Abstract
The methods of adjusting gear shaving to meet both the gear shaving profile and the undercut requirement when gear hobbing undercut is insufficient. By meticulously examining a specific case, this paper provides practical insights into addressing this manufacturing challenge.
Introduction
In the gear manufacturing process, achieving the desired undercut during gear shaving is crucial for maintaining gear tooth root strength. An undercut that is either too large or too small can compromise the gear’s structural integrity, leading to potential failure. This paper presents a detailed analysis of a specific case where the gear shaving process resulted in the complete removal of gear hobbing undercut, and outlines the steps taken to rectify the issue.
Case Study Background
During the shaving process of an NMT active fifth-gear in August 2016, it was observed that there was no undercut on the shaved gear tooth root. Instead, the entire hobbing undercut was removed by the shaving cutter. As specified, the finished gear tooth root should have an undercut ranging from 0.01 to 0.05 mm. Any deviation from this range can adversely affect the gear’s tooth root strength, increasing the risk of tooth breakage.
Figure 1: Gear Root Completely Shaved
<img src=”https://example.com/gear_root_shaved.jpg” />
Analysis of the Root Cause
Upon encountering this issue, the initial focus was on gear hobbing and shaving pairing, attempting to minimize the shaving removal by adjusting the respective M-values. However, this approach yielded no significant results. Further investigation revealed the following root causes:
- Insufficient Hobbing UndercutThe gear hobbing undercut was measured to be less than 0.04 mm, falling short of the required 0.05-0.08 mm range. This was attributed to a tooling quality issue, specifically the manufacture of gear hobbing cutter. Long-term corrective measures involve regrinding the hobbing cutter, but this was not feasible at the time due to production constraints.
- Mismatch in Pressure AnglesThe actual pressure angle (fHa) of the left tooth flank after shaving was 17.8°, close to the upper limit of the specified range of 15±0.03°. Some individual teeth even exceeded the upper limit, reaching 20.2°. Conversely, the pressure angle of gear hobbing process remained near zero, deviating from the specified range of 5±10°. This mismatch resulted in excessive shaving allowance at the tooth root.
- Excessive Tooth Profile CrownThe tooth profile crown of the shaved gear was measured to be 6.5° on the left tooth flank and 7.2° on the right, exceeding the specified range of 4±3°. The combination of the mismatched pressure angles and the excessive tooth profile crown led to excessive shaving allowance at the tooth root.
Table 1: Summary of Root Causes
| Root Cause | Description | Impact |
|---|---|---|
| Insufficient Hobbing Undercut | Less than 0.04 mm, below the required 0.05-0.08 mm range. | Weakened tooth root strength. |
| Mismatch in Pressure Angles | Actual fHa of 17.8°, exceeding the specified range. | Excessive shaving allowance at the tooth root. |
| Excessive Tooth Profile Crown | Measured at 6.5° (left) and 7.2° (right), exceeding range. | Increased shaving allowance at the tooth root. |
Corrective Measures
To address the identified root causes, the following corrective measures were implemented:
Measure 1: Adjusting the Shaving Cutter
The shaving cutter was regrinded to increase its pressure angle to approximately 7.5°, deviating from the specified range to optimize the match between the gear hobbing and shaving pressure angles. This adjustment aimed to minimize the shaving allowance at the tooth root while ensuring proper shaving of the tooth tip and pitch circle.
Table 2: Adjusted Shaving Cutter Parameters
| Parameter | Original Value | Adjusted Value |
|---|---|---|
| Pressure Angle (fHa) | 15±0.03° | ~7.5° |
| Tooth Profile Crown | 4±3° | Reduced |
Measure 2: Minimizing the Tooth Profile Crown
The shaving cutter was further regrinded to reduce the tooth profile crown. Experiments showed that, with the same pressure angle and pitch circle shaving allowance, a smaller tooth profile crown resulted in less shaving allowance at the tooth root. In this case, the tooth profile crown was adjusted to within the range of 0-2°.
Adjustment Methods
After implementing the above measures, the shaved gear profile showed significant improvement, particularly on the left tooth flank. However, a slight step was still visible at the tooth root. To eliminate this step, the following adjustment methods were employed:
(1) Adjusting Hobbing and Shaving Pairing
Adjusting gear hobbing and shaving allowances is an effective means of addressing minor shaving steps. In this case, the hobbing M-value was reduced by 0.05 mm on the middle tolerance, and the shaving M-value was increased by 0.01 mm on the middle tolerance. This adjustment reduced the shaving allowance by 0.01 mm in the tooth thickness direction.
Table 3: Adjusted Hobbing and Shaving Pairing
| Parameter | Original Value | Adjusted Value |
|---|---|---|
| Hobbing M-Value | – | Reduced by 0.05 mm |
| Shaving M-Value | – | Increased by 0.01 mm |
(2) Adjusting Helix Angle and Fine-Tuning
After adjusting the shaving cutter and hobbing and shaving pairing, the shaved gear showed an undercut, but the left and right undercuts were asymmetric. Additionally, the tooth profile crown was slightly small. To address these issues, the tooth flank deviation (fHb) of the shaving machine was fine-tuned. This adjustment allowed for slight changes in the pressure angle, symmetry of the left and right undercuts, and tooth profile crown. Repeated adjustments were made until the desired tooth profile shape was achieved.
Conclusion
The issue of the gear tooth root being completely shaved due to insufficient hobbing undercut was successfully resolved through a combination of measures. The root causes, including insufficient hobbing undercut, mismatch in pressure angles, and excessive tooth profile crown, were identified and addressed. By adjusting the shaving cutter, minimizing the tooth profile crown, fine-tuning gear hobbing and shaving pairing, and adjusting the helix angle, the desired gear profile shape was achieved. This experience highlights the importance of closely monitoring and adjusting gear manufacturing parameters to ensure optimal gear performance.
In gear manufacturing, parameters such as fHα, fHβ, Cα, Cβ, and the tooth profile root shape are critical. Any deviation in these parameters can significantly impact gear performance. Therefore, constant monitoring and adjustment are necessary to maintain gear strength and reduce meshing noise.