Trebuchet SIG 2016 Build (season over)

Looks like the dual-release trigger system is common on floating-arm trebuchets and has been used at slingfest before. I just hope it synchronizes well.

I mentioned the prototype scale model and they suggested that we bring it as as an additional entry into one of the lesser categories - breakdown is by weight: <1lb, 1-5lb, and >5lb.

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I am so sorry; I let you down. Something came up and I did not go. Thankfully, Mark was able to be there.

We may not need to tie it down. The machine is top heavy when ready to fire, but unless there is a serious wind, the forces are mostly in the downward direction until the arm begins its swing in earnest, but by then, the weights are low enough that the center of gravity works in our favor. The only thing which would concern me is the waggle at the end of the firing cycle as the arm settles into its final vertical resting position rocking the frame forward and backward; we may benefit from lengthening the base of the frame too make sure it does not try to tip.

Some people mount traditional trebuchets on wheels so that the frame can roll and allow the counterweight to fall mostly vertically for more efficient energy transfer.

With our floating arm, the weights fall vertically anyway.

If you have not seen the video from the Instructables page yet, I strongly suggest you do so: https://youtu.be/AX83nT9WeuA

Not only will you get a feel for how a floating arm design works; you can imagine ours in action. Because the frame will want to try to move backward in reaction to the arm movement, we might get slightly more velocity if the frame was anchored. Maybe put the prototype on a dolly and fire it to see where the frame wants to go?

Paracord stretches - nearly 10% according to the numbers I found (not to mention that it doesn’t always hold knots reliably). That might defeat any tuning or optimizing we do on our sling.

The current base is only 3’ x 2.5’ wide and was designed with additional supports in mind, stakes would have just been the quickest option.

Since it’s accelerating a projectile horizontally as fast as possible (hopefully at a 45 degree angle, so half of the speed is in horizontal component), the whole base will want slide/flip backwards during the firing, I don’t think we can depend on the mass of the base (~500 pounds, with a CG ~4 feet from the ground) to stay stationary, will need extensive anchoring.

My current idea is 4 6’ long 2"x2" 14 gauge steel tubes connecting to the ends of each of the rails, would be detachable for transport and storage. Each square tube would have a foot welded on the bottom at the appropriate angle, connecting each foot would be steel cable with turn buckles, the steel cable would be highly tensioned to keep the leg stationary and for adjusting vertical, would cost around $150 (50 for square tube, 25 for steel for feet, 25 for cable, 50 for various turnbuckles and attachment hardware). I will need to check to see if we have $150 left in the budget


Or we just attach a strap to a ~250 pound weight at each corner, but that means hauling another 1000 pound to the site.

Continuing the sling discussion …

I found these D-rings online. 70 cents each (plus $5 for economy delivery - although if we want these then we might need expedited shipping).

At 0.25" wire diameter, they look a lot more robust than the ones you were considering (and we could order enough to make a couple spare slings, too). They also appear to be already welded - I’ve emailed them to ask for clarification.

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Design question … do we have a “launch trough”? From what I read, "The launch trough is a surprisingly important part of the system. It ensures that the thrown object a) is kept on a straight path in the early stage of the launch and b) accelerates up until lift-off from a smooth surface. If this piece is neglected the thrown object will bump along (probably hitting the carriage frame) and will not make much of a flight. "

If you have the weight distribution you can calculate the torque required to tilt the engine about a corner (integrate mgrdr where m is the mass per unit length).

Compare this to the max torque given by the arm. I believe mg*r sinB would be a reasonable approximation where m is the mass of the weights (and arm to be conservative), r is the distance from the arm pivot to the corner, and B is the angle between that line and the vertical (mg x r in vector notation)

Not yet, was going to just throw something together last minute, interesting to note that as the arm falls through a 45 degree angle (starts at 36 degrees) the tip will only be about ~6 inches from the ground, so trough needs to be shallow.

