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Old 06-11-2017, 09:56 PM   #94
popgun pete
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Re: Alemanni Demultiplied Rollerguns

If you want to compare band gun configurations then you need to start with the band energy storage, that is why I created all those “green triangle” energy diagrams. What the diagrams don’t show is the effects of inefficiency which absorb some of the energy during the shot, so a gun that starts off with more energy storage may not shoot as well if the extra energy is gobbled up by the movement and drag of all the extra moving parts used in that gun. There is no magic involved in any of this, a rollergun will always be less efficient than a classic band gun, but the roller system can allow longer bands than those which could be accommodated on the classic form of band speargun in that same stock length. The mobile sub-pulley systems can halve the length of these longer bands, but then the number of bands have to be doubled to achieve the same energy storage because you have halved the stroke of the band battery in what is now a “cable rollergun”. Every inch that the band battery contracts in length moves the wishbone by two inches as the cable wraps the sub-pulley and is therefore doubled up in the length of cable being moved. What the latter arrangement allows, as the bands are now shorter than classic bands, is that they can be drawn out with a pre-stretch to use up more of the now available gun body length. This pre-stretched or tensioned band battery approach uses the top end of the band energy storage for the shot (the thick end of the triangle) with the energy balance remaining stored in the gun. If you leave the bands in that state then they will eventually crack due to ozone cracking (oxygen preferentially attacks stressed rubber), therefore such guns need a demounting system to take the stress off the bands when the gun is not in use. Without pre-stretch such guns will not outperform classic guns because the bands are too short, having been halved in length and are also propelling moving pulleys which need to be added to create the drive train to the shaft.

The idea of the energy storage diagrams is you view them in sequence as they build up a story, they assume you look at the prior ones rather than just go to the layout that you are currently interested in. Any band speargun can be simplified down to its energy storage diagram, but thicker diameter bands will increase the slope of the “green triangle” used for that band.

This diagram shows how the triangles were sized for the stretch length shown on the various guns depicted in the diagrams.
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Last edited by popgun pete; 06-11-2017 at 10:07 PM. Reason: added a diagram
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