Pulley system -speed testing

Rubber has an inherent speed limit which  for Theratube silver for example is at 55-60m/s.  If you use more rubber you can shoot heavier ammo but it will not go any faster. Very fast speeds can be had with extremely tapered bands and butterfly draw but these do last only a few shots. 
To get faster you need to employ some kind of mechanical advantage like levers or pulleys. But some of the MA is eaten away by inertia mass and friction from the added parts this will take.
I want to test different pulley systems for speed and see if there is a gain to be had. If I find something reasonable that works I want to build a rubber powered crossbow comparable in draw weigh and no larger in size than  commercially avaliable high end crossbows and hopefully get it as near as possible to contemporary arrow speeds. Lets see what rubber can do! 



I omitted the roller setup because with thicker rollers at front the string would have been riding too high over the rail. This would have needed a redesign of my test rig so I will do this last. I just build a simple stationary test rig from some scrap wood I had in the shop. Nothing fancy or elaborate  just enough to test some pulley systems on it.

I started the test with the 2:1 pulley system, here is the setup:


Spanned:



The Arrows:


Draw length is 70cm and draw weight on the trigger is app. 80lbs I use coil chrono to record the speed.

Results:

The 37g(571gr) arrow (equals a 21mm steel ball)  was shot 5 times and the speed recorded:

1.  221,66fps
2.  222,31fps
3.  222,46fps
4.  223,16fps
5.  223,24fps

Average: 222.56fps


The 22g(339,5gr) Arrow (equals a 17,4mm steel ball)

1.  272,73fps
2.  273,01fps
3.  272,84fps
4.  270,77fps
5.  272.56fps

Average: 272,58fps


The 12g(185,2gr) Arrow (equals a 14,3mm Steel ball)

1.  346,46fps
2.  348,28fps
3.  346,45fps
4.  345,83fps
5.  347,25fps

Average: 346,58fps




I am using Theratube silver. According to other peoples findings its max speed if used directly is about 55-60m/s =180-197fps 

So this shows that ist possible to get some more speed from a simple pulley system.
The gain in heavier projectiles is as expected - not so very much  but you can see that using lighter arrows the speed gain is noticeable.

This indicates that to shoot a large 37g arrow we will have to increase the draw weight considerably from what its now. But if the system was already maxed out or pulley systems would not work at all it would not have shot the light arrow at higher speed.
Try shooting a 14mm steel ball at 346fps using theratube silver directly- not possible I think  even if you could do a butterfly draw with it.

You may also notice that I did not stretch the rubber to its maximum length - I only get about 170-180% elongation from the slightly prestreched loops here. 

222fps for a 571gr arrow is a decent speed but not overly impressive so far. But keep in mind that high end crossbows  usually range in between 180-200lbs in draw weight to shoot  very fast arrows.  We are nowhere near that here.


It would be nice to test this setup with 200lbs and see what it can do then, but unfortunately  the ugly test rig is nor sturdy enough to handle such draw weights. I am hesitant to try  because bad things may happen.
I will probably increase the draw weigh  about as much as I dare but  will do the other tests before.

The 346fps on the lighter arrow was nice but now we need to do this with the large one At the moment with this setup its more like the "raptor" slingshot - only able to shoot light ammo fast. But I think we can shoot even heavier ammo much faster if done right.

So there is room for improvement. I can use higher draw weight, shorten the string or the rubber a bit(there is still some room) or I could use Theraband gold which is reportedly faster than slow silver tube. 
But first I will do some more tests on the other pulley setups to see if we can get better than the 2:1 without having to resort to insanely high drawweights.


Using pulley systems with hand drawn slingshots is  in my opinion not a viable way to use but employed in a really powerfull (150+ lbs) crossbow I think they can really increase the speed .
These are my first findings on the subject of rubber*pulleys and I am curious what the next one will do  


Best Regards

hi,
please show how you did the chronometer

I got this idea from the archerytalk forum.
Its 2 coils of thin laquered copperwire wound on 2 rings and connected to a mic plug to go into the laptop. A guy over there even made a programm for it to calculate speed.
Instructions on how to build and how to operate the sofrware can be found here:


http://www.archerytalk.com/vb/showthread.php?t=1485253

http://www.archerytalk.com/vb/showthread.php?t=1485256

Instructions call for 50 windings per coil but to make it more sensitive you can make more windings or make a smaller diameter(harder to hit though)

I tested it with slingshots and arrows. It works for slingshots also but produces more error messages with steel balls. Works better with arrows I get almost zero error messages. I had to fiddle a bit with the threshohold settings in the beginning but if you get it right it works perfectly.

Downside is it needs the arrowtip to be magnetised to induce a small current signal while passing the coils. So thats while I think it does not work so good with steel balls. With arrows the North/south poles stay oriented always the same when passing the coils, With steel balls they can spin in flight and the N/S orientation is not always thesame when passing the second coil. So thats why you need sometimes a few shots with steel balls to get a good reading.

