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Basic Rollergun Energy Storage Diagrams

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Another rollergun type is the "Fusion" which is a hybrid of the other rollergun types and is distinguished by having a short set of bands which remain on the gun’s top deck and which are connected to the end of the drive cables that wrap the muzzle rollers. That means the bands do not travel around the muzzle rollers at any time, most of the propulsion being derived from the bands on the bottom deck. If the bands top and bottom are the same rubber type and size then the upper set don’t have to be stretch limited, but if the lower bands are thicker or in multiple sets with a bifurcated connection to the drive cable then the top bands need an inner cord to stop them being over stretched. Being stretch limited they add nothing to cocked gun cable tension until the spear is released at which time the top band set can start to contract. The diagram below compares a standard rollergun (if there is such a thing) to the simplest form of a Fusion rollergun.
Fusion rollergun.jpg
 
Because the same band rubber connected in series with a load applied to it always undergoes the same proportion of stretch in the various segments I had to slightly modify the diagram accordingly to work out the energy storage diagrams. This is the result and you can see the Fusion rollergun stores less energy for the shot all things being equal than the standard rollergun. Ergonomically it may be easier to load just because you have something more substantial to grab onto i.e. those short top bands, but giving away wishbone travel has its consequences. Using piggy back bands to pile on more rubber can change the situation and that is usually the purpose of composite band guns that link more bands onto a given run of cable by splitting the rear ends to run to separate anchors on the rear underside of the gun. One advantage of the Fusion rollergun is the bands don't get bent around the rollers where some energy must be lost during the shot.
Fusion rollergun energy comparison.jpg
 
Here is a diagram showing the stretched limited front band and Y connector multi-band battery rollergun. These rollerguns have extra band battery strands arranged in parallel with a series connection to the front band that holds the wishbone. An inner cord in the front band prevents that band being overstretched as all the thicker band battery strands transmit their combined force through it. The advantage of this system is the band battery can be cocked in stages or the gun fired without all bands hooked up on their anchors. Usually one of the band battery strands is permanently anchored as you to have something to pull against when setting the wishbone on the shaft notch or tab.
limited stretch band rollergun.jpg

Basically the stretch limited band is like a gun bungee with the line or cable coiled up inside it. This band will only stretch as far as the inner cable allows it. By using this “variable length” cable wishbone it provides space on the gun for a rear band battery, as otherwise the stroke of the band battery would not drive the spear for the full length of the gun, which is the whole idea of a rollergun. The alternative is the sub pulley system as that doubles the stroke of the band battery as seen by the spear, which means the band battery then takes up only half of the lower gun deck. The downside of the latter is the forces are halved, one half goes to the spear and the other half to the muzzle cable anchor position.
 
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This is Salvimar’s “Tomahawk” which uses this principle, the thinner black band when cocked wraps the muzzle rollers and occupies the top deck while the thicker red band stretches out on the bottom deck stopping just short of the muzzle rollers. The black band has an inner core so that at a certain point it no longer stretches but starts dragging the red band forwards as you cock the gun. The red band can then be dragged to further back anchoring points to increase band tension in the red band which transits via the black band inner core to the wishbone engaging the shaft. Additional rear bands can be added to the band battery operating in series with the black band and in parallel with the red band.
salvimar tomahawk stretch limited series band roller gun.jpg
 
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Note that if we took the fully cocked "Tomahawk" gun and ran a razor blade over the black band causing it to split simultaneously on both sides then the tension on the shaft wishbone would not change as once pulled to its limit the black band just sits there acting as a sleeve over the taut inner cable. The black band cannot transmit any force to the inner cable until it is able to start hauling on the cord knots that act as limits to its stretch where the rubber is clamped on the inner cable at either end. Only when the tension falls to its own inherent stretch value can the black band begin to contract and that occurs as the band battery force drops as it contracts.
 
My 2 cents from a scientific point - are rollers really a thing?

Why I cant wrap my head around them is rather simple - when loading bands is not an issue why would I use a roller setup over simple gun if one can accelerate the shaft to its terminal velocity at a desired shaft weight?

After seeing some russian guy scientific tests some time ago on the pneumatic gun where he compared load pressures to shaft weight, the conclusion was fairly simple - adding more power does nothing if you reach terminal velocity (was it around 27-28m/s?) at desired shaft weight.
Once you reach terminal that the only way to increase range of the gun and shaft penetration would be to increase shaft weight while maintaining the terminal velocity untill you cant add more power to the bands/pressure.

