Rollergun basic speargun 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.

  • Thanks Pete, John Waren and I have a napkin form late 80's with a linear power graph like that.We both had a lot of fun even before the Niko Emails.


    Cheers, Don

    ''Great mother ocean brought forth all life, it is my eternal home''
    Don Berry from Blue Water Hunters.
    Speardiver Gear

  • Neat diagrams Pete.


    The "can be eliminated" drawing doesn't say the whole truth. The roller doesn't allow the use of a 2nd band. So that shorter gun with two bands would be no less powerful than the roller.


    Also the spear wouldn't be as long unless you wanted MASSIVE overhand :)

  • Neat diagrams Pete.


    The "can be eliminated" drawing doesn't say the whole truth. The roller doesn't allow the use of a 2nd band. So that shorter gun with two bands would be no less powerful than the roller.


    Dan, Jeff and I spent a couple weeks figuring it out. The roller translates to about the same work (force times distance) as 1.6 to 1.75 bands, but you can increase the thickness of bands. There are other factors the diagrams don't show; the power delivery of the bands are not exactly linear like that, but can be closely approximated. I can't remember if Jeff created a thread, but here it is if not.




    I also have the spreadsheet calculator if anyone is really interested. The results do not seem to match what people experience in the field; rollers have a lot of power. Maybe it's because people don't actually cut their bands to 3.5x stretch.

    The post was edited 1 time, last by JimCuda ().

  • 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.

  • 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.


    I like the significant recoil reduction for around the same power as 2 bands.


    What happens when the roller band is pulled even further back on the bottom? Like to 4x stretch?


    C

  • It depends on the capability of the shooter. You could keep adding preload and keep increasing the power as long as the shooter could pull the bands back on the second stage. With my lever action, you make the band length such that the shooter can just barely load the first stage because you have a mechanical advantage (for my gun it's 2:1) that always allows you to load the second stage.

  • This was the first discussion in French that I have from 2000. I had already removed the rollers from one of my home builds around 86 as it could not out preform the same gun with just 2 bands yet alone 3. Every pulley has power losses.
    I'm not anti roller, I just don't hunt with one .


    Cheers, Don

  • 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.

  • 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).

  • 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.

  • Makes sense Popgun Pete. Any difference in roller friction between the sides could force the side with the slower roller or sticking point into compression and then kinking.

  • Very interesting Pete. I hate cable. I find it brittle and kink prone and just temperamental
    I understand it has purposes but I hope to avoid them.


    Great thread btw. I have learned a lot.

    i like to spear fish

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