Basic Rollergun Energy Storage Diagrams

You can see claims of rubber being taken to 600 or even 800 percent elongation as a material, but that is not realistic for speargun power bands. At higher elongations rubber begins to yield and will not return fully to its original length or strength. Spearguns use around 300 percent elongation, if you use much more then the bands get tired and don't perform like they used to. What you need in a speargun are reproducible shots. Using rubber bands at around 300 percent elongation the results are consistent. The stretch limited bands are to preserve the elasticity of the rubber from being overstrained in the hybrid rollerguns. Truly elastic materials don't lose their shape and return energy from being deformed, in that sense rubber is not truly elastic, but we use the term in that sense.

An example of elastic collisions is those metal balls on strings in a suspension frame swinging back and forth clicking into each other in groups. They continue swinging due to the high elasticity of steel balls. Also think of those very hard "superballs" that you could drop from a high vantage point and bounce right back up to you, which is elastic behaviour.

In order to have our power bands perform elastically in the technical sense we use low stretch factors so that they operate for a long period as reliable energy storage mediums. For a time I worked in the rubber industry and know that there were many laboratory tests used to routinely assess the characteristics of different blends or compounds of rubber, one of them being their elongation under load and repeated cycling.

If we look at the Salvimar Tomahawk which is a production rollergun using stretch limited front bands we can estimate the elongations used by laying out the static band length against what it is stretched. Note that a band with 100 percent elongation is stretched by a factor of two, a band with 200 percent elongation is stretched by a factor of three and so on. We can see from the photo that Salvimar keep the band stretch factors from being too large, remaining below 300 percent elongation or a stretch factor of four. Bear in mind that when stretched the front band wraps the muzzle rollers.

Note that a second rear band, not shown here, has to stretch further onto the rearmost anchor point.

Looking from underneath the Salvimar Tomahawk with all bands cocked.

Black_Reef_Co said:

Question how do you interpret band stretch %? i.e is 300% 3x the original length of the band or 4x?

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The definitions are not mine. People offer confusing descriptions of stretch. When you want to express how much rubber is stretched by then you use elongation, which you then add to the original length to come to the total length. When comparing the new length to the original length it is the stretch factor, a multiple of the original length.

Another look at gun comparisons.

Bands being used outside their linear stretch zone are useless for spearfishing. Rubber compounders try to find a formulation that gives consistent performance, but as their ingredients change from year to year with natural rubber depending on growing conditions they have to tweak their recipes so to speak. Some finding a good batch of band rubber will store a huge roll of the stuff in sealed conditions rather than take a chance on the next batch. Of course eventually they have to as even stored rubber slowly deteriorates. Bulk band rubber can be stored in tyremakers talc to stop sticking. Oxygen is the enemy of rubber and ozone, a form of oxygen, is the worst as it causes rubber cracking. Once produced band rubber is under attack from oxygen from the day that it is made, new bands are often protected to a certain degree by a coating, but with use that wears off. The protection is not absolute, even unused new old stock bands will go off as the rubber out gases and oxidizes.

Black_Reef_Co said:

Totally! But the problem I see is most people cannot load the necessary tension in let's say a single roller to achieve the necessary stretch % to truly take advantage of the roller platform. The power goes parabolic after 400% (graph 1 pictured) Load assists help but I would say that the majority are better off going with a two banded traditional over a single roller. This is just my opinion as a manufacturer tailoring to the niche dive crowd of south FL.

I am a huge fan of "mono rollers" which breaks the load into two stages. Yet what nobody speaks of is that rubber also resists returning to its original form (graph 2 below) so it loses significant power having to redirect its direction of momentum round a wheel at the muzzle. So I went with my design pictured in the last comment for this reason with dyneema around the pulleys like an inverter

pics courtesy of the spear guru John at Linghunt

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Testing methods have a bearing on the results because rubber relaxes with time. At a fixed elongation the force in the band begins to decrease until it hits a stable value. Dangling a band with a fixed weight the band gradually lengthens then stops. To get the most out of band cocking you should fire your gun immediately as half an hour after cocking some energy has already been lost. Practically we cock our gun and swim around looking for a victim, so we have to tolerate the band soak. When we fire the band retracts instantly, not in stages, so static figures taken in lab measurements of stretch versus force are only a guide. In the rubber industry a machine called an Instron (the manufacturer) has a motor driven screw drive that pulls rubber at a constant rate and a load cell in the puller measures the force at any instant. The results are speed dependent, so tests are done at set speeds, none of which resemble band retraction velocity. The latter is not an absolute, it depends on the load being pulled, heavy loads slow the band down.

The testing machine can also compress rubber as well as stretch it. Rubber samples are made in set sizes to be tested using special moulds and the rubber compound is mixed in small batches or a sample withdrawn from production batches which are then used for quality control. It is not like baking a cake as the ingredients can vary even though nominally it is the same stuff. I remember when the rubber testing lab was looking at samples taken during the day and noted a strange fact, the samples placed end to end all matched like a jigsaw, so were in fact one long lump that the sampler had cut up and submitted as individual samples. The culprit was fired on the spot.