Simplified cardan-gear mechanism

          A cardan-gear mechanism converts rotary motion into straight-line without the need of drives or slideways. Cardan gearing works on the principle that any point on the periphery of a circle rolling on the inside of another circle describes, in general, a hypocycloid. This curve degenerates into a true straight line (diameter of the larger circle) if the diameters of both circle are in the ratio of 1:2 (mechanisms and mechanical devices sourcebook, CHIRONIS-SCLATER). Usually on a cardan mechanism an input shaft causes a small gear to roll around the inside of a fixed gear (internal). A pin located on the pitch circle of the small gear describes a straight line. On the simplified cardan there is no need for the expensive internal gear. The input shaft rotates a carriage around the fixed gear. On the carriage an idler gear rolls on the periphery of the fixed gear. This idler gear rotates a second gear (on the carriage). The end of a lever located (through a shaft) on the second gear describes the straight line. This straight line is always passing through the center of the fixed gear. The total stroke of the mechanism is 4D, where D is the center distance between the fixed gear and the second gear of carriage. The distance of the lever (center of rotation to point) is also D. The mechanism works only in one direction of rotation and the ratio between gears must be 2:1. The mechanism is very useful and produces a very smooth reciprocation motion.