Pnuematic Question?

[AlexD744]

Hi, in our robotics class we were challeneged to make a device to throw an orbit ball as far as we can using only pnuematics, PVC, wood, rope, twine, string and fasteners. We are designing a ballista, in which the pnuematic actuator will pull back a rope and then release it. However, I fear that in our last calss period, when we are building like the whole system (minus pneumatics), we will make a hook that pulls back the rope and then dosen't let it go. Does anyone know of a sure-fire way to design a hook that will stay on during the pull back and release when it stops?

Thank You.

[gorrilla]

well you could buy one..

Generally, a hook on a pivot is used, such as some sort of block with a small hole through it,in which the hook uses as its fulcrum, so when you pull the line(whatever you are using to launch the balls) tight, and then hit the end of the hook so it levers(?)upward and releases the line...

Also, a hole-pin is used, A small loop or equivelent(thing) is attached to the end of the line, And a hole is made in a predetermined position(eg.between the two guide boards of a ballista), So when the line is pulled tight, A pin is inserted thorugh the holes and line,and when ready to fire the pin is removed(quickly)....

the latter could be considered safer since you dont have to be as close to the ballista when you want to fire it(attaching long string to pin) dont know if this would be a problem for you though....

this might be helpfull,
http://www.instructables.com/id/SLT34AXFRXUSX66/

you should post a video when your done....

[AlexD744]

Ok so instead of building a ballista we built a trebuchet (don't know how to spell that sorry) because we realized the hook would have to be another input to the device, which was not allowed. So we whipped up a trebuchet in 70 minutes (the fastest we've ever worked) and it launched the orbit ball 18.5 feet (with rolling). We hade a 22" X 26" base and had a pivot made of two PVC pipes around each other. We taped the arm to the pivot and held it up on the counter weight side with 2 X 4 (about 4 feet tall). The pnuematics then push the 2 X 4 out from under the counter weight and it worked really well. All I have to say after that is thank goodness for duct tape.