Self Launching Portable Trebuchet

My Brother-in-law came to me a month ago and asked if I could make him a Trebuchet. He is the head custodian at one of our local schools. An MST teacher wanted to know as she was teaching her class certain principles of science through ancient weapons. I jumped at the chance, I have always wanted to build one.

So to the Google's!!!

I researched trebuchet's for a week or so and stumbled upon Tom Heck, Human-powered-portable-water-balloon-launching-Trebuchet. This was even better than a standard trebuchet as the children had to launch the projectile and not a weight. But Tom had not updated his page and given the world the plans. But through modern technology Prime Ruler and an access pass on a picture on his site I was able to make a reasonable facsimile of the one Tom built.

Step 1: Shopping List, Tools and More...

Lumber

    • 2 @ 2"x4"x8'
    • 1 @ 2"x4"x10'

    PVC40

    • 1 @ 1-1/2"x10'
    • 1 @ 3/4"x10'
    • 1 @ 1/2"x1'
    • 2 @ 1-1/2" cap
    • 8 @ 3/4" cap

    Nuts & Bolts

    • 1 @ 12"x1/2"-13 threaded bolt
    • 2 @ 3/8"-16x3-1/2" hanger bolt
    • 1 @ 1/4"-20x2" hanger bolt
    • 2 @ 1/2"-13 nylon locking nut
    • 6 @ 1/2" washer
    • 2 @ 3/8"-16 nylon locking nut
    • 2 @ 3/8" washer
    • 8 @ 2"x1/4"-20 eye bolt
    • 8 @ 1/4" washer
    • 8 @ 1/4" lock washer
    • 8 @ 1/4"-20 nut
    • 6 @ #6 screw eye
    • 2 @ 5/16"x1-15/16"x4-5/8" u-bolt
    • 2 @ 1/4" quick link
    • 3 @ 3/4"x3-11/16" round swivel snap eye bolt
    • 2 @ 2" deck screw
    • 8 @ 2-1/2" deck screws or 3-1/4" framing nails
    • 1 @ 3/8"x2" Zinc-Plated Welded Ring

    Misc

    • 24' - 5/32" Braided Polypropylene Rope
    • 1 - 18" bungee cord
    • 1 - can PVC cleaner
    • 1 - can PVC glue
    • Asst. sandpaper
    • 2 - 1/2" grommets
    • 1 - 12"x18" denim

    Tools

    • Drill / Drill press
    • Hand Saw / Chop Saw
    • Sabre Saw
    • Hack saw / Metal Chop Saw and or Saws All
    • Assorted Drill Bits
    • Ratchet Set / Wrenches
    • Hammer
    • Wood clamps
    • Scissors
    • Flame to seal cord
    • Sewing Machine / Needle and Thread
    • Compass
    • Small framing square
    • Vise and or Vise Grips
    • Grommet installation tool

    Tools if available

    • Framing gun and compressor
    • 4" electric grinder

    Reference web sites

    Step 2: Lets Cut Stuff...

    I like to use my tools in an order making it easier when assembly time rolls around.

    Wood first.

    Lets get started. Using what ever means to cut your lumber, halve the 2"x4"x10'. So if your lucky and its a little longer than 10' make two at 60" or in my case two at 59-15/16", these are the uprights. Next out of one of the 2"x4"x8' cut a piece at 70" and one at 24", these will be part of the base and the swing arm. The last 2"x4"x8' cut two at 46". Which will complete the pieces for the base.

    On the 60" uprights on one end of each, measure from the end in on the face 1-3/4". Draw a line across the face using the framing square parallel with the end. Find the center and mark it. Using the compass from this point draw a half circle from one end of the line past the end of the board to the other end of the line. Cut along this line to round off one end of each upright.

    PVC next.

    Like the 2"x4"x10' halve the two 10' pieces of the 3/4" PVC so you have four 60" pieces. The 10'@1-1/2" piece of PVC no cut is needed.

    Rope.

