It is primarily composed of three basic components:

　　(1) A rigid gear with an internal gear ring (rigid gear);

　　(2) A flexible gear (flexspline) with an external gear ring;

　　(3) Wave generator H.

　　When used as a reducer, it typically employs a wave generator as the driving element, with the rigid gear fixed and the flexible gear serving as the output.

　　The wave generator H is a rod‑shaped component with rolling bearings at both ends, forming rollers that press tightly against the inner wall of the flexspline 1. The flexspline is a thin‑walled gear capable of undergoing substantial elastic deformation, with an inner bore diameter slightly smaller than the overall length of the wave generator. The wave generator is the element that induces controlled elastic deformation in the flexspline. When the wave generator is inserted into the flexspline, it forces the flexspline’s cross‑section to change from its original circular shape to an elliptical one; the teeth near the major axis are fully engaged with the teeth of the rigid gear, while the teeth near the minor axis are completely disengaged. In other regions along the circumference, the teeth are in a transitional state between engagement and disengagement. As the wave generator rotates continuously in the direction shown, the flexspline’s deformation continually varies, causing the meshing condition between the flexspline and the rigid gear to alternate repeatedly: engagement, meshing, disengagement, and then re‑engagement—repeating cyclically—thereby enabling the flexspline to rotate slowly in the direction opposite to that of the rigid gear, relative to the wave generator H. During operation, the rigid gear is fixed, the wave generator is driven by an electric motor, and the flexspline, acting as the driven member, outputs rotational motion to drive the load. In the transmission process, for each complete revolution of the wave generator, the number of deformation cycles experienced by a given point on the flexspline is called the wave number, denoted by n. The most commonly used configurations are two‑wave and three‑wave types. Two‑wave drives exhibit lower stress in the flexspline, simpler structural design, and easier attainment of high transmission ratios; thus, they are widely employed. In harmonic gear drives, the flexspline and the rigid gear have identical tooth pitches but unequal numbers of teeth; typically, the difference between the number of teeth on the rigid gear and the flexspline equals the wave number, namely…

　　z2-z1=n

　　In the equation, z2 and z1 represent the number of teeth on the rigid gear and the flexible gear, respectively.

　　When the rigid gear is fixed, the generator is driven, and the flexible gear is driven, the transmission ratio of the harmonic gear drive is

　　i = -z1 / (z2 - z1)

　　In a two-wave drive, z2 − z1 = 2, and the flexspline has a large number of teeth. The negative sign in the above equation indicates that the flexspline rotates in the opposite direction to the wave generator. From this, it can be seen that harmonic reducers can achieve very high gear ratios.