Another small HCW trebuchet!
My small HCW prototype
This trebuchet came about cos I haven't done any treb work for over a year, and
got the bug again. And, I'd been meaning to build a 6 feet tall HCW treb to throw
golf balls (or similar) but wanted it to look traditional. This meant lots of
angles in pieces of wood and lots of wood crossing and joining. I wasn't sure that
I could achieve the accuracy I needed using hand tools alone, so this was a good
test. It turns out that I can't achieve the accuracy I need with hand tools,
at least not for a treb this small. Perhaps things get easier for bigger trebs?
I decided that I wanted this treb to throw 5 grams, a 66% increase of projectile
mass when compared to my other small trebs. I wanted to wage war on my old
boss's collection of 3 inch high plastic daleks (the ultimate force for evil in
the multiverse, he'll have you know) and he sits some 30 feet away from me. I
then put the following stats through my treb size calculator
to give the rough dimensions I should be using:-
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5 gram projectile
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500 gram counter weight
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9144 mm required range (30 feet)
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efficiency 40 out of 100 (i.e. 40%)
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upper to lower arm ratio of 4 to 1
This yielded the following critical dimensions:-
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counter weight drop of 114.3 mm
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lower arm of 66.96 mm (rounded up to 67 mm)
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upper arm of 267.84 mm (rounded up to 268 mm)
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axle height of 189.36 mm (rounded up to 190 mm)
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a height to the tip of the machine at rest of 457 mm (roughly 18 inches).
I've made a sling from a carrier bag, and it's working well. I've got
my heart set on a 'proper' net sling pouch, though that looks like a project for
the future. I've seen a good
net making site, but it's something that you can't do overnight so the carrier
bag sling pouch will have to do for now.
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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about 465mm (I've not actually measured, this is just taken from the Visio drawings
that I did for the design).
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height to pivot
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195mm
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drop of counter weight
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114mm
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counter weight
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500 to 650 grams lead shot
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projectile
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5 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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268mm / 67mm
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base dimensions
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315mm long, 220mm wide including buttresses
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sling length
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260mm on free end of the sling, shorter on the captive side.
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And here are the results of the best
throw that I will have measured:-
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counter weight
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projectile weight
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theoretical max throw
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actual throw
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efficiency
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notes
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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.
The photos
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(treb07pic01.jpg 41kb)
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The treb at rest.
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(treb07pic02.jpg 59kb)
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The treb cocked, front 3/4 view.
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(treb07pic03.jpg 66kb)
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The treb cocked, rear 3/4 view.
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(treb07pic04.jpg 74kb)
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The projectile in the sling pouch.
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(treb07pic05.jpg 63kb)
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A view down the track, the sling pouch slides along the track when the treb is
triggered.
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(treb07pic06.jpg 86kb)
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A close-up view of the sling pouch cupping the projectile.
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(treb07pic07.jpg 56kb)
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The counter weight carrier fitting snugly between the top plates.
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(treb07pic08.jpg 54kb)
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The lower arm, counter weight carrier and top plates/cartridges that contain
the bearings for the arm.
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(treb07pic09.jpg 49kb)
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Here you can see the portion of the top plates that was removed to allow the
counter weight carrier to fit snugly between the inside top plates. Without
this modification to the inside top plates the counter weight carrier would have
been propped as it does not fit between the inside top plates - I neglected to make
room for the width of the counter weight carrier when making my plans.
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A test firing sequence
The sequence below shows the treb
during a test firing, mainly to show the action involved. The counter
weight carrier is about half full, and the projectile is a 1 inch diameter wooden
bead that weighs 5 grams.
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(treb07fire01.jpg 15kb)
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The treb is cocked and ready to fire.
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(treb07fire02.jpg 15kb)
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The trigger arm is removed, and the arm starts to rise.
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(treb07fire03.jpg 15kb)
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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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(treb07fire04.jpg 15kb)
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The projectile is released, though the arm appears to have stalled. This
point in the throw can be make or break. If the projectile has left the sling
before the arm stalls then the throw should be good. If the projectile is
still in the sling at this point then the arm will stop providing energy to the
projectile and range will suffer. The stall is particularly noticeable in the
small animation of the throw below.
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(treb07fire05.jpg 15kb)
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The empty sling carries on in front of the arm, and the arm starts moving again
as the counter weight continues to move at the bottom of it's swing.
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(treb07fire06.jpg 15kb)
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The counter weight swings under the arm, dragging the lower end of the arm with
it and forcing the upper end of the arm forward.
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(treb07fire07.jpg 15kb)
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The counter weight completes its
swing under the arm, and starts to return.
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(treb07fire08.jpg 15kb)
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And the arm starts rocking back and forth...
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A small animation of the above (112kb)
If you're interested in seeing more about trebuchets, return to the index for
some links.
If you'd like to build one of your own, then there's a guide available.
return to trebuchet index
This page last updated on April 13th, 2004
Created by hand using 1st Page 2000
