Rollergun basic speargun diagrams

This is the "Z" layout rollergun, it has no preload unless you can drag the rear roller axle back allowing some tension to be developed in the band and then lock the axle into a new position. Extra barrel length would be required. The idea was discussed in the Rollergun Group, but not followed up. Another idea was to run the band through a labyrinth of pulleys to shorten its linear length, but the band going around extra pulleys gobbles up energy from the system.

The simplest reverse band layout cable gun is the "Arrow Gun", so called because there is a US patent on it of that name. In an actual "Arrow Gun" the band is cocked first and then the projectile is fitted, unlike the speargun version shown here. However very early spearguns experimented with cocking the bands first and then fitting the spear and were in a sense true "Arbaletes" or crossbows. The prospect of an early and unreliable trigger mechanism letting go while fitting the spear soon got rid of that idea, plus the spear could fall off the gun, especially as spears are heavier than arrows. Except for the reversed gun being shorter by swapping the slack band's mounting length from the front of the stock to the extended cocking stock at the rear and hence shooting a shorter spear, the prospect of long cables and muzzle rollers seemed to have been a turn-off for a speargun version. The "Arrow Gun" used a folding stock, band cocking system which pushed the "bowstring" back (there was a rigid metal narrow bow frame at the front with roller equipped tips in addition to the muzzle rollers) in order to cock it, so the reversed band idea had some merit in that application.

I have just been thinking about the inverted (and reversed) rollergun compared to the reversed rollergun seen in the second last diagram that I posted. If you are going to bury the band system in a gun which has a timber stock (or even a carbon fibre composite stock) then the stretched band track accommodation slot cut in the stock will take strength away from the front end of the gun, especially if the stock is tapered towards the muzzle. By "inverting" the rear band anchor this slot is now angled under the stock rather than along the longitudinal spine of the gun and parallel to the spear guide track, so the gun stock will be that much stronger. Early cable guns and concealed band guns were box section metal tubing based, I doubt that any of them were made from timber. Other rollerguns usually had external band systems so that bands could be inspected and replaced, not always so easy with the bands hidden inside the gun. The idea of the cable gun was to remove the stretched bands that vibrated as the gun was swung around in the water and hide them inside the gun, not such a problem when spearguns were usually sinkers and no one bothered with buoyancy chambers inside guns that were produced with the lathe, bench vise, hacksaw, assorted files and a drill press.

Rollerguns for the same stock length are more powerful because the band elongation takes up the same linear space where the standard gun has its entire band run, including the slack band length, but is it worth it? If you have a number of guns then you can grab another one if one gun fails, but how quickly can that gun be returned to service? The more complicated the gun then the repairs like changing bands (and cables) will take longer. Historically spearguns got more complicated initially, then they got simpler as divers wanted to spend more time shooting fish than fixing their guns. The crossbow inspired "Arbalete" replaced an early arsenal of rifle barrel inspired guns, you could throw sand in it and even slightly bend the spear and it still functioned. The KISS principle in action.

I own a short rollergun myself, but my pneumatic guns will do everything that it can do, as long as they don't leak air. Not always listening to my own advice I now have to get back to my speargun repairs.

Dual axle, twin band rollergun allows the three band standard gun to "overtake" it with respect to energy storage unless the rollergun uses more preload in its two bands. This is because the rollergun stock is made longer with the dual axle spacing, but there are no changes to its band lengths or elongation which are now simply offset due to the axle spacing, while the standard gun receives longer bands proportional to its added stock length (to match the length of the rollergun).

However if the axle spacing is small, compared to the wishbone travel length on the gun body, then the rollergun stores more energy than the three band standard gun of the same length as both guns are now much longer in the barrel and the standard gun's band length does not change appreciably with the addition of a slightly longer section to its barrel.

This diagram shows the missing loading steps that had not been shown before, plus it shows how the band stretch comparisons were generated. The rollergun is now longer compared to the ones shown in earlier diagrams.

