1.First,choose the speed ratio that you need.
Second,multiply the rated torque of the servo motor you select by the speed ratio,the resulting value, in principle,should be less than the rated output torque of the similar reducer you select in the catalog book,at the same time, the overload capacity of the drive motor and the maximum operating torque required in practice should be considered.The maximum operating torque required is less than 2 times the rated output torque,if meeting above conditions,please select the minimum volume planetary reducer , generally,small planetary reducer, cost is lower accordingly.
3.The next step is to consider the backlash of the planetary reducer.The smaller the backlash, the higher the accuracy and the higher the cost.The user has to choose the reducers that meet their accuracy requirements.You also need to consider transverse/radial forces and average lifetime. Reducer with bigger Lateral/radial force,the reliability is better during the installation and using, not easy to go wrong,generally, the average lifetime is much longer than that of the matched servo motor.
4.Finally, you need to consider the motor weight.A reducer is only allowed to be matched with a motor of less than a certain weight. If the motor is too heavy, the input flange of the reducer will be damaged if it runs for a long time.