Here is the second project, a floating arm straight drop trebuchet. I started design and build after seeing pictures of a real machine that would throw 12lb balls 300 feet, just from a 470lb counterweight. The tip of the arm stands about 9" above the floor. It's currently got about 9 1/2 ounces or 270 grams counterweight, and throws a wooden bead about an inch in diameter some 20 feet. It's not been tuned yet, so I'm expecting to improve the range. The wheels are made of brass, and replaced a set of wooden test wheels that were really just discs mounted on a flanged bearing. The brass seems to have quite an effect on the range, so I'd like to investigate plastic wheels. The sling in the photos below could be a lot better, but it's the first sling I made for the machine. The current sling has a much better pouch, but has only made the machine better at throwing lighter projectiles.
Here are some stats:-
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height to top of arm when at rest |
9 inches |
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height to top of counter weight guides |
7 1/4 inches |
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height to wheel tracks |
4 1/8 inches |
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drop of counter weight |
3 7/8 inches |
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counter weight |
about 9 1/2 ounces or 270 grams |
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projectile |
4/5 inch or 20mm wooden bead, about 1/9 ounce or 3 grams |
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ratio of counter weight to projectile weight |
about 90 to 1 |
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upper arm/lower arm length |
4 9/16 inch (115mm) / 2 3/32 inch (53mm) |
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wheels |
5/8 inch diameter, 9/32 inch wide plus flange 1/16 inch |
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base dimensions |
6 inches long, 1 7/8 inches to outside of counter weight guides |
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sling length |
4 1/2 inches |
And here are the results of the first few throws that I've measured:-
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counter weight |
projectile weight |
theoretical max throw |
actual throw |
efficiency |
notes |
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140 gms/5 oz |
3 gms/ 1/9 oz |
349 inches |
192 inches |
55% |
wooden test wheels |
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140 gms/5 oz |
3 gms/ 1/9 oz |
349 inches |
156 inches |
44% |
'proper' brass wheels |
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270 gms/9.7 oz |
3 gms/ 1/9 oz |
676 inches |
244 inches |
36% |
'proper' brass wheels |
The efficiency is calculated as the ratio of actual throw distance to the theoretical maximum, expressed as a percentage. The theoretical maximum can be approximated as 2 * counter weight * drop of counter weight divided by projectile weight. For the 244 inch throw we get the theoretical maximum of 2 * 9.7 oz * 3.875 inches divided by 1/9 oz = 676 inches.
Here's a small graphical representation of the big throw, just so you can get an idea of how big the treb is compared to how far it's throwing. It's more or less to a scale of 2 pixels per inch.
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A view from the danger end of the trebuchet. You can see the brass wheels on the arm and the counter weights on the outside of the machine. You can also see the trough that the projectile sits in, a V shaped channel mounted on the base of the machine. The photo doesn't show the recent addition of spacers between the wheels and the arm to stop the wheels from moving towards the arm. |
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A side elevation of the machine at rest. |
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A side elevation of the machine cocked and ready to fire. |
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A close-up view of the wheels sitting on the tracks. |
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A view of the counter weights at the bottom of their travel. The weights are suspended from the lower end of the arm, and have a bar connecting the bottom of each weight carrier to aid in the stability of the weights and the arm during use. The weight carriers are capable of accepting 5 pieces of lead sheet in each half. The 9 1/2 ounce / 270gm weight means putting only 4 pieces of lead in each half, so I can up the weight yet should I need even more range. |
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A view of the trigger mechanism. A small piece of dowel pierces the upper end of the arm near the tip. The fixed end of the sling is attached to this, and a small loop is also attached to allow the arm to be held while the machine is loaded. The free end of the loop is passed between two small eye hooks, and a piece of paper clip on a lanyard holds the loop between the eye hooks. Pull the clip out to fire the machine. |
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A view of most of the bottom of the frame. |
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A view of the major components, disassembled. |
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treb at rest before firing. |
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treb triggered, arm starting to rise. |
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arm approaching release point, you can just see the sling as a blurred bluish arc against the background. |
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projectile released, a whitish blur rising from the sling. |
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arm moves to upright position and rests almost immediately, while the sling passes over the top of the treb. |
A small animation of the above.
If you're interested, some links can be found on my trebuchet index page
If you'd like to build one of your own, then there's a guide available.
This page last updated on May 27th, 2005
Created by hand using 1st Page 2000