Note that the diagrams shown previously were about the energy stored in the guns, not what comes out with each shot. Bands reversing their travel direction and being bent in a half loop around rollers or pulleys require some energy to be removed from the system in order to do so. During cocking of the bands the energy used to turn the rollers and, if applicable, bend the bands around the rollers is not stored in the bands but immediately dissipated as heat. During the shot that energy has to be resupplied from somewhere and that will be from the remaining energy reserve that is actually stored in the bands. That lowers the efficiency of a rollergun, but not its effectiveness.

The Omer "Cayman ET rollergun" is a speargun that has a variable position rear band anchor provided on the lower deck of the barrel so that you can adjust the total band stretch, the length of band fitted on the gun being fixed (unless you change it separately). This rear anchor adjustment allows the operator to change the energy stored in the gun for different shooting requirements. Any rear anchor location positional change alters both the preload (or residual band tension after the shot) as well as the amount of energy which will be stored to propel the shaft. In the "Rollergun Figure 13" diagram shown here there are three alternative rear anchor positions illustrated, i.e. Max, Medium and Low. The idea is to use the higher force derived from extra band elongation by harvesting only the energy in the upper portion of the energy graph (green area) and not utilizing the rest of the energy (blue area) which is lost when the bands are unhitched for reloading. The first band draw on the top deck, where you reach further forwards on the gun, requires less pull than the second band draw on the lower deck, but when the gun shoots the energy is used in the reverse order to that in which it was applied to the gun.

From the historical record it appears that rollerguns were originally designed as a means to achieve lever-loading of the rubber bands as you can then move the band anchor which is otherwise fixed by moving the lever and drawing the bands backwards underneath the barrel after they go around a set of pulleys or rollers mounted at the muzzle. The "United Service Agency" and "Hurricane" both made lever-loaded rollergun patent applications in France in 1949, the lever operation in a sense "supercharging" the gun with "easier loading" also being a key factor due to the mechanical advantage provided by using a cocking lever. There do not appear to be any rollerguns mentioned prior to these developments that go on to become production spearguns, but users began to realize that the cocking lever could be eliminated and more stretch could be used in the bands than was actually used for the shot by tapping only the "fat end" of the energy graph.

There has been a recent patent to provide an extension of this principle by utilizing another set of pulleys at the muzzle, I have attached modified diagrams from the patent to show how some of the various concepts work. It will be interesting to see if anyone produces any guns, but with more crowding of available space on the gun body the utility of these designs is suspect as there are more things to go wrong. Slack band length and where it fits on the gun is a key factor as bands can only be stretched so far before they yield, so the bands cannot be too short or they will soon be unusable.

The international patent number is WO2012150387A1 published on 8 November 2012 and the inventor is Gueye Lamine; priority date is 2 May 2011 in France. International application was a year later, 2 May 2012.

The bands don't wrap any pulleys at all, what you see on those diagrams is cords or cable winding around back and forth on the gun. The reason these ideas have not been patented before is probably because no one thought that it was worth doing! There is not enough slack band length mounting space on the guns to work with, although the second set of muzzle pulleys idea could add preload to the side-pulley rollergun, but the lower cable draw could not be the length of the gun as shown (equal to the top draw) as it would stretch the bands out too much (unless the gun was very short). The full length cable draws top and bottom each translate to half lengths on the side-pulley (as the cable doubles up on itself), so that is a combined full length draw on the side-pulley with very little room left for the slack bands on the rear end of the gun. Hence the lower cable draw has to be shorter than full length or the bands will be overstretched. Line snagging potential would go up and you have taut bands covering the sides of the gun from end to end, so I don't see these "improvements" having much appeal. Added here for interest as someone else passed the info on to me, being surprised to see such a design at this late stage for rollerguns.

The "Alemanni" rollerguns are a variation on the sub-pulley theme, this is their long range shooter, the Vela 135 Special. As this gun is presented on the "Alemanni" website in plan views (top and bottom) and side elevation we don't need a sketch to show how it works in storing energy.

