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Trebuchet
(page 3)
Update 19-October-2005
It's
been almost a year since I last touched this thing. Two weeks
ago, we took it outside and launched all sorts of projectiles
- mostly apples, potatoes, tennis balls and a orange. We learned
a few things and brought the contraption back in for its final
touches.
(78,595
bytes and 94,080 bytes respectively - the projectile basket
and adjustable sling)
I
was going to use an old pair of jeans to make the sling. My
wife handed me a spool of belting - a lining material used
on the inside of belts. She inserted grommets on both ends
and the result is a very strong and flexible sling. The basket
which holds the projectile is from the old jeans. It is sewn
slightly cupped to help hold the projectile. The belting snakes
through a center channel. To allow some level of adjustments,
the belting ends in a metal D-ring and doubles back. As the
sling is lengthened or shortened, the basket is moved along
the belting to the desired location. A single staple locks
it to the belting.
(79,916
bytes and 82,132 bytes respectively. Connecting and releasing
the sling)
On
the "arm" end, the sling is attached to the arm
on both sides of the arm. This arrangement gives a balanced
force on the sling so it doesn't swing left or right too much.
The D-ring connects to a small metal bar screwed to the end
of the arm. The shape of this metal bar is important - it
determines the release angle of the sling. Release the sling
to early and the projectile may go straight up. Release it
to late and it will drive straight into the ground. The hard
part is getting an angle that can still catch the D-ring yet
release it early enough for a proper projectile flight path.
(60,524 bytes, 54,746 bytes and 59,700 bytes respectively)
The
trigger mechanism also uses belting material. A piece of belting
with grommets on each end is stretched across the path of
the arm. One side is screwed into the wooden support beam.
The other side is held in place by a cut-off bolt. A short
piece of 1/2 inch plumbing pipe is inserted into the support
beam; screws hold it in place from both sides. The threaded
portion of the bolt has been cut off and the end ground round
(for safety). To arm the device, the bolt (key) is placed
through the grommet and into the plumbing pipe. Pulling the
rope on the key releases the belting and thus the arm. Automotive
grease is used to lubricate the key. To make it easier to
arm, the trigger belting does not have to be extremely taut
when the key is inserted. An additional spacer (a block of
wood) is inserted between the arm and the trigger to force
the arm down to the channel where the D-ring can stay hooked
onto the release bar. A blue shoelace hold this spacer block
to one side of the device so that when it releases, it won't
fly over to the other side of the arm and get tangled up with
the sling's path of motion.
 
(64192
and 66,669 bytes respectively - two weights and safety bar)
It
became obvious from the trial runs that a single 29+ pound
weight just wasn't enough. So the second weight was sanded
down and painted (black this time; ran out of silver paint!).
One weight is dangerous enough; two is even more so. A safety
bar is installed at the bottom of the arm's path to prevent
accidental firings. Metal 1 inch pipe clamps loosely hold
it in place.
(133,667 bytes; completed unit)
That's
it !
19-October-2005
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