Generally (but not quite always) if something breaks, the scary bits stay roughly in the plane of rotation. If the arm breaks for example, the projectile could easily go straight up or backwards, along with chunks of the arm and sling. Its pretty unlikely for anything to go straight sideways, though things that go almost straight up can fall down next to the treb. I generally envision the danger zone as an hourglass shape: thin around the center of the machine (axle) but widening the further down range or behind it you go.

Thus generally I try and keep almost as much space free behind as in-front, always stay to the side when working on a machine with any stored energy (counterweight not at the bottom for trebs). When ever possible avoid putting anything in the path of the counterweight or arm except when absolutely necessary, and have a safety in place when doing so (ex: loading the sling) that physically blocks the arm from moving in case of a trigger or winch failure (this is usually required by the rules of competition, and for good reason: its great practice to have a redundant safety). When ever possible (and for sure when firing) stay far enough back to the side that there is no chance the sling could even come close if whipping around.

Another thing to watch our for are winches. If you have a home made winch, if the ratchet breaks, or gets set to free wheeling mode, or you slip off the handle, you can very easily get a winch handle spinning really fast breaking you hands. Having a high quality ratchet, and be ready to just pull your hands back and back away to the side should anything go wrong while winching is a good idea. Always remember where the stored energy is, and how it could break loose and get you so you are prepared to just back away and let what ever happens happen if something goes wrong instead of having to think about it when something starts failing.

Should something go wrong (or right when actually firing it), hopefully you are to the side. Generally back away further to the side if you can so that if anything went straight up, you have more clearance before it comes down.

For small trebs, safety glasses are all the PPE I used: shattering wood or metal could easily take out an eye. Similar to working with a lathe, I always felt gloves might be more of a risk to get caught in something that to protect me. For larger machines a hard hat is a great choice. While it likely won't save you from an errant projectile direct hit, it helps communicate to others that its dangerous and the area nearby is only for people who are protected and know what they are doing, and it can save you from some things (like a ring from the sling to the head, or a glancing blow from a projectile). I did have a metal pulley shatter once: safety and a hard hat are pretty good against that kind of debris.

For more specifics, its really hard to give clear safe answers. Starting with low counter weights, safer projectiles, no spectators and ideally smaller model machines and/or simulations can really help you get a sense for the risks while keeping safe. Work your way up to higher risk situations carefully so you will be able to evaluate them better. Hopefully such an approach can get you a better sense of how big of margins are needed. Most large trebuchets tend to have low enough mass ratios that 300 arm lengths is easily enough, but it really depends on the details (drop height, mass ratio, how much roll is possible etc), so in some extreme cases that might not be enough, and in some other extreme cases is far more than necessary. Looking at total stored energy in the treb and computing how far it could throw if 100 percent efficient could give you a rough upper bound (after accounting for rolling), though most designs won't even get close to that.

I personally haven't operated any really large trebuchets, and I can never really know how lucky I got to come out of almost everything safely (one assistant got a bloody finger when hit by a broken pulley once), so I can't really give any high confidence or athoriative advice, but hopefully this, and some careful thinking and planning, can help you operate safely. Good luck, and may the stored energy leave your systems in a safe manner!

1. Range needs to be 300?X the arm length deep. More, if the projectile can bounce and keeps going.
2. How wide?
3. Will people wander through the range?
4. How much room behind? Do projectiles or trebuchet parts ever get thrown that way? How far?
5. Block spectators out of the area the sling will whip through (traffic cones? Posts and cords? A moat?)
6. When trebuchets, er, rapidly disassemble themselves, how far and in what directions do bits generally go?
7. Any need for PPE (Personal Protective Equipment)?

From the description I can infer that if it achieves CW or arm stall it would have to be with the CW well in front of the axle (or it would hit the ground), and has a short arm rotation (angle wise), which explains why it only works well for low mass ratios.

It shouldn't be very hard to make one in a simulator if you want to see how they work. To help you get started I added a really horrible simulation of one to the baka page.

Thanks in advance!

Good luck!

Thanks for your help!

If you are looking to build something efficient and compact, thats very different from a trebuchet that may take the same amount of work to build, but is simpler and larger with the same range. Until recently (and maybe even currently), the longest ranged trebuchets were just very large HCWs. If you haven't built trebuchets before, a HCW or FCW is a good starting place if you want a design thats not too hard to get right and gets good range for the effort. All the other designs are more complex and mainly efforts to get more range without simply increasing size. I also recommend building a small model first. If that goes well, you can try building models of a few more complex designs if you wish, and scale up which ever you like best.

Most common mistakes I for school project trebuchets: really heavy untapped throwing arms, crummy slings, and axels that are longer then needed (and then end up bending a ton).

Also, "Full Sized". Full sized generally means one of 2 things: medieval scale and can throw 200+ pounds a few hundred yards, or 8-10 pounds competition pumpkins. That said, I mainly work with stuff that fits on my desk.

Anyway, thats what I have to offer. Feel free to browse around the site and ask some more specific questions if you have any. If you want a higher volume of advice, the majority of the hurling community hangs out at TheHurl Message Board. You will get a ton of fantastic advice from those folks!

Looks good. Hopefully I will have more information about them as my bigger one gets built.

BeardedOne

Thanks for the help,

BeardedOne

Slings can be very accurate. Trebuchets used to hit castle walls consistently from hundreds of yards away with slings, and onagers are no different (I beat several trebuchets in an accuracy competition with one of my onagers). Onagers can be pretty inaccurate if you aren't careful though. Its good to make sure the sling is releasing before the arm hits the stop, or you can get inconsistent releases, and be sure that you either re-aim very well every shot or weight it down really well because onagers tend to jump around a lot when throwing. I should note that is you don't get everything right, you can end up with an inconsistent sling, I've heard of it, and seen people have issues with it, but I personally have never had the issue, and you can copy my sling design :)

Be very careful when tensioning your onager up to its breaking point, they can hurt you. As long as you arn't in the plane of rotation of the arm, and have safety glasses, you will generally be quite safe, but it varies on the design. Onagers if they break the arm, it usually goes in the plane of rotation (I'm thrown chucks of the arm backwards, but I have heard of them going forwards as well). If the rope breaks, its generally not exciting at all, my little onager has had a broken strand it its bundle for years. I've broken ropes in every single one of my onagers. If the frame breaks, it will usually just cave in a bit (then there is no more load on it), I've never had that happen though.

Some random notes: Since you have a length limit, you probably want to get the bundle pretty close to the front to allow a longer arm. Mine has a 24 cm arm and throws further than you need though, so thats not critical. You also have a pretty big height limit, you might want legs (like this little guy I just put up a page for: Tiny Onager) so you can cock the arm pretty far back while still having a long sling. Make sure you are using a good type of rope. Nylon string is an excellent choice, and cheap too (I use it on all my torsion devices).

And by the way, an Onager is what I would recommend for this project. They tend to have the most power for their size. Good choice.