Siege Engine.com: How to Build a Centripetal Machine

Treatises Engines Of War Engineering Battles Spare Parts Plunder
How to Build a Centripetal Machine
Share this page
Search

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.


Drive Train

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.


To capture the human input, this Jack-Shaft box would transfer the power from the bicycle (on the right) closer toward the machine so it could travel up the frame via a very long bicycle chain.


Eventually, all we ended up with was a bicycle with the rear tire bracket bolted right onto the machine. This greatly simplified the design which used to include various types of platforms that had to be disassembled for transport. Having the bicycle bolt directly to the machine provides maximum stability and removes a lot of wobble and compression while attempting to apply maximum force while pedaling.


The chain then drives up the frame to an upper chain-ring. Just below and left of the chain ring you can see a very large derailleur hooked up to a very large spring (out of an old oven door.)

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.


Triggering a Centripetal Machine

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.






Pouch Release


Our first throwing pouch was a great idea that Kevin came up with. The blue straps held in the projectile, and a cable cinched the pouch closed. When the cable tension was release, the projectile would push open the pouch and release the projectile.

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.


This is our Super-Spring powered basketball pouch. To provide the easiest egress for the pumpkin, we moved to this spring-powered over-lock pouch. It took a long time to design and fabricate this thing.

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.


The final pouch.

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.


Buy a Trebuchet
Buy a Ballista
 
 
Home
 
Treatises
About
Philosophy
Members
Logs
In The News
Sponsors
Contact Us
Copyright
 
Engines Of War
Micro Treb
Baby Trebuchet
Treb Jr
Juggernaut
Juggernaut 2
Higgs
Pumpkin Putter
Baby Ballista
Ballista Jr
Mista Ballista
Baby Hatra Ballista
Hatra Jr
Baby Onager
Onager Jr
UTM
Baby ASOK
Cardan Treb
Scissor Treb
Kid-A-Pult
Vacuum Bazooka
Air Cannon Jr
 
Engineering
Frames
Slings
Triggers
Torsion
Winches
Centripetal
Advice
 
Battles
Extreme Chunk 2016
Extreme Chunk 2015
Punkin Chunk 2013
Higgins Siege The Day 2013
Punkin Chunk 2012
Punkin Chunk 2011
Punkin Chunk 2010
MathWorks 2010
Punkin Chunk 2009
Zukapult 2009
History Chunk 2009
Punkin Chunk 2008
Zukapult 2008
Punkin Chunk 2007
Flying Chunk 2007
Punkin Chunk 2006
Zukapult 2006
History Chunk 2006
Punkin Chunk 2005
Zukapult 2005
Punkin Chunk 2004
Punkin Chunk 2003
Bennington HS 2002
Punkin Chunk 2002
Mohonk 2002
Mt. Snow 2002
Bennington HS 2001
Punkin Chunk 2001
Busti Hurl 2001
History Chunk 2001
Punkin Chunk 2000
Punkin Chunk 1999
Punkin Chunk 1998
 
Spare Parts
Random Picture
All Videos
Art
Seuss Trebuchet
Seuss Onager
Night Before Chunk
Pillsbury Cartoons
Cartoons
Dave Gets Married
Roger Gets Married
Geek Calculator
Browser Hurling
Search
 
Plunder
Catapult Kits
Catapult Plans
Books
 
Contact: Team Tormentum
Copyright © 2000-2016 Eric M. Ludlam All rights reserved.
Twas' brillig, and the slithy toves did gyre and gimble in the wabe...
Last Modified: 11/26/16