We have decided on the ASOK trebuchet. the fewest moving parts. The cam of this type of trebuchet will allow us to make quick changes to the design, looking for its optimal position e curvature. In this way we will be able to achieve the max force, minimize the friction and reach the biggest throw range.
Modelling the trabuchet, it was helpful to break down its different parts into 6 basic categories, each with their own set of challenges. We got inspired by existing models, but finally the parts were all made from scratch, and thought to be easily printed.
1) Vertical Track
It’s the sliding tree of the trabuchet: it needs to be strong enough to support the counter weight and sustain the arm movement, and at the same time have low friction and allow and easy fall. It’s a long and fla part, we don’t really mind about sharp edges, hence main problems could be related to warpage of the part.
2) Cam/Cam mounting
As said before, cam must be movable. Problems are hence related to the assembly with the frame. These two parts will have different dimensions and shrikange behaviors, barely predictable. We will probably need a few try to find the best configurations.
For what concern curvature and position, several cam parts with different arcs will be designed and printed. In this way we will be able to find the best situation.
3)Trigger
Trigger is a tricky part of the design. It will be designed after testing the first produced assembly. We don’t want a removable pin as a trigger because we could interfere with the counterweight movement and at the same time we want something that is easy to access and activate.
Edit.
Trigger was thought to be an hinge on the vertical frame (an image is shown below), activated by a rope. First dimensions were guessed following the guidelines for press fit design.
We didn’t have particular problems with it: we expected that it would have reduced the strenght of the vertical arm, but that did’t happened. We tried also to avoid sharp edges and corners in the contour to avoid stress concentrations.
4) Pin/Hook (end of arm)
Must be found the best shape for an easy assembly and release. There are no particular problems in such a small part.
5) Frame
It needs to keep all the parts together in a stable condition. Vertical track has to be kept in position and the cam must be movable. Same considerations made in 1 and 2 therefore apply.
6) Counter Weight
We want it to be a pair of boxes removable and fillable with heavy material. Problems are related to the minimum thickness achievable and the long building times. Further modifications of design are required to optimize them.
In general, all the parts were thought to be small as possible, without losing in strenght.
