Forum: Catapult Message Board

Great looking site!

(EDIT) I was commenting on the size of the axle but, after looking at more photos, I see you're going to keep the actual span very short. It still looks kinda thin though matey and I'd hate to see your machine self destruct.

Ripcord has a lot of options for triggers, scaling up to big Trebs on his site. It's certainly worth a look.

I'm not sure what you are looking for in terms of a physics/dynamics simulator but, if you mean Treb related, A-Treb is the best out there and Win Treb Star gives a good grounding. Both have free demo versions, which you can unlock to allow you to enter all the parameters. A-Treb is $19.95 for the full version and well worth the cash in my opinion.

Yes, we're hoping that by keeping the span short, we won't have any problems. That axle is a 2", 6 ft. long sold iron rod. The span is only about 11" though. I think something else will fail before the axle does.

As far as software goes - I am aware of those treb simulators, but I was looking for something that could be used for more general applications. It's no big deal - just curious.

Thanks for the comments.

Carl

You wrote "I'd love to hear any suggestions from you experienced flingers. " I may not qualify but can reply to your questions.

1."Does anybody have any better ideas for making loops at the end of some real thick cable? I think it's about 3/4". Those cable clamps are too expensive for us."

Cable clamps or clips are very inexpensive. Most hardware store carry this size. For only 5000 pounds, assuming you have two cables supporting the load with a vertical load, you only need 1/2 inch cables.

2. "How about the quck-release mechanism? I know there's lots of options, but for a trebuchet this big, we haven't decided exactly how we'll make the trigger. Any ideas?"

Pelican hooks come in all sizes. Marine supply companies have them.

Tom

Thanks very much for the suggestions.

About the cable - I'm not sure exactly what size it is. It might only be 1/2". I haven't seen it in a while. My friend's dad had this cable from a crane that used to be at a factory in the town where we're building the trebuchet. We plan on using a total of six strands going from the weight to the arm, and for a couple reasons. First, we're worried that because this cable has been sitting outside for some time, there could easily be some weak spots. Second, when the weight completes its swing, I understand that it might bounce around rather violently. When it's doing all that bouncing, that's a lot more than just the 5000 pounds being absorbed. We have two lenghts of 40 ft' or so each, so it's not like it's costing us anything more. Except of course for the clamps. I've attached a picture showing how the weight will be hung from the arm.

About the quick-release dealy - Pelican hooks are, so far, my #1 choice. However, the whole price thing is an issue. I was watching Modern Marvels today, and I saw a guy using a sort of clip with a crane. The crane was lifting something and this guy moved this c-shaped type thing over a bar, flipped a lever, and the crane lifted it away. Anybody know what that is? I'm just trying find the cheapest possible way of doing it.

Thanks again.
Carl

Carl, one important thing to watch out for with this kind of weight is that you can't control the orientation of it. It WILL swing sideways, no matter what. If the longest (diagonal) dimension of the drum won't fit through the uprights with ample room to spare, you'll have a disaster — maybe not on the first throw, but soon. You get that 4000-5000 pounds moving, it isn't going to stop for a mere pole.

When you get your cable clamps, be sure to put them on the right way. The base of the clamp goes on the continuous leg of the loop, while the u-bolt part goes on the free end.

That's a good point - one that I haven't given enough consideration. The two uprights have about 6 ft of clearence between the two of them. The tank is 47" in diameter and 6 ft. in length. So no, there is not enough room for it to swing between the poles diagonally. We had talked about what would happen if it were to collide with a pole, and we're all in agreement that it would pretty much be the end of our trebuchet. However, so far, the plan has been that we'll take our chances with a foot of room on each side of the tank for swinging. However, I'm thinking we might want to build some sort of basket for the tank to sit in to keep it from twisting. I'll have to play around with that idea for a while.

Thanks.

Carl, as your at college and an engineering student, you may wish to due a beam analysis on that axle. Talk it over with one of your Prof's and I'm sure they'll be able to point out the relevent factors and equations. But also keep in mind the "times 10" rule-of-thumb. This 'rule' states that the CW load can increase by a factor of 10, under dry-fire conditions, putting an unusually high stress on the machine, the axle particularly but the hanger system as well of course. Even under normal hurling conditions, the CW is a dynamic monster and can wreck havoc if not properly supported. (Off hand, I wouldn't be surprised to see your axle bending a bit, even at rest.) Double check what the axle material is. You commented that it was Iron, but find out for sure and find out what grade of material it is, as those numbers are critical for beam loading calculations.

Given the tank dimensions, I show a diagonal length of just over 7 feet. Yep, better keep that puppy lined up! Your graphics show the CW suspension lines being supported on the ends of a cross-bar through the TA. That will certainly help keeping the CW aligned properly, but may not be enough. Once that much mass starts twisting it does not stop easily and it takes surprisingly little to get it twisting. For example, if the CW support axle is not perfectly aligned at all times (throughout the rotation cycle of a hurl) that alone could put a twisting effect into play. The CW may pass through on the first swing but may strike the frame on a later oscilation.

