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|10-24-2012, 08:10 PM||#11 (permalink)|
Join Date: Feb 2011
Location: PEI, Canada
Safety first! Even fairly small and trivial looking machines of this nature can do some serious damage to a human being should you act stupid around them. (And the big ones can do some really serious damage to a large group of people.)
Don't Screw Around!
Trust me, guys are the ER give you funny looks when you try to explain to them what a trebuchet is, and why you need that piece back when they're done digging it out of you. (Really wish I had gotten video of a high school buddy trying that. Sadly my high school times predated HD video cameras in everyone's pocket.)
Think before you do something, think before someone goes near the device. Wear eye protection at minimum around all sized devices. (Really really big ones don't actually require you to wear much safety gear. If something goes wrong then there is a good chance you're dead anyway.)
Design issue wise:
Two biggest design failures I've seen in Trebs tend to be their sling, and their sling release. These are vital to the functioning of your machine, and demand great attention to detail. They are also the most fun to experiment with and research.
Remember that there is actually a great deal of tension pulling outward on your throwing arm. I've seen a good number of sling-arm designs that failed because they used a truss design expecting all the needed strength to be resisting flex. Flex really isn't all that bad of a thing, and you can get excellent power transfer results out of a bending arm. Do not get stuck on designing it to avoid bending, or you may actually over weight the far end of the throwing arm and eat energy that would otherwise have been transferred in a whipping action.
Two main groups of trebs that I have seen: Fixed link weights, and split link weights. Fixed links are generally easier to construct, especially if you go with a fixed pivot. (Where the axle is mounted such that it doesn't move, aka the classic Medieval design.) Rolling pivots on a fixed link are a little harder to design well (And make safe) because the pivot becomes a wheel that rolls forward, but can offer increased efficiency.
Split links are an option that some don't consider a treb at all, but potentially offer even greater force transfers for a limited construction. Generally consisting of a counterweight attached to a pulley instead of the throwing arm, and allowing the weight to drop straight down. This motion is then transferred to the throwing arm in some manner, sometimes simple, others overly complex. If you're not trying to be clever and getting around material/weight limits, then these options usually don't offer much from what I have seen. But they can be really fun to use when trying to break rules. (Saw a few really fun designs using 1kg counter weight limits. One being a massive tower that allowed a super long drop... off a three story building. Another really skirting the rules by using multiple weights that tripped each other, one falling at a time. Forget how the rules were worded, but perfectly legal. They won, judges smiled, and reworded the rules for next year.)
|10-25-2012, 12:54 PM||#13 (permalink)|
MCBs armed pacifist
Join Date: Jun 2009
Location: West Michigan
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