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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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I plan to have rubber for the power source ( only about 40mm in length for each is my guess) these are stretched as the eccentric cams roll over. Even largish eccentrics over 100mm are only going to give 300mm of draw length, this has always been the problem with a compound speargun. The eureka moment was realising that we could use pulleys to increase this draw length. I have not decided on where or how I will locate the eccentrics yet but I am favouring having two pairs of them in sheave boxes in a vertical position. The beauty of reversing the power stroke isnt just in its efficiency of throwing a spear, it should be able to throw a thinner spear as the softer start will cause less chance of whip or porpoising. Once I have these current demultiplied guns finished & taken for a swim I think I will have a better idea on how to best use the avaliable real-estate on the speargun stock for ,rubbers, eccentrics & pulleys.
 
That is way too complicated for me. What I am wanting to do next is very simple in comparison. I have no intentions of trying to produce something at a comercial scale & am just curious to see what is going to be the result of this. As I have mentioned a little over 20 years ago I did have a basic compound speargun, it worked, but its draw length was very short, too short to be practical, that problem & the concept has stuck in my head & when I got to see & hold a demultiplied gun I thought that combining the two concepts might be the solution. I still have those eccentrics from decades ago, but If I am going to use two pairs I will try to source four new identical ones. I really am very hands on & I need to have working models to see how things work & then work through & solve any problems. I dont mind making several pairs of spearguns & also seeking peer review & honest appraisal from everyone on each new concept. I hope you dont mind if I post photos & discuss them here on your thread? It may not work, but I'm sure I will come away a bit wiser about eccentrics & pulleys. Hopefully I will get a day this weekend to shape up the de multiplied guns & start to fit them out.
 
Here is the up date on the construction of some of the demultiplied guns discussed so far. The basic carpentry is done, its just a matter of fitting & making some of the hardware. Its the first time I have played with a cuttlefish shape & it feels different, not in a bad way just different. I'nm calling these "claymores" because the shape, size & feel reminds me of the large Scottish broadsword not a speargun. Hopefully these will be getting wet by Christmas & given a fair comparison & appraisal with guns of equal utility. This exercise has been helpful & given me several options on how to fit true eccentric cams on a speargun. Hopefully its a step in the right
direction.

 
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This is how simple it is to set up the pulleys. I have used two D shackles with one bolt through both to hold the pulleys. The rubbers will be tied to the D shackles & I expect it to be reasonably neat even with four rubbers on each. The gun on the left shows the position of the pulleys in the loaded, ready to shoot position. The pulleys on the right are in the fired position.

 
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It feels like its taken forever, although it is a prety hectic time of the year. The "claymores" now have four coats of "2 pack" & just need a few bits of hardware made & fitted. They should be wet by the new year if I can get back to the coast from the farm & the weather stays good. These are "big" guns, those are a couple of my largest 4 rubber guns on their right & all those spears are 1.8m long. I havent thought about ballst ATM, just wanting to see if the concept really is worth pursuing before going any further.
 
This work really needs its own thread as someone glancing at the front page here will not realize what follows is not more of the same diagrams and may not bother to go to the next page.
 
Pete,
Thanks for writing all this up. Aren't you assuming for the Vela 135 Special that the photo shows the bands in a slacked state when there is probably some existing preload on it? You can see in this video:
that the stretch on the non-roller bands is about 3.5x

Basically
 
I suppose you can take any gun design and store as much as energy as you want with increased number/thickness of rubbers and stretch. At some point you exceed how much a particular shaft can handle before it starts whipping but then you can just go up to the next diameter. Pete, what do you think is the limit for energy storage for a given gun? Recoil? Available space for bands?
 
A gun can be powered up to shoot very heavy spears as was shown with the "Supergun" built by John Warren, that was an exercise in seeing how powerful a band gun can be and was inspired by the 10 band "lapped band" gun built by Jack Prodanovich (2 x 5 band muzzles spaced longitudinally on the gun). Neither of those guns were rollerguns and shot very large diameter spears. In practical terms there is no need to power a shaft beyond what is required to do the job, plus you start to enter a region of diminishing returns where more and more energy is needed to buy each extra increment of performance. Increased gun mass can get rid of recoil, but then you are either swimming with a log or the gun is a sinker, plus there is considerable reloading time and effort if the gun has many bands. The "Alemmani" cable rollerguns make use of available real estate on the gun body to cram on many bands, but with that comes the prospect of increased maintenance when checking all the parts and the time required for any repairs and band replacements.

One advantage of cable rollerguns is the rubber mountain of cocked bands on a standard multi-band gun has been redistributed around the gun, mainly being stowed underneath the gun or inside its body contours which can improve the drag when turning and swinging the gun. That gives you a better sighting line on a top deck largely free of clutter. The downside is the efficiency is less due to the movement and hydrodynamic drag as all those extra parts are spun around or pulled through the water.

Practicality and utility should determine the characteristics of a gun rather than how much energy can be stored in search of maximum power, especially in a gun put to general spearfishing use. A balanced gun which shoots reliably and consistently is what most divers need and the patience to use it to its best advantage in the situations it was designed for.
 

It seems like the cable rollerguns have gotten complicated enough that some kind of a pneumatic would provide the same low recoil and low profile benefits. Air, unlike rubber, doesn't have a practical limit for failure stress and also weighs very little. So the limit to energy storage for pneumatic is only the strength of the components subjected to the differential pressures. The implication of this is you can have a gun that provides adequate power for the situation with a smaller footprint and mass.

Why do you think people are reluctant about using pneumatics assuming you can use something like a Mirage or one of the Russian guns with the lever pump to split the loading effort into multiple stages? Is your go-to gun a pneumatic or rubber gun?
 
