Slings and Pouches


 
Many questions out there about building or designing a pouch and sling.  The place to start is by knowin Brot g what your going to be using for a payload (missile, projectile, ammunition, hunk-o-stuff or whatever you want to call it.)  How much mass does it have?  How big are the dimensions?  What is it made of and does it have any projections that might get caught in things.

The most common items hurled in competition are pumpkins.  These are usually selected for their firmness as well as size and uniformity of shape. They also make a nice orange mess.  If your not doing the competitions, then a whole world of possibilities opens up.  1 gallon milk jugs full of water do interesting things, so do bags of flour.  Old computer towers are VERY therapeutic!  Really big machines can hurl cows, sheep, horses, cars, mother-n-laws...

Smaller machines may be hurling tennis balls, small bricks, 1 liter soda bottles, balloons full of water, etc.  Even smaller machines (table top models) can throw penny rolls, marbles, ball bearings, lumps of silly putty, toy soldiers, wedding rings...uh, well almost anything small and light.

The problem facing the designer is to decide what the 'standard' payload is going to be and how much mass it has (how heavy is it.)  The first and most important thing is to select materials for the pouch that will NOT break under the forces involved.  Nothing like watching a red brick going through your neighbors window.  Try this to get an idea of what I'm talking about;  Pick a clear area that will allow some 'elbow room' and pick up a shoe or boot by the end of one shoe lace.  Start spinning the shoe around your head, no need to get real energetic about this though.  Notice how much harder it is to hold the shoe lace while watching the orbiting shoe as compared to simply holding up the shoe by the lace.  Now imagine what swinging 1 gallon of water or a bowling ball must be like if your arm was 8 or 15 feet long!  My point is to be sure that whatever you use for sling material, be sure it's strong enough.

Another consideration is the weight of the sling.  Your trying to get some speed out of all of this, for the payload, which requires energy.  Moving a heavy sling (and pouch) requires more energy than a light weight one.  So, staying consistent with the strength issue, try to keep the whole sling/pouch combination light weight.

Friction due to air drag is also a point you might want to consider.  For the sling lines themselves there is probably little you can change but the pouch is another matter.  Too large a pouch will create a fair amount of air drag, not only will this reduce the range of your payload but interfere with the timing of release and hence the launch angle.  On the other hand, too small a pouch won't hold the payload sufficiently. A good pouch will hold the payload while at rest in such a manner that the payload doesn't tumble out just sitting there, during the first bit of movement of the sling (when the sling lines get tight) and of course during the full arc of the hurl.  It should also LET GO!  At the appropriate moment of course.  Many a trebuchet has been rendered into a fractured mess by having the payload swing around and run right into the frame or throwing arm, simply because the pouch didn't want to part with the payload.  So there is a bit of a narrow margin there you have to walk down between too tight and too loose.  So for now, concentrate on a pouch that works well with your payload, whatever it is, and worry about drag later on once you've gain more experience and knowledge.

If you want a place to start in designing a pouch, here is a simple one suitable for small to medium Trebuchet.

Note the two V cuts, the edges of these are sewn together which cause the whole thing to warp, creating a bit of a pocket for the payload.  You can leave the V cuts out of your design, but you increase the risk of the payload coming out too early, possibly through  a window or somebody's chest. (Remember...Safety First!)  Grommets can be added to the holes at the long axis ends, but may not be needed for the smaller machines, depending on what material you use for the pouch.  Leather is nice, but ages and weathers badly.  Nylon, dacron and many of the other man-made fibers work well. No, I haven't included any numbers for this because I don't know what size your payload is.  But you can see the general diamond shape and go from there on your own.

Here is a design we use on the 8 foot Trebuchet for throwing bowling balls up to 16 pounds. It works very well and is easy to make.

The straps are 2 inch nylon taken from car seat belts, which can be found in almost any junk car laying around.  There are the 2 steel rings, which can be purchased at many hardware stores at little cost.  Be sure to get the welded rings only and not the ones that are simply bent!  You'd be surprised how easy they open up when firing a trebuchet.  The nylon is in 3 pieces, the 2 cross straps and the 1 'border' strap.  The black lines you see are where we put in nylon thread stitching. You shouldn't use cotton thread because it doesn't have nearly the same strength plus the cotton and nylon materials will chew on each other.  A zig-zag stitch allows for a bit of motion and stretch without breaking the thread.  Be generous with the stitches!  The single border strap has an overlap of 2 inches, well sewn of course.  Near the steel rings is more stitching, this keeps the whole sling from rendering through the rings, it doesn't have to be tight but just close enough to close most of the slack.