Hard to calculate torque when I don’t yet know the acceleration of the weights over time, I gave up trying to predict / model the movement, and will just see what happens during the first test. My guess is a max of 3,000 pounds of force on end of arm 38" away from the pivot. But I would think the only force actually acting sideways on the base would have to come from the weight bar in the vertical slot (which will be heavily greased but otherwise just steel on steel action)

With no math to support it, both of these arrows makes me assume it isn’t stable without something more


https://cad.onshape.com/documents/b7b97484037a9bbd5eaf277c/w/d6f61758db8d30258783dd5b/e/26644813b1664c4d277d6d30

IIRC optimal will be around 37* from the ground to counter air resistance. It’s been a while since I’ve had to integrate the projectile motion equations…

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But 90 degrees would make a better video…

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I’ve been researching the sling/pouch and subsequent tuning. I’ll describe what I found, and if the team is already past this point, then I apologize in advance. Not trying to second guess here …

POUCH DESIGN:
It seems that @Brandon_Green open webbing pouch is a good idea. According to Urban Siege, and their world championship quest, “big solid pouches tend to act as pretty good sails, slowing down the launch speed, and thus killing range. Therefore, solid pouches usually are not used for launching anything rather large, such as a pumpkin (like the 8-10 pounders used in competition).” It clearly needs to cradle the pumpkin, so again … good design on the webbing prototype.

TWO-PART SLING/POUCH:
The few pictures I could find suggest that the pouch and the sling “rope” (sorry - don’t know what else to call it) are frequently two separate pieces. This one from [Yankee Siege] (https://www.pinterest.com/pin/79587118385357648/) shows something very similar to our webbing prototype, using tubular webbing as the “rope”.

I may have misunderstood, but I think we had considered having our pouch webbing extend out and be the “rope” that connects it to the arm, but that would make it difficult to adjust length. We would need a discrete pouch/sling for each projectile size/trajectory combination.

I recommend we find a way to have a separate pouch to attach to our sling “rope”, if we haven’t already done this. This also makes installation easier if we have a field failure of the pouch.

ADJUSTABLE LENGTH:
The larger/more elaborate trebs have a way to adjust the length of the sling to “tune” for different sizes of projectiles/trajectories. This is compatible with the separate sling/rope design concept.

I found this interesting photo. It shows a pouch that has a movable position along a rope. They pass the rope through the fabric sling, which IMO doesn’t seem to be a good idea because of both the fragility and the aerodynamics of a fabric sling. I like the idea, but I’m not sure how we would implement that with a pouch on D-rings.

SLING LENGTH:
Opinions differ on the optimal length of the sling, but these should provide us a starting point:

• The length of the sling is equal to the length of the long arm of the beam (on the payload side) (attributed to Donald B. Siano, in his analysis of trebuchet physics (Trebuchet Mechanics, March 28, 2001).

  • The general ratio to start at is a sling length (arm connection to tip of pouch) 80% the length of the throwing arm. Longer length slings have a lower angle of release, good for line drives … Shorter lengths will result in a higher angle of release, best for lobbing projectiles over tall objects.

There is some opinion that higher trajectories (i.e., shorter sling) give more precise shots, in which case an adjustable sling might be useful for the accuracy category. This conceptually makes sense - in the case of a vertical shot the variation in the flight path would be mostly in the Z-dimension so the X,Y variation would be less.
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I volunteered to sew up the pouches, and as you can probably tell, I’m chomping at the bit to get started on this. But I don’t want to do this until we have a little better idea exactly what we’d like to make. I’d love to find a way to test the webbing prototype to see if it’s suitable as a pouch. I’m optimistic that if we make the pouch separate from the sling “rope” then I could be working on the pouches, which I suspect will take longer to make than the “rope”.

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I think the following would work, not shown is the detail of how it attaches to base in a removable way, there would be 2 pieces of steel angle coming out at a 45 degree angle from the base, and a single bolt would go through the steel angle and a hole drilled through the square tubing, so the legs can pivot on one axis. Also not shown is there would be a foot contacting the ground that connects to the tube the same way, with a bolt going through the square tubing.

The end of each foot is connected via a ratchet strap to the base, and there is a piece of steel angle connecting two legs together on an end.

