Input by the crank on the right, output at the left. The 57t gear is bolted to a bushwheel by 6 Elektrikit contact studs, with 2 gaps. It is driven by the input crank (through 3:1) and turns continuously. The next bushwheel is fixed to the output shaft. The third is free on the shaft but turns with the second because of a pair of screwed rod adapters on the second, and is pushed away by a compression spring. Mounted in this assembly are two spring loaded plungers made of two pivot bolts joined in a threaded boss. The last bushwheek is pushed forward by the cam follower. In this demonstration piece, the cam has 8 positions at which rollers can be placed. It is driven by 3:1 bevels and 4:1 spur gears from the input so it turns one roller position for each half turn of the 57t gear. In operation the cam pushes the bushwheel forward, compressing all 3 springs and pushing the plungers against the studs on the 57t gear. When the gap comes round (which must be timed to the peak of the cam) the plungers snap over causing the output to be driven. Barely visible in the picture is a 1 1/2" flat girder just below and outside the third bushwheel. It does two things. First, once the output starts turning it keeps one of the plungers held forward for the full half turn, even as the cam passes the peak. Second, after the half turn the plungers can snap back and the flat girder bears against the heads of the pivot bolts so the output cannot turn. In a real application the cam can have as many positions as required, provided the gearing is adjusted to keep it synchronous at the rate of one position for each half turn of the 57t gear.