The Team Work Trebuchet

I'd like to finally post something regarding the Trebuchet that we have been building with the Cub Scouts for Scout-O-Rama.

Last year we built one that basically followed one found at the makezine.tv site.  This design by John Park utilized PVC piping.  I immediately saw a flaw or safety risk in that design in relation to using this with young boys.  There was room for someone to pinch their hand or fingers due to the format of the throwing arm.  This is very evident from the diagrams and pictures in his documentation and video Makezine.tv PVC Trebuchet Documentation

So we took his design as a basis and made some modification in the area of safety and design.
The first issue resolved was the throwing arm and how it was activated.  To avoid pinching hands a cross beam was added.  This also forced the trebuchet to be used two individuals.  There have been some that wanted to be smart and use it solo (Not recommended).

Next we resolve the issues with the payload.  In John Park's design you have a simple peg or hook at the end of the throwing arm.  All the balloons you launch have to have a sting tied to them that goes with the hook.  Anyone want to tied strings on to 400+ balloons?
So a simple bucket/bowl was attached to the back end of the throwing arm.

This arrangement works find for 6-10 yr boys and even some adults.
It does not work very well for 14-17 yr old Scouts or adults looking to set a distance record or not paying attention.

Mainly what became clear is the fact that a lot of stress is place on the cross beam and throwing arm supports as they make contact.  John Park's original design does not have the issue of stress since he has no cross arm that contact with the supports.  Fix one problem or issue to create another right?

With the physics involved you find that the more downward force you can put on the arm the more motion and momentum you have at the payload site.  This is especially true in this case since we are dealing with human force and not a counter weight.


In a real trebuchet you have an optimum level of counter weight to projectile weight and throwing arm length.  Siege engineers back in the day could work out the proper proportion to achieve the designed distance and effect.  Does no good to be able to fling your projectile 1500 ft. if you have to use a 1 pound stone to do it.

So we were stuck with force provided by 2 participants (lopsided at times) in the place of our counter weight.  This caused a lot of variation in the distance the water balloon would fly.  But it allows a lot more participation compared to the one you only pulled on a rope.

In the end the first break came when we had 2 volunteers break off the cross beam at the juncture to the the throwing arm.  We were able to temporarily fix this with duck tape and apiece of pipe we had laying around.  Then the final unfixable stress came when we had an over zealous scout of about 15 yr tell his buddy "the harder you force it down the further it will go".  True in theory but there are limits.  You don't want to cause so much force that the equipment can't handle it.
The arm finally snapped around the area for the bolt that holds the arm to the supports and acts like an axle.  At that point the day was shot.  We did have the pleasure of making the scout stand around for 20-30 minutes trying to figure out how he was going to pay to fix the launcher. Then we forced him to pic up all the spent balloon pieces in the shooting range.

So why bother to fix and make it out of pipe again.  It would not work to restrict usage on a size or age basis.  Try explaining this to some 13 yr old you are afraid might break it.

I liked the basic design of the PVC pipe trebuchet and the modification to have it used by 2 people. I liked also the idea of it being portable in pieces.  However, I needed to make sure the new design would hold up against some fairly large stress and strain.  I also wanted to keep things fairly portable so I could put it all in the trunk of my Ford Focus.  Cost was also a factor.

So I set about changing things to be built with wood.
Now the most common wood is your standard 2X4 stud.  They come in various types so I'd just have to pick a harder wood and not use pine even though that probably would hold up better then PVC piping.  Utilizing this common size would also cut down on cost - about $2.50 per 8 Ft. stud.  The longer the board and the harder the wood the more you pay.


Also since this whole machine in wood would be much heavier it helps with  issues of bucking when in operation.  No more need to have an adult with a foot on the back to hold it down.


You can find the full documentation on the new revised Team Work Trebuchet here.


One of the main issues I dealt with was how to connect the throwing arm support to the base.  They are essentially at a 45º angle.  I could have built in notches or done some type of fancy dove tail like fitting which leaves you high and dry if something break.   So "L" brackets were chosen.  One of these on each side of the support should hold it into place.


The only odd variable in all this is to find a good cross beam.  Most black iron or galvanized steel pipe that is the requisite 10ft length is very expensive.  When 1/2 the cost of your project is just one bar you have major issues.

That is one thing I'll have to find an alternative to that is sturdy and not bendable like cooper.


Again the full documentation for your use can be found HERE

I'll post additional information on how the build went when we finish it in a couple of week.
So far the cost on the project is ticking towards $100.00.  Not to bad for a portable human powered water balloon launcher.


Buaidh - NO - BAS