Forum: Catapult Message Board

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?