As the core component of industrial robots, precision reducers for robots require characteristics such as short transmission chains, small volume, high power, light weight, and easy control compared to universal reducers.
There are two main types of reducers widely used in articulated robots: RV reducers and harmonic reducers.
The principle, advantages and disadvantages of RV reducer and harmonic reducer:
RV reducer:
Used for the three joints of the legs, waist, and elbow of robots with high torque, and for industrial robots with high loads, RV is used for the first, second, and third axes. Compared to harmonic reducers, the key to RV reducers lies in their machining and assembly processes. RV reducers have higher fatigue strength, stiffness, and lifespan, unlike harmonic drives, which significantly reduce motion accuracy over time. Their drawbacks are heavy weight and larger overall dimensions.
Harmonic reducer:
Used for industrial robots with small loads or large robots with several shafts at the end, harmonic reducer is a type of harmonic transmission device, which includes harmonic accelerator and harmonic reducer. Harmonic reducers mainly include: rigid wheels, flexible wheels, bearings, and wave generators, all of which are indispensable. Among them, the number of teeth of the rigid wheel is slightly greater than that of the flexible wheel. The characteristics of harmonic reducers used in small robots are small size, light weight, high load-bearing capacity, high motion accuracy, and large single stage transmission ratio.
Both are meshing with small tooth differences, but the difference is that a key gear in harmonics is flexible and requires repeated high-speed deformation, so it is relatively fragile, with limited bearing capacity and lifespan. RV usually uses cycloidal pinwheels, while harmonics used to use involute tooth profiles. Some manufacturers now use double arc tooth profiles, which are much more advanced than involutes.
The two giants of speed reducers are Nabtesco and Harmonica Drive, which almost monopolize the global robot speed reducers. Both types of reducers have machining accuracy at the micrometer level, making it difficult to achieve high reliability during mass production alone, let alone high-speed operation of several thousand revolutions, and also have a long lifespan.
The harmonic reducer is composed of four basic components: "flexible wheel, wave generator, rigid wheel, and bearing".
The outer diameter of the flexible wheel is slightly smaller than the inner diameter of the rigid wheel, and usually the flexible wheel has 2 fewer teeth than the rigid wheel. The elliptical shape of the wave generator determines that the tooth contact points of the flexible wheel and the rigid wheel are distributed on two opposite sides between the center of the ellipse. During the rotation of the wave generator, the contact part between the flexible gear and the rigid gear teeth begins to mesh. The wave generator rotates 180 degrees clockwise, and the flexible wheel is equivalent to a rigid wheel rotating counterclockwise by one tooth difference. At two 180 ° symmetrical locations, more than 30% of the total number of teeth mesh simultaneously, which also creates high torque transmission.
Compared to harmonic reducers, RV transmission is a newly emerging type of transmission, which is developed on the basis of traditional needle pendulum planetary transmission. It not only overcomes the shortcomings of general needle pendulum transmission, but also has a series of advantages such as small size, light weight, large transmission ratio range, long service life, stable accuracy, high efficiency, and stable transmission.
RV reducers are composed of cycloidal pinwheels and planetary supports, and are widely used in industrial robots, machine tools, medical testing equipment, satellite receiving systems, and other fields due to their advantages such as small size, strong impact resistance, high torque, high positioning accuracy, low vibration, and large reduction ratio.
The housing and cycloidal gear of the RV reducer are transmitted through solid steel, thus having strong load-bearing capacity. The flexible wheels of harmonic reducers can continuously deform to transmit torque, which determines the limited ability of harmonic reducers to withstand large torque and impact loads, so they are generally used in the front end.
Advantages and disadvantages of both RV and harmonic reducers
The harmonic reducer has a simple and compact structure, suitable for applications with miniaturization, low and medium loads.
RV reducers have good rigidity, strong impact resistance, stable transmission, and high accuracy, making them suitable for applications with medium to heavy loads. However, RV reducers need to transmit a large amount of torque, withstand a large amount of overload impact, and ensure the expected working life. Therefore, relatively complex over positioning structures are used in design, making manufacturing processes and cost control difficult. There are no elastic deformation load-bearing components inside the RV reducer, so it can withstand a certain amount of torque. The bearing of the RV reducer is its weak link, and it is easy to exceed the bearing capacity limit when subjected to force, resulting in abnormal wear or fracture of the bearing. This problem is more prominent during high-speed operation, so the rated torque of the RV reducer decreases significantly with input speed.