Dry fire disaster

by ian
Categories: DIY
Comments: 2 Comments
Published on: September 6, 2008

Jake and I worked on the trebuchet some more today and were actually able to dry fire it.  Here are a couple of pictures after we got it put together.

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We then did some adjustments to make sure the arm was swinging how we wanted.  Every looked good.  So we loaded it with some weight.  We used a couple of cinder blocks.  In hind sight I think we should have used something lighter since the blocks were about 50 lbs each.  So with 100 lbs or more we lifted it up as high as we could which was only about eye level.  That should have been our clue.  We counted to 3 and let it drop.  It dropped much much faster than we expected.  The arm rollers unfortunately went off the front platform and onto the ground with such force it split the arm in half lengthwise.  It was a little scary how fast it moved.  We thought the weight would keep the arm from going too far forward.

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The next step is to rebuild the front with longer 2x4s and replace the broken arm with a 2×6.  I think there was enough force to throw a bowling ball a long ways.  Live and learn.

2 Comments
  1. hurlinthom says:

    First, as you found out, dry fires are rough on the machine. Second, if the tracks are just long enough to support the wheels you should be OK; longer isn’t necessary, shorter is asking for trouble. Third, you need reinforcement around the areas where the axles pass through the arm. Making the arm wider (2X6) won’t help much. a square of 3/4″ plywood on either side of the weight axle hole (with matching holes in the squares, of course) will help more.

    You were kidding about the bowling ball, right? Your frame looks narrow for that.

  2. ian says:

    Yeah, our rails need to be longer so that the arm can’t roll forward off the end. I think the 2×4 would have worked for the most part as it broke when it flipped forward and impacted with the concrete on the ground.

    I wasn’t kidding about the bowling ball. We could fit one pretty easily between the cinder block base. We have much more testing to do before we try something with that kind of damage potential. We will also need to move to a safer test site as well.

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