I. Harmonic Reducer
A harmonic gear reducer consists of a fixed internal gear, a flexible gear, and a wave generator that causes radial deformation of the flexible gear. It is a novel transmission structure within gear reducers, utilizing the controllable elastic deformation wave generated by the flexible gear to induce relative tooth misalignment between the teeth of the gear and the flexible gear, thereby transmitting power and motion. This transmission method differs fundamentally from conventional gear transmission, exhibiting unique characteristics in meshing theory, assembly calculations, and structural design. Harmonic gear reducers offer advantages such as high precision and high load-bearing capacity. Compared to ordinary reducers, they use 50% less material, resulting in a reduction in volume and weight of at least one-third.
Similar to planetary gear drives, harmonic gear drives also consist of three basic components: a fixed internal gear, a flexible gear (i.e., an elastic thin-walled sleeve whose base is connected to the driven shaft, with a toothed ring formed on the generatrix at the beginning of the flexible gear), and a wave generator that causes radial deformation of the flexible gear. Teeth of the same module are cut into both the rigid and flexible gears, but with different numbers of teeth; that is, the flexible gear has two fewer teeth than the rigid gear. The difference in the number of teeth in a harmonic drive characterizes the wave number of the deformed flexible gear. The most common type is the harmonic drive with a wave number w⁻². In the free state (without the generator), the two gears are concentric, and the tooth gaps between the rigid and flexible gears are uniform. The generator inside the flexible gear causes it to deform radially, becoming elliptical.
At this point, the teeth mesh along the entire working height on the major axis of the ellipse, while a radial clearance is formed between the tooth tips on the minor axis. During the generator's rotation, the shape of the flexure remains close to the shape described above.
A harmonic gear reducer consists of three main components: a rigid gear, a flexible gear, and a wave generator. The wave generator is the driving element, while either the rigid gear or the flexible gear is the driven element. The fixed rigid gear is a rigid internal gear, and the flexible gear is a easily deformable thin-walled cylindrical external gear. Both have triangular (or involute) tooth profiles and equal peripheral pitch, but the rigid gear has a few more teeth (usually two) than the flexible gear. The wave generator consists of an elliptical disk and a flexible ball bearing, or a rotating arm with rollers at both ends. Typically, the wave generator is the driving element, one of the flexible gear or the rigid gear is the driven element, and the other is the fixed element.
II. What are the advantages of harmonic reducers?
1. Small and lightweight
Compared with ordinary gear reducers, harmonic reducers can reduce their volume by 2/3 and their weight by 1/2 while maintaining the same output torque.
2. High transmission accuracy
Because harmonic drives involve a large number of teeth meshing simultaneously, errors are averaged out; that is, the multi-tooth meshing has a mutual compensation effect on errors, resulting in high transmission accuracy. Under the same gear accuracy grade, the transmission error is only about 1/4 that of ordinary cylindrical gear drives. Furthermore, by slightly altering the radius of the wave generator, the deformation of the flexspline can be increased, resulting in very small backlash, even achieving backlash-free meshing. Therefore, the transmission idle distance is small, making it suitable for reverse rotation.
3. Large transmission ratio
The speed ratio range of a single-stage harmonic drive is 70-320, and can reach 1000 in some devices. The speed ratio of a multi-stage drive can reach over 30,000. It can be used not only for deceleration but also for speed increase.
4. Simple and compact structure, easy to install
Because it has only three basic components, and the input and output are the same, the structure is simple and compact, and easy to install.
5. High load-bearing capacity
This is because there are many teeth meshing at the same time in harmonic drives. In dual-wave drives, the number of teeth meshing at the same time can reach more than 30% of the total number of teeth. Moreover, the flexible wheel uses high-strength materials, and there is surface contact between the teeth.
6. Can transmit motion into enclosed spaces
The flexibility of flex wheels gives wheel drives a unique advantage that is unmatched by other existing transmission methods.
7. High transmission efficiency and smooth operation
Because the flexible gear teeth move radially uniformly during transmission, even at high input speeds, the relative sliding speed of the teeth remains extremely low (approximately one-hundredth of that of ordinary involute gear drives). Therefore, tooth wear is minimal, and efficiency is high (reaching 69%-96%). Furthermore, since both sides of the gear participate in the engagement and disengagement processes, there is no impact, resulting in smooth operation.
Due to the superiority of the harmonic drive principle, harmonic reducers possess the seven advantages that other ordinary gear reducers cannot match, thus enabling them to find important applications in critical components across various important fields.
