In the maintenance of standard multiple unit trains, we encountered instability in the bearing clearance of the small gear shaft during assembly and running-in processes. This issue directly impacts the performance and reliability of the gearbox, which utilizes a combination of cylindrical and four-point contact ball bearings on the gear shaft. The clearance of the gear shaft bearings, particularly those involving cylindrical and ball configurations, is critical for ensuring optimal load distribution and minimizing wear. Through theoretical analysis and experimental investigations, we identified that the assembly of the sealing cover significantly influences the bearing clearance. This paper presents our comprehensive study on the factors affecting the gear shaft bearing clearance, the optimization of assembly processes, and the validation of these improvements through comparative measurements before and after running-in tests.
The gearbox in question features a small gear shaft with a cylindrical roller bearing on the motor side (PM side) to handle radial loads, and a combination of a cylindrical roller bearing and a four-point contact ball bearing on the wheel side (PW side). The cylindrical roller bearing on the PW side is installed with an interference fit, while the four-point contact ball bearing, which accommodates axial loads, has a clearance fit with the bearing housing. The inner ring of the ball bearing is secured by a pressure plate, and the outer ring is fixed by a sealing cover. During high-level maintenance cycles, such as the fourth-level repair, the gearbox is disassembled to its smallest components, with bearings replaced and reassembled. The clearance of the gear shaft bearings must be measured post-assembly and after running-in, with requirements for minimal variation and adherence to standards. However, we observed inconsistencies in the gear shaft bearing clearance, prompting a detailed investigation into the root causes.
We hypothesized that several factors could contribute to the instability of the gear shaft bearing clearance, including the measurement methodology, improper assembly of components, and the radial distribution of the ball bearing outer ring. To test these, we conducted a series of experiments under controlled conditions. The assembly process for the gear shaft involves multiple steps, such as thermal fitting of bearing outer rings, assembly of inner rings, and installation of sealing covers, with critical measurements taken at various stages to ensure proper fit. For instance, dimensions labeled X, Y, P, and Q are measured to verify the compression of the ball bearing rings, where X and Y relate to the outer ring, and P and Q to the inner ring. The difference X-Y must fall within specified limits to guarantee stable clearance. Our initial trials revealed that the assembly of the sealing cover introduced unpredictable variations in the gear shaft bearing clearance, leading us to focus on this aspect.
In one experiment, we evaluated the effect of measurement orientation—horizontal versus vertical positioning—on the gear shaft bearing clearance. We measured the clearance of multiple gearboxes in both configurations and found negligible differences, as summarized in Table 1. This indicated that the orientation during measurement was not a primary factor affecting the gear shaft stability.
| Gearbox ID | Horizontal Clearance (mm) | Vertical Clearance (mm) | Difference (mm) |
|---|---|---|---|
| 0000 | A | A + 0.001 | 0.001 |
| 1111 | B | B – 0.001 | -0.001 |
Another experiment examined the impact of spacer ring misalignment on the gear shaft bearing clearance. When spacer rings were intentionally installed with tilt, the clearance measurements deviated from specifications. However, reassembling the sealing cover often corrected this, suggesting that the sealing cover assembly could compensate for minor misalignments. We verified this by disassembling and inspecting the spacer rings, ensuring a gap of less than 0.03 mm with a feeler gauge around the entire circumference. After proper alignment, the gear shaft bearing clearance remained stable post-assembly and after sealing cover reinstallation, as shown in Table 2.
| Step | Spacer Condition | Clearance Status (Sample Gearboxes) |
|---|---|---|
| 1 | Misaligned | All不合格 (Out of spec) |
| 2 | Post-cover assembly | All合格 (Within spec) |
| 3 | Aligned (no tilt) | — |
| 4 | Post-reassembly | All合格 |
| 5 | Post-cover reinstallation | All合格 |
We further investigated the relationship between the radial position of the four-point contact ball bearing outer ring and the gear shaft bearing clearance. By inserting shims of varying thicknesses (e.g., 1.2 mm, 0.98 mm, and 0.73 mm) between the outer ring and the bearing housing, we manipulated the radial distribution. The X-values, representing the distance from the outer ring to the bearing housing, were measured at four points to ensure uniformity. The results, summarized in Table 3, demonstrated that larger shim thicknesses correlated with greater clearance variations, emphasizing the importance of concentricity between the outer and inner rings for the gear shaft.
