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How to Build a Centripetal Machine
Centripetal throwing machines don't fit the classic concept of "Catapult", but they sure are fun. Our machine, Pumpkin Putter, has been through several iterations over several years and we learned quite a bit about tuning these machines.
All the engineering advice on this page relates to light duty, direct human drive centripetal catapulting.
To hold the drive train together, it is important to have a sturdy frame. This is our earliest frame, and it was pretty stiff. We eventually moved to a system cables that were pinned into the ground in place of the long beams going from the top of the frame toward the outer base.
The chain rose 14 feet from the ground, and while everything was moving, the 10" derailleur arm would move up and down about 6 inches. The amount of wobble in the system this large derailleur accounted for is truly amazing.
A Centripetal trigger has at least three major pieces. Part1 is a frame-mounted actuator. The purpose of this actuator is to push into the path of an axle mounted actuator. The axle mounted actuator then causes a final triggering system on the throwing pouch to let go of the projectile.
The following diagram shows the first two parts.
This device was very simple, and worked well. Video inspection caused us to ponder that it may be preventing a quick release, making the timing of the release difficult to gage.
A basket ball was cut in half to provide a stiff shell to hold the pumpkin. A spring from an old stove was attached to the two angled cables which go toward the top of the image from the sides of the pouch. On top of the pouch, is a bar-lock using a similar technique to vice-grips. In this picture, a bit of pink nylon line is tied to the bar-lock central hinge. Pulling this line cause the bar-lock to open, and allowed the spring to pull the pouch completely open with great rapidity.
With what the other pouched had with complexity, this made up for in simplicity. The actuator would just tug lightly on the vice-grip release lever, and POP, it would let go of the ring that cinched in the pumpkin.
A cinch strap allowed for a wide variation in projectile size, and allowed for a super-tight fit against the pumpkin platform. The pumpkin would be put in loose, and the cinch strap would hold it in place.
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Twas' brillig, and the slithy toves did gyre and gimble in the wabe...
|Last Modified: 05/10/09|