Trebuchet 3

 

After building two smaller trebuchets, I decided to attempt a full size trebuchet. The new treubuchet would be built on a 30 foot trailer donated from a friend. The majority of the structure would also be built from 2X10's, as I had found a barn full of them with no use.

 

A 30 foot trailer was donated from a friend for the trebuchet to be built upon. The first step of construction was to build two segments composed of 3 2X10's each. These segments will serve two important functions. First, they will provide an area to secure the A-frames to. Second, when the A-frames will be raised, they will pivot on the segments. Alignment was thoroughly checked, and their positioning was set so that the center of each segment was directly over a steel cross frame of the trailer. This is important, as the A-frames will exert massive force in the center of each segment.

A-Frames

The second step of construction was to build two massive A-frames. These A-frames, in combination, must be able to support the full weight of the throwing arm and counterweight. In reality, when the counterweight falls, it likely exerts a force of about twice its weight. This amounts to about 2000 pounds of downward force per A-frame. The center of each A-frame is composed of 5 2X10's glued together. Side braces were then constructed. A 2'' diameter hole was then drilled out from the top, along with two steel plates and two bearings. Each A-frame weighs 750 pounds. The next step in the process would be to raise these massive A-frames into place.

A winch on the front of a jeep was used to pull the A-frames up into position. They were then secured onto the trailer deck with various braces.

The Throwing Arm

For the throwing arm, I decided to glue 5 2X10's together. A hole for the axle and the counterweight was then drilled out. Two case hardened 2'' steel axles were also used. One shaft is for the throwing arm pivot, and the other for the counterweight pivot. The tops of the A-frames have 2 bearings each, and the end of the throwing arm has 2 bearings. These bearing will allow for easy movement and rotation of the two pieces. The throwing arm weighs 300 pounds, and is 19 feet long. A hoisting structure was then built across the two A-frames. This structure would aid in the process of raising the 300lb. arm.
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Counterweight Box

A hinged counterweight box was chosen, as it provides better performance than a fixed counterweight. The frame of the box was constructed using mortise and tenon joints. The finished product weighs 500 pounds. The counterweight box can hold a maximum of 2000 pounds of bricks.

Here is the finished counterweight box, currently holding 120 pounds of human counterweight. The ratio of the short end to the long end is 1:2.5.

 

Final Construction Steps

The trebuchet is nearly ready for a first firing. All that remains is to construct a system for firing the trebuchet, and then bringing the arm back down. For obvious reasons, the throwing arm must be brought down with a mechanical device, rather than human force. To accomplished this, I purchased a winch, and mounted it on the front end of the trailer. It is attached to the top of the throwing arm through a pulley. The winch cocks the throwing arm down and simultaneously raises the massive counterweight. Once the throwing arm is pulled into position, I attach it to my triggering mechanism.

The trigger holds the arm down and allows for a single person to fire the trebuchet. The trigger also must be able to hold the nearly 700 pounds of upward force the arm exerts. An arrangement of two levers on the trigger allows one person to fire the trebuchet with a light pull. In fact, one person would be able to grab the rope with a single finger and fire the machine.

Finally, a rope pouch was created to hold the first bowling ball during the first launch. The total sling lenth is roughly 10 feet. A release pin and its pull line were also made. The trebuchet is fired by pulling on a rope from 150 feet away.

 

 

First Firing

For the very first shot, only several hundred pounds was put into the counterweight box, and a 12 pound bowling ball was selected for the projectile. I nervously pulled the release pin, and to my amazement the trebuchet actually worked. The very first shot was roughly 300 feet with a 12 pound bowling ball.

Final Results

The ultimate record was 621 feet with a 16 pound bowling ball.

With the full 2500 pounds of counterweight, here are the machine's results. For the stranger objects, the optimal release angle (45 degrees)was not often achieved.

Object

 

Distance/Comments
16 pound bowling ball 621 feet. (this is the record distance)
Microwave half filled with dirt 320 feet
20 pound lump of concrete 150 vertical feet, only 100 feet away
50 pound flat barbell weight 120 feet (the weight is terrible from an aerodynamic standpoint)
Large Television (around 30 pounds) The sling was cut too short and the television just flew straight up, more than 100 feet.
Small Computer Monitor 330 feet

Final Photos

Youtube Video of the trebuchet throwing a bowling ball at 1/3 speed

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