Steel cable can be tricky stuff but it is a well studied topic and information about it can be found on the 'Net. Take a look at what you have:
Check the surface of the cable for flats, where the cable may have been abraided smooth.
Check for 'splinters', which are broken wires poking out and can indicate a more serious condition inside.
Check for dirt of any kind, even paints. This may be inside the cable and drastically reduce the load holding capacity.
Check for uniform bending. Simply coil the cable and see if any section does not bend as much. This indicates internal rusting or similar problems. If a section bends more at an angle in one spot, that indicates a break.

Even a single strand of 3/8" steel cable could hold up your CW, under static conditions, so you should have a lot of safety factor built in with 3 pairs supporting it under dynamic conditions. But that assumes (a dangerous approach) that the cable is in A-1 condition.

Avoid ANY sharp bends in the cable! This calls for the use of 'thimbles', which are pretty cheap. Just be sure to use the right size. As noted by BobP, rope clips should be mounted correctly (in the manner he described), but they should also be spaced out correctly, using the appropriate number of them. Again, check with some 'Net source (steel rope companies for example) to determine what you need. Also bear in mind that the clips require a specific torque value for the nuts, lest one crush the cable too much or not have sufficient holding power. Simple stuff, really, you just need to check it out.

Triggers: Lots of options! I suggest that you consider how your going to cock that monster first though, then determine how that may play into your trigger selection, along with where and how the trigger base will be located. Some folks incorporate the trigger into the haul-down, some don't, but it's something for you to consider. Take a look at my "Triggers" page (Ripcords Trebuchet Stuff), both at about half-way down (the "W.M." entry) and at the bottom for the Kip Foss design. For the "W.M." entry I can pass along better details on request....uh...in a few days. (I'm going to be away for a bit.) Also, try to determine what the tension load will be for the trigger, as that will play a big role in your trigger design/selection.

Okay, I'm hungry...who's for lunch?

I've been working on some ideas for keeping the tank from swinging, and I think I've decided on one. I even used a can of soup and some string as a model to see how things work. I attached a picture of the latest design. I think it should keep it pretty steady.

I have a midterm on Tuesday night in my mechanics of materials class — bending moments in beams is one subject on there. Unfortunately, I don't have any idea what type of steel the axle is made out of. Here's a picture of it to see if anybody has any ideas as to how to best figure it out: https://mywebspace.wisc.edu/markestad/web/treb/images/gallery/09-18/tn_09060006.JPG
I'm most worried about the support brackets we have on there though. I'm afraid that the bolts holding the brakets on the pole will snap. According to a quick statics solution, those bolts will be supporting 1/4 of the total force on the structure. That's quite a bit.

We haven't decided how to cock this thing yet. It's going to take a lot of force to bring it down. Also, we need a way to hook on to the top of the pole easily while it's in the relaxed position. So far, we're thinking it'll be easiest to just leave a cable or rope attached to the very top of the arm, running all the way to the ground, and let it flail around when it fires. Otherwise, we'd have to climb up the pole to hook on a rope or cable every time we want to fire. Any ideas in that subject? However we cock it though, we'll probably be planting two more small chunks of telephone pole in the ground right near where it meets the ground, and holding it down with those somehow. Lot's of decisions to be made . . .

Rather than let the haul-down rope flail around, just put an eye-screw or similar in a reachable position where you can hook it up after you're cocked. Note that it needs to be reachable in the cocked position — on our recent pumpkin pitcher we initially put it right near the CW attachment then discovered we didn't have a tall enough ladder in the cocked position.

As for cocking power, aren't you on a farm? I'd suggest a tractor, pulling a cable through a pulley.

I'm afraid I'm still not too comfortable with your drum support, although it looks like it could do the job. One option would be to hang the drum vertically, if you have sufficient space.

Hanging the barrel vertically would work. An example of this would be Butch's treb which hurled bowling balls like yours is going to. Although his hurled a lot of other... flammable things. I'm going to try and attach a picture of it, but I haven't attached a pic yet.. so bear with me if this doesn't work.

For the haul-down: The only problem with using an eye-hook on the pole is that if it's anywhere reachable while it's cocked, then it's not reachable when it's relaxed. It would require climbing up the arm itself (without a ladder). If was moved down to a reachable position when relaxed, then it would require a ladder when cocked as well. Both options seem pretty scary to me. We probably will be using a tractor somehow. We were also thinking along the lines of a hand-operated winch, but will probably fall back on the easier tractor option.

I was thinking about hanging the tank vertically, but that would significantly reduce the effective distance between the arm and the counterweight. The shorter that distance, the more the weight moves around. I'm still considering it as an option, however. Also, it'll be much easier to water-proof if in the horizontal position. As it is now, the tank has about a 1 square foot hole in one circular end, and a few pipes sticking out of the top. Furthermore, it would add some difficulty in actually suspending it from the arm. There's much less surface area to grab when it's vertical.

