A small hinged counter weight trebuchet for the garden
My 6th trebuchet
This trebuchet came about when I
realized that the frame I had made for #5 wouldn't accommodate a hinged
counter weight arm, and I had to build a second frame that would allow
me to use the arm I had already made. This is the design I originally
had in mind while building #5, I just did #5 wrong that's all. It's
intended to throw a ball from a computer mouse, and uses a 1Kg counter
weight.
Here are some stats. Note that
all heights are from the top of the track that guides the sling:-
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height to top of arm when at rest
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480mm
|
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height to pivot
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200mm
|
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drop of counter weight
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138mm
|
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counter weight
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2 x 500gm discs
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projectile
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mouse ball, 10 grams
|
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ratio of counter weight to projectile weight
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100 : 1
|
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upper arm/lower arm length
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280mm / 80mm
|
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wheels
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not yet decided, but likely to be about 40 to 50mm diameter
|
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base dimensions
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310mm long, 160mm wide
|
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sling length
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fixed side 250mm, free side 245mm
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And here are the results of the best
throw that I've measured:-
|
counter weight
|
projectile weight
|
theoretical max throw
|
actual throw
|
efficiency
|
notes
|
|
1000 grams / 2.2 lbs
|
10 grams / 1/3 oz
|
27.6 metres / 90' 6"
|
11.6 metres / 38'
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42%
|
mouse ball
|
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 11.6m throw we get the theoretical maximum of 2 * 1000g * 138mm
divided by 10g = 27600mm. The efficiency compares well with that
of my other trebs, so I must be doing something consistently.
I've used WinTrebStar by Donald B
Siano to simulate a throw based on the dimensions of the treb. According
to the program's calculations I'll get the best range if I make the sling
320mm instead of 250mm. I'll give it a try and post the result here.
The photos
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(treb06treb1.jpg 50kb)
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A view of the machine at rest
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(treb06treb2.jpg 56kb)
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A side view of the machine cocked and ready to fire.
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(treb06treb3.jpg 45kb)
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A view of the trigger mechanism,
consisting of two eye hooks screwed into the base and a lanyard on the
arm with a washer at the free end. The washer is placed between the
two eye hooks and a paper clip on a lanyard retains the washer. Simply
pull the lanyard attached to the clip to fire the treb.
|
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(treb06treb4.jpg 30kb)
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A close up of the tip of the arm
whilst the machine is cocked. Here you can see the small magnetic
strip glued to the end the arm to hold the slip ring in place, 'cos gravity
can be a real nuisance.
|
A test firing sequence
The sequence below shows the treb
during a test firing, mainly to show the action involved. The counter
weight is just one 500gm disc, and the projectile is a large ball of aluminium
foil.
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(treb06fire1.jpg 26kb)
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The treb is cocked and ready to fire.
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(treb06fire2.jpg 26kb)
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The trigger pin is removed, and the arm starts to rise.
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(treb06fire3.jpg 26kb)
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The arm accelerates, the projectile can be seen as a silver coloured arc
leaving the track at the bottom of the frame.
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(treb06fire4.jpg 26kb)
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The projectile is released (a silver line can be seen against the right hand
side of the door frame), the arm continues to move.
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(treb06fire5.jpg 26kb)
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The empty sling carries on in front of the arm.
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(treb06fire6.jpg 26kb)
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The counter weight swings under the arm as the arm stalls.
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(treb06fire7.jpg 26kb)
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The counter weight completes its
swing under the arm, and starts to return. The arm has been carried
further forward by the weight, indicating that on this firing the projectile
did not take enough energy from the counter weight. Maybe it's time
to start tweaking...
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A small animation of the above (59kb)
If you're interested in seeing more about trebuchets, return to the index for
some links.
return to trebuchet index
This page last updated on April 13th, 2004
Created by hand using 1st Page 2000
