Zero-tooth-gap planetary gear reducer with double transmission chains

The invention discloses a zero-tooth-gap planetary gear reducer with double transmission chains. The zero-tooth-gap planetary gear reducer is used for transmitting medium and small loads and eliminating the transmission tooth gap of the reducer. The zero-tooth-gap planetary gear reducer comprises a gear gap eliminating mechanism, a sun wheel A, a sun wheel B, a planet wheel (10), an inner gear ring (1) and a planet carrier (2), wherein the gear gap eliminating mechanism is composed of a gear (4), a spring (5) and a gear (6); the sun wheel A is composed of a gear (7) and a gear (11); the sun wheel B is composed of a planet wheel (12), a gear (8) and a gear (9). The planet wheel (12) and the planet wheel (10) share the planet carrier (2). The transmission chain composed of the gear (4), the sun wheel A, the planet wheel (12) and the planet carrier (2) is used for transmitting input of an input shaft (3) in the positive direction. The transmission chain composed of the gear (6), the sun wheel B, the planet wheel (10) and the planet carrier (2) is used for transmitting input of the input shaft (3) in the negative direction. The spring (5) enables the gear (4) and the gear (6) to be tensioned so as to eliminate the transmission gap. The sun wheel A and the sun wheel B are of a coaxial transmission structure.