Siege Engine.com: Storing Energy in a Machine

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Storing Energy in a Machine
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Siege engines that store energy and use a throwing arm typically need some sort of winch. There are many forms of winch, and they are actually somewhat uninteresting. Of greater interest is how to use them most effectively. Tricks for tall trebuchets can greatly enhance cocking time. Often, an in line trigger is quite useful, though we haven't used one.


Winches

A boat trailer winch is a mighty fine thing if you are in a hurry. You can get them at any marine hardware store, they are cheap, and can handle vast loads. Be very careful when picking out a winch. They are typically rated in terms of how big a boat you can haul out of the water with it. If a winch can haul a 5000 lb boat out of the water, it cannot handle a 5000 lb torsion load. This is because the winch isn't suspending the boat in the air. Most of the weight is on the trailer wheels, or floating in the water.

This image is from Juggernaut 2.


You could also roll your own winch with a gear and pawl. This image from Baby Treb shows a gear and pawl scavenged from U-Mass. I don't know what it's previous life purpose was. The dowel it is attached to runs through the frame of the machine, and a rope wraps around it as the dowel is turned.


You could also just rely on manly prowess as does Roger here pulling back on Ballista Jr. We had lots of issues with Ballista Jr, but getting the bowstring pulled back wasn't one of them (Unless you were Kevin, and you were holding the bowstring in place while someone else tried to use our totally sucky trigger.)


Trebuchet Adjusting Linkage


On Juggernaut 2 we had a cable that ran down the length of the arm. Over this cable we had a pulley with a hook. We attached the winch to the hook, and as the arm came down, the pulley would move from near the fulcrum, to the other end of the arm. When the arm is vertical, we had little leverage, but this is ok since the counterweight is being pulled left to right more than up and down. As the arm came down, and the pulley slid out toward the hook, we gained more leverage. This is also when you need the most leverage as the wieght started to tilt itself.

In the below image sequence, you can see how the pulley slowly moves from one end of the arm to the other.




Onager Winch


We recycled the Juggernaut 2 winch on Onager Jr. In order to get enough leverage, we added a 2-1 pulley system. We hid the winch inside the frame so it wouldn't get in the way while traveling. A pulley at the base of the trigger is then used to keep the line straight while feeding into the winch.

The observant may notice that the handle has moved on this winch. It is fortunate that most boat winches can have their gear system reversed or transposed with ease.


Here you can see the additional pulley attached to the arm.


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Last Modified: 11/26/16