how to make a moving piece that will move both ways with one crank?

im going to make this knex thing, basically an automatic knex gun, where there is a cross bow like thing at the front, and a slide has a catch that will get the bow, and when it gos backward far enough, an arrow will automatically load, and the catch ill go down, so te rubber band will snap back, and shoot the arrow.  i can put a pic in the comments. anyways, what i need, is a way to make the slide go back and forth, but only spinning the crank one way.

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William9306 years ago
like here? https://www.instructables.com/files/deriv/FMH/8T5L/GINORUSH/FMH8T5LGINORUSH.MEDIUM.jpg
the poodleo (author) 6 years ago
oh
instructables auto knex gun.jpg
Like this?
Picture 1.png
the poodleo (author)  William9306 years ago
u have to press the awnser button at the top, so i can select it as top awnser, thats good 4 ur profile.
the poodleo (author)  William9306 years ago
thank you thats perfect.
you could have a gear attached to the rotating piece and a gear on the trigger if you want it to be trigger powered.
the poodleo (author) 6 years ago
heres the pic
you could try to use a kind of wincherster repeater like pump, im not sure if that is what you mean or if this helps
the poodleo (author)  William9306 years ago
well basically i need a sort of machine that when you turn the crank, it will go forward, and then it will go back ward, and repeat, even though your only turning the crank or whatever one way. (it dosnt have to be a crank)