How big should it be?  Well, that really depends on what your throwing, but for the pouch above we did something like this:

Leaving out the 2 cross straps and without having sewn anything yet, we positioned the border strap so that it was straddling the ball with about a 45 degree separation, added 2 inches for overlap and made a mark for cutting to length.  We used masking tape to hold it there while we added the cross straps and marked off their length.  The steel rings (red here) were not part of the fitting.

One thing to keep in mind about the length (long axis) of the pouch is how far apart are the two lines for you sling?  I mean, from where the fixed end of the one line is attached to the throwing arm and the other end as it sits on the release pin.  During firing the sling lines will rotate about these two points, causing the pouch to 'roll' around the payload (yes, the payload rolls too) so that if your pouch is too short it may actually roll the payload out, not a good thing.

You also need to decide how your going to fasten the pouch to the sling, the sling to the slip ring (if any) and the other end of the sling to the throwing arm.

Weathering and age are also items to keep in mind when selecting materials.  Obviously if your making a 'fair weather' machine, your probably not going to worry too much about mildew or rot (unless your building or storing your machine in a damp area.)  A day of high humidity though can make the sling/pouch combination a bit heavier, so perhaps you'll want to choose a material that doesn't absorb moisture.

Net Pouch

In response to many requests and due to an apparent lack of information elsewhere, I now give you my description on how to build a net pouch.  Don't expect a lot, since I had no idea how to make one myself until I started playing with some string with only a little idea of what I wanted.  I started with the idea that this couldn't be too hard, nets are all over the place, even on some of the fruit we buy at the grocery store.  I was to learn otherwise though.

The objective was to design and build a trebuchet pouch that would allow secure holding of the payload during firing, release when expected to, be light weight, strong, simple to make, use common materials and as an added bonus to impart a spin to the payload.  I don't worry too much about the spin factor myself, but others do and I do recognize it as a valuable component with certain payloads when attempting to tweak out those last few inches, feet or yards from a hurling attempt.

This net pouch design of mine has a bit of a 'belly' to it, in other words, pulling on the ends does not make all the strings tight, rather the ones in the middle will still be loose.  This means that has the projectile (a round one) lays in the net during a hurl, the sides of the net come up along the sides holding the projectile in.  Yet the whole thing 'lets go' very nicely.  I've yet to have any problems at all using tennis balls.  Someday I'll make a smaller version for golf balls.

I think the results meets all these objectives with the possible exception of being simple to build.  It's simple enough, just some strings and knots, simple knots too.  It was getting the lengths between knots that proved difficult.  On the smallish net I've built, being off on the knot location a little bit makes a big difference in how the net lays.  On a larger net this problem would be greatly reduced, something I'll try when I get some heavier line.

So here it is, my net pouch.  Feel free to pass the idea around, play with it, adjust or modify it anyway you wish.  If you come up with something better or know a better way, let me know!  I'm learning this stuff too.

I originally started using a nylon line that is really heavy thread, 3 twist, suitable for sewing canvas.  The problem with this material was that my poor thick fingers and poor eyes just could not handle it well enough at this scale.  So I moved up to cotton twine, 3 twist again, same stuff used by carpenters for framing layouts.  Usually available in most hardware or notions departments, it is about 0.070 inches thick as it sits on the spool or tube from the store, but fluffs up a bit when handled. Pretty strong too, although I've yet to find a breaking strength number for it.  A strong pair of hands might be able to break it, but I can't.  It turned out the the nylon string was useful also, for whipping the ends of the net pouch to finish it.  You could use almost any small diameter string or heavy thread.

I've used only one knot for construction, a Bailers Knot.  I whipped the end of the net (those big red arcs on the ends) to hold the ends of the strings together and to give some abrasion resistance.  The Bailers Knot is very simple, take two strings laying side-by-side (touching), pick them up as if they were one string, now tie a simple overhand knot, that's it.  Getting the position of the knot is important, so be careful as you tighten them down.