Parts list
4x 60" long, 2"x2" 14 gauge steel tube
4x ratcheting tie down straps
2x 72" long, 1" x 1/8" steel angle (connects two feet together rigidly)
4x 12" long, 2" x 1/8" steel angle (support to base mount)
4x 12" long, 2" x 1/8" steel angle (support to foot mount)
8x 12" long 2" x 1/8" steel angle (foot)
(and miscellaneous fasteners found around dms)

Materials order
4x 5’ 2" x 2" 14ga square tube, $46
2x 8’ 2" x 2" 1/8 steel angle, $30.60
2x 6’ 1" x 1" 1/8 steel angle, $18.00

$104.06

But looking at expenses so far

Sep 5, 2016 Westlake Hardware ‡2 COMMITTEES:PR $10.94
Sep 6, 2016 The Home Depot ‡3 COMMITTEES:PR Trebuchet $60.62
Sep 19, 2016 SERVICECAST ‡8 COMMITTEES:PR Trebuchet $71.12
Sep 20, 2016 Amazon ‡5 COMMITTEES:PR Trebuchet $16.35
Sep 20, 2016 Amazon ‡2 COMMITTEES:PR Trebuchet $37.50
Sep 20, 2016 Amazon ‡4 COMMITTEES:PR Trebuchet $45.89
Sep 20, 2016 Amazon ‡3 COMMITTEES:PR Trebuchet $54.32
Sep 20, 2016 Amazon ‡1 COMMITTEES:PR Trebuchet $75.03
Sep 20, 2016 Metals 4 U Inc Dallas ‡7 COMMITTEES:PR Trebuchet $646.83
Sep 22, 2016 The Home Depot ‡6 COMMITTEES:PR Trebuchet $34.88
Sep 25, 2016 Amazon ‡2 COMMITTEES:PR Trebuchet $8.54
Sep 25, 2016 Harbor Freight Tools 357 ‡4 COMMITTEES:PR Trebuchet $98.12
Sep 25, 2016 Metals4uonline ‡3 COMMITTEES:PR Trebuchet $79.08
Sep 25, 2016 Westlake Hardware ‡5 COMMITTEES:PR Trebuchet $25.96
Sep 27, 2016 Amazon ‡5 COMMITTEES:PR Trebuchet $21.92
Sep 27, 2016 Amazon ‡7 COMMITTEES:PR Trebuchet $33.77
Sep 28, 2016 Amazon ‡6 COMMITTEES:PR Trebuchet $3.24
Sep 29, 2016 The Home Depot ‡2 COMMITTEES:PR Trebuchet $95.80
Sep 30, 2016 The Home Depot ‡1 COMMITTEES:PR Trebuchet $26.02
Oct 2, 2016 The Home Depot ‡1 COMMITTEES:PR Trebuchet $20.97

We have spent $1,466.90 of our $1,500 budget, so I can either spend auto money on this or start asking for donations

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I’ll toss in a hundo. :heavy_dollar_sign:

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The weights turned out fairly consistent in mass, all around 110-115 pounds, looks like I overshot the target by 20% and we will have a max of 1,200 pounds including the weight axle

Also did some more welding on the end of the arm, this is where winch connects to to lift the 1,200 pounds

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Sorry I failed to get down there Wednesday.

Your welding looks great from here though!

And that isn’t saying anything about how nixe your weights turned out.

Well done! What remains to be done now?

Heat / relief welds on outside of uprights
Remove temporary spacers
Test motion of arm
Weld arm end cap and release pin and angle adjuster
Assemble trigger assemblies and weld to top
Design and weld winch mount
Design and build spring loaded trigger (remote end)
Order, pickup, build detachable legs
Get new spanner bushing for new arm pivot wheels and replace gray wheels
Design and build secondary safety
Final welding of rail tracks
Attach test sling
Test fire with min weight
Test fire with half weight

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I am talking to a fellow right now who could help me get my O2 tank refilled and then I will be able to heat up the sides of the trebuchet to try and draw it back to dimension.

Might be next week though. If we cannot wait that long we could rent a rig or put out a request to borrow one.

If time is of the essence, welding just to grind off the welds is going to be very labor intensive.

Hopefully he needs to draw in a direction where the welds can be ignored.

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That would be ideal, but I do think he wanted them where the counterweights were going to slide, which demands a flush surface.

If I am wrong and we can get away with it I would be thrilled to do a surface weld on it.

We could wait for the o2, but I’m not good at waiting, if we run out of other things to do on sunday can do the surface welding then grinding it smooth