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RV Reducer or Harmonic Reducer, Principle, Advantages and Disadvantages

2018-12-25 11:00:28 clicks:175
As the core part of industrial robots, precision reducer is required to have the characteristics of short transmission chain, small size, large power, light weight and easy control compared with general reducer.
There are two kinds of reducers widely used in articulated robots: RV reducer and harmonic reducer.
RV reducer:
The RV is used for the three joints of the leg, waist and elbow of the high-torque robot, and for the industrial robot with heavy load. Compared with harmonic reducer, the key of RV reducer lies in processing technology and assembly technology. RV reducer has higher fatigue strength, stiffness and life, unlike harmonic drive, with the increase of service time, motion accuracy will be significantly reduced, its shortcomings are heavy weight and large size.

Harmonic Reducer:
Harmonic reducer is a kind of harmonic drive device, which includes Harmonic accelerator and harmonic reducer. Harmonic reducer mainly includes rigid wheel, flexible wheel, bearing and wave generator, all of which are indispensable. Among them, the number of teeth of rigid wheel is slightly larger than that of flexible wheel. Harmonic reducer used in small robots is characterized by small size, light weight, large carrying capacity, high motion accuracy and large single-stage transmission ratio.

Both of them are meshing with few teeth difference. The difference is that one of the key gears in harmonic wave is flexible. It needs repeated high-speed deformation, so it is relatively fragile, and its bearing capacity and life are limited. RV usually uses cycloidal needle wheel. Harmonics used to use involute tooth shape. Now some manufacturers use double circular arc tooth shape. This tooth shape is much more advanced than involute.
The two giants of the reducer are Nabtesco和Hamonica Drive,They almost monopolize the global use of reducers for robots. These two reducers are micron-scale processing accuracy. It is difficult to have high reliability in the stage of mass production, let alone thousands of revolutions of high-speed operation, but also to have a long life.
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. Usually, the flexible wheel has two fewer teeth than the rigid wheel. The elliptical shape of the wave generator determines that the contact points of the teeth of the flexible and rigid wheels are located on two opposite sides between the center of the ellipse. When the wave generator rotates, the contact part of the flexible and rigid gear teeth begins to mesh. Whenever the wave generator rotates 180 degrees in a clockwise direction, the flexspline is equivalent to the difference in the number of teeth when the rigid wheel rotates counter-clockwise. More than 30% of the total teeth mesh simultaneously at 180 degree symmetry, which also results in its high torque transmission.

Compared with harmonic reducer, RV transmission is a new type of transmission. It is developed on the basis of traditional cycloidal transmission. It not only overcomes the shortcomings of conventional cycloidal transmission, but also has a series of advantages, such as small size, light weight, large transmission ratio, long life, stable precision, high efficiency and stable transmission.
RV reducer is composed of cycloidal pinwheel and planetary support. It is widely used in industrial robots, machine tools, medical testing equipment, satellite receiving system and other fields because of its small size, strong impact resistance, large torque, high positioning accuracy, small vibration and large deceleration ratio.

The shell and cycloidal pinwheel of RV reducer are driven by solid steel, so the bearing capacity is strong. The flexible wheel of harmonic reducer can continuously deform to transmit torque, which determines that the ability of harmonic reducer to withstand large torque and impact load is limited, so it is generally used in the front end.

The structure of harmonic reducer is simple and compact, and it is suitable for the application of miniaturization, low and medium load.
RV reducer has good rigidity, strong impact resistance, smooth transmission and high precision, which is suitable for the application of medium and heavy loads. However, RV reducer needs to transmit large torque, bear large overload impact, and ensure the expected working life. Therefore, a relatively complex overload positioning structure is used in the design, and the manufacturing process and cost control are difficult. RV reducer has no force element with elastic deformation, so it can withstand certain torque. The bearing of RV reducer is its weak link. It is easy to break through the bearing stress limit and cause abnormal wear or rupture of the bearing. This problem is more prominent when running at high speed, so the rated torque of RV reducer decreases obviously with the input speed.

3、Whether there is substitution relationship between reducers
Square view:
The RV reducer has much higher fatigue strength, stiffness and life than the harmonic drive commonly used in robots, and its backlash accuracy is stable, unlike the harmonic drive, its motion accuracy will be significantly reduced with the increase of time. Therefore, RV reducer is widely used in high-precision robot transmission in many countries. Therefore, RV reducer is gradually replacing harmonic reducer in advanced robot transmission.
These products do have substitution relationship in some models, but these types of reducers can only achieve partial substitution. In most cases, it is difficult to replace all kinds of reducers. For example, in the aspect of speed ratio, the speed ratio of harmonics and RV is much larger than that of planets, so the field of small speed ratio is planetary. Of course, the planet's speed ratio can be large, but it's hard to replace harmonics and RV. For example, in terms of rigidity, the rigidity of planets and RV is better than that of harmonics. It is difficult for harmonics to perform well under rigid operating conditions.
Harmonic reducer is characterized by light and small, in this respect, planets and RV are difficult to achieve. So all kinds of reducers can only be replaced in some cases, but it is unrealistic to replace one product with another in all directions.
Counter argument:
All kinds of reducers can not replace each other, but a complementary relationship.

RV and harmonic drive complement each other, but it does not rule out that after structural design optimization and manufacturing process breakthrough, local competition will be formed in low and medium load applications.