Finished

This is the finished product, just to give you an idea of where I'm headed.  Excuse the graphic quality, some of you will see this better than others, depending on your browser settings.  I simply whacked the thing up here and crossed my fingers.  Let me show you some details now.

Fig. 1
There are four colors involved, representing eight strings.  Each pair of strings of the same color, such as blue, are the same length.  There is gray, yellow, blue and red.  The black dots represent the knots.  If the net is laid flat, then the distance between knots varies due to the string length between the knots.  For example, A-B is longer than C-D.  A-C is shorter than C-D.  However, C-D is the same length as E-D.

The net I have made measures as follows:

A-B = 2-1/2 inches
A-C = 1-1/4 inches
C-D = 1-3/4 inches

You will probably want to try other dimensions of your own.  I should note that my lengths did not always come to these exact numbers, seldom in fact, but the net I ended up with does work well, despite the uneven lengths I accrued.

Fig. 2

In Fig. 2 I'm showing how the lengths of lines are all the same, once you get past the central area.  From AA, BB, CC and DD towards the right to the first knots, are all double strings, continuations of the lines from the net area.  I did this for strength, but I've since decided it is not needed.  You can cut out one of each pair of stings, down to the first knot, to make it a little lighter and easier to to make the whipping in the next step.

Fig. 3
What I haven't shown in Fig. 3 is that the ends of AA, BB, CC and DD overlap eachother under the (red) whipping.  AA and BB are brought together into one bundle.  CC and DD the same.  Now take the two bundles and overlap them by 2 inches.  This over lap region is where the (red) whipping is.

Whipping is simple, just start wrapping a smaller string around the area in question.  Keep the turns close and tight.  Getting the end tied is the only problem, but easy to do as well. Hopefully this graphic will show how...

So there it is, my net pouch. I hope this helps somebody out.  If nothing else, perhaps it will get you started on something better.  Drop me a line and let me know how it works for you, I'd really appreciate some feed back on this.

Two more items to include in your designing brainstorm;
One way to avoid the entire pouch 'thing' is to not use one at all!  How?  Use what is referred to at times as a disposable sling.  This is simply a single line firmly attached to the payload with a loop or ring or something to slid over the release pin at the other end.  The sling goes with the payload!  Of course it's not very efficient but...
The second thing have in mind is the fact that coming out of a pouch will usually put a back spin on the payload.  How much depends on the payload itself, friction coefficients between the payload and pouch, etc.  Back spin can increase the range, but don't get overly worried about it, it isn't THAT much of a gain for most machines.  Still, if you want to investigate it further, look up the Magnus Effect in your search engine or physics book or public library.  Once you understand all of that...let me know too! :)

No matter what material you choose or how you design your machine keep one thing uppermost in your mind...SAFETY!  Frequent inspections of ALL components (like maybe every shot?) along with safe firing procedures will help to make for a safer, happier hobby. (Fewer law suites too!)

I've received a lot of e-mail concerning payloads falling out of the pouch too early, usually as soon as the pouch starts to lift from the trough or ground.  There can be many reasons for this.

1.  Is any part of the machine that is in motion during firing hitting anything it shouldn't be?  Even if it is only momentary, the sling and pouch are often so light that they stop or slow down very quickly even though the payload (which is more massive) keeps going.  You can also check the axle bearings to see if they may be binding at some point in the rotation.  This will have the same effect on the payload.  Also check that the sling lines and pouch are not getting caught on a sliver of wood in the trough, a nail head or something.

2.  Very unlikely, but are the sling lines too stretchable?  Some folks have tried bungie cords or rubber bands for sling lines, which usually doesn't work.  One problem could be that the slings are initially getting stretch until the payload starts to move, then it moves a lot allowing slack to form and then the payload either stops or slows down causing a misfire.  Then the sling lines get strutted again and things continue to get ugly.

3.  The most likely cause though is that the pouch is not appropriate to the payload.  This is often caused by a pouch that is made of a flat material which allows the payload to slip out of the side.  While in the trough, the trough walls may keep the payload in the pouch, but once the pouch starts to lift it loses the support of the trough walls and the payload rolls out the side.  The usual answer is to create a pouch that has a bit of a 'belly' or 'pouch' in it.  Careful though, too much and the payload may not release at all!  Take a look at the first graphic at the top of this page, use that as a rough guide line.  For heavier payloads, try the strap type pouches seen below it.