Band guns are easier to load as you load from the rear and you can do it in multiple stages with separate bands and wishbone notches or shaft tabs. Pneumatic guns are all or nothing and easier to load when shorter, but muzzle loading becomes more awkward as the guns get longer. Bigger fish, longer distances and I use my band guns, particularly in rougher waters as when being moved around on the surface it is hard to muzzle load a pneumatic gun unless the gun is short to medium length. I thought the lever-loaded hydropneumatic gun was an answer to my prayers as you can load a long gun, much longer than a pneumatic, without the dangers of muzzle loading. The spear can match that of any band gun, unlike most pneumatic guns, because it is muzzle loaded, but not cocked. Powerful, quiet and straight shooters, they seemed the perfect gun, but they don't float after the shot and that is their biggest drawback. You can fit them with a buoyancy stock so the gun looks like a rifle, but then it loses the slim form that made it very handy to swing around in the water. Then there is the maintenance, it is not arduous, but if you don't do it then the gun can fail, plus there is no guarantee that you can get parts. The advantage of all the pneumatic guns is no bands to check or replace, if the gun still has its pressure then you are good to go. They are generally reliable, but when you get a leak it is not necessarily a quick fix, whereas with a band gun you just change the band, assuming that you have a spare to hand. Band guns are pretty foolproof and can be abused, up to a point, and still work, which suits a lot of spearfishermen who don't spend much time on their guns.

By having a number of guns of different types you can keep your options open, but too many guns and you may be a lousy shot with all of them. I knew a guy who only had one band gun, but was absolutely deadly with it and seemed to never miss, but he always picked his moment of when to shoot and knew his fish and their behavior. When they seemed to least expect it is when he let them have it.
 
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I guess that another use of pulleys on a gun could be to reduce the effort needed to load a very strong rubber or multiple rubbers. I used a pulley on a Ray Odor spear once and I liked the way it worked but decided that it was totally overkill and unnecessarily complicated for my part of the world. In the right conditions though I can imagine the principle applied to pole spears could be very effective. The only downside is that you get nothing for nothing so reducing the load strength by half meant double the draw length of rope through the pulley so a really short rubber is required if using the principle on a pole spear............. that's what i found anyway
 

There has been a new expression pop up in the spearfishing vernacular in the last few years "dialing it in". This refers to the tunning required to get the roller & demultiplied guns to shoot correctly. Rubbers have to be changed & cables wear. I have seen a reluctance from people who use these machines to do maintenance on them, not because they are lazy but because its not as simple as just changing a rubber or cable, it requires tunning again. These guns are great for people who love to tinker with their kit, but from my experience, if you are going on an extended spearfishing trip to remote locations, make sure you have lots of spares & a fallback option so your trip isnt ruined.
For those who really do like to tinker; one of the problems with pulleys is that doubling of draw length of the cable to get the let off & speed, this means that there isnt often enough real estate on the gun stock; if using eccentric cams (a proper compound speargun) the draw length is dependent upon the circumference of the cams, (you cant really use 300mm eccentrics on a speargun) & there isnt enough draw length; used together the problems from one becomes the solution for the other. It also helps with the problem of needing twice as many rubbers on a demulpiplied gun, by using eccentric cams the rubbers can be as short as 40mm & of course the power stroke is reversed like with a compound bow which means spears not only are faster but they can be thinner as they aren't starting in 5th gear so acceleration is much smoother.
I made a compound speargun over 25 years ago, but it was the drawlength & the need for large cams which stumped me from going further. The first time I saw a demultiplied gun I thought "eureka" that is the solution to the problems with both. I have been fiddling a wee bit with them again over the last 12 months, but actual spearfishing has gotten in the way a bit, also I have been turned off by the falures & increassed maintenance required of some very expensive roller type guns on extended spearfishing trips. It would however be nice to see others attempts at doing this as I have no doubt that eccentric cams will be incorporated into demultiplied guns in the future.
 
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The most likely candidate for using cams is the "Dreamair" speargun which is a cam and cable drive speargun, but it is not really a rollergun as there are no rollers in it. As the cams are spiral track wound drums they provide variable gearing from a cable running inside the gun to external cables which are in a sense long wishbones. The length of the cable drive inside and outside is different as the spiral track length of the drums determines the length that can be wound on and off in each case. There is a long running thread on this gun here: https://forums.deeperblue.com/threads/infinitengines-dreamair-pneumatic-speargun.101674/

Also discussed here: http://www.spearfishingforum.gr/viewtopic.php?f=19&t=41714&start=345
 
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For those who cannot be bothered clicking on links here is the "Dreamair" cable gun with CVT action winding drums. Instead of a "band battery" the "Dreamair" employs a closed system pneumatic energy storage accumulator that never faces ambient pressure. This is because when its piston moves forwards in the gun a vacuum inside the gun’s barrel develops behind it. The piston does not drive the spear directly, instead it operates via two sets of cables, inner and outer, thus operating somewhat like a high speed winch with inner and outer winding drums mounted by keyways on a common axle. The axle and drums rotate as a combined unit, thus there are no rollers in the gun, with pressure seals isolating the inner drum and barrel bore from the environment with only the axle shaft crossing a high pressure boundary.

An analogy of how the “Dreamair” speargun functions would be an aircraft carrier steam catapult system for launching fighter jets off the relatively short carrier flight deck. The CVT aspect is due to the inner and outer drums functioning as variable cams as at any given point of rotation the inner and outer lever arm lengths change and hence the gearing or leverage changes. The varying torque on the axle is realized as different forces in the respective cables as torque T equals force F inner cable times lever arm radius inner drum which also equals F outer cable times lever arm radius outer.drum.




https://www.facebook.com/video.php?v=1868325896747000
 
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