The main components include the wave generator, the flexspline, and the circular spline. Among the three components, one can be fixed arbitrarily, and the other two can serve as the input and output components respectively, thus achieving the function of reduction or increase (fixed transmission ratio). Additionally, they can be configured as two inputs and one output to form a differential transmission.
The flexspline is designed very thinly with a specially equipped complete gear ring (covering 360°), and the modulus of the gear teeth is relatively small, usually ranging between 0.2 and 1.5mm. The wave generator, acting as a cam component, is tightly pressed against the inner wall of the flexspline at both ends. The flexspline, capable of significant elastic deformation, has an inner diameter slightly smaller than the total length of the wave generator. When the wave generator is inserted into the flexspline, it forces the cross-section of the flexspline to change from its original circular shape to an elliptical shape.
During this process, the teeth at both ends of the long axis fully engage with the teeth of the circular spline, while the teeth at both ends of the short axis completely disengage from the circular spline; at the same time, the teeth on other sections of the flexspline perimeter are in a transitional state of engagement and disengagement. If the wave generator rotates clockwise, the engagement area of the flexspline and the circular spline changes accordingly, with the gear teeth gradually engaging and disengaging. Due to the symmetrical harmonic characteristics of the flexspline deformation process, this transmission method is called harmonic gear drive.
For double-wave transmission, its uniqueness lies in the fact that for each rotation of the generator, the flexspline rotates relative to the circular spline by an arc length of two tooth pitches, forming two engagement areas. This design results in lower stress on the surface of the flexspline during deformation, making it easier to achieve large transmission ratios, with a relatively simple structure.
The three-wave transmission, characterized by a tooth number difference of 3, involves three engagement areas. The advantage of this transmission is the smaller radial force acting on the shaft, more balanced internal stress distribution, and relatively high precision. However, during the deformation process of three-wave transmission, the surface stress of the flexspline will be slightly higher than that of double-wave transmission, and its structure is relatively more complex.
When the wave generator continues to rotate, the deformation state of the flexspline continuously changes, which in turn causes the engagement state between the flexspline and the circular spline to change accordingly. This process will keep recurring, from entry, engagement, exit, disengagement, to re-entry, thus achieving a slow rotation of the flexspline relative to the circular spline in the opposite direction of the wave generator.
In the harmonic transmission process, for every rotation of the wave generator, a specific point on the flexspline undergoes a complete deformation cycle, known as the number of waves, typically denoted by n. In practical applications, double-wave and three-wave are two commonly used wave numbers. Double-wave transmission, with its low flexspline stress and simple structure characteristics, while achieving a large transmission ratio, is widely applied in various scenarios.
Harmonic gear drive is flexible, as it can be used either as a reducer or as an increaser. During the transmission process, we can fix any one of the components among the flexspline, the circular spline, or the wave generator, and the remaining two components will serve as the input and output components respectively.
Harmonic gear drive has a significant advantage in transmission ratio, with relatively high load-carrying capacity. The single-stage transmission ratio range can reach 50 to 500; if using a planetary wave generator, the transmission ratio will increase to 150 to 4000, while multi-wave transmission can achieve an astonishing transmission ratio of 10^7. Additionally, the engagement manner between the flexspline and the circular spline involves face contact and multi-tooth engagement, resulting in low sliding speed, which makes the gear wear uniform and further extends the service life.
Harmonic gear drive boasts excellent transmission accuracy. The side clearance of the gear teeth between the flexspline and the circular spline can be flexibly adjusted, and due to the multi-tooth engagement characteristic, errors can be averaged and mutually compensated. With the same gear precision level, its transmission error is only about a quarter of that of conventional cylindrical gear transmissions. When the torsional stiffness of the flexspline is sufficiently high, it can achieve zero-backlash high-precision engagement, thus reducing transmission play, which is especially suitable for applications requiring reversal rotation.
Harmonic gear drive not only has high transmission efficiency but also operates smoothly. During the transmission process, the gear teeth of the flexspline perform uniform radial movements, even if the input speed is very high, the relative sliding speed of the gear teeth remains very low, which reduces gear wear and improves transmission efficiency to a high level, even up to 92%-96%.
Harmonic gear drive has a compact structure and lightweight. Under the same output torque condition, compared with conventional reducers, its volume can be reduced by two-thirds, and its weight can be halved. This advantage makes harmonic gear drive outstanding in terms of space-saving and weight reduction.
Harmonic gear drive features smooth motion characteristics without impact phenomena. This is because during the engagement process between the flexspline and the circular spline, the gear teeth can uniformly contact each other, and the engagement and disengagement of the teeth occur gradually and smoothly following the deformation of the flexspline.