It needs something magnetised to travel through the coils so no lead balls, marbles or all-wood projectiles. But you can glue a small magnet to the projectile if necessary....
Since I am measuring mostly arrows its no problem for me.

Any diameter wire will work but very fine wire is better because it makes a thinner coil and you can measure the distance between the coils better and I think it also gives a more exact signal for the moment of passing. Wire needs to be insulated or coated. Blank wire will not work because the layers connect and you are not making a coil with multiple windings  but its just one winding then.

I like it because its easy to make and it was very cheap. I only had to buy the wire which was  6,35€ . You will also need a computer but if you had none you wouldnt be asking me this here  


I like it over the purely audio based  method using only microphone because with these its sometimes hard to determine which sound represents the arrow leaving the string since A multiply of other sounds are produced by trigger, bow etc With the coil chrono you get a signal the exact moment the arrowtip is passing the coil so I think its more accurate.

So if you build your own, let me know how it works for you 

Thank you !

Impressive test. I am waiting for other pulley system test.
 I have question about coil chronograph. I don't have standard chronograph so  I can't check up  coil chronograph's value is accurate. can I trust coil chronograph that i made? I don't care about little  error(5-10fps). 
I read build instructions and they said distance between chronograph and arrow tip at least 4'-6'. But you're chrono looks closer than 4'-6'. Is it okay if i use chrono at 2'-3' distance?

just rubbing magnet on point works well? or should i glue magnet on the point

Well, I cant tell about accuracy because I have no "real" chrono to test it against. Wouldnt habe bui8lt this thing otherwise. But the people on archerytalk forum have tested it against a real chrono and from what I have read, accuracy is good and error was less than 1% if you do it right.

Distance in the picture is decieving. It it so far away that the arrow has left the string before entering the first coil.  
Just rubbing with a magnet is enough. It gives a weaker signal so you have to set "threshhold" value accordingly but the signal is clear and distinctive.  The wood arrow has a magnet glued into the tip.
It also works with magnetised steel balls (12mm bb tested) but because these can spin in flight  the North/south poles orientation is not always the same when passing the second coil and you get more error messages. With arrows no problem at all because  orientation is always the same when passing the coils.


I made a test with more rubber  Draw weight is now 120lbs at 70 cm drawlength. Its Faster now  

I took some photos of the results from the laptop , so you can see how the curves look:



Unfortunately the 12g wooden arrow broke on the first shot so I made a even shorter Aluminium version which is a really close 11,9g in weight.






The Results

37g (571gr) Arrow

1. 270,70fps
2. 269.39fps
3 268,57fps
4. 269,11fps
5. 269,18fps

Average: 269,39fps




The 22g (339,5gr) arrow

1. 327,14fps
2. 322,74fps
3. 329.59fps
4. 328,12fps
5. 327,38fps

Average  326,99fps



The best for last

the 12g (185,2gr) Arrow

1. 415,75fps
2. 415,75fps
3. 408,67fps 
4. 418,38fps
5. 414,89fps

Average 414,68fps

oooo I like where this is going!

Thanks for sharing.  Personally I do not love rubber powered weapons because it is often cold here... but your set-up seems very adaptable to other energy storage options.

Where did you find your little pulleys?

Yes , Rubber has its drawbacks, especially when its cold. Thers` also the hysteresis issue. But its cheap, makes for an easy build and you can do large draw lengths + its easy to adjust power by adding just more rubber...


As for the pulleys - I made them myself  because I could not find what I was looking for in the shops. I needed something strong yet lightweight and low friction. I made them from Delrin(POM)

Heres a little WIP how I make them with my minimal Toolset (no lathe - this would have helped) But I managed and its possible with some care, frequent measuring and some experience to make them to within similar specs(weight/dimensions).

Sorry for the many pics but I kept them  low in filesize and it spares me some overly complicated lengthy  explanations...  
Cutting the blanks

Mounting on makeshift mandrel

Turning to size


Turning the groove

The tools (Made from Drill rod, hardened+tempered, need to be sharp!)
After some wet sanding+polishing


Mini-string (because of weight issue, even carbon fiber sideplates were heavier))






This should be fairly self-explanatory , but  if anything is unclear feel free to ask.

From the other thread

rolynd wrote:I'd like to see someone tackle this one:




I briefly experimented with this kind of setup in my rubber thingy and noticed it had almost no recoil. It was also little faster than the same amount of rubber set up in a "single" bow. But I dont know if that readily translates to one made with "real" bows.

I was going to suggest you try this. This doubles transmission ratio without any added mass. So with massless string it could in theory be twice as fast.
Should work even better with normal bows, as the spring material usually has more inertia than rubber. Certainly works well . I have tried it.