Now that I think about it a little more - after all that weight is attached to the arm, we'll be able to actually lean a ladder against it above the axle. We had ruled out that option before because we were thinking we could only lean a ladder up to the axle. So that option should work.

Axle:
I seriously doubt that it is Iron. (One can't really tell from a photo, but it appears to be some form of hot-rolled material, which is fairly standard for round bar stock.) For a rough guesstimate, assume it is 'standard steel' with Yield Strength of about 42 ksi. Almost any other steel will be higher then that. If your college labs have the facilities, saw a small section off one end of the axle, take it into class and analyze it. This is a project many teachers love to help out on! Just be sure to analyze the appropriate areas of the sample.

CW Hanger:
You new design doesn't really improve stability that much. Note that the craddle lines still lead up to 2 points that are pretty close together. It is that region that needs to be better stabilized, where the Hanger lines come close together. The simple answer is to keep them farther apart, doing so though is another matter.

Support Brackets:
A possible alternative, if you can come up with the material, is to replace the angle-iron brackets. Instead of each pair, use a 'U' shaped band of material instead. This band would run from the center leg bolt, up over one corner leg, then back down to the other side of the center leg. Do this again for the other corner leg, with both bands overlapping each other in the middle and a single through monster bolt holding them there. See attachment for a rough idea.

Haul Down:
Permenately attache a line to the beam tip. To avoid having it flop around though you can cock the arm, then run the haul down line back up next to or over the axle. (You did bring your ladder, right?) For a little more security, wrap the line once around the TA first. Another method is to use a traveling pulley arrangement. A trolley line is fastened parallel and to the 'bottom' side of the TA. On this rides a pulley. With the TA at rest, the pulley falls towards the axle, hopefully within reach, at least by ladder. To the pulley you attach your haul down line. As you start cranking her down, the traveler pulley begins to move out towards the beam tip. Once cocked and safety locked, you can then unhook the haul down line and move the traveler pulley back towards the axle and tie it off there, just to reduce its moment arm effect on the TA rotation. (Okay, who has the link for that one?)

Looking at your new design for the cables holding the drum, if you replaced those cables with wood (it wouldn't cost any more than the clamps you intend to use probably) you would have a much more rigid structure, which is unlikely to twist. I'm suggesting a kind of wooden cradle, which is no different in essence than a lot of hangers.

Ripcord -
Those are some very helpful suggestions. I'll see if we have the material to do the U-shaped connectors. I'm guessing that we'll end up testing it the way it is (without much counterweight), and if it doesn't appear to be strong enough, we'll add those. I'll also bring up the idea with the haul down method you suggested to the my cohorts.

Knapweed -
I was thinking something along those lines. Suspending it with the cables and letting them do the work of holding the tank up, and building a sort of wooden frame to keep it from twisting. The big stumbling point I have with that idea is how to hang the wood from the arm. I'll be looking into that a bit more.

Thanks very much for the suggestions everybody. They're extremely helpful. When there's only one or two people working in this, our vision starts to get narrow. Your ideas are a great help.

I had a little idea for which I've included a sketch. Pardon the crudity of it but it illustrates the point. The wooden guides don't have to be that long, in fact half that length should suffice but, the longer they are, the more they would reduce the chance of twist. You wouldn't need much length for the axle and a few washers should be enough to keep the guides away from the beam.

About your axle material. Google for spark + test + steel and you can get a rough idea of what you have using a grinder. Basically you grind a little bit on the material and see how the sparks look and compare that to pictures of other sparks. Otherwise Ripcord is probably right about the material but I'd be a lot more chicken and call it A36 and give it a strength of more like 32 Ksi.

If I managed to get the pic attached here you should be able to see a brown beam, a black tank, and and a red "something rigid". I'd use steel like maybe old pipe from a salvage yard or angle iron or tubing. Make two of the triangles one for each side of the beam. Tie them together at the bottom corners and in two places just below the beam. The less space between the two the less CW axle you need. Attach the tank anyway you want, welding the bottom piece along the tank would work well and spread the load but makes it all harder to take apart.

One thing I haven't heard mention of is the fact that the tank is going to try to slide off the cables. After CW stall the CW and hangers are rotated and practically in freefall for a time. That was where my CW pile always slid apart. You need to put some thought into that.

> phsstpok:
> After CW stall the CW and hangers are rotated and practically in freefall for a time. That was where my CW pile always slid apart. You need to put some thought into that.

Freefall or even negative G! When we built our N.E.R.D.S. treb for the recent Pumpkin Pitch, we set it up and dry fired it a couple of times in Toolman's yard. There was no weight in the box, but there was some scrap wood and stuff which had been stored there. All of that stuff ejected on the first dry fire. We also discovered that even with a reasonably rigid CW hanging system, we hit the legs because the whole thing wasn't leveled.