    Cut the rope into two pieces at 5' and two at 7'. Using what ever flame source you have, heat each end to seal them from fraying. I used our home stove which is gas.

    Lastly some eye bolts.

    I could not find #6 screw cup hooks. So I took four of the #6 eye screws and cut them to make cup hooks. Put it in a vise or vise grips and hacksaw/electric grinder off enough of the eye screw so another #6 eye screw can click in. Do this on all four #6 eye screws.

    Step 3: Lets Drill Stuff...

    Now that we have cut our supplies lets drill a few holes.

    You will need a 1/2" bit, 3/8" bit, 5/16" bit, 1/4" bit and 1/8" bit.

    Starting with the 1/2" bit take the swing arm, which is the 24" piece of 2"x4". Find the center over the length on the face and carrying it over to the side using the framing square. Now find the center their and drill a 1/2" hole all the way through.

    Take the two 60" up rights and clamp them together so they are even on all sides and the rounded ends are on the same end. Measure from the none rounded end in 12" along the face and find the center at that point. Using the 1/2" bit, drill through both pieces of the uprights. On the rounded end using the 3/8" bit drill through both pieces of upright at the center point you used for the rounding of the ends.

    Now we will drill holes for the u-bolts. On each end of the swing arm measure in six inches and draw a line parallel with the end using the framing square. Take one u-bolt and lay it down on one end of the swing arm at the six inch line centered on the line. Mark the center of each side of the u-bolt on the six inch line. Repeat on the other end with the other u-bolt. Using the 5/16"" bit drill those four holes all the way through.

    Still using the 5/16" bit, take the two 46" pieces of 2"x4" for the base and measure in from one end 12" on each piece and find the center. Drill a hole through each piece, this is for the hanger bolts for the swing arms.

    Now using the 1/4" bit, drill a hole in the center of all eight 3/4" caps. Also a 1/4" hole in the center of one of the 1-1/2" caps.

    And we finish off our hole drilling using the 1/8" bit, on the square end of an upright measure down 18" and find the center. Drill approximately 3/4" deep hole. On the rounded end measure in 3/4" find the center and drill a 3/4" deep hole. Repeat on the other upright. Lastly on the 70" piece of 2"x4" on an end find the center and drill a 1/8" hole about 3/4" deep. This is for an eye bolt for the cables for the wings.

    Step 4: Now Lets Assemble...

    Lets start by putting the base together.

    Take the two 46" pieces and the 70" piece. Using what you allotted screws,nails and or nail gun to assemble these pieces place a 46" piece with the hole at 12" on the opposite end of the 70" with the hole on the end. Repeat with the other 46" on the other side of the 70" so it makes a cradle.

    Take the two 3/8" hanger bolts and screw them into the holes on the sides of the 46" pieces of the cradle leaving enough length for an 60" upright the washer and lock nut. I have a saws-all so I was able to cut the hanger bolts that came through into the cradle. You probably don't have to I don't think the sling will catch on them.

    Take one of the 60" up rights and bolt it on, not to tight so it will swing freely. Repeat on the other side.

    We need to measure and cut the 1/2" PVC for the swing arm. Measure the inside width between the two uprights. Measure the thickness of four of the 1/2" washers. Take the width between the uprights minus the thickness of the washers minus the width of the swing arm and divide that by two. Now using that cut two pieces of the 1/2" PVC. Mine was 1-5/16". Put one of the locking 1/2" nut on the 1/2-12" piece of threaded rod using a ratchet or a wrench, so one or two thread protrude through and a 1/2" washer. Slide it into one of the uprights so it sticks out enough to put a 1/2" washer a piece of the cut 1/2" PVC then another 1/2" washer. Then put it through the swing arm just enough to get another 1/2" washer, the other piece of 1/2" PVC another 1/2" washer then through the other upright. Put the last 1/2" washer and put the other 1/2" locking nut on using a ratchet or wrench. Same as the other side one or two threads but also not to tight that the swing can swing freely. Using the metal cutting saw of choice, cut off the excess 1/2" threaded rod.

