Case Study of High-Speed Shaft Fracture in Gearbox

Below are two case studies.

Case Study 1,

Gearbox Model: H3RV4B desilification tank gearbox,

Fault: High-speed shaft fracture.

Cause Analysis :

The shaft fracture of this gearbox occurred at the outer end of the bearing. In the high-speed shaft transmission, the outer end of the bearing, the inner end of the coupling, the keyway, and the shoulder are all areas where stress is easily concentrated. This is a defect that cannot be perfectly overcome by mechanical design. Even if optimization measures are adopted in the later design, it cannot be absolutely improved. Therefore, the common fracture locations are either the keyway and shoulder or the inner end of the coupling and the outer end of the bearing.

The high-speed fracture of this gearbox is mainly due to fatigue fracture. From the observation of the fracture, in addition to stress fatigue, the surface of the static fracture area is rough, with one or more textures, indicating that the forging and heat treatment of this part also have room for optimization. The main cause of the fracture is still stress fatigue, which is directly related to the manufacturing accuracy and installation of the pulley.

Points to note in the future: 1. Select high-quality and reputable pulleys; 2. Pay attention to the wear of the pulley installation position; 3. Pay attention to the appropriate tension of the pulley; 4. Regularly replace the V-belts, all four must be replaced together. If the pulley groove is worn or slips, it is recommended to replace it with a new pulley.

Pulley transmission itself has a relatively high risk of shaft fracture. This is a transmission defect, and there is no good way to completely avoid it. What we need to do later is what is mentioned above. Four points.

  Case Study 2

Gearbox Model: DCY355-25-4N belt conveyor gearbox.

Fault: High-speed shaft fracture.

 

Cause Analysis:

This is a typical fracture at the root of the keyway, due to design reasons, but mainly caused by poor alignment of the brake wheel and brake or excessive deviation of the centrifugal force. This situation is common on high-speed shafts with brake wheels, requiring that the forces acting on the shaft on both sides during braking of the gearbox must be as equal as possible. It is also necessary to pay attention to the timely cleaning of dust accumulation in the recesses of the brake wheel. Dust accumulation will increase the centrifugal force of the brake wheel, which is harmful.

Several inducing factors for high-speed shaft fracture with brake wheel:

1. Poor coupling alignment, leading to shaft bending and stress concentration. This mainly manifests in the weak positions of the bent shaft shoulder, shaft platform, and keyway.

2. Brake wheel damage or brake wheel dust accumulation, leading to increased centrifugal force of the brake wheel and greater甩出的力更大.

3. Poor alignment of the brake wheel. Most maintenance personnel may not pay attention to the alignment relationship between the brake wheel and the brake. In fact, this poor alignment is the cause of most fatigue shaft fractures. In each braking contact process, the brake shoe and brake wheel must be consistent in color, consistent in force, and consistent in wear on both sides of the brake shoe. The consistency of these three points is the indicator for checking the alignment of the brake and brake wheel.

4. Poor pre-tightening of the brake wheel installation position, specifically manifested as loose brake wheel installation or too tight tolerance, resulting in stress generation in the alternating loose and tight area.

5. Improper cooling position or method of the brake wheel after hot installation, resulting in excessive cold and hot alternation in one area.

6. Loaded start or heavy load operation, emergency stop.

From the fracture surface, this gearbox belongs to fatigue fracture. From the initiation area to the propagation area and then to the instantaneous fracture area, the entire section is relatively obvious. However, the fracture propagation area is relatively rough and has certain edges and corners. Therefore, it can be analyzed that the time from the occurrence of the fracture to the fracture is relatively short, and the instantaneous fracture area is large, indicating that the stress affecting the shaft is not small. Based on this phenomenon, poor alignment or rocking force should be the main reason for the formation of this fracture. Judging from the two locations of the instantaneous fracture area in the picture, forming a 180-degree angle, the alignment between the brake wheel and brake shoe should be problematic. The fracture location is exactly at the shoulder chamfer, around the chamfer, indicating that there is still room for optimization of the shoulder, and higher requirements need to be put forward for the processing of the shaft.

In the future, we need to pay attention to the alignment of the two positions, the installation of the brake wheel, the cooling method, the cleaning of dust accumulation, and the wear and replacement of the brake shoe.

 

 

As shown in the figure above, the shaft fracture in Case Study 1 occurred at B, and the shaft fracture in Case Study 2 occurred at A. The fractures at these two locations are common locations for shaft fractures and are typical damage cases. The following suggestions are provided on how to avoid high-speed shaft fractures in gearboxes.

1. Shaft design: Fully consider stress relief, shaft diameter, tolerance matching, chamfering, material, and flaw detection.

2. Installation considerations: Alignment, wear, dynamic balance, weight should not be too large, parts quality, and installation base calibration.

3. Usage notes: Load, emergency stop, regular measurement and verification of data, cleaning, and regular inspection and fixing.

 

Thank you for reading. !