As a core component of industrial robots , precision reducers , compared with general-purpose reducers, require reducers for robots to have characteristics such as short transmission chains, small size, high power, light weight, and easy control.
There are two main types of speed reducers widely used in articulated robots: RV speed reducers and harmonic speed reducers.
1. The principle, advantages and disadvantages of RV reducers and harmonic reducers
RV reducer:
RV reducers are used in the legs, waist, and elbow joints of robots with high torque, and in industrial robots with heavy loads, where the first, second, and third axes are all RV reducers. Compared to harmonic reducers, the key to RV reducers lies in their manufacturing and assembly processes. RV reducers have higher fatigue strength, rigidity, and lifespan, unlike harmonic drives which experience a significant decrease in motion accuracy over time. Their disadvantages are heavier weight and larger size.
▲RV-E type reducer▲
Harmonic reducer:
Harmonic reducers are used in industrial robots with small loads or for the end effectors of large robots. They are a type of harmonic drive device, which includes a harmonic accelerator and a harmonic reducer. A harmonic reducer mainly consists of a rigid wheel, a flexible wheel, bearings, and a wave generator; all four are indispensable. The rigid wheel has slightly more teeth than the flexible wheel. Harmonic reducers used in small robots are characterized by their small size, light weight, high load capacity, high motion precision, and large single-stage transmission ratio.
▲Harmonic reducer▲
Both are low-tooth-difference meshing, but the difference lies in the fact that a key gear in a harmonic drive is flexible. It requires repeated high-speed deformation, making it more fragile and limiting its load-bearing capacity and lifespan. RV typically uses a cycloidal pinwheel, while harmonic drives used to use involute teeth. Now, some manufacturers are using double circular arc teeth, which are much more advanced than involute teeth.
The two giants in the reducer industry are Nabtesco and Hamonica Drive, who almost monopolize the global market for reducers used in robots. Both types of reducers are manufactured with micron-level precision. This alone makes it difficult to achieve high reliability in mass production, let alone high-speed operation at thousands of revolutions per minute, while also maintaining a long lifespan.
Harmonic reducers consist of four basic components: a flexible wheel, a wave generator, a rigid wheel, and bearings.
The outer diameter of the flexible wheel is slightly smaller than the inner diameter of the rigid wheel, and the flexible wheel typically has two fewer teeth than the rigid wheel. The elliptical shape of the wave generator determines that the tooth contact points of the flexible and rigid wheels are distributed on two opposite planes located between the center of the ellipse. During the rotation of the wave generator, the tooth contact portions of the flexible and rigid wheels begin to mesh. For every 180° clockwise rotation of the wave generator, the flexible wheel rotates by the equivalent of one tooth difference counterclockwise on the rigid wheel. At two symmetrical points within 180°, more than 30% of the total number of teeth are engaged simultaneously, which results in its high torque transmission.
Compared to harmonic reducers, RV transmission is a newly emerging type of transmission. It was developed based on the traditional pinwheel planetary transmission. It not only overcomes the shortcomings of general pinwheel 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 smooth transmission.
The RV reducer consists of a cycloidal pinwheel and a planetary support. With its many advantages such as small size, strong impact resistance, large torque, high positioning accuracy, low vibration, and large reduction ratio, it is widely used in industrial robots, machine tools, medical testing equipment, satellite receiving systems and other fields.
The RV reducer's housing and cycloidal pinwheel are made of solid steel for transmission, thus offering high load-bearing capacity. In contrast, the flexible wheel in a harmonic reducer continuously deforms to transmit torque, which limits its ability to withstand high torque and impact loads; therefore, it is generally used in the front end.
2. Advantages and disadvantages of RV reducers and harmonic reducers
Harmonic reducers have a simple and compact structure, making them suitable for miniaturized, low- and medium-load applications.
RV reducers offer high rigidity, strong impact resistance, smooth transmission, and high precision, making them suitable for medium to heavy load applications. However, RV reducers need to transmit large torques and withstand significant overload impacts to ensure their expected service life. Therefore, they employ relatively complex over-alignment structures in their design, making manufacturing processes and cost control more challenging. RV reducers lack elastically deformable load-bearing components, thus enabling them to withstand a certain amount of torque. The bearings in RV reducers are their weakest point; under stress, they are prone to exceeding their bearing capacity, leading to abnormal wear or breakage. This problem is more pronounced at high speeds, resulting in a significant decrease in the rated torque of the RV reducer with increasing input speed.
3. Is there a substitution relationship between the speed reducers?
affirmative viewpoint:
RV reducers offer significantly higher fatigue strength, stiffness, and lifespan compared to harmonic drives commonly used in robots. Furthermore, their backlash accuracy remains stable, unlike harmonic drives which exhibit a marked decrease in motion accuracy over time. Therefore, many countries utilize RV reducers for high-precision robot transmissions, and RV reducers are increasingly replacing harmonic reducers in advanced robot transmissions.
While these products do offer some substitution opportunities in certain models, these types of reducers can only provide partial replacement. In most cases, it's difficult to completely replace any type of reducer. For example, in terms of speed ratio, harmonic and RV reducers have much higher ratios than planetary reducers, making planetary reducers dominant in the low-ratio range. While planetary reducers can have larger speed ratios, they are unlikely to completely replace harmonic and RV reducers. Furthermore, in terms of rigidity, planetary and RV reducers are more rigid than harmonic reducers, making it difficult for harmonic reducers to perform well in applications requiring high rigidity.
Harmonic reducers are characterized by their light weight and small size, which is difficult for planetary and RV reducers to achieve. Therefore, various reducers can only be replaced in certain situations, and it is unrealistic to expect one product to completely replace another.
Opposing viewpoints:
Different types of speed reducers cannot replace each other; rather, they are complementary.
RV and harmonic drives are complementary, but it cannot be ruled out that after structural design optimization and breakthroughs in manufacturing processes, they may form local competition in the field of medium and low load applications.
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