    Take the cut #6 eye screws and screw them into the holes on the outside of the two uprights.

    Take an uncut #6 eye screw and screw it into the end of the base. And the cradle is done.

    PVC assembly, take a 1/4"-20 eye screw and put it in an 3/4" PVC cap with a washer, lock washer and nut on the other side and snug it up. Repeat on the other seven 3/4" PVC caps. Take the 1-1/2" cap with the 1/4" hole and screw in the 1/4" hanging bolt, about halfway up the screw side of the bolt.

    Using the PVC cleaner and glue, glue each 3/4" cap onto each end of the four pieces of 60" 3/4" PVC pipe. Take the two 1-1/2" caps, using the PVC cleaner and glue again and put them on the 10' piece of PVC pipe.

    Once the glue sets, take two of the 3/4" PVC pipes and connect them together with a 1/4" quick disconnect and repeat the process on the other two pieces of 3/4" PVC.

    Take two of 3/4"x3-11/16" round swivel snap eye bolt and the two pieces of 5' rope and tie one end of the rope onto each swivel snap eye. Take each set of 3/4" PVC pipes and connect them to the back of the cradle. Lift up the the uprights and attach the other 3/4" PVC to the eyelets on each side so the 3/4" PVC and upright form a triangle. Clip each of the snap eye bolts to the 1/4" disconnect at the end of the joined 3/4" PVC. Measure from the 1/4" disconnect to the #6 eye bolt at the end of the cradle and the both should be out 4'". Tie each of the ropes to the #6 eye bolt at the end of the cradle. This stops the wings from flying out during launch.

    Now we can assemble the throwing arm, place the piece of 1-1/2" of PVC onto the swing arm. The end with the hanger bolts in it is on the side where the swing arms attach to the base. Put the u-bolts over it and into the holes drilled earlier and tighten. Not to tight, tight enough to keep it in place but able to slide the pipe back and fourth. Now make sure you have enough ceiling clearance and swing the arm up slowly and make sure the pulling end clears the cradle. If not, slide the pipe up until it does then tighten. Also not to tight to crack the PVC and trim off the extra bolt length.

    On the inside of each of the uprights measure down 2" and find the center. In each of those spots screw in the 2" deck screws half way in. These are for the bungee cord. See the photo of how you should put the bungee cord on.

    I made my pouch for the sling from this site Sling Pouch Design we used a baseball which is about 3". Also this is a good size pouch for apples, oranges and medium sized water balloons.

    Lastly the sling, take the two 7' pieces of rope and tie one to the last round swivel snap eye bolt and the other rope to the 3/8"x2" ring. At the end of the PVC pipe with the 1/4" hanger bolt, drill a 3/16" hole in the side of the 1-1/2" end cap and screw the last #6 eye bolt in. Slip the ring over the hanger bolt and clip the snap eye bolt to the #6 eye bolt. Draw the two strings back through the cradle. Put a baseball into the pouch and lay it into the cradle as far back as you can but not outside of the cradle. Attach the two ropes to the pouch. You are now ready to fling things.

    Step 5: The Throw and the Break Down...

    This is a nice trebuchet in that it breaks down and sets back up in a few short moment.

    The break down...

    • Disconnect the sling,wrap it neatly and set it aside.
    • Disconnect the bungee cord and set it aside.
    • Disconnect both wing ropes.
    • Holding up the swing arm/up-rights and disconnect both wings at the top laying them down, and set the swing arm/uprights down.
    • Disconnect the other end of the wings and place on top of the trebuchet.
    • Place the sling assemble on top of the trebuchet centered on the upper wing hooks.
    • Cross the wing ropes over the sling assemble through the wing hooks and back again through the wing hooks and clip them together..
    • Using the bungee cord at the other end where the uprights bolt on wrap the bungee cord.

    To set it up reverse the break down.

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