Note that this cable rollergun has five band groups, but the first four are the bands that perform the propulsion stroke on the spear. Band group 5 is a short band applied to steady the shaft in the gun, then the single wishbone that connects to the other band groups is pulled back to the desired shaft tab position. The moving sub-pulley system halves the tension of the bands going into that wishbone, hence it will be easier to cock than it may at first appear. The stretch factor is low on the bands if you compare the slack (yellow) and elongation (red) lengths, with only one group, band group 2, having any preload (blue) which is shown here only as an estimate, being the longest band group on the gun.

Some rollerguns which are of the band wrapping type In terms of the band going around a roller use a connecting link in some of the bands so that they can branch out into two bands. This means that at one end the band has the wishbone and at the other or rear end there are two anchor points, or four if you count the other two for the paired band on the other side of the gunthat forms that particular band group. You can see these linked or composite bands on the Alemmani "Bluewater Express" which is a double rollergun. The composite bands are indicated by red markers and use the outer set of muzzle rollers on the gun. The central band group is of the top and bottom deck window pulley wrapping type where the bands pass through a window in the stock that contains pulleys or rollers. Thus these bands have their anchor on the top deck forward on the gun body and their run to the muzzle on the bottom deck via the windows in the stock. The Alemmani "Vela 135" also has such bands, but there they are part of a cable rollergun where the front end of the bands are connected to a moving shackle set of rollers which the long cable wishbone passes around and then runs forward to the muzzle rollers which are sized for cable, not rubber.

The composite bands are there to aid loading as the bands can be cocked by drawing the wishbone back with one of the rear anchors engaged and the other one free. The gun can then be flipped over and the second anchor engaged by pulling the second band pair back. In the "Bluewater Express" the rear band pairs are a wraparound band and a tied end band which is essentially a short wishbone to hook over the rear anchor point, this being a pulley mounted in a horizontal drum fashion just in front of the trigger finger guard. In the above diagram we can see the rear anchor B for the composite bands and the forward anchor A for the window wrapping band set. Note that window wrapping band sets are always under prestretch as they are used to operate further up the band stretch gradient of that band set in order to harvest more force from the bands while still having enough real estate on the gun body to occupy, otherwise they would never fit on the gun.

Make up of Composite Bands: Here we will just consider one side of the gun. If the bands were just rubber with no limiting cords inside them then at a node where two bands join onto one band the force each side has to be the same as the node does not move when the gun is cocked. If the two bands on the bifurcated side are at 200% elongation then the force is doubled (as there are two bands) and if the band they are joined onto is of the same diameter then it bears the force that it will produce at 400% elongation which would be too much for it. The limiting cord inside the single band can be sized to stop the band at 200% elongation after which it in effect becomes simply a connecting cable. Taking that cable length you calculate the lengths from the node at the front cable stretched back to the rear anchors. Those lengths will be the stretched lengths, hence if the rear bands are at 200% elongation then those lengths need to be one third of that when you cut the bands to size, allowing for the node length and the wishbone length.

When the gun shoots the bands give up their energy in the sections, so you add up the work energy triangles, not the forces as the contributions from each band are not constant over the wishbone draw length. This is demonstrated by the use of those green triangles on the rollergun Work Energy diagrams. Here I used 200% elongation and the same rubber diameter, but the logic is the same whatever you scale it up to.

Here is another Alemanni rollergun, the "Carbon Stilo Express 115" that uses composite bands on what is a double rollergun. Note that this is a rollergun and not a cable rollergun as the muzzle rollers are sized for stretched rubber. This gun basically uses up all available space on the bottom deck of the gun to stow the bands before they are cocked for shooting. Cocking requires two wishbone draws followed by the setting of the second set of bands of the outer composite bands on the mid anchor point on the gun. For some reason there is a connecting link on the central or inner band group that may be there to attach an optional set of bands in a piggy back fashion to create another rear band set for anchoring on the mid anchor provided by the larger of the two drums.

Briante make some interesting rollerguns, but this gold leaf wrapped gun might bring some hoots and catcalls down at the dock.

This one is more conservative. B3 Vastator 130 cm