The question is, has anyone really ever done something similar with bands setups to determine when the shaft velocity goes down/its energy based on its weight to power ratio of rubber elengation, AND how do roller guns cope in this comparison to simple guns and where are the limits of each setup to trully be able to say that X or Y is better.

The closest tests Ive seen that have scientific value for rubber guns are majdq8/spearo8 tests on YT.
However so much more could be done in this topic, but I understand its time/money.

What I think above all, that pretty much any gun brand will try to convince me that they are awsome with little to no real value research. I am sorry, but I wont trust someones 'feel' that X or Y is better.
Bring me the research, prove me your gun can shoot further, more precise and/or with bigger momentum.

Its like comparing 2 race cars on a 1/4 mile race. No clock, no timing and each driver saying he was faster.

Ill gladly be put away with my doubts, but I havent found a satisfying information and I really tried. I think its time that we stop the marketing nonsense.


Sometime I wish I was an engineering student able to access all needed equipment and tools to run such tests becouse all in all, no real equipment is needed to do such tests but just simple and heavy cast with shaft guide firmly fixated and trigger mech to test the correlations of band power to shaft weight in first place. Second thing would be to compare roller setup(s) to above.
Im pretty sure a uni student could do it writing their finals
 
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Terminal velocity is a notion that has crept into speargun discussions, but there is no such thing. What they are really talking about is the maximum stable launch velocity out of a band gun. An unsupported spear can be free to buckle under acceleration and bounce off the gun deck sending the shaft anywhere, whereas a similar sized shaft can be blasted at much higher velocities out of a hydropneumatic gun as the spear is enclosed in a barrel. The attraction of rollerguns is that inherent losses in the propulsion system take the edge off the acceleration which technically is the “jerk”, jerk being the change of acceleration with time, just as acceleration is the change of velocity with time.

If you look through the diagrams on this thread for equivalent sized bands the rollerguns are not necessarily more powerful, in fact they are often less. But a standard gun piled up with bands has such a massive jerk when you pull the trigger that unless the gun is a ballasted monster it is not really that useful to shoot with. Such large guns can also be a pain to swim with, although often banded up and ready to go the diver often drops in beside a teased up whopper with one and blasts the fish from relatively close range.

The more elaborate rollerguns allow much longer bands to be installed on a gun body, yet bending bands around roller and hauling moving pulleys on cables all contribute to extra losses which make the gun easier to shoot at the expense of taking longer to reload. This is all shown in the diagrams by those green energy triangles, you just have to study them carefully.

The inverted rollerguns still recoil as is shown by these photos. Here the gun is tilting up with the shot and moving rearwards, lifting the spear up slighty as it departs.
DB1-2XRW.jpg
 
Terminal velocity is a notion that has crept into speargun discussions, but there is no such thing. What they are really talking about is the maximum stable launch velocity out of a band gun. An unsupported spear can be free to buckle under acceleration and bounce off the gun deck sending the shaft anywhere, whereas a similar sized shaft can be blasted at much higher velocities out of a hydropneumatic gun as the spear is enclosed in a barrel. The attraction of rollerguns is that inherent losses in the propulsion system take the edge off the acceleration which technically is the “jerk”, jerk being the change of acceleration with time, just as acceleration is the change of velocity with time.

If you look through the diagrams on this thread for equivalent sized bands the rollerguns are not necessarily more powerful, in fact they are often less. But a standard gun piled up with bands has such a massive jerk when you pull the trigger that unless the gun is a ballasted monster it is not really that useful to shoot with. Such large guns can also be a pain to swim with, although often banded up and ready to go the diver often drops in beside a teased up whopper with one and blasts the fish from relatively close range.

The more elaborate rollerguns allow much longer bands to be installed on a gun body, yet bending bands around roller and hauling moving pulleys on cables all contribute to extra losses which make the gun easier to shoot at the expense of taking longer to reload. This is all shown in the diagrams by those green energy triangles, you just have to study them carefully.

The inverted rollerguns still recoil as is shown by these photos. Here the gun is tilting up with the shot and moving rearwards, lifting the spear up slighty as it departs.
View attachment 58707

I think we have to talk about both terminal velocity and stable one and separate well the discussion on the scientific level hence why I think the tests should be done on a fully stable platform.

Unfortunately in my eyes A LOT discussion is on the feeling not backup by any research.


