hi and thanks for the replys!
as you can see in the picture from my first post i already use a rotary encoder from a mouse (this thing named "mousewheel"). But this litte thing from the mouse is way to fragile for my needs. I'm afraid the little rotary encoder from the site which pylon posted isn't solid enough as well. So i thought i can build a Rotary Encoder by myself which is very solid - and thats my question here 
I need a rock-solid one, because its a big button (like a "buzzer") which is in heavy use...
"rotary encoder" is a good keyword, i'll do some research now! thanks!