definition
Harmonic drive reducers are gear transmissions that use a wave generator to cause controllable elastic deformation of flexible gears, which then mesh with rigid gears to transmit motion and power. They are mainly composed of three basic components: a wave generator, flexible gears, and rigid gears.
advantage
(1) Large transmission ratio. The transmission ratio of a single-stage harmonic gear transmission ranges from 70 to 320, and can reach 1000 in some devices. The transmission ratio of a multi-stage transmission can reach more than 30,000. It can be used not only for deceleration, but also for speed increase.
(2) High load-bearing capacity. This is because there are many teeth meshing at the same time in harmonic gear transmission. The number of teeth meshing at the same time in dual-wave transmission 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.
(3) High transmission accuracy. This is because the number of teeth meshing simultaneously in harmonic gear transmission is large, and the error is averaged out. That is, the multi-tooth meshing has a mutual compensation effect on the error, so the transmission accuracy is high. Under the same gear accuracy grade, the transmission error is only about 1/4 of that of ordinary cylindrical gear transmission. At the same time, the radius of the wave generator can be slightly changed to increase the deformation of the flexible wheel, so that the backlash is very small, and even backlash-free meshing can be achieved. Therefore, the harmonic gear reducer has a small transmission backlash and is suitable for reverse rotation.
(4) High transmission efficiency and smooth motion. 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 transmissions). Therefore, tooth wear is minimal and efficiency is high (reaching 69% to 96%). Furthermore, since both sides of the gear participate in the operation during engagement and disengagement, there is no impact phenomenon, resulting in smooth motion.
(5) Simple structure, few parts, and easy installation. It has only three basic components, and the input and output shafts are coaxial, so the structure is simple and easy to install.
(6) Small size and light weight. Compared with general reducers, the volume of harmonic gear reducers can be reduced by 2/3 and the weight can be reduced by 1/2 when the output torque is the same.
(7) It can transmit motion into enclosed spaces. Utilizing the flexibility of the flex wheel, this valuable advantage of wheel transmission is unmatched by other existing transmission methods.
Transmission principle
Figure 1 on the right shows the basic structure of a simple harmonic drive reducer, and Figure 2 shows the working principle diagram of harmonic drive.
Figure 1 Basic Structure
Figure 2 Working principle diagram
It mainly consists of three basic components: (1) a rigid gear (rigid wheel) with an internal gear ring, which is equivalent to the central wheel in a planetary system; (2) a flexible gear (flexible wheel) with an external gear ring, which is equivalent to a planetary gear; and (3) a wave generator H, which is equivalent to a planet carrier. When used as a speed reducer, it usually adopts the form of wave generator as the driving force, rigid wheel as the fixed force, and flexible wheel as the output force. The wave generator H is a rod-shaped component with rolling bearings at both ends to form rollers, which are pressed against the inner wall of the flexible wheel 1. The flexible wheel is a thin-walled gear that can produce large elastic deformation, and its inner diameter is slightly smaller than the total length of the wave generator.
A wave generator is a component that causes controllable elastic deformation of a flexible wheel. When the wave generator is installed on the flexible wheel, it forces the cross-section of the flexible wheel to change from a circle to an ellipse. The teeth near the ends of its major axis fully mesh with the teeth of the rigid wheel, while the teeth near the ends of its minor axis fully disengage from the rigid wheel. The teeth in other sections of the circumference are in a transitional state of engagement and disengagement. When the wave generator rotates continuously in the direction shown in the figure, the deformation of the flexible wheel changes continuously, causing the meshing state between the flexible wheel and the rigid wheel to change continuously as well, from meshing, meshing, disengaging, disengaging, and meshing again, repeating cyclically, thereby achieving a slow rotation of the flexible wheel relative to the rigid wheel in the opposite direction of the wave generator H. During operation, the rigid wheel is fixed, and the wave generator is driven by a motor to rotate. The flexible wheel, as the driven wheel, outputs rotation, driving the load to move. During the transmission process, the number of cycles of deformation at a point on the flexible wheel for one revolution of the wave generator is called the wave number, denoted by n. Double-wave and triple-wave types are commonly used.
Dual-wave transmission has lower flexural stress, a simpler structure, and is easier to achieve a large transmission ratio. Therefore, it is currently the most widely used type. In harmonic gear transmission, the flexural and rigid gears have the same tooth pitch but different tooth numbers. Typically, the difference in tooth number between the rigid and flexural gears is equal to the wave number, i.e., z2 - z1 = n, where z2 and z1 are the tooth numbers of the rigid and flexural gears, respectively. When the rigid gear is fixed, the generator is the driving force, and the flexural gear is the driven force, the transmission ratio of the harmonic gear transmission is i = -z1/(z2 - z1). The wave generator causes the flexural gear to undergo elastic deformation, resulting in an elliptical shape. Therefore, the major axis of the ellipse is fully engaged with the rigid gear, while the minor axis is completely disengaged.
With the rigid wheel fixed, the wave generator rotates clockwise, causing the flexible wheel to elastically deform and the parts meshing with the rigid wheel's teeth to move sequentially. When the wave generator rotates 180 degrees clockwise, the rigid wheel moves one tooth counterclockwise. In a full rotation (360 degrees), the flexible wheel moves two teeth counterclockwise because it has two fewer teeth than the rigid wheel. This motion is typically transmitted as the output.
Application Scope
Harmonic gear transmission is increasingly widely used in aviation, aerospace, energy, navigation, shipbuilding, bionic machinery, common military equipment, machine tools, instruments, electronic equipment, mining and metallurgy, transportation, hoisting machinery, petrochemical machinery, textile machinery, agricultural machinery, and medical devices. In particular, it demonstrates its superiority in high-dynamic-performance servo systems.
