Posts by popgun pete

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.

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.

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.

Quote from tromic

Pete, from your last sketch we can see two important facts regarding inverted roller:

1. Moving of the shaft is in opposite direction from moving the bands. Result is much lower recoil, almost no recoil at all so the gun could be very precise.
2. Speed of bands is 1/2 of shaft speed. That means lower frictional and drag losses of bands in water.

Bands usually have starting tension pretty high so shaft is being accelerated with high force all the time (similar to pneumatic guns).

The term "inverted roller" seems to have different meanings when looking around the web, including guns with the entire system reversed in the gun to place the "muzzle" pulley at the rear end. I can only assume they use the "Z" arrangement when seen in profile.

Yes, the cable rollergun seems OK in this configuration as the bands themselves do not wrap anything, they just have a straight pull.

However the problem with cable guns is the continued reliable tracking of the cables. In the old days (home-built) cable guns proved to be unreliable, they would have used steel wire cables back then to connect to the rubber band pack hidden inside the gun body and any kink or set in the cables would have had them jump the muzzle rollers and snag. The cables could then create a tangle of loops if some part of the cable stopped while other parts kept moving so that the cable in a sense caught up with itself. I don't think the old cable guns used the "block and tackle" system, but they certainly used rollers. Lighter synthetic cord cables are less likely to develop kinks and have lower mass than steel cables so they have less tendency to overshoot when going around the rollers, but on a long run of initially high speed moving cable any vibration in the cable could cause the cables to jump their roller flanges which are much smaller than they are for band rollers. There is a potential for the small cable rollers not to turn under high loads, so the cables will skid on the surface of the rollers. Any slip then stick action, even with the presence of water, may cause the cable to jump the rollers. I don't know the actual mechanism, but cables have a history of not being reliable in this application in what were then home-built guns. When new everything probably works OK, it is when things start to wear out that the problems can develop.

Although the intention of this thread was to show the simple relationship between slack band length, band stretch and energy storage in the rollergun, I guess it was inevitable that it would become a design permutation thread. However the same basic "rules" apply, so once you understand the principles you can understand all of them, no matter how complicated they look. The cable gun, or the cable rollergun, has long wishbones that run the full length of the gun's top deck, so if the rear end of the cable wishbone pushing the spear traverses the full length of the gun then so must the front end of the cable where it attaches to the band, only it will be travelling in the opposite direction. That means there will be nowhere to put the bands on the gun unless the bands wrap forwards again, in a sort of a "Z" arrangement (for the combined run of the cables and bands) which really adds nothing to the gun. In order to halve the lower deck (or sidewalls) cable travel compared to its operating stroke on the top deck a moving secondary pulley is used that creates a "block and tackle" arrangement connecting to the bands, hence for every 2 cm of cable travel on the top deck this results in 1 cm of travel on the lower deck (or sidewalls) as the cable has to double up on itself going around the secondary pulley. That means the bands can now be half the length that they normally would be on a speargun, but the secondary pulley system halves the tension going into the wishbone cable, so the usual response is to double the sets of bands being pulled by the secondary pulley in order to restore the gun's energy levels. Note that the energy stored is not necessarily all used for propelling the shaft, so the gun may be less powerful than it looks as a roller system always introduces some additional losses.

Something else to think about.

As Dan raised the issue of a double band gun, I tried the same diagrammatic approach to comparing one to a rollergun of the same spear and stock length.

In a revised diagram the "equivalent" single band standard gun has the same length spear as the rollergun, but then the spear is too short with the speartip sitting rearwards of the muzzle.

Here is the third diagram in the set. The intention was just to look at the single band case, the drawings are schematics and not meant to be prescriptive for gun design, but allow some appreciation of the forces at work. Spearguns are simple devices, but getting the designs to work properly needs some attention to detail with respect to the components and their layout. The standard single band gun is simply used as a reference in the diagrams.

They say a picture is worth a thousand words, so here is the rollergun in a couple of diagrams. Most rollerguns are variations on the themes shown here, they just change the method of second stage loading. Meanwhile fish depart while you struggle with getting it ready for the next shot, but of course it depends on what and where you are hunting.

The notch spacing was probably to allow bands to stack without the wishbone ferrules overlapping. A lot of European spearguns used the ferrule system and so did Australian spearguns. Georges Beuchat patented the ferrule system in 1943, then everyone copied it! Photo is of the "Sea Hornet" bands, "Undersee" bands were very similar. If you went into a store to buy bands then this is what was available for decades. In the USA the less bulky "wedding ring" band system was used, so bands could be used stacked closer together. The notch spacing on the shaft may also be to provide knuckle room when pulling successive bands back with palms up, that is how I load my bands.

