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I need some ideas for designing a catapult that runs on a motor. Basic requirements:

  • able to launch a cookie 30m,
  • capable of continuous operation,
  • fits within a 0.4 m × 0.6 m × 0.4 m high box,
  • allows 2 seconds minimum between each launch to load,
  • self-sufficient (once the motor is turned on it can't be touched)
  • made of steel,
  • springs, bearings, and gears can be used, but no hydraulics.

Does anyone have any ideas?

I was thinking of a gear attached to a motor connected to another gear on the launch arm that would wind the launch arm horizontally. The gear on the motor would have teeth grinded off so that when the arm became horizontal it would release. The problem with this is that I think that when the gear came back around it would interfere with the arm launching the cookie and/or that the teeth on the gears would break because of the large torque required by the motor to compress a spring with a large enough stiffness to give the cookie a flight distance of 30 m. Anyone have any other ideas or possible fixes to mine?

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3  
How to build a cookie launcher? Best question evar! –  tyblu Jan 8 '11 at 3:42
    
Can you use pneumatics? (Probably not...but worth a shot :P) –  Nick T Jan 8 '11 at 4:17
    
I wish, but no, no pneumatics haha –  joe Jan 9 '11 at 1:00
    
Maybe this could inspire you? youtube.com/watch?v=Q0OTX4IwSOo –  endolith Jan 11 '11 at 20:59

2 Answers 2

This sounds like a description for an automatic clay pigeon thrower.

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this wouldnt work... It has to run continuous without human intervention which means no trigger mechanisms or anything like that –  joe Jan 9 '11 at 1:01
    
So hold down the trigger? Set up something like a 555 driving a relay / microcontroller to periodically fire the trigger? –  W5VO Jan 9 '11 at 2:17
    
How uniform are your cookies? Oreo's or something easy like that? Something like this would be a great method. You could probably actually use a real one (though they are too big) if you came up with some insert for the clay holder and shim for the arm :P –  Nick T Jan 9 '11 at 16:45

Not difficult, but without drawing tools, this may be hard to describe.

  1. Your launch mechanism is a coil spring with a rod through the center to keep it from bending. We'll assume there is a plate of some kind on the end that the cookie will sit on. A tab extends off the end of this plate. You'll need to drill a hole in the plate for the rod to pass through.
  2. Almost(1) parallel to the spring is a belt/wire that runs on two pulleys and is driven continuously by a motor. Gear down the motor as appropriate for the torque you need. Attach a hook to the belt.
  3. Arrange these two so with the spring extended, the hook is around the tab, nearest the plate.
  4. Remember we said the belt was "almost" parallel? Well, it's at enough of an angle so when the spring is fully retracted, the hook will slide off the end of the tab and the spring will fire.

In operation, the belt runs continuously. The hook grabs the tab and pulls the spring back, and as it does so, the hook is sliding down the length of the tab until the spring is fully compressed and the hook slides off the end of the tab, releasing the cocked spring. Now the hook just runs along the bottom and when it comes back around, it catches the tab again and the whole cycle repeats. There are other ways, but this seems to be the easiest one to build.

[edit] A simpler idea may be to use a disk that can fit inside a PVC tube with the spring behind it. Then just milling a slot along the length of the tube will allow the tab to protrude and make it easier to build. This is becoming interesting enough that I might make one for the heck of it.

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