Speed reducer

A speed reducer, comprising a drive gear, at least two planetary gears and a fixed internal gear that are arranged in sequence in an inside-out direction and mesh with one another, a movable internal gear being coaxially arranged beside the fixed internal gear, the number of teeth of the movable internal gear being different from the number of teeth of the fixed internal gear, the planetary gears each having deformation elasticity, the planetary gears extending into the movable internal gear and meshing with the movable internal gear respectively, the drive gear being arranged along an axis for driving the planetary gears to rotate between the drive gear and the fixed internal gear;

• wherein due to the difference in the number of teeth between the fixed internal gear and the movable internal gear, when the planetary gears are rotated, the movable internal gear is pushed and move by the planetary gears to perform a reduction output.