A roundel structure for a five-compressing-chamber diaphragm pump includes a cylindrical or inverted frustoconical eccentric roundel in an eccentric roundel mount. The cylindrical or inverted frustoconical eccentric roundel includes an annular positioning groove or indentation, a vertical or inverted frustoconical flank and a sloped top ring extending from the annular positioning groove or indentation to the flank. The sloped top ring eliminates the oblique high frequency pull and squeezing phenomena that occurs in a conventional tubular eccentric roundel arrangement.