Abstract: A new type of planetary indexing cam mechanisms called type Ⅳ is presented. The transmission principle of type Ⅳ is discussed. Based on the forming principle of conjugate tooth contour, the equations of the cam’s pitch curve are derived. The continuity and symmetry of the cam’s pitch curve is also analyzed. It can be concluded that：This type has no output institution so the output precision can be improved; asymmetrical two-ring structure is used to overcome dead point and its phase angle is ; The condition that the cam’s pitch curve is continuous is that the average driver ratio is –n in an intermittent motion period.

Key words: Cam mechanism Three-ring gear transmission, Intermittent mechanism indexing