The differential gear FDS series is divided into the combined type and the component type. The combined type aims to integrate the differential mechanism into a single unit for easy assembly into equipment, and its housing allows direct mounting of transmission gears, pulleys, etc. The component type pursues a flat and thin design, enabling customers to design their own encapsulation housing and optimize device space according to their structural requirements. Utilizing Harmonic Drive's unique operating principle, it is an ultra-compact differential device capable of performing phase and timing fine-tuning during operation. The combined type of the FDS series shares the same four core components as the pancake component type, except that the combined type includes additional front and rear sealing flanges and support bearings. The FDS series operates on the same working principle as the cup-type FSG series. The shape of the pancake flexspline is identical to the cross-section of the cup flexspline's base, and the wave generator bearings are arranged in two rows, with the tooth width of the circular spline and flexspline increased to handle higher torque capacity. Additionally, the pancake type incorporates an extra circular spline with the same number of teeth as the flexspline for connection to the output shaft.