The present invention discloses an internal three-ring decelerator, which consists of a bearing frame, an output axle, an input gear axle, two
side tooth ring plates, a middle tooth ring plate, an internal tooth ring, and three eccentric
crank axles, etc., wherein three eccentric
crank axles are mutually paralleled; synchronous
planet gears are respectively arranged on three eccentric
crank axles, and arranged in a triangle, the
small teeth number difference transmission is realized in the meshing process of two
side tooth ring plates and the middle tooth ring plate with the internal tooth ring, the
phase difference is 180 degrees finally, the qualities of two
side tooth ring plates are respectively arranged into one seconds of the quality of the middle tooth ring plate; the bearing frame and the output axle are connected as a
rigid frame structure, the circular disks at two end of the frame support three eccentric crank axles, the positions of the input gear axle and the output axle are positioned in an identical axial line, and the input gear axle is meshed with three synchronous
planet gears. The present invention has the advantages that the reliability is high, the
transmission ratio is large, the movement is steady, the
carrying capacity is strong, and the
layout is reasonable, thus being particularly suitable for being used under the working condition that the large power and the large torque are required.