Trebuchet – Throwing Arm

11 11 2008

So in explaining on how to size the primary axle, I realized something.  The same force that the primary axle experiences will also be experienced by the secondary axle!  This is bad, considering that I was planning on using something a lot smaller on the secondary axle.

However, this is not a complete disaster!  We had a couple of different options.

  • Resize the axle
  • make the axle strong (I don’t know how exactly)
  • or distribute the forces closer to the edges

So that’s what we did!  Instead of having the two 2×8’s right next to each other we decided that we would put a 2×4 in between the board in order to space out the forces that the 2×8’s are putting on the secondary axle.



The forces are acting on the basket, throught the axle to the throwing arm.  The closer we bring the 2×8’s to the basket arms the less of a moment arm there is, which means less bending.  Just think for a minute.  We’ve all seen those large cat machines and all of those joints are made in a similar fashion to how we are making the basket joint.  If we look at the figure below the pin there is a tremenous amount of forces on that pin, but it completely in shear forces, and not bending.  It’s a lot easier to deal with shear forces, than it is to deal with bending forces.

So that what we did by separtating the beam and distributing the forces.

Other Resources

Trebuchet Design

Cement Blocks


Sizing the Axle (Part 1)

Sizing the Axle (Part 2)


Car Throwing Trebuchet

Cool Trebuchet Pictures

The bucket