The total empty return of the gear transmission chain caused by the factors such as the increase of the gear center distance and the shaft runout is defined as the return difference, and the symbol is recorded as B. There is transmission backlash in the gear because there is side clearance in the gear. There are two types of backlash: constant backlash and variable backlash. In the process of gearing transmission, the constant backlash keeps a fixed value, while the variable backlash, on the contrary, changes continuously with the gear transmission.
There are many factors that cause return difference. For constant return difference, it can be divided into primary source and secondary source. The main source is gear size accuracy. Compared with the primary source, the secondary source leads to a smaller return difference and is not obvious. In addition, there are many secondary sources, mainly divided into three categories:
(1) Eccentricity of outer ring of rolling bearing
When installing the rolling bearing, the outer ring of the bearing will be eccentric and the center distance of the gear will change, resulting in backlash.
(2) Radial clearance during part installation
When the gear does not rotate, there is radial clearance on the mating surface of the parts.
(3) Motor installation error
For the precision gear, the servo motor is usually connected with the gear box by bolts, and is matched by two concentric cylindrical surfaces. Therefore, there is a radial gap between the motor and the box, resulting in the transmission backlash of the input gear.
The variable backlash is mainly generated after the gear is installed on the shaft. However, the influence of the deflection caused by the high-speed operation and the thermal deformation caused by the gear material when the temperature changes greatly on the transmission backlash is not obvious, so these two factors are not considered temporarily.
The eccentricity of the bearing inner ring is similar to that of the fixed outer ring. The radial runout of the rotating inner ring causes the change of the rotating center and the variable backlash. The radial runout of the shaft refers to that the radial runout of the shaft segment matched with the gear and rolling bearing will cause backlash and change with the rotation of the shaft. Because of the size error between the gear hole and the matching shaft section, the assembly is eccentric, and the pitch circle of the gear runs out radially, resulting in variable backlash.