Forum: Catapult Message Board

Shadow Dragon,

I can't picture what you mean. Are you saying that the triangle frame shifted out of shape?
Or did the intact frame move as a whole?

Wish I could see a viddy of it.

Walker

On the original treb, the cross-brace was nothing more than a 2x6 bolted up to the side of the legs, so it provided no real structural support. The legs simply sat in pockets on the base, and were held together at the top by a welded steel frame... so essentially the legs were just sitting in place, free to move slightly.

What happened was after we fired, and the CW was still rocking back and forth, as the CW swung to the front, all 6 legs also tended to shift forward about 2-3 inches. If you were watching the top of the A-frame where the three legs came together, it would appear (because the legs were shifting forward) that the center leg leaned forward, the front leg rose up inside the steel frame, and the back leg dropped down.

I'm not sure if I have any video of this happening, but hopefully the description helps?

The whole frame was kind of hopping-skidding?

If I understand this correctly, the force of the mass was greater than the resistance of the frame's mass.
When the mass of the CW reaches the stall position and is not in a vertical alignment with the axle, then the CW will jerk the frame to follow it into alignment. Which is in keeping with Newtonian Laws of Motion.

Although additional bracing may be a good idea to prevent the frame from being ripped apart, the frame's position can be maintained by adding sand bags to the corners or driving steaks into the ground fore and aft of the legs so that they push against the steaks and can't move.

Does this address your problem?

How far forward of the axle does the CW stall anyway?
Attached is an old drawing I made formoving the stall position closer to the axle axis. The dimensions are not perfect but it does express the idea. Could something like this work for you?


Walker

I was trying to visualize the problem. Are you saying the joints are slipping? Other than that, a triangle is about as stable as a shape gets.

Another type of bracing you might consider is having boards going from where the crosspiece meets the legs diagonally downward to meet the upright at the frame bottom, sort of resembling an upside-down W. That shoud really stabilize things, though it might compromise access to the sling for loading.

Clumsy Paint drawing to illustrate the idea is attached.

Thomas

Walker:
No the frame is not shifting on the ground, it's the top of the frame, where the three legs come together at the top, that things are slipping. I have not tested the new treb yet, waiting to see what it does before I change anything...

Thomas:
What you drew is essentially what I meant by additional bracing, except I would put those diagonal braces *above* the crosspiece, rather than below it.

I think the real issue is going to be how well my wooden framework is braced to prevent unintentional movements. There are metal straps buried under the layers of wood to help hold things together, but that doesn't prevent the top joint from slipping around. Another issue I've been struggling with is, if I do add these diagonal braces, HOW am I going to bolt them into place? The crosspiece was built right into the framework. Because of the angles of the envisioned extra brace, I have no idea how to actually attach it without doing a major deconstruction of some of the existing framework.

Is plywood allowed?

Craig

I think I agree with SpringTreb, if I understand his intentions correctly. Plywood gussets at the top of the frame would help. The larger and thicker, the better, assuming they are well connected to the everything.
As Thomas pointed out, triangles are one of the most stable shapes going. The forces acting at the apex though can cause the beams to attempt to rotate around their common connection.

There are a lot of different solutions to this kind of problem, one of which is to avoid it altogether. This can be done by having the forces acting through the center of the connection and all legs centered and in-line with that same point.

So goes the theory at least...

Well I took a good look at things today, and came to the conclusion that I might just be OK. The construction of this treb is certainly different from the first one, and the legs are more interconnected this time around.

Picture of the day... http://www.madsplatter.net/2010/pics/2010-07-28/index.php?show=IMG_1744.JPG

At the top of the frame, you can somewhat see that I have layers of boards going both vertically and horizontally through the top joint. What you can't see is that I have also embedded a couple of metal construction straps to tie the legs together, held in place by the three lag bolts (you can see their heads on the second horizontal board from the top). I think as a final bit of reinforcement, once I get the trim work done, I'm going to make a wedge to bolt inside the top joint between each leg (a 60-degree triangular piece going between the center leg and each outer leg, with a bolt shot through the wedge into each leg). Hopefully the wedge will give just enough extra support that the legs are not able to slide out of their alignment.

A couple other things of note from today's picture...
1) I sunk some bolts into the four outer legs to act as anchors for the straps. However even with four straps crossed and tied directly to the trailer, the A-frames were still able to rock sideways a couple of inches just from me rocking it by hand. So I will be using the 2x4 cross in addition to the straps for road travel, and see what happens.
2) I completed wrapping the center legs today, my effort to give the illusion of solid beams for each leg. I used the router to round certain edges, trying to make it look like the legs are actually notched out for the crosspiece. I'm not sure how well you can really see the details in that picture, but I'm excited about how it turned out. I have some help coming on Saturday to do final sanding and staining, and then we'll really see if my efforts paid off.

You may wish to get some larger washers under both ends of any bolts. A little movement of the frame can put HUGE tension on the bolts. This can cause standard sized washers to bury themselves into the wood, thus loosening them up as well as damaging the wood. 'Fender' washers would be a good place to start, but be sure they are thick enough as well as having large diameters. They're cheap enough.

At the rear frame/trailer, I see what appears to be a round shaft. Is that the intended main axle?

A picture's worth 100 words, counting for Inflation.
Thanks, Now I get it.

100% agree with Ripcord: Gussets!
Large Gussets at all corners.
Either plywood or steel.

The sturdy triangle shape is lost because the sides of the triangle are not tied into each other but into a Cap block which changes things into a trapazoid.
Using gussets corrects that by providing an out-of-linier solid attachment.

Love the work you've done on it.
That's classic Shadow Dragon craftsmanship.


Walker

Ripcord:
Yes, that's the main axle. It was originally a tractor axle, so it's worked well for me. Now I'm sizing it up to weld a heavy ring to each end, to be used for tying a rope on each side to haul the arm up to the top of the A-frame. Making a bit change in how we do this part of assembly, but anticipating making the job much easier! There are definitely places where I could use larger washers, just need to pick some up on my next trip to Ace.

Walker:
Yeah gussets would be a benefit, I just need to see how it reacts to a firing, and figure out what needs reinforced.

Pictures from today: http://www.madsplatter.net/2010/pics/2010-07-30/index.php
Took the trailer off the jacks, and even sitting on a slope on the road, I could rock the whole thing, but the treb stayed solid and simply rocked the trailer. Looks like it'll be road-worthy. These are the last pictures of the bare wood — tomorrow I finish cutting the boards that cover the ladder rungs, then we'll unload the whole thing onto the street and stain it all. Oh, and the final resting height from the top of the treb to the ground = 13'10 ... about 6 inches shorter than what I originally anticipated!

Next up... finish assembling the boards for the outriggers, try to forge a new trigger, and spend the rest of the time re-working the winches to the new framework.

Got a bunch of new pictures today! Staining is mostly done, but more importantly, we did a test-run of unloading the whole frame off the trailer. Took 30 minutes to completely unload, including a lot of head-scratching as we figured things out. I can see getting this down to 15-20 minutes easily once we know what we're doing!

Lots of pictures, so here's a link to the thumbnails. Enjoy.
http://www.madsplatter.net/2010/pics/2010-07-31/