :)

    2. Yes, but at some point the bands will tear.

  9. If I remember correctly, did you not make a slingshot with a heating system a while ago? I think it used propane or something to heat the bands.

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