According to Brian Davis' lexicon, a great ball contraption step feeder has a series of vertically oscillating ledges or steps, each just a little bit higher than the preceding step. The first step lifts a ball out of the bin, and at the top of the lift the ball rolls onto a static step with another moving feeder step on the other side, which repeats the process. Steps can be driven by cams on a central drive shaft at the base (or partway up)Davis. Steps in fact can be any width or length down to the nominal size of a single ball.
- single-acting: odd-numbered steps oscillate up and down synchronously, while even-numbered steps are stationary (passive "holding pens"). Since all the moving steps are synchronous, they can all be part of one structure, moved by a cam at the base. It is also possible, if the step is the width of a ball, for half the step to oscillate and the other half to be static.
- double-acting: every step moves up & down, with alternate steps 180 degrees out of phase, potentially much faster.
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Davis - The Great Ball Contraption