Siege Engine.com: How to Trigger a Siege Engine

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How to Trigger a Siege Engine
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Different types of machines store their energy in different ways. While we have been successful in adapting different styles of triggers between machines, the type of trigger you construct will depend on the load and structure of the machine.

In particular, if you build small models first, it is nearly impossible to use similar construction techniques between the small and larger scale, though similar design is usually possible.


Autonomous Counterweight

Traction trebuchets use Autonomous Conterweights (humans.) Because of this, their idle position is waiting to be fired. If you are thinking of building a trebuchet, this is an ideal design because you don't need to haul lots of counterweight around, and you don't have to design a trigger.


Simple Hook Trigger


Hook triggers are the simplest to conceptualize and build, and the hardest to get right. At a small scale (as with this trigger on Baby Onager) the strength of the operator can overcome most engineering flaws.

The design is quite simple. An L or T shaped bar is constructed. One bar rides over the arm (as in the case for our onagers) or through a loop on the bottom of the arm. The opposing bar goes down to a pivot beneath the arm.

The forces applied to the trigger will attempt to straighten everything out. Your pivot must be rotating in a plane that is perpendicular to the force applied by the arm. Our small onager was not designed this way, but the materials in the trigger are much stronger than the torsion bundle.


When building this type of trigger on the larger scale, the forces applied to the trigger are much greater. In this case Onager Jr's trigger was extended upwards quite a bit. This is because the force of the arm against the blocking bar creates a lot of friction. This leverage is necessary to overcome that friction and launch the machine. As it is, kids around the age of 7 to 8 have a hard time operating the machine.




Balancing the Hook Trigger


It is important to balance the trigger over the arm. The bar that locks the arm down must be perpendicular to the desired motion of the arm.

In this image, if the trigger were tipped more to the left, it would launch itself. If the trigger were tipped more to the right, it would be nearly impossible to fire the machine.

Often balancing this trigger is done after cranking the machine down. Inexperienced users have been known to set things up so that they can't get the trigger to pull over.


Dealing with Damage for the Hook Trigger


Onager Jr's trigger was not designed to be in a perpendicular plane with the arm before firing. A side effect is that the arm bent the trigger at it's hinge so that it was. This was only about 5 degrees, but it was enough to break the hinge eventually.

The hook mechanism was replaced with a barn door style hinge. The trigger still isn't in line, but I was hoping the hinge would be stronger. It did last well, but when we added a little more power, it too has become quite bent, though it is still functional.


Opposing Tooth Gear Trigger


An Opposing Tooth gear is easily made starting with a single circular piece of wood or metal. Two notches are then removed with the faces 180 degrees across from each other. One tooth will hold back the stored energy of the machine. The other will be blocked by a trigger pawl.

When triggering a trebuchet such as Juggernaut 2, or other machine where the trigger is attached directly to an arm, it is necessary to balance the force of the lead-rope directly against the trigger tooth. If the arm has potential to wobble, it is important to add a guide or governor to make sure the line stays steady.


This style of trigger was first seen by our team for use with crossbows and ballista. The wheel on this image has a gap in the center of the bowstring tooth. This allows an arrow style projectile to rest inside the trigger, and abut the bowstring more tightly. The rotation of the wheel doesn't jog the arrow, permitting it to be launched cleanly.




Shimmed Hook Trigger


A Shimmed Hook Trigger holds a line or rope between a solid piece of the machine frame, and a hook. The hook pivots on one end, but is prevented from releasing the line by a shim or block. When the block is removed, the hook pivots, releasing the triggering line.

On Baby Treb some credit-cards are used to shim the hook. The hook has it's point in the side of the machine. When the cards are removed, then the pull on the chain forces the hook to pull.

Shimmed hook triggers were common for spear shooting ballistas as well. These types of machines had a sliding trigger mount called a Chelonium. This provided the base for the hook to press against.




Inline Quick Release Catches

An inline trigger is a device used to release a line under load. Commonly used on sailboats, or other marine applications, they are also useful for triggering a catapult. A cheap way of doing an inline trigger is to use a sacrificial line, meaning that the arm is tied down, and the rope holding it is cut to launch the machine.


Commercial Inline triggers always have a load rating. You will need to know what your load is to safely use these small devices.


The Sea Catch


Despite the fact that I get money if you buy one of the triggers in the previous section, my team are big fans of the Sea Catch toggle release trigger. This gizmo comes in many sizes and is used by several of the championship machines in the various catapult divisions at the Punkin Chunk.

The big Sea Catch pictured here is used on our machine Mista Ballista and can handle over 7000 pounds. The small one will be used on a medium sized model and can handle over 700 pounds. We opted to go with a Sea Catch due to the high load rating (some models handle over 70 tons) and the recommendations of our peers.

If you call to order one, be sure to let them know where you first read about them.


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Last Modified: 11/26/16