Bungee Ballista Part 5 PDF Print E-mail
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Tuesday, 27 October 2009 11:39

Bungee Ballista 5 - Construction Cont



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 Ballista Construction - mounting the wheels

 wheel closeup

  The wheels will have a tremendous amount of force on them.  If there is 150 pounds of tension in the rope, then there is also 150 pounds of tension in the bungee also.   Both are pulling the wheel in the same direction.  So at the point where the shuttle is pulled all the way back, the wheel will have 300 lbs on it.

  In addition, having the wheels be aligned with the top of the track is important to keep the rope from jumping out of the rims.   For these reasons, two steel plates were mounted on the wood, with a 5/16" hole for the wheel.   This provides a very strong attachment which does not give over time, despite the large forces involved.

   There is a block underneath the wheel to bring it to the proper height vertically.   The plate is held in by 1/4" carriage bolts going all the way through the lower beam and the block.   There is another steel plate on the top of the wheel.


Basic Construction   

  Here the main beam has been attached to the crossmember with 3/8" bolts.   These bolts are two short.  Later, those bolts will be replaced with much longer ones because they will go through two pillars and into the top piece.


Crossmember Assembly

   In this picture the pillars are shown.   Because each wheel has a force of 2 times the rope tension in it, and there are two wheels, the total force trying to tear off the crossmember will be 4x the rope tension.   So if one of the multiple-bungee cord springs is pulling with 80 lbs, the total force on the crossmember is 320 lbs.     The peak force may be even higher.  It is important to take care to build this part carefully.

  The wheel alignment is tricky.  The plates for the wheel mounts were fastened to the bottom crossmember first, and then the wheels were leveled and aligned with the top fastened.  When the top steel plates were in the right place, and short pencil was used to mark where the wholes should be.   Then the top was removed and flipped upside down to drill the wholes for the carriage bolts.


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Last Updated on Thursday, 29 October 2009 01:44

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