Anyway you should record the stored energy of your system. After all energy economy is the whole point of this experiment. Also the efficiency figure would give virtual mass for independent bolts to check for other sources of inneficiency than system inertia.

Well, I actually have tried it , it looked like this:






I had to move the trigger a bit forward because I ran out of space for drawing the rubber much further so its about 160lbs with a 60cm draw resultig in 277fps for the 571gr arrow.
Compared to the 120lbs , 70cm draw at 269fps I do not get much more speed from this setup. Maybe a little more if the draw length were the same but  the increase is not as great as I expected. What I noticed was that the assembly has almost no recoil, which is a good thing but I get almost the same speed with 40lbs less drawweight. Maybe this slef-dampening effect thats responsible for the low recoil is also the cause for the low speed over what was expected with mor drawweight.

I really like your double bow design with the levers, though! 
If I should tire about playing with pulley systems I think I'll switch to levers or Inswinger designs....

But there is still some things I want to know about the pulleys so I carry on with these for the moment.

Hallo, im interesting in rubber crossbows too. Very simple way to increase ratio arrow/rubber is increase lenght between places of attachment of string. Actually make bow wider, this doesnt add any masses. Do you think the same ? I want make the same on my crossbow.

Hello Prorock, welcome to the forums!
I have thought about increasing the  distance between the frontal rollers, but I am not sure if this would really increase the speed. Rubber pulls back with a certain  max velocity - this doesnt get greater when I move the rollers further apart.

In a traditional bow  if you increase/decrease the limb length you change the leverage/angle and thus the limb tips travel at different speeds but with rubber set up like I did -with the frontal rollers-  it doesnt change  anything  besides giving me a bit mor draw length. Maybe its different for slingbows  with rubber instead of a standard string.  The only way I have found so far that works in overcoming rubbers inherent speed limit is the use of simple machines like either pulleys or levers.

No, it has effect, when you move the rollers further apart. This changes the angle between arrow and string if you have the same drawlenght (powerstroke). And due this change of angle, it decrease part of force acting on arrow from back. As well it decreases the track of string througt the rollers - powerstroke and track of string isnt the same it forms a triangle (look at it like in trigonometry). The smaller the angle on side of roller the string has (the wider bow), the smaller is the track of string to overcame the same powerstroke. You can make small test with string, like me.

Ok, I try to wrap my head around it. I know it has effect in bows and crossbows but I was able to convince myself  it would not work the same with rubber , but was not 100% sure -since I have not tested it - I will try it!
Do you have some pictures of your setup? Drawlength, draweight, speed?

My crossbow has now 2 levers and 4 pulleys-two static and two on levers - this makes ratio 1:3 without efect of triangel. Drawlength is 57 cm, draweight is about 32 pounds, speed i cant measure, is slow with 22 g arrows but sufficient for shooting at target from 15 m. I have problems with torsion of levers from plywood and the lenght between  front pulleys is very short - i want to rebuilt it. It has bad angle between levers and rubber band as well. It will aproach the setup of man here(topic Inswinger crossbow) after rebuild, but the string will forms a loop that hangs on pulley near trigger. The pulley is fixed by rubber - the string is silented this way.

great tutorial on lathe-less pulley making, thanks

rolynd wrote:Well, I actually have tried it , it looked like this:


Maybe a little more if the draw length were the same but  the increase is not as great as I expected.

OK. Problem is those fixed router pulleys. This is not the Ming set up. The front pulley should be moving backwards - that is what gives the leverage.

Your set up above needs longer string and the resulting fast moving extra mass is ruining the efficiency.

Fix the fatter rubber with pulleys to front and thin ones back. Run the string only through the moving pulleys. This way the string will be shorter & lighter.
This naturally means some 15 cm idle length in front of the system, so you may want to move the bolt track back to get full power stroke.

Yes, I get you, its not exactly the setup  as in the chuanzi nu because of the frontal rollers.

This is what you talk about I guess: Green= string, red = rubber



It would do away with the frontal rollers and uses a shorter string.  

Well, it does not work on my test rig because with this setup the distance between the frontal attachment points is too small. It would work if  I could widen the frontal "fork".  If I use the attachment points of the frontal rollers  what happens is  that the pulley wheels are pulled to the middle until they touch. It looks then like some sort of X  with the pulleys touching in the middle.

It has merit though and is closer to the original setup  but I would need a wider "fork" in the front and as you said it wastes a bit of stock length in the front. But it lightens the whole setup for a few grams and does away with inertia and friction of the frontal rollers. Should be faster but how much I cannot tell without testing.

Testing is on hold because I accidentally shot my trusty chrony and destroyed the back ring.   (note to self: check alignment of chrony after bumping into it!) I am constructing a new one now using photodiodes amd IR LEDs  so I can do away with magnetised tips...



Thers always a little setback but first results were relatively promising in overcoming the rubber speed limit so  I`ll carry on - but slowly at the moment....