Among the speed reducers, planetary speed reducers are widely used in the transmission systems of servo, stepper, and brushless DC control motors (hereinafter referred to as drive motors) due to their advantages such as small size (basically the same diameter as the motor), high transmission efficiency (85-90%), wide reduction range (1 : 3-100), and high precision (small hysteresis). While ensuring precise transmission, they can reduce speed, increase torque, and lower the ratio of load to the drive motor's moment of inertia. However, in practical use, improper installation often leads to failures, with broken shafts in the speed reducer and drive motor being one of the main types of failures. Analyzing the mechanism of shaft breakage helps customers understand how to correctly install planetary speed reducers and better utilize their capabilities.
I. Shaft breakage due to misalignment
Some users have experienced their drive motor's output shaft breaking after the equipment has been running for a period of time. Why does the drive motor's output shaft break? When we carefully observe the cross-section of the broken drive motor's output shaft, we will find that the outer ring of the cross-section is brighter, while the color of the cross-section becomes darker towards the center of the shaft, finally showing the breakage mark (dot-like marks) at the center. This phenomenon is mostly caused by misalignment between the drive motor and the reducer during assembly.
When the concentricity of the drive motor and reducer is well ensured, the drive motor output shaft only bears rotational force (torque), resulting in smooth operation without pulsation. However, when there is misalignment, the drive motor output shaft also bears radial force (bending moment) from the reducer input end. This radial force forces the drive motor output shaft to bend, and the direction of bending changes continuously as the output shaft rotates. If the concentricity error is large, this radial force causes localized temperature increases on the motor output shaft, continuously damaging its metal structure, eventually leading to breakage due to localized fatigue. The greater the concentricity error, the shorter the time it takes for the drive motor output shaft to break. Simultaneously with the breakage of the drive motor output shaft, the reducer input end will also bear radial force from the drive motor output shaft. If this radial force exceeds the maximum radial load that the reducer input end can withstand, it will also result in deformation or even breakage of the reducer input end, or damage to the input end support bearing. Therefore, ensuring concentricity during assembly is crucial!
From an assembly process perspective, if the drive motor shaft and the reducer input end are concentric, the drive motor shaft surface and the reducer input end hole surface will fit together perfectly, with their contact surfaces tightly pressed together, without radial force or deformation space. However, if they are not concentric during assembly, the contact surfaces will not fit together or there will be gaps, resulting in radial force and providing space for deformation.
Similarly, the output shaft of the speed reducer can also break or bend, for the same reasons as the drive motor shaft breakage. However, the output force of the speed reducer is the product of the drive motor output force and the reduction ratio, which is greater than that of the motor. Therefore, the output shaft of the speed reducer is more prone to breakage. Thus, users should pay extra attention to ensuring concentricity during the assembly of the speed reducer's output end!
II. Shaft breakage due to insufficient reducer output
If it is not the drive motor shaft that is broken, but the output shaft of the reducer that is broken, in addition to the possibility of poor concentricity of the reducer output end assembly, there are several other possible reasons.
First, incorrect selection leads to insufficient output power from the selected reducer. Some users mistakenly believe that as long as the rated output torque of the chosen reducer meets the working requirements, it's sufficient. This is not the case. Firstly, the rated output torque of the drive motor multiplied by the speed ratio should, in principle, be less than the corresponding rated output torque provided in the reducer's product catalog. Secondly, the overload capacity of the drive motor and the maximum working torque required in the actual application must also be considered. Theoretically, the maximum working torque required by the user must be less than twice the rated output torque of the reducer. This principle must be strictly adhered to in some applications, not only to protect the internal gears and shafts of the reducer, but more importantly, to prevent the reducer's output shaft from breaking. If these factors are not considered, and the equipment is installed incorrectly, causing the reducer's output shaft to be jammed by the load, the drive motor's overload capacity will still cause it to continuously increase its output until the force on the reducer's output shaft exceeds its maximum output torque, resulting in shaft breakage. If the reducer's rated output torque has a certain margin, then the terrible situation of the output shaft breaking can be avoided.
Secondly, if the instantaneous impact torque on the reducer's output shaft exceeds twice its rated output torque during acceleration and deceleration, and this acceleration and deceleration is too frequent, it will eventually cause the reducer shaft to break. If this occurs, careful calculations should be made to increase the torque margin.
III. Correct Installation of the Gear Reducer
Proper installation, use, and maintenance of the speed reducer are crucial for ensuring the normal operation of mechanical equipment. Therefore, when installing a planetary speed reducer, please strictly follow the installation sequence below and assemble it carefully.
Step 1: Before installation, confirm that the motor and reducer are intact and undamaged, and strictly check whether the dimensions of all parts connecting the drive motor and the reducer match. This refers to the dimensions and fit tolerances between the positioning boss and shaft diameter of the drive motor flange and the positioning groove and hole diameter of the reducer flange; wipe away dirt and burrs from the mating surfaces.
Step 2: Unscrew the plug on the process hole on the side of the reducer flange, rotate the input end of the reducer to align the locking hex socket head cap with the process hole, and insert an Allen wrench to loosen the locking hex socket head cap.
Step 3: Hold the drive motor and align the keyway on its shaft perpendicular to the clamping screw in the input hole of the reducer. Insert the drive motor shaft into the input hole of the reducer. During insertion, ensure that both are concentric and that the two flanges are parallel. If concentricity is inconsistent or the flanges are not parallel, the cause must be identified. Furthermore, never use a hammer during installation. This is to prevent excessive axial or radial force from damaging the bearings and to allow you to judge the fit by feel. The method to determine concentricity and flange parallelism is: after insertion, the flanges should be roughly flush with each other, with consistent gaps.
Step 4: To ensure even stress distribution on the flange connection, first loosely screw on the drive motor mounting screws, but do not tighten them completely. Then, gradually tighten the four mounting screws diagonally. Finally, tighten the clamping screw at the input hole of the reducer. It is crucial to tighten the drive motor mounting screws before tightening the clamping screw at the input hole of the reducer.
Note: The correct installation of a speed reducer with mechanical equipment is similar to the correct installation of a speed reducer with a drive motor. The key is to ensure that the output shaft of the speed reducer is concentric with the input shaft of the driven component.
IV. Conclusion
With the continuous development of control motor applications, planetary gearboxes will be used more and more in the field of motion control transmission. We hope you will ensure the planetary gearbox is installed correctly before use to guarantee the reliable and safe operation of your equipment!
