It consists of two flanged half-coupling bodies with external teeth and an inner ring with integral structure of front and rear inner teeth. The front and rear inner teeth of the inner ring mesh with the outer teeth of the flanged half-coupling body respectively. The front and rear end faces of the inner ring are provided with annular end caps. The advantages of the utility model are as follows: removing the original flange connection form of inner ring gear, realizing that one ring gear combines the two inner teeth of the input end and the output end, eliminating the connection flange and bolts, changing the traditional split structure, on the one hand, greatly reducing the material consumption of the inner ring gear, saving the production and manufacturing costs, and installing more. For convenience; on the other hand, structural changes make the shape and size of products with the same specifications smaller, simple and compact structure, occupying less space, thus meeting the needs of some special industry machinery working in narrow working conditions, such as road maintenance engineering vehicles and other special machinery.