Quote from marginatus

did you inquired the factory why they were made that way? It could be one of those cases "We always did it like this... and nobody asked for it differently"

Maybe not always, check the close-up photo of the notches on speargun number one!

Prodanovich_Potts_Early_Guns

Notches were easy to cut with a hacksaw, so that is how shafts were made originally.

Here is another "professional" fishing gadget from eBay a few months back, spear never leaves the "gun", so these devices are intended to be short range fish stickers and are probably used by people wading in the shallows hunting flatfish. At least this one from Korea has raised the bar in terms of quality, not that it had far to lift.

Band "power" as in loop rubber band from the stationery cupboard. The progression from dive bin to trash bin will be rapid. I doubt the rubber would last with UV and saltwater and as Dan says rust will soon claim the rest of it.

The rubber plugs are tapered bungs, so they are just driven in and the tight fit keeps them in place. If you glue them in then they will be hard to remove. Pressure at depth will try to push them in further, so you need the right size for your barrel tube. In the old days they used corks of a similar shape and you pushed them in with a rod that just fitted in the tube and with blows from a soft-faced hammer. Wet the bungs slightly to aid knocking them in.

Quote from Don Paul

Here is a vintage twin barrel Submarine.
The gun came to me with flaking black paint that has preserved the twin blue anodized barrels.
The shaft can handle two power bands, which are centered along the axis of the shaft.
I will need Pop Gun Pete's help with patent details as I know very little about the gun.
More images later. I usually don't restore my vintage guns, other than a soft wash, but may remove the paint on this one.

Cheers, Don
PS
I have a lot more guns to post here as time go's on.

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I have seen a photo of one of these before, the barrels are blue and the grip is grey anodized under that black paint. The upper and lower bands are connected on either side to a special drawbar or wishbone that is pulled back under the tension of all bands simultaneously. The twin slim barrels are intended to replace one fat one, in terms of the required strength, and the spear is controlled top and bottom by the barrels, so this is a variation on the side-slotted barrel idea, but instead of long window slots cut in a tube with a spear running inside the tube, which will weaken it, there are two separate barrels and the space between them is effectively the slot which can run the full length of the barrel rather than just for a section of it. Too much lateral drag on turning and an exercise in making something different to everyone else, the side-slotted barrel idea had already been killed off by the Champion "Arbalete". Note the slide action trigger. I have never seen a patent for it. The other one I saw had the same tri-prong tip and was in better condition than this one.

There is another band gun with a very similar layout, but with plastic handle and muzzle moldings called the "Match", however there the sear box is right down the back end of the gun with the slot between the upper and lower barrel running the full length of the gun and over the top of the rear-biased grip handle.

PVC is a hard plastic often softened with plasticizers, otherwise it tends to be brittle. Items made from it may not be durable and will be affected by sunlight and UV exposure, but you could always make another one. PVC parts on spearguns tend to be small items and not load bearing, like buttons or covers or "silencers" on line releases.

Before you expend too much effort check the alloy out for rusting by putting the pieces you cut off (the header) into a cup of saltwater. Also some castings can be brittle, so see how a piece responds to bending by putting it in a vice. Then you can proceed with the detail finishing of the components.

To check if the pump works put your thumb over the end of the pump and push the pump handle, you should feel air pressure on your thumb. The pump draws in air through the breather hole in the top of the pump body, so the pump handle has to be pulled right back before you push it down. If the seal in the rear of the gun charging port is missing then air will just flow out around the connecting screw thread between the pump and gun and not go into the gun, you need to check if that seal is in place. Hand pumps usually don't work if the pump body is not establishing an air-tight connection with the gun, so you need to check that next after you are sure that the hand pump is working on its own. If the hand pump does not work then the seal on the plunger is faulty, it is located at the end of the pump rod opposite to the end with the pump handle.

If a shaft falls off a gun with an open muzzle then the spear tail can lever against the sear box because that is the only thing holding it. During reloading is the time the shaft might be free to do this and the weight of the shaft will apply a strong leverage on the tail notch. Note the shaft fractured off from one side, then the last section parted giving that grey appearance due to the fine grain structure in the metal. A heavy shaft, it applies a lot of twisting load on the tail if it swings to one side. The damage may have been done earlier, then the tail snapped at a later time as the remaining metal could not take the tensile load.