The russian guy tested various shaft sizes going from small diam to big and high pressures and he was not able to surpass some speeds regardless of power setting. So there is sth to it if lets say scenarios that were tested was sth like this (unfortunately cant find the research anymore):
7mm - 27m/s PSI X (chamber pressure - call it power factor)
6.5mm - 27m/s PSI X or PSI+1X (going down in diameter and/or extra pressure added 0 benefit to muzzle exit and or Joules in
penetration.

So if we operate under this concept that majdaq8 is also mentioning and can be seen in his video that often addding power does not mean energy. This can be both atributed to terminal velocity and lost energy due to unstability where the whip takes the energy off the shaft.

Proper tests should be done couse literally all the discussion here and there IS purely theoretical. We might understand how thing work more or less yet engineers are often surprised in areas where there isnt enough testing done and all things are hypothetical.

Ive tried to look at band stretch equations and most of them are done dry, water as medium changes the results becouse of both density properties, cavitation and water springyness(albait the letter one we can almost nulify as it should be within marginal error as at 100PSI its just ~1%

I mean where else to start from if not from basics and go further into theorethical applications?

Regarding the muzzle lift on release - I feel this topic is so complicated that videos like this might not really reflect that real behavior. There is SO MANY FACTORS involved that I would not look into it (simple video) so much yet unless we can do a very good aplication of band strentch/power ratios in first step.
Not saying it is not a thing (becouse it is, I did use an omer cayman 100 with HF head with double 380% 14mm setup and 6.25mm shafts at some point xD), but we should take it step by step instead of fighting with a behavior that can be probably minimalized if we have the proper tool to judge the power to weight/diameter ratios.

I am thinking about simple tool we could use/make to actually measure the shaft muzzle exit velocity on a daily basis to judge band wear or just simple have a way to adjust band lenght properly. Imagine we had a simple and cheap tool to measure this things, a lot good would come out of it and so many myths could be trashed.

On totally side note, talking about proper science, CARBON guns for deep spearfishing in theory are a BAD material. Why?
Carbon has great tensile strengh but its crap at compression, I have seen at least 2 tubes brake at 30m+ ;) .
 
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The hydropneumatic guns were shooting with 1,500 psi, which is way more than any pneumatic speargun. Expellable gas weapons have used even higher pressures, but lose pressure with every shot. If the shaft column bends under a high drive force on its tail it can bounce off the gun deck, which is the stability we are talking about. Band guns don’t have the ability to shoot at those levels, but float after the shot and are loaded with the diver’s muscular effort. All the gas guns are sinkers, the floating pneumatics are basically spring guns using an air column as a spring and are tail end spear drivers, unlike band guns. There have been tail end drive band guns, but these had other problems, such as this one.
rebikoff advert.jpg
 
We are a bit lucky ;)
Trying to find some research it looks like Rob Allen started doing some proper testing with (not) so high speed camera but its enough to extrapolate the results. Only downside I dont like they do not use numerations on their graphs, instead just plain graphs with XY but no adnotation of the values. Seems like they will test further upcoming days including the multi pulley systems to get more travel on the line than the band stretch itself.

Interesting part said, not sure if it was said here earlier, is that the rubber has its contraction speed limited to Z regardless of the stretch, so indeed in theory multi pulley system is the only way to get more speed out of the rubber. There were some hints in comments by RA that this might be a smart solution to the above.
 
Retraction velocity of band rubber depends on the load it is pulling. Moving side pulley inverted cable rollerguns have the pulley on the end of the band battery moving at half the speed of the wishbone, but the load imposed by the spear and the folded cable system on the band slows it down and the shaft is only being driven by half the force from the band battery. There are no energy multipliers, everything the bands have to move takes energy from the system, and that includes drag on all moving parts being pulled through the water. The more parts, more drag.
gun diag b.jpg
 
According to some data from spearfishing GR max band contraccion speed with no load is around 50m/s (180km/h).
Medical sources (accidents) - estimate the muzzle velocities for spearguns at around 45m/s.

Spearfishing GR were able to reach around 33-35m/s (125-133km/h) pretty much across the diameters, with fastests shafts being the lightest. There is room for slight improvement maybe but indeed, at those speeds everything we do to improve might require such a hassle due to diminishing returns that its simply not worth it.

I feel more focus should be on finding the sweet spots with band elongations/type in regards to shaft diameters and weight to end a lot of this nonsense and urban legends.

