The principles, advantages and disadvantages of RV reducer and harmonic reducer
RV reducer:
RV is used for the three joints of the waist and elbow legs of robots with large torques, and for industrial robots with large loads, RV is used for the first, second and third axes. Compared with harmonic reducers, the key to RV reducers lies in the processing technology and assembly technology. The RV reducer has higher fatigue strength, stiffness and lifespan. Unlike harmonic transmission, the movement accuracy will be significantly reduced as the use time increases. Its disadvantages are heavy weight and large dimensions.
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▲RV-E type reducer▲
Harmonic reducer:
Used for industrial robots with small loads or the end axes of large robots, the harmonic reducer is a type of harmonic transmission device. The harmonic transmission device includes a harmonic accelerator and a harmonic reducer. The harmonic reducer mainly includes: rigid spline, flexspline, bearing and wave generator, all four of which are indispensable. Among them, the number of teeth of the rigid spline is slightly larger than that of the flexspline. The characteristics of harmonic reducer for small robots are small size, light weight, large load-bearing capacity, high movement accuracy, and large single-stage transmission ratio.
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▲Harmonic reducer▲
Both mesh with a small tooth difference. The difference is that a key gear in harmonics is flexible. It requires repeated high-speed deformation, so it is relatively fragile and has limited load-bearing capacity and service life. RV usually uses cycloidal pinwheels. Harmonics used to use involute tooth shapes. Now some manufacturers use double arc tooth shapes. This tooth shape is much more advanced than involute.
The two giants of reducers are Nabtesco and Hamonica Drive. They almost monopolize the world's robot reducers. Both of these reducers have micron-level processing accuracy. It is difficult to achieve high reliability in the mass production stage, let alone high-speed operation of thousands of revolutions, and a long life.
The harmonic reducer consists of four basic components: flexspline, wave generator, rigid spline, and bearing.
The outer diameter of the flexspline is slightly smaller than the inner diameter of the rigid spline, and usually the flexspline has 2 fewer teeth than the rigid spline. The elliptical shape of the wave generator determines that the tooth contact points of the flexspline and the rigid spline are distributed on two opposite sides between the center of the ellipse. During the rotation of the wave generator, the contact parts of the flexspline and the rigid gear teeth begin to mesh. For every 180° clockwise rotation of the wave generator, the flexspline is equivalent to a counterclockwise rotation of the rigid spline with a tooth number difference. At two 180° symmetrical locations, more than 30% of the total number of teeth are engaged at the same time, which also contributes to its high torque transmission.
Compared with harmonic reducers, RV transmission is a newly emerging transmission. It is developed on the basis of traditional pin-pendulum planetary transmission. It not only overcomes the shortcomings of general pin-pendulum transmission, but also has the characteristics of small size, light weight, It has a series of advantages such as large transmission ratio range, long life, stable accuracy, high efficiency, and smooth transmission.
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The RV reducer is composed of a cycloidal pinwheel and a planetary bracket. It is widely used in industrial robots, machine tools, medical equipment and many other advantages due to its small size, strong impact resistance, large torque, high positioning accuracy, small vibration, and large reduction ratio. Testing equipment, satellite receiving systems and other fields.
The housing and cycloidal pinwheel of the RV reducer are driven by solid steel, so they have strong load-bearing capacity. The flexspline of the harmonic reducer can continuously deform to transmit torque. This determines that the harmonic reducer has limited ability to withstand large torque and impact loads, so it is generally used at the front end.
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Advantages and disadvantages of RV reducer and harmonic reducer
The harmonic reducer has a simple and compact structure and is suitable for miniaturization, low and medium load applications.
The RV reducer has good rigidity, strong impact resistance, stable transmission and high precision, and is suitable for medium and heavy load applications. However, the RV reducer needs to transmit a large torque and withstand a large overload impact to ensure the expected working life. The design uses a relatively complex over-positioning structure, which makes manufacturing process and cost control difficult. There are no elastically deformed stress components inside the RV reducer, so it can withstand a certain torque. The bearing of the RV reducer is its weak link. When force is applied, it is easy to exceed the bearing stress limit and cause abnormal wear or rupture of the bearing. This problem is more prominent during high-speed operation, so the rated torque of the RV reducer decreases significantly with the input speed.
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Is there a substitution relationship between reducers?
proposition view:
The RV reducer has much higher fatigue strength, stiffness and lifespan than the harmonic transmission commonly used in robots, and the backlash accuracy is stable. Unlike the harmonic transmission, the motion accuracy will significantly decrease as the use time increases. Therefore, high-precision robot transmissions in many countries mostly use RV reducers. Therefore, RV reducers have a tendency to gradually replace harmonic reducers in advanced robot transmissions.
These products do have substitution relationships on some models, but these types of reducers can only achieve partial substitution. In most cases, it is difficult to replace various types of reducers. For example, in terms of speed ratio, the speed ratios of harmonic and RV are much larger than those of planets, so the small speed ratio field is dominated by planets. Of course, the planetary speed ratio can be made larger, but it is difficult to replace the harmonic and RV. For another example, in terms of rigidity, the rigidity of planetary and RV is better than that of harmonics. Under usage conditions that reflect rigidity, it is difficult for harmonics to perform well.
The characteristic of harmonic reducer is that it is light and small, which is difficult for planetary and RV to achieve in this regard. Therefore, various types of reducers can only be replaced in some cases, but it is unrealistic for one product to fully replace another product.
Opposition point of view:
Various types of reducers cannot replace each other, but have a complementary relationship.
The two transmissions, RV and harmonic, are complementary, but it does not rule out the formation of local competition in the field of medium and low load applications after structural design optimization and manufacturing process breakthroughs.
