Hurl those stones...

Anyone who has played Age of Empires (this includes me, yes), will know exactly what a trebuchet is, and what it does. A trebuchet exists as a medieval rock throwing machine, but quite different to a catapult.

Here is an oversimplified diagram of an armed trebuchet. CW stands for counterweight, and the release pin... well nevermind you will see what that's for later. Don't you love it when I explain things?

Gravity pulls the heavy counterweight down towards the ground. Because the counterweight is attached to the throwing arm on one side of the axle, the projectile side of the throwing arm is pulled upwards. The rate of movement on the right hand side is faster than the left hand side, due to the lever physics.

The sling carrying the projectile is carried forwards by the lifting arm.

As the counterweight falls some more, the sling is brought off the ground by the throwing arm, and travels at a tangent to the circle traced out by the throwing arm (at any instant).

The throwing arm is now nearing perpendicularity with the ground. However, the release pin is angled differently, and the loose release string on the sling is starting to slip off the pin. Note that the projectile is held tightly by the sling so that it can't just fall out.

At a critical angle, the release string is well... released from the release pin. The momentum of the throwing arm makes sure that the released string lifts up high and wide of the projectile. The projectile is thus released from the pouch, and follows a tangential path at that instant.
Momentum carries the throwing arm through, and everything swings and sways and does whatever it has to do before collapsing into its unarmed state.

The Penguin's Lab Trebuchet!

This was the result of a rushed project using materials in the near vicinity (the shed), and materials were definitely limited, so please spare my ugly construction methods.

The trebuchet originally consisted of one microwave oven transformer as a counterweight (trust me, I have alot of these). This, however, was increased to two microwave oven trasformers after it was decided that the range was not adquate. The counterweight now measures in at around 15kg.

A view of the 2 MOT (microwave oven transformer) counterweight. I had previously attempted to secure these to the throwing arm using rope, but the rope snapped on the first go.

When the trebuchet is fired, you can hear the fatigue in the metal fastenings. But who cares, it works!

The triggering pin is shown here. Two curtain hooks attached to the end of the base serve as the holders, while a camping peg is the triggering pin. The string attached to the peg is pulled to fire the trebuchet.

A closer view of the axle. Aluminium tubing is passed over a decking spike (which serves as the axle) to reduce frictional losses between wood and metal. The two wood pieces bound to the throwing arm are reinforcement pieces in case the throwing arm snaps at the axle (which is unlikely, but with such a heavy counterweight, you never really know).

The throwing arm was configured for a 4:1 ratio between sling side and counterweight side respectively.

A stroboscopic shot of the trebuchet firing a blank shot. From here you can see the maximum deviation of the throwing arm on the left side. Unfortunately, the strobe light was unable to capture the pouch releasing from the release pin.

The current maximum range of the trebuchet for a 50 gram projectile has been recorded as ~15m.

Here is the trebuchet in action (slow motion).

Some lightweight statistics- the trebuchet has:

  • Bent one axle (made of solid steel), and competely destroyed another (made of copper)
  • Had its support frame modified many times due to massive amounts of stress on screw joints, which caused splitting of wood.
  • Had three different counterweights attached; 1 MOT, 2 MOTs, and 2.5 MOTs (which is in use now).
  • Tipped itself over a million times after the heavier counterweight was attached, even with three heavier transformers securing the base to the ground. (I told you I had alot of transformers).

This image of a trebuchet hastily drawn by Nick





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