Bungee Ballista Part 7 PDF Print E-mail
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Wednesday, 28 October 2009 23:51

Making a Bungee Ballista  Part 7 -- Constructing the trigger

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  The trigger is nothing but a steel pin which pops up in the center of the track, back near the winch.   The shuttle gets winched back beyond the trigger hole, then the pin is raised, and the shuttle is unwinched until it is held by the pin.  At that point, the winch can be unattached.   Then we have a simple lever which can make the trigger pin retract into the hole, and the shuttle is then free to get pulled forward at great speed and force by the bungee.

  The pin was created by taking a 1/2" carriage bold, and sawing off the head, and the threads.   A metal file was used make a flat spot in the side so the drill didn't wander while we drilled.

 Drilling the trigger pin

A poplar 2x2 was used for the trigger actuating lever.     A slot was created by drilling two holes about 1.5 inches back from the end, and a hand saw.   Then a hole was drilled through the side and a nail put through and bent over.    The trigger had a little too much play in it because all the holes involved were too big.  But it worked well enough.

 Trigger Lever Assembly

 This photo shows how the trigger mechanism will work.  The poplar trigger handle pivots about a point about 5 inches from the end.  When the lever is down, the pin will be pushed up and will block the shuttle from traveling in the track.   To fire the trigger, the end of the poplar board is raised, thus pulling the pin back into the hole.   In the picture below, the poplar board is temporarily attached to the side of the frame so we can check that the pin will go high enough, and also that it will go totally into the pin hole when raised. 

   This design has the advantage that gravity will tend to keep the trigger up.  The firing of the ballista will take place when one lifts the lever.  It is not likely to be triggered by accident by bumping, etc.

   In addition, the location of the pin hole was determined by marking the side of the main beam, and then using a carpenter's square to get an accurate position in the middle of the track for the pin hole.


Estimating the position of the trigger pin hole

   The shuttle comes zooming up the track, but then must stop, while the projectile continues.   To prevent damage, some kind of rubber stop is needed.  From the local farm supply store, two of these were purchased.   It comes apart quite easily, leaving a nice rubber bumper with a big hole for mounting.

 Rubber Pickup Truck Thingy

  Some long thin screws were put to hold them in place.  The screw was much thinner than the hole, because the hole will move when the rubber squashes.  So the thin tall screws were put on the far side of the hole to hold it securely.

  Note that in the end, the rubber was not good enough.   The shuttle is moving so fast when it struck the stops that it was damaged.    The steps taken to make the stop more gentle will be covered in the following pages.

 rubber mount mounting

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Last Updated on Thursday, 29 October 2009 01:44

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