A speed reducer is an independent component consisting of gear drives, worm drives, or gear-worm drives enclosed in a rigid housing. It is commonly used as a speed reduction transmission device between a prime mover and a driven machine. It plays a role in matching speeds and transmitting torque between the prime mover and the driven machine or actuator, and its applications are extremely widespread in modern machinery.
Gear reducers are generally used in low-speed, high-torque transmission equipment. They reduce the speed of electric motors, internal combustion engines, or other high-speed power sources by meshing a gear with fewer teeth on the input shaft with a larger gear on the output shaft. Ordinary gear reducers may also have several pairs of gears that work on the same principle to achieve the desired speed reduction effect. The ratio of the number of teeth on the large gear to the number of teeth on the small gear is the transmission ratio.
In industrial robots, the speed reducer is one of the three most important components, accounting for up to one-third of the robot's total cost. Because industrial robots have very high requirements for speed reducers, the selection of a precision speed reducer must consider factors such as torsional stiffness, starting torque, transmission accuracy, transmission error, and transmission efficiency. Currently, only three types of speed reducers can meet the requirements of industrial robots: precision harmonic speed reducers, precision planetary speed reducers, and precision RV speed reducers.
RV reducer
The RV reducer was developed based on the cycloidal pinwheel transmission and features a two-stage reduction and a central disc support structure. Since its market launch in 1986, it has been the go-to reducer for robots due to its advantages such as large transmission ratio, high transmission efficiency, high motion accuracy, low backlash, low vibration, high rigidity, and high reliability.
Harmonic reducer
Harmonic reducers consist of three parts: a harmonic generator, a flexible wheel, and a rigid wheel. Their working principle is that the harmonic generator causes the flexible wheel to produce controllable elastic deformation, and the power is transmitted by the meshing of the flexible wheel and the rigid wheel, thereby achieving the purpose of speed reduction. According to the different types of harmonic generators, there are cam type, roller type and eccentric disc type.
Planetary gear reducer
As the name suggests, a planetary gear reducer is a gear reducer with three planetary gears rotating around a sun gear. Planetary gear reducers are small in size, lightweight, have high load-bearing capacity, long service life, smooth operation, and low noise. They feature power splitting and unique multi-tooth meshing characteristics; they are a widely used industrial product, with performance comparable to other military-grade planetary gear reducers, but at an industrial-grade price, making them suitable for a wide range of industrial applications.
Development of Gear Reducers in China
In 2017, hundreds of speed reducer companies emerged in my country, but now the industry has entered a phase of survival of the fittest.
Planetary reducers are not widely used and can be ignored; the main types are RV reducers and harmonic reducers.
In the RV reducer market, the largest supplier is Teijin (Nabtesco), which holds 70% of the global RV reducer market and has virtually a monopoly in heavy-duty transmission machinery. Therefore, we can say that this is a champion company in Japanese precision engineering. In the RV reducer sector, virtually no major domestic reducer manufacturers have been able to challenge Teijin. Nantong Zhenkang is technically rigorous and has a good reputation in the industry; however, as a company established in 2002, its experience is relatively short, and it has only shipped around 20,000 units so far. Qinchuan Machine Tool, on the other hand, has already established a significant business share in the reducer industry, thanks to the continuous output of research institutes. Furthermore, we have noticed that the cooperation between Shanghai Electric and Teijin (Nabtesco) is gradually leading to the mass production of RV reducers in China.
In the field of harmonic reducers, the largest player is Harmonic Drive Systems (HD Harmonic Reducers), a Japanese company that was also an early inventor of this type of reducer, having used it during the Apollo moon landing. However, in the domestic market, Suzhou Green Harmonic Reducers has emerged as a strong competitor, holding about half of the domestic market share. Therefore, domestic manufacturers have an advantage in this area. However, Green Harmonic Reducers' international sales are not high, accounting for only about 10% of its total revenue, while another Japanese company, Shinpo, holds the second position globally.
Difference between RV reducers and harmonic reducers
RV reducers are generally used in industrial robots with heavy loads, while harmonic reducers are mainly used in industrial robots with light loads. Nabtesco and Murata are representative of RV reducers, while the harmonic reducer market is basically monopolized by Harmonic Drive Systems of Japan.
A harmonic reducer is a type of harmonic drive device, which includes harmonic accelerators and harmonic reducers. A harmonic reducer mainly consists of three components: a rigid wheel, a flexible wheel, and a wave generator; all three are indispensable.
The number of teeth on the rigid wheel is slightly greater than that on the flexible wheel. The length of the wave generator is larger than the inner diameter of the undeformed flexible wheel: when the wave generator is inserted into the inner circle of the flexible wheel, it forces the flexible wheel to undergo elastic deformation and become elliptical, so that the teeth of the flexible wheel at its long axis are inserted into the tooth grooves of the rigid wheel, forming a fully engaged state; while the teeth of the two wheels at its short axis are not in contact at all, and are in a disengaged state.
Between engagement and disengagement, the device is either engaged or disengaged. When the wave generator rotates continuously, it forces the flexure to deform continuously, causing the teeth of the two wheels to constantly change their working states during engagement, disengagement, and engagement, resulting in the so-called tooth-shifting motion, thereby realizing the motion transmission between the active wave generator and the flexure.
RV transmission is a newly emerging type of transmission. It was developed based on the traditional pendulum planetary transmission. It not only overcomes the shortcomings of general 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 smooth transmission.
RV reducers, composed of cycloidal pinwheels and planetary carriers, are widely used in industrial robots, machine tools, medical testing equipment, and satellite receiving systems due to their advantages such as small size, high impact resistance, large torque, high positioning accuracy, low vibration, and large reduction ratio. Compared to harmonic drives commonly used in robots, they offer significantly higher fatigue strength, stiffness, and lifespan, and their backlash accuracy is stable, unlike harmonic drives which show a significant decrease in motion accuracy over time. Therefore, many countries worldwide use RV reducers for high-precision robot transmissions, and thus, RV reducers are gradually replacing harmonic reducers in advanced robot transmissions.
Reasons for the slow development of China's speed reducer industry
Reason 1: Industrial robots are integrated machines, but most domestic reducer suppliers are single-component providers, lacking comprehensive industrial robot companies. Therefore, even if your reducer is good, you'll be blamed if other components malfunction.
Reason 2: The workload, working environment, and even operating temperature of industrial robots in China affect their precision. There's a common misconception that industrial robots are indestructible, but in reality, they also require maintenance and downtime.
Reason three: The inconsistent quality of domestically produced speed reducers has led to some stereotypes about them. Sometimes, one bad apple spoils the whole bunch.
Reason 4: Insufficient technological accumulation and high turnover rate of technical personnel. These precision engineering fields require practical experience and time to accumulate expertise. We have seen some companies focusing on R&D, but compared to Japanese companies, they are still too young.
