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How to optimize the performance of a hand-made invert roller?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Hello Zbanko,

Was not able to log in for a while. It is quite nice to see that great discussions are still ongoing. Leander forwarded me a website with calculations on different setups. Assuming that they are correct, it is a nice tool to check not only the output energy but also to compare other parameters.

The video you shared is stunning. However, the rubber combination is much higher than I expected. My gun has 4 x 14mm, I do consider 3 x 16mm. (on both sides). The visibility is up to 10m usually. So a range of 6m with high accuracy is more than enough for me. I have limited equipments so my idea is to find the best of that. Therefore, I would go for 150cm x 7mm Pathos spear.

Here, I would like also to refer to your former post regarding the calculation. It has been quite enlighthening for me. I try to make a comparison between regular guns and invert rollers with more or less same rubber combination. According to my understanding, the energy you can get from invert roller with same rubber combination (thickness and stretch) is 3/4 of the regular one due to the distance between wishbone and notch.
Let me give an example to make it clear:

1. A regular gun with 2 x 14mm rubber, stretch of 350%
2. An invert roller with 2 x 14mm rubber (on each side), stretch of 350%

Lets assume that the force1 in the rubber bands is F. Force in the second variation will be F/2 due to the movable pulley.

Energy = Force x Distance. The distance1 between wishbone and notch is 2x/3 whereas it is x for the second gun. Eventually, the energy in the regular gun will be 2Fx/3 whereas it is Fx/2 for the invert roller.

Energy1 = 4Fx/6
Energy2 = 3Fx/6

If the assumption is correct we can exceed the power of a regular gun by adding the third band to the roller. This is much easier for me to digest

My next statement goes to momentum, a function of mass and speed. The faster the spear goes out, the higher counter momentum will be. However, due to the bands moving to the opposite direction, the recoil of the gun is likely to be less. In the end total momentum needs to be zero which is a compromise between components moving forward and backward.

Of course, friction is also an important parameter but neglected in my statement, assuming that this is quite similar in both cases.

What do you think?

BR

Eray
 
Reactions: Aldo Gjipali
Hello Eray
I am just like you trying to figure out the benefit of inverted rollers. The thing that the force at the top is half the force of the rubbers is true and so is the wishbone speed being double the speed the rubbers contract and this has been fucking with my head for a couple of days now. Your calculation is a bit wrong. Well like you said if the bamds are stretched to 350% the distance from the wishbone to the notch would be a bit more than 2x/3. From my quick calculations it would be 7.14x/10.
Also one thing you haven't thought about is the preload that all guns woth rollers will have. You can see the different sustems in the pictue below. The force would not start at 0 like in a normal. So we can calculate the total energy by the area under the graph. Also the energy equation in rubbers is 1/2FX and not Fx. So without knowing the preload and and the total force of the rubbers it is impossible to actually have the correct ratio of energies.
 

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Andrew the fish said:
this one here

Technica Spearguns

www.technicaspearguns.com
Click to expand...

I am in the same place as original author of this topic and trying to get my head around inverted and eventually deciding what do I do with my 2 Custom 140cm builds for bluewater.

I have read the paper and science behind it which I love but I BELIEVE the simulator is doing the inverted rollers math wrong but I hope its me still not understanding it properly.

If you compare gun
X = Double 2x14mm 350%
Y = Inverted 1x14mm 350%

X=Y with very small difference. There is no WAY I can believe that!

Same story if you double the bands in X and Y by 2. All results end up pretty much equal with minor differences.
It is possible that the inverted calculations are multiplied by 2 instead diveded by 2?

Do we have an author of the sim here or contact? I will defo try to contact them.

EDIT: DID FURTHER TESTING
Inverted 1 window - X=Y
Inverted 2 window - X = 1/2Y

So sth is defo not right!!!
 
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When I built and computed my invert power, I based everything off of Work calculations and determined the Velocity of the shaft in each scenario to compare. I was comparing things to my 2 banded (14 mm) gun, which shot well for me. I made sure when I designed my invert, I wasn't not overly exceeding the calculated velocities from my 2 banded gun.

I did not read this thread, but I cannot believe that X would equal Y in your post. A single pair of 14 mm bands for an invert, at 350%, would be quite under-powered. Even with two pairs, I'd have to be quite close to the target. I use two pairs often for holes or point blank shots when I don't need much power and don't want to damage my shaft much.
 

If you look at it when you simplify non existant preload
1 circular bands = 2 pairs of inverted roller

The X=Y being impossible is correct in my post, but what it was refering was that I just overlooked that in Inverted 1 window in the technicaspearguns.com simulator there is 'doubled - yes' option by default that basicly doubles the amount of bands in bands number selector.

an inverted gun needs at least 4 pairs of bands to match 2 double banded gun under assumption there is no preload and equal diam and %.

interesting enough a double roller has either more power or less recoil in the sim at comparable power or recoil efficiency levels to inverted rollers depending what you go for.

As to gun designing rules and the sim I can say your way is pretty decent, personally I'd put recoil levels above and 120j+ as excessive and would personally keep it below it as thats where I find it still at controlable levels when I did input my various guns and some of them were completly overpowered giving decent reference together with more balanced guns.
120j+ excessive
60-70j - very low/low recoil.

I wish all the producers would start to put buoyancy information information (positive/negative in grams or kg) and item weight. I would be extremly more easy to design guns on paper instead of guessing some values or redoing my shafts buoyancy/weight based on material, diam and lenght.
IT would be very much possible to basicly design the gun ON PAPER without testing in the water to be very close to desired levels!
 
Reactions: musubi
My brain gets tired just reading this, and it seems to be fairly typical. Even people who buy roller guns rather than build them often go through hell getting things right. I admire you, but I think I'll stick to my old fashioned non-roller guns.
 
Reactions: musubi
I thought the author in this following page did well in showing the reduction in recoil for an invert roller:

Roller guns and why they’re different…

**Updated: Please see additional information at the bottom.** Hey Folks. So, in the real world, I’m a mechanical engineer, and this same discussion has come up a number of times in a number o…
okolehupo.wordpress.com

All resultant velocities being equal, an invert roller has a 60% reduction in recoil! The reduction in recoil is, to me, the main benefit of using any roller. It's not that it adds range or power (not saying anyone said that here).

My bands are pre-loaded about an inch (3 pairs). I use various band thicknesses with my band pairs which keeps me at 3 band pairs, and not 4. This setup allowed me to play around with higher shaft velocities with very little recoil.
 
3x14mm is a light setup for a 100cm polyspast with a 140cm 7mm shaft. The typical setup for power is 1x19mm (or 20mm) + 2x17,5mm. Stretch is around 320-330% in order to get a bit more prestrech (~1.6+).

3x14mm @ ~3.5 would work fine for a 120-125cm gun but not a medium length one. Also 3x14mm works better with lighter shafts, not thicker than 6.75mm. Think of it closer to a single roller with a 17.5mm band.

Another solution would be to go with 4x14mm @ ~3.3+.
 
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