For the coaxial reducer, the input shaft and output shaft are on the same rotary shaft. The coaxial reducer is usually designed as a gear reducer. At present, various types of gear reducers exist in industrial applications.
In a cylindrical gear reducer with a simple structure, the shafts of the cylindrical gears are parallel to each other and are fixed. The rotation direction of the input shaft and the output shaft is determined according to the number of gear pairs, and the direction can be the same or opposite.

Although cylindrical gear reducers are not coaxial reducers in principle, coaxiality can also be achieved through two gear pairs (four gears in total). Two of the gears are parallel to a shaft.
A special type of coaxial reducer is the planetary reducer. Unlike the cylindrical gear reducer, not all shafts of the planetary reducer are fixed. One of the cylindrical gears is designed as an internal tooth. When using a planetary reducer, multiple cylindrical gears move around a concentric circle between the inner gear and the outer gear. Therefore, the planetary reducer is usually also called a planetary gear reducer. The special structure of this type of coaxial reducer specifies the coaxial layout of the input shaft and the output shaft.
Another special feature of the planetary reducer is that it does not require all cylindrical gears to rotate around their own rotary axis to transmit torque. Generally, the internal tooth cylindrical gear is fixed, so the input and output shafts run in the same direction.
Compared with a simple cylindrical gear reducer, the power density of a planetary reducer is extremely high. By distributing the torque to the rotating cylindrical gear, multiple tooth meshing can be performed to achieve extremely high power density. In this way, a compact structure can be realized and an ultra-high efficiency can be ensured. In addition, multi-stage transmission ratios can be realized through a simple serial arrangement in the same internal tooth cylindrical gear. According to the single gear ratio of each planetary grade, the total gear ratio can be obtained.
The conversion of speed and torque is similar to a cylindrical gear reducer. For example, when the gear ratio is 10:1, if the torque increases by 10, the speed decreases by the corresponding factor.
Advantages of coaxial reducer:
Input shaft and output shaft are coaxial;
Extremely efficient;
Very high power density;
Compact structure;
Wide range of applications;
Multi-stage reducer can be realized with extremely high transmission ratio.