Once we have the data of terminal velocity in least power setup there could be done tests comparing the recoil of any setup and the differences between simple/roller design.
It would be interesting to see if difference in recoil is not as bad as our guts tell us at optimal controled enviroment.

PS. Its fun and stupid how much chaos one can put in huge spanish spearfishing whatsap group by providing scientific research LOL. I need more popcorn.

Not really on the topic exacly but it seems salvimar released tables what im asking regarding bands/shafts but the data looks quite off what majdaq8 tested in the pools and they quite convenietly forget to add shaft speeds.
If someone requests I may add the tables.
 
If we transpose the bands and their stretch onto a standard three band gun then we can see the only real difference if we neglect the stretchy wishbone (the orange band) is that for the rollergun the band battery power goes through a band being squashed and bent in a half circle around a roller which takes energy out of the system and acts as a sort of shock absorber. On the standard gun the band power goes straight into the spear, there being no shock absorber, and that is what makes the difference to the felt recoil.
limited stretch band rollergun vs band gun.jpg
 
The problem with rollers will always be the loading strength translated to power output. What's going for rollers is they possess linear power delivery, which if put on a graph would equate to more meat on the power to distance volumetric output. But most people can't load enough tension in the rubbers to make the roller worth it

Unless you run the rigging as such for mechanical advantage in the loading process. Which was my goal
 

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The problem with rollers will always be the loading strength translated to power output. What's going for rollers is they possess linear power delivery, which if put on a graph would equate to more meat on the power to distance volumetric output. But most people can't load enough tension in the rubbers to make the roller worth it

Unless you run the rigging as such for mechanical advantage in the loading process. Which was my goal

What do you mean cant load enough tension in the rubber? We are not constricted/limited to loading capabilities but rubbers stretch limits most often than not!
 
If we transpose the bands and their stretch onto a standard three band gun then we can see the only real difference if we neglect the stretchy wishbone (the orange band) is that for the rollergun the band battery power goes through a band being squashed and bent in a half circle around a roller which takes energy out of the system and acts as a sort of shock absorber. On the standard gun the band power goes straight into the spear, there being no shock absorber, and that is what makes the difference to the felt recoil.
View attachment 58732

Pete I think spearfishing GR sums the less recoil part better.

Due to shaft being accelerated in longer distance and over longer time in a roller, the kick is not as bad as in simple gun.
They also explain that a simple 100cm gun with rubber anchor 100cm ahead of the head would have same recoil as a roller. Its not the back movement or rollers acting as a shock obsorber but simple power*time/distance difference is what makes the recoil difference


Pete I cant understsnd the other topic claim - same stretch ratio a roller will have a lower load on a trigger than simple gun?
 
As an applied physicist I know what I am talking about, but the answer is when a band does more than contract it loses energy as heat in the rubber rather than using it all to propel a spear. Basically schoolboy physics for that matter. Such as bending and squashing it out of shape during the power stroke. Actually bands always lose some energy as when you first load the gun the force in the band drops off slightly as the rubber relaxes, which is termed band soak. Band energy for practical purposes can be represented as a linear output with distance which is what those green triangles are about. When I did the diagrams I used a standardised stretch diagram, this one which has been shown here before.
rollergun band lengths.jpg

And this one.
rollergun band stretch.jpg
 
Pete as not physics It woulld be great if you could give a small summary of results, that is in order for someone to not missinterpret the diagrams.

Im doing my best:
So the F (force) is always the same for same band stretch regardless of lenght of the rubber. This is ofc not counting any energy disipations/lost on rollers or other means etc.
 
If you stretch a piece of rubber to the same elongation or stretch factor it develops exactly the same force, provided that rubber is exactly of the same stuff. But there is more energy stored in the longer piece of rubber as work is the force, which declines as the rubber contracts, applied over the displacement of the rubber or its travel distance. Work and Energy are the same thing. Work is defined as force times distance (F = M x A [acceleration which itself is L/T^2]) which is ML/T^2 x L = ML^2/T^2. Energy is 0.5 x MV^2 where Velocity V is L/T and velocity squared is L^2/T^2, thus Energy is ML^2/T^2, the same units as work. M is mass, L is length, T is time. This is how Physics defines such quantities.
 
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This one completes the story as far as the energy storage goes. Assuming all the bands here are the same.
Rollergun combined or composite bands.jpg
 
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