| Shim Thickness (mm) | X-value Uniformity | X-Y Difference | Clearance Change Post-Assembly |
|---|---|---|---|
| 1.2 | Within tolerance | Within spec | Positive deviation (A1 – B1 – C1 > 0) |
| 0.98 | Within tolerance | Within spec | Positive deviation (A1 – B1 > 0) |
| 0.73 | Within tolerance | Within spec | Positive deviation (A1 > 0) |
The radial clearance of the four-point contact ball bearing, which directly affects the gear shaft performance, can be described by the formula: $$G_r = 2 D_W \cos \beta$$ where \(G_r\) is the radial clearance, \(D_W\) is the ball diameter, and \(\beta\) is the contact angle between the balls and the raceways. This formula highlights that variations in \(\beta\), influenced by the concentricity of the outer and inner rings, can lead to clearance instability. In our tests, when the outer ring was not concentric due to assembly forces, the contact angle \(\beta\) changed, altering the effective clearance. The assembly of the sealing cover applied additional radial forces, which could either stabilize or destabilize the gear shaft bearing clearance depending on the magnitude and distribution.
Based on these findings, we optimized the sealing cover assembly process to minimize its impact on the gear shaft. The revised procedure involves a two-stage assembly with a three-step tightening sequence: first, the sealing cover is installed without sealant and tightened in steps (pre-tighten to compress the spring washer, then to half the final torque, and finally to the full torque, e.g., A2 N·m). After assembling the PM and PW side bearing covers, the sealing cover is removed, sealant is applied, and it is reinstalled using the same tightening sequence. This method reduces the introduction of uneven forces and promotes better alignment of the ball bearing outer ring, thereby stabilizing the gear shaft bearing clearance.
To assess the effectiveness of this optimization, we analyzed the X-value measurements, which indicate the concentricity of the ball bearing outer ring. Prior to optimization, the variation in X-values around the circumference was up to 0.010 mm, leading to significant clearance changes between initial assembly and running-in. After optimization, the X-value variation reduced to 0.004 mm, and the clearance difference pre- and post-running-in decreased to 0.004 mm, demonstrating improved stability for the gear shaft. Comparative data from multiple gearboxes are presented in Table 4, showing the reduction in clearance variability.
| Condition | X-value Variation (mm) | Clearance Change Pre- to Post-Running-In (mm) | Number of Gearboxes Assessed |
|---|---|---|---|
| Before Optimization | 0.010 | -0.06 to 0.10 | 16 |
| After Optimization | 0.004 | -0.03 to 0.04 | 16 |
Our results confirm that the optimized assembly process effectively controls the gear shaft bearing clearance by ensuring better concentricity and reducing external forces during sealing cover installation. The relationship between the outer ring position and the contact angle \(\beta\) is crucial; maintaining a stable \(\beta\) through precise assembly minimizes clearance fluctuations. The formula for radial clearance underscores that even small changes in \(\beta\) can amplify variations, which is why controlling the assembly parameters is vital for the gear shaft integrity.
In conclusion, through systematic experimentation and process refinement, we have addressed the instability in the gear shaft bearing clearance in standard multiple unit trains. The key insight is that the sealing cover assembly plays a pivotal role in influencing the radial distribution of the ball bearing outer ring, which in turn affects the contact angle and clearance. By implementing a two-stage, controlled tightening process, we achieved a more consistent gear shaft performance, with clearance measurements remaining within acceptable limits after running-in. This approach not only enhances the reliability of gearbox maintenance but also provides a framework for future improvements in high-speed train components. As mileage accumulates and more components undergo advanced repairs, data-driven optimizations like this will be essential for sustaining high-quality standards in both manufacturing and maintenance of the gear shaft systems.